1. The induction stroke takes air in ideally at constant volume, pressure at temperature. 2. The compression stroke takes place from A to B. The air is compressed adiabatically to about 1/20 of its original volume. It gets hot. 3. From B to C fuel is injected in atomised form. It burns steadily so that the pressure on the piston is constant. 4. From C to D the power stroke moves the piston down as adiabatic expansion takes place. 5. D to A cooling and exhaust occurs.
• Course Learning Objective: Familiarize students to the basic cycle and design features of modern marine diesel enginesSpecific Objectives:- Define the theory and principle of Internal Combustion Engine- Describe basic operations of working cycle- Identify the engine timing diagram - Describe differences and advantages of 2S & 4S
Principle of I.C.E• An internal combustion engine is one in which the fuel is burnt within the engine -> usually of the reciprocating type.• It involve system where combustion of the fuel and the conversion of the heat energy from combustion to mechanical energy takes place within the cylinders (ICE)
I.C.E. CATEGORIES– Spark ignition engines use gaseous or volatile distillate fuels -> work on a modified Otto cycle -> operate on the 2 or 4 – stroke cycle.– Compression ignition engine use distillate liquid fuels -> work on either 2 or 4 – stroke cycle and normally designed to operate on the dual-combustion cycle (Otto and Diesel cycle)
Otto cycle• In the Otto cycle the theoretical pressure – volume diagram is formed from : two constant – volume and two adiabatic processes.• The air in the cylinder is compressed adiabatically.• Heat is added to the air at constant volume -> Work is done during the adiabatic expansion and -> then heat is rejected at constant volume
Otto Cycle Pressure (Pa) C P2 D P1 B A V1 V2 Volume (m3)A–B : Adiabatic compressionB–C : Heat received at constant volume (combustion)C–D : Adiabatic expansionD–A : Heat rejected at constant volume (exhaust)
Otto Cycle• 1. The induction stroke takes place at A. Although in theory the pressure should be the same as atmospheric, in practice its rather lower. The amount of petrol air mixture taken in can be increased by use of a supercharger.• 2. A to B is the compression stroke. Both valves are closed. The compression is adiabatic, and no heat enters or leaves the cylinder. • 3. Ignition occurs at C. The gases resulting from the ignition expand adiabatically, leading to the power stroke.• 4. D to A the gas is cooled instantaneously. • 5. At A the exhaust stroke occurs and the the gases are removed at constant pressure to the atmosphere.• 6. Strange as it may seem, the piston does half a revolution at A. Actually its slightly in practice, as the the valve timing is more complex.
Diesel cycle• In the diesel cycle the theoretical pressure- volume diagram is formed from two adiabatic operations, one constant-pressure and one constant-volume operation.• Air is compressed adiabatically, then heat is added at constant pressure. Adiabatic expansion takes place and then heat is rejected at constant volume
Diesel Cycle Pressure (Pa) B C P D A V1 V2 Volume (m3)A–B : Adiabatic compressionB–C : Heat received at constant pressure(combustion)C–D : Adiabatic expansionD–A : Heat rejected at constant volume
Diesel cycle• 1. The induction stroke takes air in ideally at constant volume, pressure at temperature.• 2. The compression stroke takes place from A to B. The air is compressed adiabatically to about 1/20 of its original volume. It gets hot.• 3. From B to C fuel is injected in atomised form. It burns steadily so that the pressure on the piston is constant.• 4. From C to D the power stroke moves the piston down as adiabatic expansion takes place.• 5. D to A cooling and exhaust occurs.
Dual cycle• In the dual cycle, air is compressed adiabatically, then heat is added, partly in a constant volume process and the remainder in a constant pressure process.• Expansion takes place adiabatically and then heat is rejected at constant volume
Pressure (Pa) C D P E B A V1 V2 Volume (m3)A–B : Adiabatic compressionB–C : Heat received at constant volumeC–D : Heat received at constant pressureD–E : Adiabatic expansionE–A : Heat rejected at constant volume
COMPRESSION IGNITION ENGINE• Compression ignition engine works on dual cycle• The fresh air enters each of the engine cylinders and is compressed by the upward movement of the piston.• The compression causes the temperature and pressure of the fresh air to increase• Fuel injectors or fuel valve will supply the fuel oil in fine spray when the piston is nearly at top dead centre
• The fuel will then be mixed with air (compressed) and burn inside the cylinder when the piston is at TDC.• The expanding gases on top of the piston (completed combustion) will push the piston moving it downward and rotating the crankshaft .• The cycle will be repeated until the engine stops
Cycle of Operations• Four strokes of CI engine are as follows:- – Suction Stroke / Induction Stroke – Compression Stroke – Explosion Stroke / Power Stroke – Exhaust Stroke
SUCTION STROKE• In which the air is admitted to the engine cylinder
COMPRESSION STROKE• In which the charge of fresh air is compressed by the piston, and fuel is injected just before the point of maximum compression
POWER STROKE• In which the air- fuel mixture is ignited by the heat produced by compression of air• The pressure rises due to fuel combustion and pushes piston downwards to drive the engine
EXHAUST STROKE• Exhaust valve opens at the end of power stroke• The expanded burnt gases are exhausted / expelled from the cylinder
• The four strokes in duel cycle of CI engine are completed in two revolutions of the crankshaft.• There are thus two piston strokes in each revolution of the crankshaft
FOUR STROKE ENGINEINLET VALVE CYLINDER HEAD FUEL INJECTOR EXHAUST VALVE PISTON CYLINDER LINER CRANKSHAFT DIRECTION CRANK PININDUCTION STROKE / COMPRESSION STROKE POWER / EXPANSION STROKE EXHAUST STROKE STROKE EXHAUSTSCAVENGE STROKE
How strokes are executed• Strokes are executed by combination of valves and gears
SUCTION / INDUCTION STROKE• Piston draws air into cylinder during downward movement or stroke through opened inlet valve. (suction effect)• Exhaust valve and fuel injector are closed• At the end of the stroke (BDC) the inlet valve close, which inside the cylinder now full with fresh air.
COMPRESSION STROKE• Stroke begins when the piston starts to move upward (from BDC to TDC).• Inlet valve, exhaust valve and fuel injector remain closed.• The air which is trapped in the cylinder is now compressed rising in temperature
POWER STROKE• Before the piston reaches TDC(approx.15 – 20o), the fuel injectors supply fuel oil in a fine spray(end approx. 15-20o after TDC)• The mixture (fuel oil and air) ignites and explodes while the piston crosses TDC• High pressure (expansion of the gases) on top of the piston push the piston downward towards BDC
EXHAUST STROKE• Stroke begins when the piston again starts to move upward (from BDC to TDC) as in compression stroke, however only exhaust valves are opened.• The exhaust gases are expelled from the cylinder through the exhaust valve ports.• At the end of the stroke (TDC), the exhaust valve closes but inlet valve is opened starting the cycle once again
Power produced• Power produced by a 4-stroke cycle engine in kW is given as PLAN Power = 2 P= Mean effective pressure, kN/m2 L= Stroke length, m A= Area of cylinder bore, m2 N= Revolution/second
4 - STROKE CYCLE• 1-2 Suction stroke ends• 2-3 Compression stroke. Inlet valve closed and piston moved upwards to compress the trapped air (Temperature rises).• 3-4-5 Fuel injector in operation. Combustion occurs (mixture of compressed air and fuel)• 5-6 Due to expansion of gases piston moves downward. (Power stroke)• 6-7-8 Exhaust stroke. Exhaust valve opens and piston moves upward removing gases.• 8-9-10 Overlapping period: both exhaust and inlet valves are open.• 10-1 Suction stroke – piston moves downward. Exhaust valve closed and inlet valve open.• 1- the rest – The cycle continues until the engine stops
Exh. v/v closes Fuel Fuel injection injection begins ends PO COMPRESSION STROKE W E ER T STROK Inlet v/v ST opens RO SUC KE RotationEXHAUS TIO N STR Inlet v/v OKE closes Exh. v/v opens FOUR STROKE TIMING DIAGRAM
VALVE OVERLAPPINGIt can be defined as the period when inlet and exhaust valve were open at the same time.E.g.,• Inlet valve opened before the piston reached TDC at the end of exhaust stroke, say 20o before TDC.• Exhaust valve remained open and will be closed at certain degree of the piston movement after TDC, say 20o after TDC.• By providing overlapping period on 4 – stroke engine, the residual exhaust gases will be expelled effectively with the rushing in of fresh air.
• Learning Objective: Know the basic cycle and design features of modern marine diesel enginesSpecific Objectives:• Describe the operation cycle process of a 2-stroke diesel engine.• Identify the 2-stroke engine timing diagram
TWO STROKE CYCLE• The two stroke cycle is so called because it takes two strokes of the piston or one revolution of crank shaft to complete the processes needed to convert the energy in the fuel into work.
Why 2-Stroke Cycle Engines• We know 4-stroke cycle engine gives only one power stroke out of 4 strokes of the piston or one power stroke in two revolutions of the crank shaft.• This makes engine’s power to weight ratio low mainly because three strokes consume power against one which produces
2S• In the two stroke engine, cycle is completed in two strokes of the piston or one revolution of the crankshaft.• Thus out of 3 power consuming strokes of the 4- stroke cycle two strokes are saved• Engine thus produces one power stroke in every revolution of the engine which is two times in comparison to 4-stroke cycle• This improves power to weight ratio of the engine and reduces its size for same power.
2S• 2-Stroke cycle is achieved by eliminating suction and exhaust strokes of the 4-stroke cycle• In order to eliminate suction and exhaust strokes, some special arrangements are required to be provided for:- -.charging air into cylinder without suction from piston - Exhaust gases must be expelled out of the cylinder without assistance from piston
Power Piston Comp stroke stroke Exst port Exst port Inlet air PistonInlet air portport
The crankshaft is revolvingclockwise and the piston ismoving up the cylinder,compressing the charge ofair.Because energy is beingtransferred into the air,pressure and temperatureincrease.By the time the piston is nearthe top of the cylinder(known as Top Dead Centeror TDC) the pressure is >100bar and the temperature >500°C
Just before TDC fuel is injectedinto the cylinder by the fuelinjector.The fuel is "atomised" into tinydroplets. Being very small, thesedroplets heat up very quickly andstart to burn as the piston passesover TDC.The expanding gas from the fuelburning in the oxygen forces thepiston down the cylinder, turningthe crankshaft.It is during this stroke that workenergy is being put into theengine; during the upward strokeof the piston, the engine is having
As the piston moves down thecylinder, the useful energyfrom the burning fuel isexpended.At about 110° after TDC theexhaust valve opens and thehot exhaust gas (consistingmostly of nitrogen, carbondioxide, water vapour andunused oxygen) begin toleave the cylinder.
At about 140º after TDC thepiston uncovers a set of portsknown as scavenge ports.Pressurized air enters thecylinder via these ports andpushes the remainingexhaust gas from thecylinder, "scavenging".The piston now goes pastBDC and starts moving upthe cylinder, closing thescavenge ports. The exhaustvalve then closes andcompression begins.
The two stroke cycle can also be illustrated on a timing diagram. 1 -2 Compression 2 - 3 Fuel Injection 3 - 4 Power 4 - 5 Exhaust Blowdown 5 - 6 Scavenging 6 - 1 Post Scavenging 1. approx 110º BTDC 2. approx 10º BTDC 3. approx 12º ATDC 4. approx 110º ATDC 5. approx 140º ATDC 6. approx 140º BTDC
• 1-2 Scavenging period, both exhaust and inlet ports are open.• 2-3 Scavenge stroke ends. Exhaust ports remain open to ensure only fresh air remains in the cylinder.• 3-4 Compression takes place. Both ports closed. The air is then compressed by the upward movement of the piston.• 4-5-6 Fuel injector is operational supplying fuel oil.• 6-7 Due to expansion of gases, piston moves downward. (Power stroke)• 7-8 When piston crown/top ring passes the exhaust ports, exhaust begins• 8-1 When the piston passes the inlet ports, Scavenging begins and fresh air fills the cylinder, thus pushing the remaining exhaust gases out
Fuel Fuel injection injection begins ends ON POW SI ES ER R MP STRCO OK Rotation E Scavenge Scavenge ports ports close open Exhaust Exhaust SCAVENGE ports ports close open EXHAUST TWO STROKE TIMING DIAGRAM
The 2 stroke crosshead engine works on exactlythe same principleand cycle as the 2stroke trunk piston engine.
The disadvantages of the two stroketrunk piston engine are that:It has a low overall height, lubricatingoil splashed up from the crankcase tolubricate the liner can find its way intothe scavenge space, causing foulingand a risk of fire.There is also the likelihood of liner andpiston skirt wear, allowing air into thecrankcase. This can supply therequired oxygen for an explosionshould a hot spot develop.The crankcase oil must have additiveswhich can cope with contaminationfrom products of combustion, and theacids formed during combustion due tothe sulphur in the fuel.
The majority of 2 stroke engines encountered at sea are of the "crosshead" type.In this type of engine the combustion space (formed by the cylinder liner, pistonand cylinder head), and the scavenge space are separated from the crankcase bythe diaphragm plate.The piston rod is bolted to the piston and passes through a stuffing box mountedin the diaphragm plate. The stuffing box provides a seal between the two spaces,stopping oil from being carried up to the scavenge space, and scavenge air leakinginto the crankcase.The foot of the piston rod is bolted to the crosshead pin. The top end of theconnecting rod swings about the crosshead pin, as the downward load from theexpanding gas applies a turning force to the crankshaft.To ensure that the crosshead reciprocates in alignment with the piston in thecylinder, guide shoes are attached either side of the crosshead pin. These shoesare lined with white metal, a bearing material and they reciprocate against thecrosshead guides, which are bolted to the frame of the engine. The crossheadguides are located in-between each cylinder.Using the crosshead design of engine allows engines to be built with very longstrokes - which means the engine can burn a greater quantity of fuel/stroke anddevelop more power. The fuel used can be of a lower grade than that used in atrunk piston engine, with a higher sulphur content, whilst high alkalinity cylinderoils with a different specification to that of the crankcase oil are used to lubricatethe cylinder liner and piston rings and combat the effects of acid attack.
SCAVENGING• To ensure a sufficient supply of fresh air for combustion by removing all remaining exhaust gases by blowing with these fresh air.• Supercharging is a large mass of air that is supplied to the cylinder by blowing it in under pressure either by electrically driven auxiliary blower or exhaust gas driven turbocharger.• The flow path of the scavenge air is decided by the engine port shape and design and the exhaust arrangements.
SCAVENGING PERIODIt can be defined as a period when inlet and exhaust are open at the same time:• Remaining exhaust gas will be expelled from the cylinder through exhaust ports or exhaust valve (if fitted).• Fresh air which has collected in the scavenge manifold rush into the cylinder• Scavenging period: Normally when piston is at BDC, (or as per maker or engine design or the location of the ports itself)
2 stroke engines do not have exhaustvalves; With scavenge ports in the cylinderliner, they are fitted with exhaust portslocated just above the scavenge ports.As the piston uncovers the exhaust ports onthe power stroke, the exhaust gas starts toleave the cylinder.When the scavenge ports are uncovered,scavenge air loops around the cylinder andpushes the remaining exhaust gas out ofthe cylinder.This type of engine is known as a loopscavenged engine. Note that the pistonskirt is much longer than that for a uniflowscavenged engine. This is because the skirthas to seal the scavenge and exhaust portswhen the piston is at TDC.
TWO STROKE ADVANTAGES• Compactness in relation to the power output. Not required to increase brake mean effective pressure or the engine speed to increase rating.(High bmep increases the stresses on engine components, greater rate of cylinder wear, whilst the alternative of higher speed, valve flutter may become a serious problem)• Each out-stroke being a working stroke gives more even turning for the same number of cranks, consequently a lighter flywheel may be employed.• The reversing operation of rotation is simplified since there is less valve gear to contend with.
OTHER ADVANTAGES• Fewer moving parts and lower maintenance• Lower specific fuel consumption• No gear loss• Simplicity in construction• Longer life time• Higher reliability (product)• Low lubricating oil consumption• Better ability to burn low quality fuel oil
FOUR STROKE ADVANTAGES• Good volumetric efficiency, good combustion characteristic and positive exhaust scavenging.• The thermal and mechanical efficiencies are slightly better than 2S engine.• Only half the quantity of the heat generated in the cylinders has to be dealt within a given time, so that efficient lubrication of the piston and cooling of the cylinder is more easily accomplished.
OTHER ADVANTAGES• Lower initial cost for equivalent power• Ease of installation• Lower weight per unit power• Saving in weight and engine room length• Increased cargo capacity• Free choice of propeller speed through gearing• Suitable for electrical power take off
Supercharging/Turbocharging• Process of pushing a higher pressure air charge into the cylinder greater than atmospheric pressure, so that extra mass of air can be delivered into cylinder to burn more fuel and produce extra power.• Turbocharging can increase power output of engine by 60%
Turbocharging• Very effective pressure charging.• Utilizes 20% of waste heat in exhaust gas which contains 35% of fuel heat.• How?
•By increasing mass of air in cylinder, more fuel can beburned and correspondingly power output will beincreased•Various methods can be adopted: –Electrically powered auxiliary blower –Utilization of heat energy from exhaust gas to drive a single stage impulse turbine directly coupled to a simple blower (free running unit) called exhaust gas turbocharger Turbocharger utilizes free energy of exhaustgases and hence improves efficiency of the engine
Typical heat balance of an engineUseful Output (Brake Power) 34%Cooling Loss 30%Exhaust Loss 26%Friction, Radiation, etc. 10% -------Total Heat Input 100%
Advantages• Increased power for an engine of the same size OR reduction in size for an engine with the same power output.• Reduced specific fuel oil consumption -> mechanical, thermal and scavenge efficiencies are improved due to less cylinders, greater air supply and use of exhaust gasses.• Thermal loading is reduced due to shorter more efficient burning period for the fuel leading to less exacting cylinder conditions.
Design consideration• Types of fuel and fuel oil system design• Types of lubricating oil and lubricating oil systems• Cooling systems• Waste heat utilization systems• Intake and exhaust valve systems• Starting air systems• Instrumentation system• Control and automation system• Installation items• Safety features
Summary• Principle of ICE• Theoretical Cycles• Basic principle of operations of working cycle• Cycle & Timing Diagram• Principles of Scavenging & Arrangements• Advantages of 2S & 4S• Structural differences• Overlap of Inlet & Exhaust
References• Introduction to Marine Engineering,• Marine Engineering , Roy L. Harrington, SNAME, 198• El-Hawary, F. (2001). Ocean Engineering Handbook. CRC Press, UK.• Calder, Nigel (2007): Marine diesel engine: maintenance, troubleshooting and repair.