Diesel Locomotive TechnologyContentsThe Diesel Locomotive - The Diesel Engine - Diesel Engine Types - Size Does Count - To V ornot to V - Tractive Effort, Pull and Power - Starting - Governor - Fuel Injection - Fuel Control -Engine Control Development - Power Control - Cooling - Lubrication - Transmission - Parts ofa Diesel-Electric Locomotive - Mechanical Transmission - Hydraulic Transmission - Wheel Slip- DMUs - More Information (Links).The Diesel LocomotiveThe modern diesel locomotive is a self contained version of the electric locomotive. Like theelectric locomotive, it has electric drive, in the form of traction motors driving the axles andcontrolled with electronic controls. It also has many of the same auxiliary systems for cooling,lighting, heating, braking and hotel power (if required) for the train. It can operate over the sameroutes (usually) and can be operated by the same drivers. It differs principally in that it carriesits own generating station around with it, instead of being connected to a remote generatingstation through overhead wires or a third rail. The generating station consists of a large dieselengine coupled to an alternator producing the necessary electricity. A fuel tank is also essential.It is interesting to note that the modern diesel locomotive produces about 35% of the power of aelectric locomotive of similar weight.The UK Class 47 is typical of the general New SD90MAC 6,000 hp heavy freight USpurpose diesel-electric locomotives introduced diesel-electric locomotives with AC drive firstin the 1960s. built in 1998Click on an image for the full size view.Parts of a Diesel-Electric LocomotiveThe following diagram shows the main parts of a US-built diesel-electric locomotive. Click onthe part name for a description.Diesel EngineThis is the main power source for the locomotive. It comprises a large cylinder block, with thecylinders arranged in a straight line or in a V (see more here). The engine rotates the drive shaftat up to 1,000 rpm and this drives the various items needed to power the locomotive. As the
transmission is electric, the engine is used as the power source for the electricity generator oralternator, as it is called nowadays.Main AlternatorThe diesel engine drives the main alternator which provides the power to move the train. Thealternator generates AC electricity which is used to provide power for the traction motorsmounted on the trucks (bogies). In older locomotives, the alternator was a DC machine, called agenerator. It produced direct current which was used to provide power for DC traction motors.Many of these machines are still in regular use. The next development was the replacement ofthe generator by the alternator but still using DC traction motors. The AC output is rectified togive the DC required for the motors. For more details on AC and DC traction, see the ElectronicPower Page on this site.Auxiliary AlternatorLocomotives used to operate passenger trains are equipped with an auxiliary alternator. Thisprovides AC power for lighting, heating, air conditioning, dining facilities etc. on the train. Theoutput is transmitted along the train through an auxiliary power line. In the US, it is known as"head end power" or "hotel power". In the UK, air conditioned passenger coaches get what iscalled electric train supply (ETS) from the auxiliary alternator.Motor BlowerThe diesel engine also drives a motor blower. As its name suggests, the motor blower providesair which is blown over the traction motors to keep them cool during periods of heavy work.The blower is mounted inside the locomotive body but the motors are on the trucks, so theblower output is connected to each of the motors through flexible ducting. The blower outputalso cools the alternators. Some designs have separate blowers for the group of motors on eachtruck and others for the alternators. Whatever the arrangement, a modern locomotive has acomplex air management system which monitors the temperature of the various rotatingmachines in the locomotive and adjusts the flow of air accordingly.Air IntakesThe air for cooling the locomotives motors is drawn in from outside the locomotive. It has to befiltered to remove dust and other impurities and its flow regulated by temperature, both insideand outside the locomotive. The air management system has to take account of the wide range oftemperatures from the possible +40°C of summer to the possible -40°C of winter.Rectifiers/InvertersThe output from the main alternator is AC but it can be used in a locomotive with either DC orAC traction motors. DC motors were the traditional type used for many years but, in the last 10years, AC motors have become standard for new locomotives. They are cheaper to build andcost less to maintain and, with electronic management can be very finely controlled. To see
more on the difference between DC and AC traction technology try the Electronic Power Pageon this site.To convert the AC output from the main alternator to DC, rectifiers are required. If the motorsare DC, the output from the rectifiers is used directly. If the motors are AC, the DC output fromthe rectifiers is converted to 3-phase AC for the traction motors.In the US, there are some variations in how the inverters are configured. GM EMD relies on oneinverter per truck, while GE uses one inverter per axle - both systems have their merits. EMDssystem links the axles within each truck in parallel, ensuring wheel slip control is maximisedamong the axles equally. Parallel control also means even wheel wear even between axles.However, if one inverter (i.e. one truck) fails then the unit is only able to produce 50 per cent ofits tractive effort. One inverter per axle is more complicated, but the GE view is that individualaxle control can provide the best tractive effort. If an inverter fails, the tractive effort for thataxle is lost, but full tractive effort is still available through the other five inverters. Bycontrolling each axle individually, keeping wheel diameters closely matched for optimumperformance is no longer necessary. This paragraph sourced from e-mail by unknowncorrespondent 3 November 1997.Electronic ControlsAlmost every part of the modern locomotives equipment has some form of electronic control.These are usually collected in a control cubicle near the cab for easy access. The controls willusually include a maintenance management system of some sort which can be used to downloaddata to a portable or hand-held computer.Control StandThis is the principal man-machine interface, known as a control desk in the UK or control standin the US. The common US type of stand is positioned at an angle on the left side of the drivingposition and, it is said, is much preferred by drivers to the modern desk type of control layoutusual in Europe and now being offered on some locomotives in the US.BatteriesJust like an automobile, the diesel engine needs a battery to start it and to provide electricalpower for lights and controls when the engine is switched off and the alternator is not running.CabMost US diesel locomotives have only one cab but the practice in Europe is two cabs. USfreight locos are also designed with narrow engine compartments and walkways along eitherside. This gives a reasonable forward view if the locomotive is working "hood forwards". USpassenger locos, on the other hand have full width bodies and more streamlined ends but stillusually with one cab. In Europe, it is difficult to tell the difference between a freight andpassenger locomotive because the designs are almost all wide bodied and their use is oftenmixed.
Traction MotorSince the diesel-electric locomotive uses electric transmission, traction motors are provided onthe axles to give the final drive. These motors were traditionally DC but the development ofmodern power and control electronics has led to the introduction of 3-phase AC motors. For adescription of how this technology works, go to the Electronic Power Page on this site. Thereare between four and six motors on most diesel-electric locomotives. A modern AC motor withair blowing can provide up to 1,000 hp.Pinion/GearThe traction motor drives the axle through a reduction gear of a range between 3 to 1 (freight)and 4 to 1 (passenger).Fuel TankA diesel locomotive has to carry its own fuel around with it and there has to be enough for areasonable length of trip. The fuel tank is normally under the loco frame and will have acapacity of say 1,000 imperial gallons (UK Class 59, 3,000 hp) or 5,000 US gallons in a GeneralElectric AC4400CW 4,400 hp locomotive. The new AC6000s have 5,500 gallon tanks. Inaddition to fuel, the locomotive will carry around, typically about 300 US gallons of coolingwater and 250 gallons of lubricating oil for the diesel engine.Air reservoirs are also required for the train braking and some other systems on the locomotive.These are often mounted next to the fuel tank under the floor of the locomotive.Air CompressorThe air compressor is required to provide a constant supply of compressed air for the locomotiveand train brakes. In the US, it is standard practice to drive the compressor off the diesel enginedrive shaft. In the UK, the compressor is usually electrically driven and can therefore bemounted anywhere. The Class 60 compressor is under the frame, whereas the Class 37 has thecompressors in the nose.Drive ShaftThe main output from the diesel engine is transmitted by the drive shaft to the alternators at oneend and the radiator fans and compressor at the other end.Gear BoxThe radiator and its cooling fan is often located in the roof of the locomotive. Drive to the fan istherefore through a gearbox to change the direction of the drive upwards.Radiator and Radiator Fan
The radiator works the same way as in an automobile. Water is distributed around the engineblock to keep the temperature within the most efficient range for the engine. The water is cooledby passing it through a radiator blown by a fan driven by the diesel engine. See Cooling formore information.Turbo ChargingThe amount of power obtained from a cylinder in a diesel engine depends on how much fuel canbe burnt in it. The amount of fuel which can be burnt depends on the amount of air available inthe cylinder. So, if you can get more air into the cylinder, more fuel will be burnt and you willget more power out of your ignition. Turbo charging is used to increase the amount of airpushed into each cylinder. The turbocharger is driven by exhaust gas from the engine. This gasdrives a fan which, in turn, drives a small compressor which pushes the additional air into thecylinder. Turbocharging gives a 50% increase in engine power.The main advantage of the turbocharger is that it gives more power with no increase in fuel costsbecause it uses exhaust gas as drive power. It does need additional maintenance, however, sothere are some type of lower power locomotives which are built without it.Sand BoxLocomotives always carry sand to assist adhesion in bad rail conditions. Sand is not oftenprovided on multiple unit trains because the adhesion requirements are lower and there arenormally more driven axles.Truck FrameThis is the part (called the bogie in the UK) carrying the wheels and traction motors of thelocomotive. More information is available at the Bogie Parts Page or the Wheels and BogiesPage on this site.WheelThe best page for information on wheels is the Wheels and Bogies Page on this site.Mechanical TransmissionA diesel-mechanical locomotive is the simplest type of diesel locomotive. As the name suggests,a mechanical transmission on a diesel locomotive consists a direct mechanical link between thediesel engine and the wheels. In the example below, the diesel engine is in the 350-500 hp rangeand the transmission is similar to that of an automobile with a four speed gearbox. Most of theparts are similar to the diesel-electric locomotive but there are some variations in designmentioned below.
Fluid CouplingIn a diesel-mechanical transmission, the main drive shaft is coupled to the engine by a fluidcoupling. This is a hydraulic clutch, consisting of a case filled with oil, a rotating disc withcurved blades driven by the engine and another connected to the road wheels. As the engineturns the fan, the oil is driven by one disc towards the other. This turns under the force of the oiland thus turns the drive shaft. Of course, the start up is gradual until the fan speed is almostmatched by the blades. The whole system acts like an automatic clutch to allow a graduated startfor the locomotive.GearboxThis does the same job as that on an automobile. It varies the gear ratio between the engine andthe road wheels so that the appropriate level of power can be applied to the wheels. Gear changeis manual. There is no need for a separate clutch because the functions of a clutch are alreadyprovided in the fluid coupling.Final DriveThe diesel-mechanical locomotive uses a final drive similar to that of a steam engine. Thewheels are coupled to each other to provide more adhesion. The output from the 4-speedgearbox is coupled to a final drive and reversing gearbox which is provided with a transversedrive shaft and balance weights. This is connected to the driving wheels by connecting rods.
Hydraulic TransmissionHydraulic transmission works on the same principal as the fluid coupling but it allows a widerrange of "slip" between the engine and wheels. It is known as a "torque converter". When thetrain speed has increased sufficiently to match the engine speed, the fluid is drained out of thetorque converter so that the engine is virtually coupled directly to the locomotive wheels. It isvirtually direct because the coupling is usually a fluid coupling, to give some "slip". Higherspeed locomotives use two or three torque converters in a sequence similar to gear changing in amechanical transmission and some have used a combination of torque converters and gears.Some designs of diesel-hydraulic locomotives had two diesel engines and two transmissionsystems, one for each bogie. The design was poplar in Germany (the V200 series oflocomotives, for example) in the 1950s and was imported into parts of the UK in the 1960s.However, it did not work well in heavy or express locomotive designs and has largely beenreplaced by diesel-electric transmission.Wheel SlipWheels slip is the bane of the driver trying to get a train away smoothly. The tenuous contactbetween steel wheel and steel rail is one of the weakest parts of the railway system.Traditionally, the only cure has been a combination of the skill of the driver and the selective useof sand to improve the adhesion. Today, modern electronic control has produced a very effectiveanswer to this age old problem. The system is called creep control.Extensive research into wheel slip showed that, even after a wheelset starts to slip, there is still aconsiderable amount of useable adhesion available for traction. The adhesion is available up to apeak, when it will rapidly fall away to an uncontrolled spin. Monitoring the early stages of slipcan be used to adjust the power being applied to the wheels so that the adhesion is kept withinthe limits of the "creep" towards the peak level before the uncontrolled spin sets in.The slip is measured by detecting the locomotive speed by Doppler radar (instead of the usualmethod using the rotating wheels) and comparing it to the motor current to see if the wheelrotation matches the ground speed. If there is a disparity between the two, the motor current isadjusted to keep the slip within the "creep" range and keep the tractive effort at the maximumlevel possible under the creep conditions.Diesel Multiple Units (DMUs)The diesel engines used in DMUs work on exactly the same principles as those used inlocomotives, except that the transmission is normally mechanical with some form of gear changesystem. DMU engines are smaller and several are used on a train, depending on theconfiguration. The diesel engine is often mounted under the car floor and on its side because ofthe restricted space available. Vibration being transmitted into the passenger saloon has alwaysbeen a problem but some of the newer designs are very good in this respect.There are some diesel-electric DMUs around and these normally have a separate enginecompartment containing the engine and the generator or alternator.
The Diesel EngineThe diesel engine was first patented by Dr Rudolf Diesel (1858-1913) in Germany in 1892 andhe actually got a successful engine working by 1897. By 1913, when he died, his engine was inuse on locomotives and he had set up a facility with Sulzer in Switzerland to manufacture them.His death was mysterious in that he simply disappeared from a ship taking him to London.The diesel engine is a compression-ignition engine, as opposed to the petrol (or gasoline) engine,which is a spark-ignition engine. The spark ignition engine uses an electrical spark from a"spark plug" to ignite the fuel in the engines cylinders, whereas the fuel in the diesel enginescylinders is ignited by the heat caused by air being suddenly compressed in the cylinder. At thisstage, the air gets compressed into an area 1/25th of its original volume. This would beexpressed as a compression ratio of 25 to 1. A compression ratio of 16 to 1 will give an airpressure of 500 lbs/in² (35.5 bar) and will increase the air temperature to over 800°F (427°C).The advantage of the diesel engine over the petrol engine is that it has a higher thermal capacity(it gets more work out of the fuel), the fuel is cheaper because it is less refined than petrol and itcan do heavy work under extended periods of overload. It can however, in a high speed form, besensitive to maintenance and noisy, which is why it is still not popular for passengerautomobiles.Diesel Engine TypesThere are two types of diesel engine, the two-stroke engine and the four-stroke engine. As thenames suggest, they differ in the number of movements of the piston required to complete eachcycle of operation. The simplest is the two-stroke engine. It has no valves. The exhaust fromthe combustion and the air for the new stroke is drawn in through openings in the cylinder wallas the piston reaches the bottom of the downstroke. Compression and combustion occurs on theupstroke. As one might guess, there are twice as many revolutions for the two-stroke engine asfor equivalent power in a four-stroke engine.The four-stroke engine works as follows: Downstroke 1 - air intake, upstroke 1 - compression,downstroke 2 - power, upstroke 2 - exhaust. Valves are required for air intake and exhaust,usually two for each. In this respect it is more similar to the modern petrol engine than the 2-stroke design.In the UK, both types of diesel engine were used but the 4-stroke became the standard. The UKClass 55 "Deltic" (not now in regular main line service) unusually had a two-stroke engine. Inthe US, the General Electric (GE) built locomotives have 4-stroke engines whereas GeneralMotors (GM) always used 2-stroke engines until the introduction of their SD90MAC 6000 hp "Hseries" engine, which is a 4-stroke design.The reason for using one type or the other is really a question of preference. However, it can besaid that the 2-stroke design is simpler than the 4-stroke but the 4-stroke engine is more fuelefficient.Size Does Count
Basically, the more power you need, the bigger the engine has to be. Early diesel engines wereless than 100 horse power (hp) but today the US is building 6000 hp locomotives. For a UKlocomotive of 3,300 hp (Class 58), each cylinder will produce about 200 hp, and a modernengine can double this if the engine is turbocharged.The maximum rotational speed of the engine when producing full power will be about 1000 rpm(revolutions per minute) and the engine will idle at about 400 rpm. These relatively low speedsmean that the engine design is heavy, as opposed to a high speed, lightweight engine. However,the UK HST (High Speed Train, developed in the 1970s) engine has a speed of 1,500 rpm andthis is regarded as high speed in the railway diesel engine category. The slow, heavy engineused in railway locomotives will give low maintenance requirements and an extended life.There is a limit to the size of the engine which can be accommodated within the railway loadinggauge, so the power of a single locomotive is limited. Where additional power is required, it hasbecome usual to add locomotives. In the US, where freight trains run into tens of thousands oftons weight, four locomotives at the head of a train are common and several additional ones inthe middle or at the end are not unusual.To V or not to VDiesel engines can be designed with the cylinders "in-line", "double banked" or in a "V". Thedouble banked engine has two rows of cylinders in line. Most diesel locomotives now have Vform engines. This means that the cylinders are split into two sets, with half forming one side ofthe V. A V8 engine has 4 cylinders set at an angle forming one side of the V with the other setof four forming the other side. The crankshaft, providing the drive, is at the base of the V. TheV12 was a popular design used in the UK. In the US, V16 is usual for freight locomotives andthere are some designs with V20 engines.Engines used for DMU (diesel multiple unit) trains in the UK are often mounted under the floorof the passenger cars. This restricts the design to in-line engines, which have to be mounted ontheir side to fit in the restricted space.An unusual engine design was the UK 3,300 hp Class 55 locomotive, which had the cylindersarranged in three sets of opposed Vs in an triangle, in the form of an upturned delta, hence thename "Deltic".Tractive Effort, Pull and PowerBefore going too much further, we need to understand the definitions of tractive effort, drawbarpull and power. The definition of tractive effort (TE) is simply the force exerted at the wheel rimof the locomotive and is usually expressed in pounds (lbs) or kilo Newtons (kN). By the time thetractive effort is transmitted to the coupling between the locomotive and the train, the drawbarpull, as it is called will have reduced because of the friction of the mechanical parts of the driveand some wind resistance.Power is expressed as horsepower (hp) or kilo Watts (kW) and is actually a rate of doing work.A unit of horsepower is defined as the work involved by a horse lifting 33,000 lbs one foot in
one minute. In the metric system it is calculated as the power (Watts) needed when one Newtonof force is moved one metre in one second. The formula is P = (F*d)/t where P is power, F isforce, d is distance and t is time. One horsepower equals 746 Watts.The relationship between power and drawbar pull is that a low speed and a high drawbar pull canproduce the same power as high speed and low drawbar pull. If you need to increase highertractive effort and high speed, you need to increase the power. To get the variations needed by alocomotive to operate on the railway, you need to have a suitable means of transmission betweenthe diesel engine and the wheels.One thing worth remembering is that the power produced by the diesel engine is not all availablefor traction. In a 2,580 hp diesel electric locomotive, some 450 hp is lost to on-board equipmentlike blowers, radiator fans, air compressors and "hotel power" for the train.StartingA diesel engine is started (like an automobile) by turning over the crankshaft until the cylinders"fire" or begin combustion. The starting can be done electrically or pneumatically. Pneumaticstarting was used for some engines. Compressed air was pumped into the cylinders of the engineuntil it gained sufficient speed to allow ignition, then fuel was applied to fire the engine. Thecompressed air was supplied by a small auxiliary engine or by high pressure air cylinders carriedby the locomotive.Electric starting is now standard. It works the same way as for an automobile, with batteriesproviding the power to turn a starter motor which turns over the main engine. In olderlocomotives fitted with DC generators instead of AC alternators, the generator was used as astarter motor by applying battery power to it.Governor Once a diesel engine is running, the engine speed is monitored andcontrolled through a governor. The governor ensures that the engine speed stays high enough toidle at the right speed and that the engine speed will not rise too high when full power isdemanded. The governor is a simple mechanical device which first appeared on steam engines.It operates on a diesel engine as shown in the diagram below.The governor consists of a rotating shaft, which is driven by the diesel engine. A pair offlyweights are linked to the shaft and they rotate as it rotates. The centrifugal force caused by
the rotation causes the weights to be thrown outwards as the speed of the shaft rises. If the speedfalls the weights move inwards.The flyweights are linked to a collar fitted around the shaft by a pair of arms. As the weightsmove out, so the collar rises on the shaft. If the weights move inwards, the collar moves downthe shaft. The movement of the collar is used to operate the fuel rack lever controlling theamount of fuel supplied to the engine by the injectors.Fuel InjectionIgnition is a diesel engine is achieved by compressing air inside a cylinder until it gets very hot(say 400°C, almost 800°F) and then injecting a fine spray of fuel oil to cause a miniatureexplosion. The explosion forces down the piston in the cylinder and this turns the crankshaft.To get the fine spray needed for successful ignition the fuel has to be pumped into the cylinder athigh pressure. The fuel pump is operated by a cam driven off the engine. The fuel is pumpedinto an injector, which gives the fine spray of fuel required in the cylinder for combustion.Fuel Control In an automobile engine, the power is controlled by the amount offuel/air mixture applied to the cylinder. The mixture is mixed outside the cylinder and thenapplied by a throttle valve. In a diesel engine the amount of air applied to the cylinder isconstant so power is regulated by varying the fuel input. The fine spray of fuel injected into eachcylinder has to be regulated to achieve the amount of power required. Regulation is achieved byvarying the fuel sent by the fuel pumps to the injectors. The control arrangement is shown in thediagram left.The amount of fuel being applied to the cylinders is varied by altering the effective delivery rateof the piston in the injector pumps. Each injector has its own pump, operated by an engine-driven cam, and the pumps are aligned in a row so that they can all be adjusted together. Theadjustment is done by a toothed rack (called the "fuel rack") acting on a toothed section of thepump mechanism. As the fuel rack moves, so the toothed section of the pump rotates andprovides a drive to move the pump piston round inside the pump. Moving the piston round,alters the size of the channel available inside the pump for fuel to pass through to the injectordelivery pipe.The fuel rack can be moved either by the driver operating the power controller in the cab or bythe governor. If the driver asks for more power, the control rod moves the fuel rack to set the
pump pistons to allow more fuel to the injectors. The engine will increase power and thegovernor will monitor engine speed to ensure it does not go above the predetermined limit. Thelimits are fixed by springs (not shown) limiting the weight movement.Engine Control DevelopmentSo far we have seen a simple example of diesel engine control but the systems used by mostlocomotives in service today are more sophisticated. To begin with, the drivers control wascombined with the governor and hydraulic control was introduced. One type of governor usesoil to control the fuel racks hydraulically and another uses the fuel oil pumped by a gear pumpdriven by the engine. Some governors are also linked to the turbo charging system to ensure thatfuel does not increase before enough turbocharged air is available. In the most modern systems,the governor is electronic and is part of a complete engine management system.Power ControlThe diesel engine in a diesel-electric locomotive provides the drive for the main alternatorwhich, in turn, provides the power required for the traction motors. We can see from thistherefore, that the power required from the diesel engine is related to the power required by themotors. So, if we want more power from the motors, we must get more current from thealternator so the engine needs to run faster to generate it. Therefore, to get the optimumperformance from the locomotive, we must link the control of the diesel engine to the powerdemands being made on the alternator.In the days of generators, a complex electro-mechanical system was developed to achieve thefeedback required to regulate engine speed according to generator demand. The core of thesystem was a load regulator, basically a variable resistor which was used to very the excitation ofthe generator so that its output matched engine speed. The control sequence (simplified) was asfollows:1. Driver moves the power controller to the full power position2. An air operated piston actuated by the controller moves a lever, which closes a switch tosupply a low voltage to the load regulator motor.3. The load regulator motor moves the variable resistor to increase the main generator fieldstrength and therefore its output.4. The load on the engine increases so its speed falls and the governor detects the reduced speed.5. The governor weights drop and cause the fuel rack servo system to actuate.6. The fuel rack moves to increase the fuel supplied to the injectors and therefore the powerfrom the engine.7. The lever (mentioned in 2 above) is used to reduce the pressure of the governor spring.8. When the engine has responded to the new control and governor settings, it and the generatorwill be producing more power.On locomotives with an alternator, the load regulation is done electronically. Engine speed ismeasured like modern speedometers, by counting the frequency of the gear teeth driven by theengine, in this case, the starter motor gearwheel. Electrical control of the fuel injection isanother improvement now adopted for modern engines. Overheating can be controlled by
electronic monitoring of coolant temperature and regulating the engine power accordingly. Oilpressure can be monitored and used to regulate the engine power in a similar way.CoolingLike an automobile engine, the diesel engine needs to work at an optimum temperature for bestefficiency. When it starts, it is too cold and, when working, it must not be allowed to get toohot. To keep the temperature stable, a cooling system is provided. This consists of a water-based coolant circulating around the engine block, the coolant being kept cool by passing itthrough a radiator.The coolant is pumped round the cylinder block and the radiator by an electrically or belt drivenpump. The temperature is monitored by a thermostat and this regulates the speed of the (electricor hydraulic) radiator fan motor to adjust the cooling rate. When starting the coolant isntcirculated at all. After all, you want the temperature to rise as fast as possible when starting on acold morning and this will not happen if you a blowing cold air into your radiator. Someradiators are provided with shutters to help regulate the temperature in cold conditions.If the fan is driven by a belt or mechanical link, it is driven through a fluid coupling to ensurethat no damage is caused by sudden changes in engine speed. The fan works the same way as inan automobile, the air blown by the fan being used to cool the water in the radiator. Someengines have fans with an electrically or hydrostatically driven motor. An hydraulic motor usesoil under pressure which has to be contained in a special reservoir and pumped to the motor. Ithas the advantage of providing an in-built fluid coupling.A problem with engine cooling is cold weather. Water freezes at 0°C or 32°F and frozen coolingwater will quickly split a pipe or engine block due to the expansion of the water as it freezes.Some systems are "self draining" when the engine is stopped and most in Europe are designed touse a mixture of anti-freeze, with Gycol and some form of rust inhibitor. In the US, engines donot normally contain anti-freeze, although the new GM EMD "H" engines are designed to use it.Problems with leaks and seals and the expense of putting a 100 gallons (378.5 litres) of coolantinto a 3,000 hp engine, means that engines in the US have traditionally operated without it. Incold weather, the engine is left running or the locomotive is kept warm by putting it into a heatedbuilding or by plugging in a shore supply. Another reason for keeping diesel engines running isthat the constant heating and cooling caused by shutdowns and restarts, causes stresses in theblock and pipes and tends to produce leaks.LubricationLike an automobile engine, a diesel engine needs lubrication. In an arrangement similar to theengine cooling system, lubricating oil is distributed around the engine to the cylinders,crankshaft and other moving parts. There is a reservoir of oil, usually carried in the sump, whichhas to be kept topped up, and a pump to keep the oil circulating evenly around the engine. Theoil gets heated by its passage around the engine and has to be kept cool, so it is passed through aradiator during its journey. The radiator is sometimes designed as a heat exchanger, where theoil passes through pipes encased in a water tank which is connected to the engine coolingsystem.
The oil has to be filtered to remove impurities and it has to be monitored for low pressure. If oilpressure falls to a level which could cause the engine to seize up, a "low oil pressure switch" willshut down the engine. There is also a high pressure relief valve, to drain off excess oil back tothe sump.TransmissionsLike an automobile, a diesel locomotive cannot start itself directly from a stand. It will notdevelop maximum power at idling speed, so it needs some form of transmission system tomultiply torque when starting. It will also be necessary to vary the power applied according tothe train weight or the line gradient. There are three methods of doing this: mechanical,hydraulic or electric. Most diesel locomotives use electric transmission and are called "diesel-electric" locomotives. Mechanical and hydraulic transmissions are still used but are morecommon on multiple unit trains or lighter locomotives.Diesel-Electric TypesDiesel-electric locomotives come in three varieties, according to the period in which they weredesigned. These three are:DC - DC (DC generator supplying DC traction motors);AC - DC (AC alternator output rectified to supply DC motors) andAC - DC - AC (AC alternator output rectified to DC and then inverted to 3-phase AC for thetraction motors).The DC - DC type has a generator supplying the DC traction motors through a resistance controlsystem, the AC - DC type has an alternator producing AC current which is rectified to DC andthen supplied to the DC traction motors and, finally, the most modern has the AC alternatoroutput being rectified to DC and then converted to AC (3-phase) so that it can power the 3-phaseAC traction motors. Although this last system might seem the most complex, the gains fromusing AC motors far outweigh the apparent complexity of the system. In reality, most of theequipment uses solid state power electronics with microprocessor-based controls. For moredetails on AC and DC traction, see the Electronic Power Page on this site.In the US, traction alternators (AC) were introduced with the 3000 hp single diesel enginelocomotives, the first being the Alco C630. The SD40, SD45 and GP40 also had tractionalternators only. On the GP38, SD38, GP39, and SD39s, traction generators (DC) were standard,and traction alternators were optional, until the dash-2 era, when they became standard. It was asimilar story at General Electric.There is one traction alternator (or generator) per diesel engine in a locomotive (standard NorthAmerican practice anyway). The Alco C628 was the last locomotive to lead the horsepower racewith a DC traction alternator.face="Times New Roman">Below is a diagram showing the main parts of a common US-builtdiesel-electric locomotive. I have used the US example because of the large number of countries
which use them. There are obviously many variations in layout and European practice differs inmany ways and we will note some of these in passing.More InformationThis page is just a brief description of the main points of interest concerning diesel locomotives.There arent too many technical sites around but the following links give some usefulinformation:Diesel Locomotive Systems - A good description of the operation of the equipment of themodern UK diesel-electric Class 60 locomotive. It written in simple terms and gives the reader abasic understanding of the technology.US Diesel Loco Operating Manuals - Copies of some of the older US diesel locomotive manualsissued to staff. Contains some very interesting details.Diesel-Electric and Electric Locomotives - by Steve Sconfienza, PhD.D. - >Includes sometechnical background on the development of diesel and electric traction in the US, an illustrationof the PRR catenary system and some electrical formulae related to different traction systems.Diesel-Electric Locomotive Operation - A general list of US diesel locomotive types, designsand statistics with a summary of their development. A useful introduction to the US diesel locoscene.Sources:The Railroad, What it is, What it Does by John H Armstrong, 1993, Simmons Boardman BooksInc.; BR Diesel Traction Manual for Enginemen, British Transport Commission, 1962; BREquipment, David Gibbons, Ian Allan, 1986 and 1990; Modern Railways; InternationalRailway Journal; Railway Gazette International; Mass Transit; Trains Magazine.
What a Modern Locomotive Is -- The Short Version This is the really simple version. Modern locomotives have electric motorsconnected to the drive axles. The electric motors receive electric power either froman on-board power source (e.g., a diesel motor) or from a central power source viaa distribution system (e.g., a thrid rail). The link between the electric motor and thesource of the electricity is called the transmission. The electrical power lines thatcriss-cross our towns and cities are called electric "transmission" lines; the linkfrom a diesel motor to the electric motors on a locomotives axles is called thetransmission. Thats it! [ back to page index ]Why it looks the way it does Why do modern freight locomotives look the way they do -- a cab at one end,lots of bulky equipment at the other? Why do Amtrak, LIRR, and other passengerlocomotives that have been recently designed without regard to any freightpredecessors have one cab with, at most, a hostlers position at the other end? The issue of cabs on locomotives has a number of "histories" that haveconverged to produce the style seen today. First, many early U.S. diesellocomotives did have two cabs, such as Baldwins built for Jersey Central (whilethe same locomotives for other roads had only one cab), as did other diesel andelectrics such as various boxcabs and the GG1, and of course the AEM7s of todayhave cabs at each end. What gnaws at ones mind, though, is really about the bigfreight locomotives (like the SD80MACs of Conrail). So . . . When the big frieght roads first dieselized, there were questions about MUsand crews. The railroads did not want to put a crew in each cab of an MUed set, sothat brought forth such oxymorons as referring to the evolutionarilly critical A-B-B-A FTs from General Motors as "a locomotive." Calling it one locomotive (onewith a cab at each end!) meant it needed only one crew (and note that the B-unitshad no cab, or if any just a hostlers position). As these evolved into F3s, F7s andF9s, and A-units unpaired and mixed with other units, the ubiquity of single-cabunits was assured. Roads taking E-unit derivatives (i.e., double-engine units, eventhose from other builders) such as the Jersey Central Baldwins (DR-6-4-20) didsometimes take two cabs when it was clear that the unit would only be operating as
a single unit; alternatively, some cab units came semi-permanently coupled back-to-back with a draw-bar instead of a coupler (e.g., PRRs DR-12-8-1500/2),essentially a single unit with a cab at each end (today, similar issues are resovled inpassenger operations through push-pull, with a cab at the other end of the train). The other key development was of the non-hood units, the "road switchers."General Motors again had the critical development with the GP-7. The unit waslogically divided into two hoods, with a somewhat centered cab for ease of bi-directional movement. Under one hood -- the longer one -- was the motiveequipment, while under the other -- the shorter one -- was usually a steamgenerator for passenger service. Local builder ALCO had similar locomotives withthe RS-1s, RS-2s, and RS-3s, leading ultimately into its Century line. Passengeroperations into the 1970s (e.g., the Long Island) took road switchers long-hoodforward to protect the engine crew from the steam generator in case of an accident,particularly a grade crossing accident where a vehicle could have flipped over theanti-climber (where the anti-climber would have been had the units been soequipped) and crushed the steam generator into the cab (shades of an explodingsteam locomotive!). Much earlier, many roads had begun turning their roadswitchers around and running them short hood forward for the better visability(who needed a long hood like a steam engines boiler, anyway?) -- except for TheNorfolk and Western [N&W] and The Southern [SOU], which seemed to delight intaking the most incrediably long hood and designating it front, with the cab soconfigured; and the Erie Lackawanna, which apparently had some SD-45sconfigured for either direction. Long-hood forward and bi-directional unitssometimes had two control stands, one on each side of the cab facing "forward,"but sometimes had just a single stand sitting parallel to the cab side for use with theengineer facing in either direction. As the time came when passenger equipment nolonger needed steam, the short hood was cut and shortened, and that gave theconfiguration of the road switcher of today (note that during the overlap whenshort hoods were being cut but some passenger steam-generators were still neededin operation, GMs SDP-35, SDP-40, and SDP-45 had the steam generators at thecoupler-end of the long hood). At this point, a second cab could have been added, but only a hand-full ofunits have ever been built that way (some electrics on mining roads come to mind).For the most part, the big roads run with two or more units anyway, so having east-west pairs is not difficult (turning facilities such as wyes and loop tracks seem tobe plentiful). The smaller roads simply dont want the expense. And, of course, itseems, N&W and SOU successor Norfolk Southern would just as soon run themlong-hood anyway!
[ Motors Graphic ][ back to page index ]Baisc DC Motor Concepts The traditional electric motor on a diesel-electric or electric locomotive is aDC motor. Internally, DC motors have two main components: the stator is thestationary outside part of a motor. The armature is the inner part which rotates. Toget the armature to rotate, electric motors require two sets of windings, the fieldwinding (on the stator) to develop the magnetic field within which the armaturewill turn, and the armature winding (which in many DC motors can maketransition between series and shunt winding). In series, current passes through boththe field and armature windings "in series", that is, one after the other, while inshunt the current is divided and passes either through the field or armaturewindings. DC motors with series windings develop high starting torque, whilemotors in shunt develop high speed. (A third category, the compound, combinesboth series and shunt windings simultaneously, with mixed properties: c.f. a motorthat can transition between series and shunt, but not use both at the same time; afourth category is the permanent-magnet motor, which not surprisingly uses apermanent magnet for the field, and is only used in relatively low-powerapplications.) DC motors turn because an electrical field rotates. The field rotates because anelectrical current passing into the armature changes polarity, with the armaturetugged forward with each change. (N.B., it is the field in the armature that ischanging, not the field in the stator.) In order to accomplish this change in polarity,the windings in the armature are connected to the outside world by means of acommutator, a conductive sheaf that allows for the current in the windings to havea change in polarity by breaking and making the connection. The electricity flowsinto the commutator through conductive brushes (usually carbon). These aresources of friction, heat, and general wear in the DC motor. Series-wound motors are also called universal motors (see below), universal inthe sense that they will run equally well using either AC or DC: simultaneouslyreversing the polarity of both the stator and the rotor cancel out, thus the motor willalways rotate in the same direction regardless of the voltage polarity. Sometimescalled "AC motors" instead of "DC series-wound motors" or "universal motors,"these are not the motors to which one refers when referring to AC traction motors.
Note: In the motor wiring diagrams, the DC motors do not have separatelyexcited windings. While some first generation diesel-electrics had an auxiliarygenerator to provide current to separately excited windings, which required manualswitching by the engineer, that design is now obsolete. Contemporary U.S. roadlocomotives do not have separately excited windings; however, there has beenother work in this direction, based in computer-control systems, that has includedwheel-slip detection and control and wheel-creep systems, in which the windingsare under separate control (see below). [ back to page index ]Transmissions When speaking of a vehicle such as a railroad locomotive, transmission is theprocess by which power is transmitted from one location and used in another.Often this implies some type of changing process, for example the manner in therotational force of a crankshaft is converted to electrical force in a generator. In a modern diesel-electric locomotive, this is a multi-staged process that goesfrom fuel oil to turning wheels. In the days of steam the intermediate process was,well, it was steam. Coal or oil was burned in a firebox. The heat generated heatedwater, turning it into steam (and continued to heat the steam), which in turn drove apiston in a back-and-forth motion, which -- through the drive gear -- turned thewheels. In diesel-electric locomotives, the fuel is burned in cylinders, drivingpistons in a back-and-forth motion, which -- through the crank shaft -- turns anelectricity generating device (a generator or alternator or both), which provideselectricity for electric motors that are connected to the axles of the trucks, whichturn the wheels.FIGURE ONE: BLOCK DIAGRAM OF A DIESEL-ELECTRIC LOCOMOTIVE diesel motor
generator electric motors [ Transition Graphic ][ back to page index ]Transition Transition is the process by which the transmission of a diesel-electriclocomoitve is brought from series wiring to parallel wiring. When in series, allcurrent in the locomotive pass through all motors: this produces maximum low-speed force in the motors, i.e., maximum starting torque. When in parallel, currentis divided among the motors: this produces maximum high-end efficency, i.e.,highest motor speed. This is just as with the wiring internal to DC motors, wherehaving the motor wound in series develops high starting torque, while placing themotor in parallel will develop high speed. Electrically, as current increases throughthe motors in a circuit with a given total current and voltage, the voltage dropacross each motor will decrease: parallel circuits apply the total voltage to eachload (i.e., in this case, motor), while series circuits apply the total current to eachload. Not all locomotives can make transition -- yard locomotives are often wiredonly for series. The motors on a diesel-electric road locomotives are often capableof making multiple transitions, with both trucks and motors on a truck capable ofbeing switched into series or parallel wiring. See the motor diagram for an example of a DC motor capable of transition andthe transition diagram for examples of two- and three-axle transition schemes. (Anote on how the construction of the motors is depicted is above). These diagramsare quite general: some locomotives would put like axles on different trucks inseries (first axle with first axle, second with second), or have other arrangements.
[ Catenary Graphic ][ back to page index ]Electric Locomotives In straight electric locomotives or M.U. cars (multiple-unit electric), the on-board diesel engine and generator and/or alternator is replaced by Niagara Falls orIndian Point or some other such central power generating source. The power istransmitted to the railroad and delivered to the trains. Electric transmission linesare generally high-voltage AC. This is because of the greater efficiency (i.e., lessloss) of AC during transmission over certain distances, and the likewise greaterefficiency of high-voltage transmission over low-voltage transmission (see belowand also the Formulas and Concepts page under Electric Power Transmission formore extensive notes on AC, DC, and long distance electric transmission). In railapplications (as in most others), the most efficient transmission of electricity fromgenerating stations to the tracks requires transmission lines of high-voltage ACwith substations to convert this to line voltage for equipment. The power isdelivered to the individual locomotives or cars generally either by a track levelthird rail (there are some fourth rail systems, too) or through overhead wiresknown as catenary). While there are some exceptions, AC systems usually usecatenary, while DC systems usually use third rail.FIGURE TWO: BLOCK DIAGRAM OF A ELECTRIC LOCOMOTIVE central generating facility distribution system on-board electrical equipment
(e.g., transformer, rectifier, motor-generator, inverter, etc.) electric motorsNotes on Electric Power Transmission and Distribution SystemsLong-Distance Transmission, Substations, and Local Distribution Two principal elements of an electric system are the transmission of theelectric power from the source of the electricity (i.e., a generating plant) to thelocal use area and, ultimately, distributing that power to the consumer (e.g., anelectric locomotive or a home). Thus, these parts of the system may be divided intothe transmission system and the distribution system, with transmission convenyingthe power long distances (at high voltages), and the distribution system deliveringthe power locally (at low voltages).Long-Distance Transmission As a general rule, in the United States, DC cannot be transmitted aseconomically as AC in transmission systems; railroads follow the practice of ACtransmission systems, with high voltage AC stepped-down to distribution voltagesat substations. Extensive notes on long distance electric transmission, including someformulas, are on the Formulas and Concepts page under Electric PowerTransmission.Local Distribution Local distribution is almost universally accomplished by third rail or overheadwires. The highest third rail voltage in use in the U.S. today is the 1,000 voltsystem on San Franciscos BART system. The highest, historically, is reputed tohave been an interurban that ran 2400 volts (historically, only for a brief period): itdid not really work very well, as arcing and leakage were such critical issues thatthe system was conveted to a lower voltage.
As for AC, it is generally delivered at higher volatages than third rail throughoverhead wires. As for AC third rail, no such systems exist (at least that I know of,certainly not in the U.S.). This may not be practicable: ACs advantage comes fromhigh voltage transmissions that can be readilly stepped up or down, withconversion to DC in the distribution system providing easy control of motorswithout including on-board rectifiers (but more on that following). (Now, with ACtraction in common usage, there may be a rationale for that to change, butdistribution is still basically low-voltage DC or higher voltage AC.)Distribution on the North East Corridor The NEC uses three different combinations: • D.C. to New York City: 11kV 25Hz • N.Y.C. to New Haven: 11kV 60Hz • New Haven to Boston: 25kV 60Hz Understandably, Amtraks engineers (the slide-rule type) want everything at25kV 60Hz, and that is the standard for new track, such as the latestelectrificationm, from New Haven to Boston. Sixty hertz has the advantage ofbeing compatible with the commercial grid (25Hz requires frequency converters,which run [reportedly] $40,000,000 each), and 25kV is not an unusual voltage, soequipment is available. Higher voltages are also more efficient to transmit than lower: in fact, whenthe voltage is doubled the amperage is halved for the same power level ( P = I * V:increasing voltage results in a linear decrease in current at the same power level).Since transmission losses are a function of amperage only (dissipated power = I2R,where R is the constant line resistance [or impedance, in the case of AC]), 25kVpower can be transmitted over twice as far as 11kV power at the same loss levels.One should note, however, that this does not solve the problem of drawing downthe current in a section by multiple trains running within it: high train density willstill require short segments (remember that this concerns only local distribution viathe catenary and its local feeders, not long-distance transmission frompowerplants). Unfortunately, upgrading track from 11kV to 25kV is expensive, because it isnecessary to rebuild catenary to accomodate the higher arc distance at the highervoltage, (e.g., better insulation and insulators).
Phase While it is theoretically possible to run an entire rail line, hundreds of mileslong, on a single, synchronized AC phase, in practice it is not practical. Instead,lines are generally broken into ten to twenty mile segments, each one running on adifferent phase (usually 120 degrees apart). The boundry of each segment is calleda "phase break." On former Pennsy track, these were marked by a phase-breaksignal, which looked like a typical position-light signal with all positions in theentire circle lit. To prevent arcing between the sections, an insulating section ofcatenary is run across the phase break. By segmenting the catenary into sections, itis a simple matter -- with respect to stringing the catenary -- to have not onlydifferent phases but also different frequencies, 25 or 60 hertz, or voltatges, 11kV or25kV, on the two sides of a phase break. Unlike former equipment, which had to stop and change internal settings,Acela trains are capable of changing both frequency and voltage while at speed.This is done as part of the normal "approaching phase break" message that is sentto trains via ACSES, which includes the frequency and voltage that will be on theother side of the break (typically no change). When the pantograph hits theinsulator between phases, the train temporarily cuts the input power, reconfiguresthe leads to the windings on the primary of the main transformer, and reconnectsthe input power -- all in less than a second. To the equipment downstream fromthe transformer, all that is visible is an AC voltage that drops out briefly every fiveto 15 minutes, as the locomotive hits the various phases. The blip is short enoughthere should be no noticeable traction-motor stutter or hotel-power disruption.FIGURE THREE: REGIONAL EXAMPLES OF MAINLINE ELECTRIC OPERATIONS • Northeast Corridor o 25kv, 60Hz AC via Catenary The PRR originally built this as 11kv, 25hz and supplied its own electricity, as this system was not compatible with commercial 60Hz systems (complete change-over not yet completed: see above). • Long Island Rail Road o 600v DC via 3rd rail • Delaware, Lackawanna, and Western (metro New Jersey lines)
o 3000v DC via Catenary • Montreal Suburban o 2500v DC via Catenary • Other Systems o there are also some 50kv, 60hz catenary systems o some DC operators generated their own 25Hz AC for distribution to substationsSubstations Substations sit between transmission and distribution systems. They are fairlystraightforward: a transformer steps down the AC voltage, then, if using a DCdistribution system, a rectifier converts the AC to DC. With their transformers tostep-down the high voltage transmission voltages to distribution levels, they arelocated periodically throughout the system. For the railroad, these are much likethe ubiquitious substations with their tranformers located throughout suburbanneighborhoods. In electric locomotive (or MU) applications, the use of DC (as on the MetroNorth [ex New York Central] Hudson and Harlem lines and on the LIRR) requiresclosely spaced substations to convert AC to DC, stations more closely spaced thanmight be required for a similar AC distribution system. This is because of thehigher line losses at the lower distribution voltages (this is explained elsewhereunder Electric Power Transmission).Rectification Because of long-established AC motor issues of low starting torque and ofpower control (more on that follows), the traction motors themselves have (up untilrecently) been DC. This has meant that at some point in the process AC has had tobe coverted ("rectified") to DC, either at the substation or in the locomotive. AC, which reverses direction 60 times a second (the U.S. standard), generallyresembles a sine wave in the distribution systems. A simple rectifier is an electricalcheck valve: flow is only permitted in one direction, while retaining thecharacteristic sine curve (one-half of the curve, just the "positive" half, lets say).This is referred to as half-wave rectification. A more sophisticated approach is toallow the negative alternations to pass also, but in the same direction as the
positive alternations (i.e., no direction change in the current). The AC thusbecomes a pulsating DC, with all pulsations of the sine wave in one direction fromzero. This is referred to as full-wave rectification.FIGURE FOUR: RECTIFICATIONSubstation-Based Rectification: The rectifiers in modern substations are solid state, sillicon diode based. Theyare efficient (the voltage drop is a fraction of a volt through the rectifier) and
reliable. Earlier systems, such as used on the Long Island Rail Road, used mercuryarc rectifiers, only slightly less efficient, but requiring much support (these wereknown as "ingitron" systems). They were quite large, housed in large structures,and required much cooling. Other systems, such as used by the New York CityTransit Authority, used rotary converter based substations - - very large, verymaintenance intensive. Substations are almost always fed with three phase AC, and the three phasesoverlap coming out of the rectifier, so the DC pulsates only slightly (filtering canremove the pulsations altogether: see figure seven below for an example). Fromhere, the DC is fed to the third rail (or catenary) by way of breakers, currentsensors, switchgear, and whatever else.Locomotive-Based Rectification: Prior to the advent of a solid-state technology for converting high-power ACto DC, massive locomotives were often the only solution to this issue when usingall AC systems: an AC motor turned a DC generator, which in turn supplied DC tothe motors on the axles. A rectifier that takes the form of an AC motor turning agenerator is called, not surprisingly, a "motor-generator." Some railroads, such as the Pennsylvania and New Haven, had MUs withignitrons on them. Both of these roads also had ingitron based locomotives (knownas "rectifiers," locomotives such as the EP-5, E-44, and E-33, but not the PRRsGG1: one of the most massive locomotives, it was actually an all-AC unit, withpower control through tranformer taps). In the mid-1960s, high-power solid-state rectifiers became feasible, andsmaller, lighter weight electric locomotives -- and AC transmissions on diesel-electric locomotives -- became available. In the straight-electric market, the lastGM (GMD [Canada]) motor-generator unit was the SW1200MG (2300v, 60hz),produced from 1963 to 1971 (1971 being well after the move to solid-staterectification, but the unit had gone into production in 1963 and was maintained foran existing customer). [ Catenary Graphic ][ back to page index ]
The conversion to AC/DC transmission An alternator is generally smaller and simpler than a generator of like capacity.This is because generators, like DC motors, are equipped with a commutator andcarbon brushes, which are what reverses the electrical current as the armatureturns, preventing the current from alternating, keeping the current direct. Thissimpler, lighter structure means that a diesel-electric locomotive using an alternatorinstead of a generator should be more economical. With the advent of economicaland compact solid-state rectifiers, which could be routinely installed onlocomotives (see above), the greater efficiency of the alternator could finally berealized in rail applications, and AC/DC transmission became a reality. In the mid-1960s, all three major manufacturers begin offing AC/DC transmission units, Alcoand GM in 1965 and GE in 1966.FIGURE FIVE: BLOCK DIAGRAM OF A DIESEL-ELECTRIC LOCOMOTIVE WITH ANAC/DC TRANSMISSION diesel motor alternator rectifier electric motors
The block diagram in figure five illustrates the AC/DC transmission. A dieselmotor turns an alternator; the AC produced by the alternator is rectified to DC forthe locomitives DC traction motors. GMs first applications of AC transmission were in the GP40 and SD40 of1965 and 1966, respectively. General Electrics first AC transmission were theU28B/U28C offerings of 1966 (earlier production of these models was straightDC). Alco offered the top-end of its Century line with AC/DC transmissions, theC430/C630, in 1966/1965 respectively. [ back to page index ]The move away from DC traction motors One of the most important advances in locomotive technology in recent yearsis the AC traction motors. AC motors have been around for many years (thekitchen clock that plugs into a recessed electrical socket directly behind it is anexample of one). However, AC motors were never able to match the starting torqueof the DC and are notoriously difficult to control in varrying load and speedimplementations. Unfortunately, while DC motors provide high starting torquethey also have critical limitations (as was noted above). These limitations havelong made it desirable that a substitute be found. Like generators, DC motors are equipped with a commutator and carbonbrushes, which are subjected to very high current loads. (In a generator, these arewhat reverses the electrical current as the armature turns, preventing the currentfrom alternating -- keeping the current direct; in the motor, the commutator andbrushes reverse the current, creating the moving magnetic force that rotates thearmature.) A DC motor that would have high current loads while not in motion orwhile moving slowly would receive major damage or burn-out if such a highcurrent were to be applied for too long a period of time. At low speeds, the highamperage damage would occur within minutes. Because of this, until recently, allDC locomotives all have minimum continuous speeds (for example, SD40 & 45 at11 to 12 MPH, SD50 at 10 MPH, GP40 at 12 MPH, some swithcers and regearedroad units, such as some CSX GP38s at 7 MPH).
Power in a DC circuit is simply equal to the voltage times the current. This isexpressed as power (in watts, "P") = voltage (in volts, "E" [for Electromotiveforce) times current (in amps, "I"), or P=E*I, and Power (in horsepower) = watts * 0.00134102 (going the other way, watts = horsepower * 745.6999) In DC motors, the power relationship is simple: at a constant voltage Ohmslaw requires more current to produce more power (watts = voltage * current). Thismeans that in DC high current levels will be needed to produce high power,lacking a good way to vary voltage on the fly. This becomes expensive, havingnecessitated heavy conductors throughout the system to carry the high current;further, the high current produce a great deal of heat, further limiting DC tractionmotors. For example, to compute the current flow in a 1000 horsepower switcher withDC traction motors at 600 volts, • 1000 horsepower = 745,699.872 watts • 745,699.872 watts / 600 volts = 1242.83312 amps. Using todays high-horsepower DC units, e.g. a 4400 horsepower, 6 axle unit,where each of the six motors contribute 733 horsepower to the total unithorsepower, one can get up to 5500 amps per motor in a 600 volts system.(Remember that in DC motors that current goes across the commutator andbrushes.) Specifically, • 4400 horsepower / 6 motors = 733 horsepower/motor • 733 horsepower/motor = 546,598 watts/motor When operating in parallel, with a 600 volt drop across each motor, • 546,598 watts/motor / 600 volts = 910.99667 amps/motor When operating in series, with a 100 volt drop across each of six motors, • 546,598 watts/motor / 100 volts = 5465.98 amps/motor
Note that when operating in parallel, 911 amps * 6 motors is 5466 amperestotal in the system. These modern units -- like their AC brethren -- use computer control toreduce (hopefully to eliminate) wheel slip, but even so they can still slip (and stall),and even with arc suppressors and damping material around the brushes, flashoversand destroyed brushes still occur, caused by low speed, wheel slip, rough track,etc., all of which contribute to the woes of a DC traction motor. Power-wize, contemporary DC traction motor size is getting very close to thepractical limits. This is based on such elements as magnetic saturation and thecurrent capacity of the electrical conductors used to build them, coupled with thephysical limits of the structure (it would be necessary to use physically largermotors to forestall magnetic saturation: see note below). In the 1960s, the Southern Pacific and the Rio Grande both acquired diesel-hydraulic locomotives. In the hydraulic transmission, a driveshaft connects thepower-plant to the axles, just as in an automobile. In 1961, both roads acquiredGerman-built Krauss-Maffei locomotives, twin-engined 3450 hp, c-c units with acowl carbody. In 1963, the SP took an additional 15 units with a road-switchercarbody. In 1964 SP acquired the Rio-Grande units. ALCO also made a forray intothe diesel-hydralluc experiments, the DH-643, a double-engine, 4300 hp, c-c unit:three units were built, all going to SP in 1964 after testing on the New YorkCentral. In 1970 SP retired its German units, while the ALCOs were scrapped in1973. The world still had two decades to wait for a better locomotive transmission. Note: Magnetic saturation is a rather abstract concept that may best be thoughtof as the limited ability of an object to be magnetized. In the case of a motor theobject is usually a piece of iron wound with wires conducting an electrical current.With an applied voltage to the wound wires, a current is caused to flow, and thatcurrent flow causes a magnetic field to be created. With more applied voltage,more current and more magnetic field in proportion to the applied voltage. At somepoint, the iron becomes saturated, increasing the current does not create moremagnetic field, and the linear relationship is broken: increasing voltage no longercauses a linear increase in current but instead creates a geometric increase incurrent -- that is, lots and lots of current, creating lots and lots of heat, burning outthe motor. For more on magnetic saturation, including some formulas, see ourFormulas and Concepts page under AC Motor Facts.
[ back to page index ]The move to AC traction motors It has long been known that AC motors can be more economical than DCmotors, just as with their near cousins, alternators and generators. Like alternators,AC motors are not equipped with wear-prone commutators and brushes,eliminating these sources of limitations of the low speed-high throttle position. ACmotors would allow locomotives to (1) have more pulling power, (2) avoid stallburns in the traction motors, and (3) have correspondingly lower maintenancerequirements. An early example of AC in a railroad application is the GG1 (designed in1934), which utilized 12 six-pole motors, 400 volts AC at 25 Hz. Each motor wasrated at 385 hp, with the 12 motors mounted in pairs over each of the six drivingaxles (see our GG1 page for details of the GG1 electricals). In June of 1989, GMbegan the modern AC traction motor era with its demonstrator, the F69PH-AC, anAC traction version of the F59, followed in 1991 by the SD60MAC. GM deliveredits first production unit to Burlington Northern in 1993. GE delivered its first AC-traction unit to CSX in June of 1994. As a brief technical aside to provide some background and standardizeterminology, series-wound DC motors (i.e., motors with commutators and brusheswhere the field winding and the rotor winding are connected in series) are alsocalled universal motors, universal in the sense that they will run equally well usingeither AC or DC: simultaneously reversing the polarity of both the stator and therotor cancel out, thus the motor will always rotate the same direction regardless ofthe voltage polarity. So a universal motor is in a sense a type of an AC motor in asmuch as it will operate on AC. The term "universal motor" differentiates it fromthe more generally thought of AC motor, the AC induction motor, which lackscommutators and brushes. Unfortunately for universal motors, the fact that they donot lack commutators and brushes means that they do lack all of the advantages ofwhat are more typically thought of as AC motors -- the induction motors -- whichis the very lack of commutators and brushes! So to say the universal motor "willrun equally well using either AC or DC" may be a slight misphrasing: perhaps oneshould say, "it will run equally badly!" Therefore, the universal motor does nothave a role to play in modern electric traction (although universal motors wereused in early AC applications in locomotives); rather it is the induction motor that
is the "AC motor" to which one refers when speaking of AC traction motors today.That means no brushes to maintain, no flashovers, no commutator to get damaged,no armatures to rewind, and less potential for damage at high power/low rpmsituations. AC locomotives are more expensive due to the control problems inherent inthe AC design. An AC motors speed is traditionally dependant on its design, but itmay be controlled by varying the frequency of the input voltage. Being able to varyfrequency has been a significant issue in the development of AC motors in high-horsepower traction applications. To deal with the power control problem, bothEMD and GE use an AC to DC to AC conversion, control taking place in the DCphase. In an AC traction motor application, the diesel engine drives an alternator,crating AC. This AC is rectified (i.e., converted to DC) and power control takesplace in this stage. This is the same place that power control would take place in aconventional AC/DC transmission. At this point, the DC (called the DC link) goesthrough a solid-state "inverter," which converts the DC back to AC. This AC thenpowers the motors.FIGURE SIX, BLOCK DIAGRAM OF A DIESEL-ELECTRIC LOCOMOTIVE WITH ACTRACTION MOTORS diesel motor alternator rectifier == DC LINK ==
inverter electric motorsFIGURE SEVEN, VOLTAGE/FREQUENCY IN A DIESEL-ELECTRIC LOCOMOTIVE WITHAC TRACTION MOTORS: OUTPUT OF ALTERNATOR TO OUTPUT OF INVERTER
Control takes place in the stages around the DC Link. The inverter converts theDC back to AC, with the conversion frequency and voltage specifically controlled(this is what then determines the motors speed). However, this is not simply aninverter, for in modern applicatons of AC motors, with a reliance made on varyingthe voltage and frequency of the AC to control power more than on simply thebrute force approach of the application of current, the inverter must do more thansimply convert DC to AC. Complex electronic circuitry in the form of on-boardcomputers now is used to control the inverter. (This has eliminated the need for
that classic of the diesel age, the ammeter, in the cab of the AC-motoredlocomotive, which has been replaced by a tractive effort display.) The inverter stage is actually a group of inverters, depending on manufacturereither one for each truck (GM) or one for each motor (GE). Each individualinverter consists of six "gated turn-on (GTO) devices," high-power thyristors (thatis, "silicon-controlled rectifiers"), three each for the positive and the negativephases of the AC wave in positive/negative pairs. Each positive/negative pairalternate turning-on, chopping the DC into a square wave AC. Each of the threepositive/negative pairs turn-on 120 degrees out of phase from each other (turning-on at 0 degrees, 120 degrees, and 240 degrees), producing three-phase AC. Whilethe phase remains constant, the frequency -- how many cycles per second this isrepeated -- is varried. Also able to be controlled at this stage is the voltage, howpositive and negative the AC becomes. Thus, the frequency and the voltage of theAC arriving at the AC motor is fully controlable, providing the speed control forthe locomotive. Since the frequency and voltage are closely controlled by onboard computersystems, motors cannot run away as they would on a DC locomotive, and the ACmotor will not be subject to damaging wheel slip. The use of AC traction motors,coupled with computer controlled wheel creep systems, has allowed AC units toachieve much higher adhesion levels than similar DC units, up to 45% adhesion,versus the 20% range on other units. This has permitted two-for-three and one-for-two replacement of units, with resulting economies in size and maintenanceexpenses that offset the added initial investment in the purchase of AC units.(Note, however, that there are other issues with such power reductions: forexample, a two-for-one reduction on a tradtionally two locomotive run means onelocomotive, and if that one locomotive develops problems enroute [not entirelyunheard of] there is no backup.) Computer control technology has also been applied to DC traction motors,including wheel-slip detection and a wheel-creep systems allowing for brief [weretalking fractions of seconds here] applications of power to facilitate very low speedoperations. While, this does not fully eliminate problems with high current flow atlow speeds in DC motors, these DC wheel-creep systems and wheel-slip detectionsystems provide dramatically increased adhesion in DC units as well as in AC unitsand have eliminated many of the operational issues with DC traction: CSX, forexample, does not place a minimum continuous speed on its DC-traction GMSD60s and SD70s and GE Dash 8 and Dash 9 locomotives, the same as for all ofits AC locomotives [see also note above].)
While the AC traction motor is less complex and has proven itself dependablein long term railroad use (the PRR GG1 used 12 385 hp AC motors ), the purchaseof new units with AC traction motors is an expensive undertaking, representing aninvestment in new technology with maintenance and operational issues notpreviously encountered, and the new generation of 1000 hp AC traction motors inrailroad use represents a new and untested technology, with some railroads stillvery reluctant to make the transition. (An interesting W3 site on AC motors ishttp://www.drivesys.com/asdis.html.) [ back to page index ]Expanded AC Motor PrincipalsThe Short Version This isnt expected to make sense, so dont worry. When an AC motor is at restand an AC voltage is first applied to it, the difference between the aramature speedand the rotating field is 100%. Under these conditions, a high current will flow atthe moment the aramature starts to turn. At the moment of starting, the torque is at0% of the full load torque, but as the speed increases the torque likewise increases.This is in part because, at low speeds, the motor reactance is high, and the currentand voltage are very much out phase. This contributes to the low power factor. Inan AC motor, maximum power will be generated when the voltage and current areclosest to being in phase, so it can be seen that when the voltage and current areout of phase the motor will not be very efficient.The Long Version(See the Formulas and Concepts page for more detail on AC motor operation and onthese formulas).Power in a DC circuit versus Power in an AC circuit.Power in a DC circuit As noted above, Power in a DC circuit is simply equal to the voltage times thecurrent. This is expressed as power (in watts, "P") = voltage (in volts, "E" [forElectromotive force) times current (in amps, "I"), or
P=E*I For example, ten volts times ten amps equals 100 watts. This relationship canbe used in reverse to analyze a circuit. A 40 watt bulb on a 12 volt DC circuit mustbe drawing 3.333 amps. Further, since Ohms law states that voltage = current timsresistance (E=I*R), it may be seen that the load here is 3.6 ohms. This is all simpleand straightforward because this is a DC circuit.Power in an AC circuit AC Motors One of the miricals of the AC motor is that in the AC induction motor, one ofthe of the two principal components (these two components in the AC motor arethe stator and the rotor), the rotor has no visable electrical contacts to the outsideworld. Instead, it has an electrical field induced into it by the electrical field of thestator -- no commutator, no brushes! (Acutally, some induction motors havebrushes and slip rings, but these are used for connecting control and startingequipment to the windings). The induction of the electrical field into the rotorhappens because of the characteristic pulsing flow of current in AC. However, thishas other affects as well. • Reactance In an AC circuit, things are different, because in addition to there being a pure resistive load in the circuit there is also reactance in the circuit. Reactance is the unique effect that is displayed in opposition to AC current flow. There are two types of reactance, inductive reactance (that is, a coil), the tendency of the circuit to absorb and store an electrical potential, and capacitive reactance (that is, a capacitor), the tendency of the circuit to absorb and store current. AC circuits can always be quantified in terms of these three forces: resistance, inductive reactance, and capacative reactance. The total oppostion to the AC current flow is called impedance, and it is the vectored sum of the circuit resistance plus the total reactance, inductive and capacitive. Since inductive and capacitive reactance are forces of opposite direction, they counter each other, thus, 1. Inductive Reactance, Xl, = 2PiFL 2. Capacitive Reactance, Xc, = 1/(2PiFC) where • Xl = Inductive Reactance in Ohms,
• Xc = Capacitive Reactance in Ohms, and • F = the frequency of the applied AC in Hertz (cycles per second), • L = the inductence of the circuit in Henries, and • C = the capacitence of the circuit in Farads 3. Impedance, Z, = (R2 * X2)1/2, where X2 = (Xl - Xc)2 where • Z = Impedance in Ohms, • R = Resistance in Ohms, and • X = Reactance in Ohms AC induction motors are primarilly inductive circuits, so effectively their impedence may be expressed by the formula Z = (R2 * Xl2)1/2• Power Factor In an AC circuit, voltage times current does not equal power; it equals the effective value of voltage and current, which is measured in "voltamperes" (VA). Correcting voltamperes for "power factor" produces the useful or actual power in the circuit, which is measured in watts. So, P = VA * pf and the value of pf is determined by how much the voltage and current are out of phase. An incandescent light bulb has a power factor of anywhere from 0.95 to 0.99; AC motors may have power factors ranging from .6 to .9; in all of these situations, the current is lagging the voltage -- inductive circuits.• Phase Angle A purely reactive circuit has a phase angle between the current and voltage of 90 degrees, which results in a power factor of 0.0. The relationship between phase angle and power factor is that power factor equals the cosine of the phase angle. Therefore, power equals the cosine of the phase angle times the voltamperes. In the above example, the cosine of 90 degrees = 0.0. So, at rest, with 90 degree phase angle (purely reactive
circuit -- the resistance of the motors windings is minimal), the useful power of the motor is . . . 0 watts! There are starting strategies, for example, any substantive resistance in the circuit will reduce the phase angle below 90 degrees, thus increasing power factor above 0 and allowing some work to get done. More typically, capacitor-based systems can reduce the phase angle and can be used to start the motor. The situation that has developed is that the power developed in an AC motor is related to the magnitude of the voltage, the current, and the internal resistance of the motor (i.e., the simple resistance of the wires), and the frequency of the AC applied to the motor, because the frequency will change the phase angle. (This concept is expanded upon on the Formulas and Concepts page under Power Facts/AC Motor Facts.) • Speed Control in AC Motors Since an AC motors speed is based on the frequency of the AC, a change in frequency directly results in a change in speed; however, the change in frequency also changes the reactance of the circuit (because a reduction in the frequency causes a linear reduction in inductive reactance). This in turn changes the impedance of the circuit, the oppositon to the flow of AC current. Speed control may be accomplished in these motors by utilizing solid-state, micro-processor control devices that vary the frequency and voltage of the AC applied to the motor. If it were intended to slow an AC motor, the frequency of the applied AC would be reduced: as frequency decreases, circuit reactance decreases, and therefore impedance also decreases. Given a constant voltage, current would increase, potentially to the point where the motor would be damaged. Therefore, a decrease in the frequency must be accompanied by a decrease in voltage sufficient to stabilize the current. As one last reminder, there is much more detailed information on ourFormulas and Concepts page under Power Facts/AC Motor Facts.GASP!
Kilo Newtons, kilo Watts, kilometres per HourSo just what do terms used to describe the performance of locomotives and multiple units like MaximumTractive Effort, Power At Rail, and Continuous Power mean? Here is a guide to such things showing howthey influence journey times and speeds.Some School Physics RevisionA few basic physical relationships link the various factors that influence the acceleration and speed of anobject, in this case a train! The following notes explain those relationships.The application of a force to a mass will cause it to accelerate as governed by one of Newtons laws ofmotion. The relationship is that the force necessary is the product of the mass and the acceleration rate.i.e. Force = Mass x Acceleration (1)Here it is useful to point out that, in strict scientific terms, weight is the force acting on a mass resultingfrom the influence of the acceleration due to gravity (which is constant for all objects).The energy consumed in moving an object over a distance is the product of the force required and thedistance.i.e. Energy = Force x DistanceNow, power is the rate of energy usagei.e. Power = Energy/TimeAnd speed is the rate of travelling a distancei.e. Speed = Distance/TimeThese relationships may therefore be combinedso Power = Force x Speed (2)This introduction provides two relationships that will reappear later on.Units of MeasurementAll physical quantities have some unit of measurement assigned to them in order to support theserelationships numerically. The standard system of units across the world is the Systeme International (SI),from which many units are known colloquially as "metric". Within this system, the quantification of units isbased on 10s, 100s etc, with the main divide points every 1000 (e.g. millimetres, metres and kilometres).Before this system was introduced, various other units were used, often referred to as "imperial", wherethe links between sub-units were not so mathematically straightforward (e.g. inches, yards, miles).
The rest of this article will use SI units for all but miles, but the following section explains the units foreach of the quantities already introduced, and shows their conversion to imperial units which may well bemore familiar to many readers.Quantity SI Unit SI Unit Imperial Unit Imperial Unit ConversionSI Imperial Symbol Unit Name Symbol Name Unit (approx.)Force Newton N Pound force lbf 1N 0.22 lb fMass Kilogram kg Pound lb 1kg 2.2 lbDistance Metre m Yard yd 1m 1.09 ydDistance Kilometre km Mile mile 1 km 0.62 mileTime Second sSpeed Metres per m/s Miles per hour mph 1 m/s 2.2 mph secondSpeed Kilometres km/h Miles per hour mph 1 km/h 0.62 mph per hourAcceleration Metres per m/s/s second per orm/s2 secondEnergy Joule JPower Watt WWith the SI unit system, a largely standard means of sub-dividing the units using a prefix is employed soas to keep the figures quoted sensible. These are broken down in intervals of 1000, although someintermediate intervals occur. The following table lists the commonly used prefixes. Note that the oneexception to these is the base unit of mass being the kilogram, with a thousandth of a kilogram being agram and a thousand kilograms being a tonne!Prefix Symbol Intervalmilli m 1/1000
centi c 1/100deci d 1/10 1kilo k 1000mega M 1 000 000Anyway, now we get to the trains at last……..Getting GoingTractive EffortTractive Effort (TE) is the name for the force applied to the rail by the wheel of the train to causemovement. The size of that force is determined by the characteristic of the power equipment installed onthe train, and how the driver uses it.By necessity, this tractive effort is not constant throughout the speed range, and most traction units havea characteristic that looks something like Fig 1.Fig 1:
In the example characteristic shown, the TE is constant up to 20 mph, therefore in this speed range, fromrelationship (1) above, the acceleration will be constant. As a result of this, speed will build up uniformlywith time as shown in Fig 2. This is the region of Maximum Tractive Effort.Fig 2:Above this speed, the TE falls, and in consequence the acceleration will start to fall and speed will notbuild up so quickly. The plot of speed with time, now starts to curve as shown in Fig 3.
Fig 3:PowerRelationship (2) above says that power is the product of force and speed. Now, if the force, or TE were toremain constant with increasing speed, the power requirement would continue to rise throughout thespeed range. Practically, this is not possible as the necessary equipment becomes unfeasibly big andcostly, so, when the maximum power capability (or rating) of the equipment is reached, the TE must startto be reduced as speed increases to compensate. This occurs at the "knee" point at 20mph on the aboveTE-speed curve (Fig 1).So, in the example given, the maximum TE of the unit is 100kN, and hence the maximum power may becalculated as follows:Speed in m/s from above table = 20/2.2 = 9.1 m/sPower = Force x Speed= 100kN x 9.1 m/s= 910kWFig 4:
As this is the power needed to actually move the train it is strictly referred to as the Maximum Power atRail.In reality, the total power drawn from the supply (whether overhead wire, third rail, or fuel tank) will begreater than 910kW, due to the need for additional auxiliary loads (for lighting, heating, cooling etc) anddue to losses in the conversion process, as nothing is 100% efficient.Further, it is highly unlikely that the equipment is capable of running at this power level continuously, andindeed for many types of service, it would offer little advantage relative to the associated cost. Again, forreasons of rating the characteristic of the equipment will not follow the curve of maximum power to topspeed, as indicated by the dip from 70mph onwards in Figs 1 & 4. Consequently a continuous powerrating will often also be quoted.This continuous power rating may be derived from a number of factors based around the equipmentcharacteristic and will including assumptions of proportion of time at a lower tractive effort demand(drivers controller) or coasting.Train ResistanceSo thats how a train is controlled to get it moving, but in practice there are a number of other forces whichact to make life difficult.Friction is always present where motion is concerned, and indeed, there is a certain minimum amountwhich must be overcome before any movement can take place (often known as stiction!).Air resistance, or drag, is another important factor which becomes increasingly significant with speed.Pointed noses help reduce this.
These factors are accounted for mathematically using results found by measurement and experience, astheoretical calculation would be far too complex.Generally train resistance is expressed as: 2R = a + bv + cv where v = speedThe factors a, b and c characterise the particular train, with a being the stiction referred to above, b arisesfrom other mechanical considerations, and c is due to the air resistance.The train resistance typically looks something like that shown in Fig 5.Fig 5:There are further factors to take into account which depend on the route. The main one of these isgradient, which brings in the effect of gravity.If the train was travelling vertically upwards (i.e. it thought it was the space shuttle at take off), it wouldincur the full effect of gravity. As explained earlier, the acceleration due to gravity is constant.Mathematically, it is known as g (as in the term g forces in also the best quality intellectual films!) and is 29.81 m/s .For example, for a 150 tonne (150 x 1000 kg) train, the gravitational force acting on it is:Force = Mass x Acceleration= 150 x 1000 x 9.81= 1 471 500 N
= 1 471.5 kNThis is the weight of the train.Now, even the Lickey incline isnt that steep, so the gravitational resistance practically encountered isntnearly so great. While its not completely accurate, for the gradients encountered by trains, it suffices todivide the weight by the gradient to obtain the value for this resistance.So, for example if the above train were climbing a 1 in 200 gradient, the resistance due to gravity wouldbe:1 471.5/200= 7.3575 kNThis resistance is constant irrespective of speed and thus simply adds to the train resistance. When thetrain is going downhill, this figure is subtracted from the train resistance - i.e. it assists the train.The effect of gradient is seen in Fig 6.Fig 6:Now, how do these forces look compared to the Tractive Effort developed by the trainFig 7:
As long as the train produces Tractive Effort greater than the overall train resistance, then it willaccelerate. The point at which the two curves cross is when it will cease to accelerate and is known asthe balancing speed and is the maximum speed attainable on that particular track. In the example here itis 95 mph on the level, but 75 mph on a 1 in 100 gradient.The force available to accelerate the train is the difference between the Tractive Effort and the trainresistance. Thus it will be realised that an earlier statement about constant acceleration, when the TE isconstant, is not strictly correct. In practice the acceleration will reduce as the resistance increases withspeed. Additionally it will be noted that train resistance becomes increasingly significant as speedincreases.The following curve shows the actual build up in speed allowing for train resistance (Actual Characteristic)compared with the theoretical build up in speed seen earlier in Fig 3 (Ideal Characteristic):Fig 8: