• Volumetric Efficiency: The ‘volumetric efficiency’ represents the degree of completeness with which the cylinder is re-charged with fresh combustible mixture and varies with different engines and also with the speed.• Combustion Efficiency: The ‘combustion efficiency’ represents the degree of completeness with which the potential heat units in the charge are produced as actual heat in the cylinder.• Thermal Efficiency: The ‘thermal efficiency’ governs the percentage of the actual heat units present in the cylinder which are converted into mechanical work.
Horsepower• Indicated horse power• Brake horse power
• Indicated Horse PowerLet, p = mean effective pressure, N/m2. D = diameter of cylinders, m. L = length of stroke, m. N = revolutions per minute. f = no. of effective strokes, per revolution per cylinder. n = number of cylinders.
Definitions• Top dead Centre : Position of the piston when it stops at the furthest point away from the crankshaft• Bottom Dead Centre : Position of the piston when it stops at the point closest to the crankshaft• Bore : Diameter of the cylinder or diameter of the piston face, which is the same minus a very small clearance• Stroke : Movement distance of the piston from one extreme position to the other: TDC to BDC or BDC to TDC
Definitions• Indirect Injection : Fuel injection into the secondary chamber of an engine with a divided combustion chamber.• Direct Injection : Fuel injection into the main combustion chamber of an engine. Engines have either one main combustion chamber (open chamber) or a divided combustion chamber made up of a main chamber and a smaller connected secondary chamber.
Definitions• Displacement volume : Volume displaced by the piston as it travels through one stroke. Also known as swept volume.• Clearance volume : Minimum volume in the combustion chamber with piston at Top Dead Centre.
Definitions• Ignition Delay : Time interval between ignition initiation and the actual start of combustion process.• Wide-Open Throttle : Engine operated with throttle valve fully open when maximum power and/or speed is desired.
Definitions• Compression Ratio: It is the ratio of the volume of the combustion chamber or the cylinder at the bottom dead center to the volume of the combustion chamber at the top dead center. i.e. Cr=VBDC/VTDC
Classification of Engines• Internal combustion• External combustion
Internal combustion Engine• It is a heat engine that converts chemical energy in a fuel into mechanical energy, usually made available on a rotating output shaft.• Chemical energy Heat energy Mechanical Energy
Classification of Internal combustion Engines• Type of stroke• Type of ignition• Type of design• Relative position of cylinders• Valve Location• Type of Fuel used• Air intake process• Method of Fuel Input
Type of stroke• Four-Stroke Cycle. A four-stroke cycle experiences four piston movements over two engine revolutions for each cycle.• Two-Stroke Cycle. A two-stroke cycle has two piston movements over one revolution for each cycle.• Six-Stroke Cycle.
Type of ignition• Spark Ignition (SI). An SI engine starts the combustion process in each cycle by use of a spark plug. The spark plug gives a high-voltage electrical discharge between two electrodes which ignites the air-fuel mixture in the combustion chamber surrounding the plug.• Compression Ignition (CI). The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression.
Type of design• Reciprocating. Engine has one or more cylinders in which pistons reciprocate back and forth. Power is delivered to a rotating output crankshaft by mechanical linkage with the pistons.• Rotary. Engine is made of a block (stator) built around a large non-concentric rotor and crankshaft. The combustion chambers are built into the non-rotating block.
Relative position of CylindersSingle CylinderIn-LineV EngineW engineOpposed Cylinder EngineOpposed piston EngineRadial engine
Single Cylinder. Engine hasone cylinder and pistonconnected to thecrankshaft.
• In-Line. Cylinders are positioned in a straight line, one behind the other along the length of the crankshaft. They can consist of 2 to 11 cylinders or possibly more. In-line four- cylinder engines are very common for automobile and other applications.
• V Engine. Two banks of cylinders at an angle with each other along a single crankshaft. The angle between the banks of cylinders can be anywhere from 15° to 120°, with 60°-90° being common. V engines have even numbers of cylinders.
Opposed Cylinder Engine.Two banks of cylindersopposite each other on asingle crankshaft (a Vengine with a 180°V).These are common onsmall aircraft and someautomobiles. Theseengines are often calledflat engines.
Sample Problem• What is the difference between opposed cylinder and V-cylinder configuration?
• W Engine. Same as a V engine except with three banks of cylinders on the same crankshaft. Not common, but some have been developed for racing automobiles, both modern and historic. Usually 12 cylinders with about a 60° angle between each bank.
Opposed Piston Engine.
• Two pistons in each cylinder with the combustion chamber in the center between the pistons.• A single-combustion process causes two power strokes at the same time, with each piston being pushed away from the center and delivering power to a separate crankshaft at each end of the cylinder.• Engine output is either on two rotating crankshafts or on one crankshaft incorporating complex mechanical linkage.
Valve Location• Valves in head• Valves in block• One valve in head and one in block
Valve in block, L head.Older automobiles and some small engines.
Valve in head, I head. Standardon modern automobiles.
One valve in head and one valve in block, Fhead. Older, less common automobiles.
Valves in block on opposite sides ofcylinder, T head.
Type of Fuel used• Gasoline.• Diesel Oil or Fuel Oil.• Gas, Natural Gas, Methane.• LPG.• Alcohol-Ethyl, Methyl.
Air Intake Process• Naturally Aspirated. No intake air pressure boost system.• Supercharged. Intake air pressure increased with the compressor driven off of the engine crankshaft.• Turbocharged. Intake air pressure increased with the turbine-compressor driven by the engine exhaust gases
Method of Fuel Input• Carbureted.• Multipoint Port Fuel Injection. One or more injectors at each cylinder intake.• Throttle Body Fuel Injection. Injectors upstream in intake manifold.
Engine Operation Cycle• Two stroke cycle• Four stroke cycle
Two Stroke Cycle• Compression Stroke• Expansion Stroke
First StrokeWith all valves (or ports)closed, the piston travelstowards TDC andcompresses the air-fuelmixture to a higherpressure andtemperature. Near theend of the compressionstroke, the spark plug isfired; by the time thepiston gets to IDC,combustion occurs andthe next engine cyclebegins.
Combustionoccurs very quickly,raising thetemperature andpressure to peakvalues, almost atconstant volume.
Second StrokeVery high pressurecreated by thecombustion processforces the piston downin the power stroke. Theexpanding volume of thecombustion chambercauses pressure andtemperature to decreaseas the piston travelstowards BDC.
Exhaust Blow downAt some point bBDC, theexhaust valve opens and blowdown occurs. The exhaustvalve may be a poppet valvein the cylinder head, or it maybe a slot in the side of thecylinder which is uncoveredas the piston approachesBDC. After blow down thecylinder remains filled withexhaust gas at lowerpressure.
Intake and ScavengingAt some point bBDC, the intakeslot on the side of the cylinder isuncovered and intake air-fuelenters under pressure.This incoming mixture pushesmuch of the remaining exhaustgases out the open exhaust valveand fills the cylinder with acombustible air-fuel mixture, aprocess called scavenging.The piston passes BDC and veryquickly covers the intake port andthen the exhaust port. The higherpressure at which the air entersthe cylinder is established in oneof two ways: supercharger andcrankcase.
Four stroke cycle• Intake stroke or Induction• Compression• Expansion stroke or Power Stroke• Exhaust Stroke
First StrokeThe piston travels from TDC toBDC with the intake valve open andexhaust valve closed. This creates anincreasing volume in the combustionchamber, which in turn creates avacuum. The resulting pressuredifferential through the intakesystem from atmospheric pressureon the outside to the vacuum on theinside causes air to be pushed intothe cylinder. As the air passesthrough the intake system, fuel isadded to it in the desired amount bymeans of fuel injectors or acarburetor.
Second StrokeWhen the piston reaches BDC,the intake valve closes and thepiston travels back to TDC withall valves closed. This compressesthe air-fuel mixture, raising boththe pressure and temperature inthe cylinder. The finite timerequired to close the intake valvemeans that actual compressiondoesnt start until sometimeaBDC. Near the end of thecompression stroke, the sparkplug is fired and combustion isinitiated.
CombustionCombustion of the air-fuel mixtureoccurs in a very short but finitelength of time with the piston nearTDC (i.e., nearly constant-volumecombustion). It starts near the endof the compression stroke slightlybTDC and lasts into the powerstroke slightly aTDC. Combustionchanges the composition of the gasmixture to that of exhaust productsand increases the temperature inthe cylinder to a very high peakvalue. This, in turn, raises thepressure in the cylinder to a veryhigh peak value.
Third StrokeWith all valves closed, the highpressure created by thecombustion process pushes thepiston away from TDC. This isthe stroke which produces thework output of the enginecycle. As the piston travels fromTDC to BDC, cylinder volume isincreased, causing pressure andtemperature to drop.
Exhaust Blow DownThe exhaust valve is opened andexhaust blow down occurs. Apressure differential is createdthrough the exhaust system which isopen to atmospheric pressurecausing much of the hot exhaust gasto be pushed out of the cylinder andthrough the exhaust system whenthe piston is near BDC. Opening theexhaust valve before BDC reducesthe work obtained during the powerstroke but is required because of thefinite time needed for exhaust blowdown.
Fourth StrokeBy the time the piston reaches BDC,exhaust blow down is complete, but thecylinder is still full of exhaust gases.With the exhaust valve remaining open,the piston now travels from BDC to TDCin the exhaust stroke. This pushes mostof the remaining exhaust gases out ofthe cylinder, leaving only that trapped inthe clearance volume when the pistonreaches TDC.Near the end of the exhaust strokebTDC, the intake valve starts to open, sothat it is fully open by TDC when the newintake stroke starts. Near TDC theexhaust valve starts to close and finally isfully closed sometime aTDC. This periodwhen both the intake valve and exhaustvalve are open is called valve overlap.
Difference between SI and CI Engine• Type of fuel used• Type of cycle used• Introduction of the fuel - In SI, through carburetor/fuel injectors in intake stroke and in CI through fuel pump in combustion stroke.• Introduction of the air - In CI turbochargers or superchargers are used where as there is no such thing in SI engines.
• Ignition of fuel - Petrol is a highly volatile liquid, but its self-ignition temperature is high. Hence for the combustion of this fuel spark is necessary to initiate its burning process.• With diesel, the self-ignition temperature is comparatively lower. When diesel fuel is compressed to high pressures, its temperature also increases beyond the self-ignition temperature of the fuel. Hence in the case of CI engines, the ignition of fuel occurs due to compression of the air-fuel mixture and there is no need for sparkplugs.
• Compression Ratio for the case of SI engines, the compression ratio of the fuel is in the range of 6 to 10 depending on the size of the engine and the power to be produced. In CI engines, the compression ratio for air is 16 to 20. The high compression ratio of air creates high temperatures, which ensures the diesel fuel can self-ignite.• Weight of the engines• Speed achieved by the engine• Thermal efficiency of the engine
• Air Fuel Ratio: The air-fuel mixture is homogeneous throughout in SI while in CI engines only air enters and mixture is heterogeneous in the cylinder.• Engine Strength: Compression ignition engine has to be built much stronger than an spark ignition engine due to the fact that the Compression Ignition engine will "Detonate" the fuel and will cause it to explode . This is why diesel engines are so much louder than gas engines. Spark Ignition system ignites the fuel and it burns in a rapid but controlled manner that “Pushes" the piston down during the power stroke and puts less stress on engine parts.
Sample Problem• Write 2 advantages of two stroke engine over four stroke engines and 2 advantages of four stroke engines over two stroke engines
Detonation in SI Engine• During the combustion of the fuel through spark plug, the end charge auto-ignites itself before the flame front reaches it, hence this abnormal combustion is called detonation or knocking.• Auto-ignition occurs at a very critical temperature and the time required by the fuel to reach this temperature is called ignition- delay.• In auto-ignition, the burning is almost instantaneous which results in extremely rapid release energy and hence causing the pressure of the end gases to rise to almost 3 to 4 times.• This large pressure differential gives rise to severe pressure wave which strikes the cylinder wall and sets it vibrating, giving high pitched metallic pinking or ringing sound.• To avoid this, a fuel should have high auto-ignition temperature and long ignition-delay in SI engine.
Engine Problems• Three fundamental things can happen: – A bad fuel mix – Lack of compression – lack of spark• Beyond that, thousands of minor things can create problems, but these are the "big three." Based on the simple engine we have been discussing, here is a quick rundown on how these problems affect your engine:
Bad Fuel Mix• You are out of gas, so the engine is getting air but no fuel.• The air intake might be clogged, so there is fuel but not enough air.• The fuel system might be supplying too much or too little fuel to the mix, meaning that combustion does not occur properly.• There might be an impurity in the fuel (like water in your gas tank) that makes the fuel not burn.
Lack of Compression• If the charge of air and fuel cannot be compressed properly, the combustion process will not work like it should. Lack of compression might occur for these reasons:• Your piston rings are worn (allowing air/fuel to leak past the piston during compression).• The intake or exhaust valves are not sealing properly, again allowing a leak during compression.
Lack of spark• The spark might be nonexistent or weak for a number of reasons:• If your spark plug or the wire leading to it is worn out, the spark will be weak.• If the wire is cut or missing, or if the system that sends a spark down the wire is not working properly, there will be no spark.• If the spark occurs either too early or too late in the cycle, the fuel will not ignite at the right time, and this can cause all sorts of problems.
• Many other things can go wrong. For example: – If the battery is dead, you cannot turn over the engine to start it. – If the bearings that allow the crankshaft to turn freely are worn out, the crankshaft cannot turn so the engine cannot run. – If the valves do not open and close at the right time or at all, air cannot get in and exhaust cannot get out, so the engine cannot run. – If someone sticks a potato up your tailpipe, exhaust cannot exit the cylinder so the engine will not run. – If you run out of oil, the piston cannot move up and down freely in the cylinder, and the engine will seize.
• Engine block Body of engine containing the cylinders, made of cast iron or aluminum.
Cylinder Head• The piece which closes the end of the cylinders, usually containing part of the clearance volume of the combustion chamber.• It is usually cast iron or aluminum, and bolts to the engine block.• The head contains the spark plugs in SI engines and the fuel injectors in CI engines and some SI engines.• Most modern engines have the valves in the head, and many have the camshaft(s) positioned there also (overhead valves and overhead cam).
Camshaft• Rotating shaft used to push open valves at the proper time in the engine cycle, either directly or through mechanical or hydraulic linkage.• Camshafts are generally made of forged steel or cast iron and are driven off the crankshaft by means of a belt or chain.• In four-stroke cycle engines, the camshaft rotates at half engine speed.
Crankshaft• Rotating shaft through which engine work output is supplied to external systems.• The crankshaft is connected to the engine block.• It is rotated by the reciprocating pistons through connecting rods connected to the crankshaft.• Most crankshafts are made of forged steel, while some are made of cast iron.
Cylinder• Portion of engine block in which the pistons reciprocate back and forth.• The walls of the cylinder have highly polished hard surfaces.• Cylinders may be machined directly in the engine block, or a hard metal sleeve may be pressed into the softer metal block.• Sleeves may be dry sleeves, which do not contact the liquid in the water jacket, or wet sleeves, which form part of the water jacket.
Piston• The cylindrical-shaped mass that reciprocates back and forth in the cylinder, transmitting the pressure forces in the combustion chamber to the rotating crankshaft.• The top of the piston is called the crown and the sides are called the skirt.• Pistons are made of cast iron, steel, or aluminum.
Connecting Rods• Rod connecting the piston with the rotating crankshaft, usually made of steel or alloy forging in most engines but may be aluminum in some small engines.
Piston & Connecting Rods
Piston Rings• Metal rings that fit into circumferential grooves around the piston and form a sliding surface against the cylinder walls.• Piston rings are made of highly polished hard chrome steel.• The purpose of these is to form a seal between the piston and cylinder walls and to restrict the high-pressure gases in the combustion chamber from leaking past the piston into the crankcase.
Piston rings Ride in grooves in piston head to seal the cylinder TYPESCompression rings : Prevent pressure leakage into the crankcase.
Spark Plug• Electrical device used to initiate combustion in an SI engine by creating a high-voltage discharge across an electrode gap.• Spark plugs are usually made of metal surrounded with ceramic insulation.
Exhaust System• Flow system for removing exhaust gases from the cylinders, treating them, and exhausting them to the surroundings.• It consists of an exhaust manifold which carries the exhaust gases away from the engine, a thermal or catalytic converter to reduce emissions, a muffler to reduce engine noise, and a tailpipe to carry the exhaust gases away from the passenger compartment.
Fuel Injector• A pressurized nozzle that sprays fuel into the incoming air on SI engines or into the cylinder on CI engines.• On SI engines, fuel injectors are located at the intake valve ports on multipoint port injector systems and upstream at the intake manifold inlet on throttle body injector systems.• In a few SI engines, injectors spray directly into the combustion chamber
Fuel Pump• Pump to supply fuel from the fuel tank (reservoir) to the engine.• Many modern automobiles have an electric fuel pump mounted submerged in the fuel tank.• Some small engines and early automobiles had no fuel pump, relying on gravity feed.
Water Jacket• System of liquid flow passages surrounding the cylinders, usually constructed as part of the engine block and head.• Engine coolant flows through the water jacket and keeps the cylinder walls from overheating.• The coolant is usually a water-ethylene glycol mixture.
Radiator• Liquid-to-air heat exchanger of honeycomb construction used to remove heat from the engine coolant after the engine has been cooled.• The radiator is usually mounted in front of the engine in the flow of air as the automobile moves forward.• An engine-driven fan is often used to increase air flow through the radiator.
Glow Plug• Small electrical resistance heater mounted inside the combustion chamber of many CI engines.• It is used to preheat the chamber enough so that combustion will occur when first starting a cold engine.• The glow plug is turned off after the engine is started.
Valves• Used to allow flow into and out of the cylinder at the proper time in the cycle.• Most engines use poppet valves, which are spring loaded closed and pushed open by camshaft action.• Valves are mostly made of forged steel.• Valve seats and are made of hardened steel or ceramic.
Smart engine• Engine with computer controls that regulate operating characteristics such as air-fuel ratio, ignition timing, valve timing, exhaust control, intake tuning, etc.• Computer inputs come from electronic, mechanical, thermal, and chemical sensors located throughout the engine.• In automobiles the same computers are used to make smart cars by controlling the steering, brakes, seats, anti-theft systems, sound systems, navigation, noise suppression, environment etc.• On some systems engine speed is adjusted at the instant when the transmission shifts gears, resulting in a smoother shifting process.
Catalytic Converter• It is the chamber mounted in exhaust flow containing catalytic material that promotes reduction of emissions by chemical reaction.• Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion carbon monoxide, unburned hydrocarbons, and oxides of nitrogen are converted to less-toxic or inert substances such as carbon dioxide, hydrogen, nitrogen and oxygen.
Engine Emissions• One of the biggest problem with the automobiles.• Can’t be completely normalized.• Compromises have to be done with the power to decrease the emissions.
Emissions• Hydrocarbons• Carbon Monoxide• Oxides of Nitrogen (NOx)
• Hydrocarbons are fuel molecules which did not get burned and smaller non-equilibrium particles of partially burned fuel.• Carbon monoxide occurs when not enough oxygen is present to fully react all carbon to CO2 or when incomplete air-fuel mixing occurs due to the very short engine cycle time.• Oxides of nitrogen are created in an engine when high combustion temperatures cause some normally stable N2 to dissociate into monatomic nitrogen N, which then combines with reacting oxygen.
Prevention• To improve the technology of engines and fuels so that better combustion occurs and fewer emissions are generated.• The second method is after treatment of the exhaust gases. This is done by using thermal converters or catalytic converters that promote chemical reactions in the exhaust flow.• These chemical reactions convert the harmful emissions to acceptable CO2, H20, and N2.