View
247
Download
0
Category
Preview:
Citation preview
ADDIS ABABA UNIVERSITY
INSTITUTE OF TECHNOLOGY
MECHANICAL ENGINEERING DEPARTMENT
DIVISON OF THERMAL AND ENERGY CONVERSION
INTERNAL COMBUSTION ENGINES1
IC Engine Fundamentals2
Engine Systems An engine must at least include the following systems to
operate
Air supply and Exhaust system
Fuel supply system
Combustion system
Ignition system (SI)
Cooling system
Lubricating system
Starting system
Electrical power supply (battery or generator)
IC Engine Fundamentals3
areaforcepressure =
force = pressure x area
IC Engine Fundamentals4
areaforcepressure =
force = pressure x area
IC Engine Fundamentals5
IC Engine Fundamentals
Ignition system
6
Crank mechanism
Ignition system
IC Engine Fundamentals7
8
Spark Ignnition System9
Distributer contains:- A rotating cam driven by the engine drive
- A set of breaker points,
- A condensor(capacitor)
- A roter and distributer cap
o The ignition coil consit of two transformer windings sharing acommon megnetic core- the primary (200 to 300 turns) andsecondary windings (21,000 turns).
o An alternating current in the PW induces magnetic field in thecoil’s core so that it serves as a step up transformer.
Spark Ignnition System10
For an ignition coil, one end of windings of both the primaryand secondary are connected together.
This common point is connected to the battry ( ussually throughthe current –limiting ballast rsistor)
The other end of the primary is connected to the point in thedistributer
The other end of the SW is connected via the the distributercap and rotor to the spark plugs
Intake system
Ignition system
Crank mechanism
IC Engine Fundamentals11
Exhaust system
Ignition system
Crank mechanism
Intake system
IC Engine Fundamentals12
Cooling system ThermostatIntake system Exhaust system
Ignition system
Crank mechanism
IC Engine Fundamentals13
14
Lubrication system
Crankcase vent
Ignition system
Exhaust systemIntake systemCooling system Thermostat
IC Engine Fundamentals15
16
IC Engine Fundamentals17
Otto cycles
Intake
Compression,
Combustion
Power
Exhaust
fuel
air air + fuel
volume
pres
sure
TDC BDC
1. INTAKE STROKE
IC Engine Fundamentals18
Stoichiometric mixture
volume
pres
sure
TDC BDC
IC Engine Fundamentals19
volume
pres
sure
TDC BDC
IC Engine Fundamentals
2. COMPRESSION STROKE
20
volume
pres
sure
TDC BDC
IC Engine Fundamentals21
volume
pres
sure
TDC BDC
IC Engine Fundamentals
3. Power stroke
22
volume
pres
sure
TDC BDC
IC Engine Fundamentals23
volume
pres
sure
TDC BDC
IC Engine Fundamentals
4. Exaust stroke
24
volume
pres
sure
TDC BDC
IC Engine Fundamentals25
Exhaust gas residual
volume
pres
sure
TDC BDC
Negative work
positive work
Work = (pressure x volume)
IC Engine Fundamentals26
IC Engine Fundamentals27
CxHxCONOx
Mixture preparation
Intake tuning
Combustion, auto ignition
Wear
Inertia
lubrication
Exhaust tuning Emissions
IC Engine Fundamentals-Some Problems
1. Performance2. Fuel economy3. Emission
28
The Diesel Engine
volume
pres
sure
TDC BDC
Negligible negative work
IC Engine Fundamentals-Diesel Engine29
10000 - 20000 psi
IC Engine Fundamentals-Diesel Engine
30
31
• Intake Stroke
• The piston travels from TDC to BDC with theintake valve open and exhaust valve closed.
• Piston draws air and fuel mixture intocylinder
IC Engine fundamental-SI Engine32
IC Engine fundamental-SI Engine33
• Compression Stroke• When the piston reaches BDC, the intake
valve closes and the piston travels backto TDC with all valves closed.
• Rising piston compresses mixture raisingboth the pressure and temperature in thecylinder.
• Near the end of the compression stroke,the spark plug is fired and combustion isinitiated. (constant-volume combustion).
IC Engine fundamental-SI Engine34
• Expansion Stroke
• With all valves closed, the high pressurecreated by the combustion process pushesthe piston away from TDC.
• As the piston travels from TDC to BDC,cylinder volume is increased, causingpressure and temperature to drop.
35
• Exhaust Stroke
• With the exhaust valve remainingopen, the piston now travels from BDCto TDC in the exhaust stroke.
• This pushes most of the remainingexhaust gases out of the cylinder intothe exhaust system
IC Engine fundamental-SI Engine
36
Intake Stroke
Intake stroke in Diesel engine is the same as the intakestroke in an SI engine.
The major difference being no fuel is added to theincoming air.
IC Engine fundamental-SI Engine
IC Engine fundamental-Diesel Engine37
Compression Stroke The same as in SI engine except that only air is compressed and
compression is to higher pressures and temperature.
Late in the compression stroke fuel is injected directly into thecombustion chamber where it mixes with the very hot air. This causesthe fuel to evaporate and self-ignite causing combustion to start.
Combustion is fully developed by TDC and continues at aboutconstant pressure until fuel injection is complete (i.e. after Cut - off)and the piston has started towards BDC.
IC Engine fundamental-Diesel Engine38
Power Stroke
o The power stroke continues as combustion ends and the piston travels towards BDC.
Exhaust Stroke
o The same as SI engine
IC Engine Fundamentals- 2 Stroke Engine39
The two stroke engine employs the crankcaseas well as the cylinder to achieve all theelements of the Otto cycle in only two strokesof the piston.
One power stroke in one revolution of thecrankshaft
The main difference between two stroke andfour-stroke engines is in the method of fillingthe fresh charge and removing the bunt gasesfrom the cylinder.
IC Engine Fundamentals- 2 Stroke Engine
40
In a two-stroke engine, the filling process is accomplished by thecharge compressed in crankcase or by a blower.
The induction of the compressed charge moves out the product ofcombustion through exhaust ports.
Therefore, no piston strokes are required for these two operations.
Two strokes are sufficient to complete the cycle, one for compressingthe fresh charge and the other for expansion or power stroke.
IC Engine Fundamentals-2 Stroke Engine
A compression stroke (intake +compression) starts by closing theintake and exhaust ports, and thencompresses the cylinder contents anddraws fresh charge into thecrankcase.
The fuel/air mixture is drawn into thecrankcase by the vacuum that iscreated during the upward stroke ofthe piston.
As the piston approaches TDC,combustion is initiated.
41
IC Engine Fundamentals- 2 Stroke Engine
A power or expansion stroke (combustion+ exhaust)
At the end of compression stroke thespark plug ignites the fuel mixture. Theburning fuel expands, driving the pistondownward, to complete the cycle.
At the same time, another crankcasecompression stroke is happening beneaththe piston.
42
IC Engine Fundamentals-2 Stroke Engine
During downward movement of the pistonfirst the exhaust ports and then the intakeports are uncovered.
When the inlet ports are uncovered, thefresh charge which has been compressed inthe crankcase flows into the cylinder.(Scavenging)
The piston and the ports are generallyshaped to deflect the incoming charge fromflowing directly into the exhaust ports and toachieve effective scavenging of the residualgases
43
IC Engine Fundamentals-2 Stroke Engine44
Upward stroke of the piston During the downward stroke
Intake Valve open Crank Case compression
Transfer port OpeningScavenging
IC Engine Fundamentals -2 Stroke Engine45
Upward stroke of the piston
Cylinder compression-another intake stroke is happening beneath the piston
Power Stroke-Crankcase compression
Down ward stroke of the piston
Exhaust Stroke-Scavenging
IC Engine Fundamentals
Comparison of SI & CI Engines
Description SI Engine CI Engine
Basic Cycle Otto Cycle Heat addition @ Constant volume
Diesel cycle, Heat addition @ constant pressure
Fuel Gasoline, Highly volatile fuel, self ignition temp is high
Diesel oil, non volatile fuel, self-ignition temp is comparatively low
Introduction of Fuel
Fuel-air mixture introduced during suction stroke (Carburetor is necessary)
Fuel directly injected to the combustion chamber at high pressure (fuel pump and injector is necessary)
Load control Throttle controls the quantity of mixture introduce
The quantity of fuel is regulated in the pump. Air quantity is not control
Ignition Required an ignition system with spark plug
Ignition system & sparkplug are not necessary
46
IC Engine Fundamentals
Comparison of SI & CI Engines
Description SI Engine CI Engine
Compression ratio
6-10, upper limit is fixed by anti knock quantity of fuel
16-20 upper limit is limited by weight increase of the engine
SpeedThey are high speed engines (light weight & homogeneous combustion)
They are low speed engines ( heavy weight & heterogeneous combustion)
Thermal Efficiency
Lower thermal Efficiency (lower compression ratio)
Higher thermal efficiency (higher compression ratio)
weightLighter due to lower peak pressure
Heavier due to higher peak pressure
47
IC Engine Fundamentals
4-Stroke Engines 2-Stroke Engine
The cycle completed in 4 strokes of piston or 2-revolution of crankshaft
The cycle is completed in twoA strokes of the piston or one power strokes obtained in every revo. of crankshaft
Turning moment is not uniform & hence a heavier flywheel is needed
Turning moment is more uniform & hence a lighter flywheel can be used
The power produced for the same size engine is less (2 revo gives 1 power)
Power produced for the same size of engine is more (Theoretically twice, actually 1.3 times) due to 1 Power stroke in 1 revo. Of Cra Sh)
Lesser cooling & lubrication requirementLower rate of wear & tear
Greater cooling & lubrication requirementsHigher rate of wear & tear
48
Comparison of 2-Stroke & 4-Stroke Engines
IC Engine Fundamentals
4-S Engines 2-S Engine
Contains valve & Valve mechanisms to open & close valves
No valves but ports (some 2-s engines are fitted with exhaust valves or reed valve)
The initial cost is very high (Coz of heavy wt. & complicated valve mechanism)
Initial cost of the engine is less (Coz odlight wt. & simplicity)
Volumetric Efficiency is more (Coz of more time for induction)
Volumetric Efficiency is low (Coz of lesser time for induction)
Thermal Efficiency is higher, part load efficiency is better
Thermal Efficiency is less, part load efficiency is poor compared 4-s engine
Used where Efficiency is important( in Cars, Buses, aero planes, etc…
Used where low cost, compactness & Light Wt. are important (in Scooters, Motorcycles
49
IC Engine Fundamentals-Wankel Engine
Alternative to the reciprocating enginegeometry
The intake, compression, combustion andexhaust process happen in different cavitiescreated between the rotor and the part ofthe housing.
In Wankel engine there are two rotatingparts: the triangular shaped rotor and theoutput shaft with its integral eccentric (lobe).
50
The rotor revolves directly on the eccentric
The rotor has an integral timing gear whichmeshes with the fixed timing gear on one sideof the housing to maintain the correct pulserelationship between the rotor and theeccentric shaft rotations.
As the rotor makes one complete rotation,during which the eccentric shaft rotatesthrough three revolutions, each chamberproduces one power stroke.
51
IC Engine Fundamentals-Wankel Engine52
A rotary engine has an ignition system and a fuel-delivery system that aresimilar to the ones on piston engines.
The Rotor The rotor has three convex faces, each of which acts like a piston.
Each face of the rotor has a pocket in it, which increases the displacementof the engine, allowing more space for air/fuel mixture.
At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber
IC Engine Fundamentals-Wankel Engine
The rotor has a set of internal gearteeth cut into the center of one side.
These teeth mate with a gear that isfixed to the housing.
This gear mating determines the pathand direction the rotor takes throughthe housing.
53
IC Engine Fundamentals-Wankel Engine
The housing is roughly oval in shape.
its shape is designed so that the threetips of the rotor will always stay incontact with the wall of the chamber,forming three sealed volumes of gas.
Each part of the housing is dedicatedto one part of the combustion process.
54
IC Engine Fundamentals-Wankel Engine
The four sections are:◦ Intake◦ Compression◦ Combustion◦ Exhaust
The intake and exhaust ports arelocated in the housing. There are novalves in these ports.
The exhaust port connects directly tothe exhaust, and the intake portconnects directly to the throttle.
55
IC Engine Fundamentals-Wankel Engine56
Output Shaft The output shaft has round lobes mounted eccentrically,
meaning that they are offset from the centerline of theshaft.
Each rotor fits over one of these lobes. The lobe acts sort oflike the crankshaft in a piston engine.
As the rotor follows its path around the housing, it pushes onthe lobes.
Since the lobes are mounted eccentric to the output shaft,the force that the rotor applies to the lobes creates torquein the shaft, causing it to spin.
IC Engine Fundamentals-Wankel Engine
ADVANTAGES
higher output for similardisplacement and physical size
simple and contain far fewermoving parts
the shape of the Wankelcombustion chamber and theturbulence induced by themoving rotor prevent localizedhot spots from forming
DISADVANTAGES
seals exposed to heating andcooling cycles in use, led to a veryhigh incidence of loss of sealing
incomplete combustion of the air-fuel charge, with the remainingunburned hydrocarbons releasedinto the exhaust.
Is difficult to expand the engineto more than two rotors
57
Recommended