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INTERNAL COMBUSTION ENGINES
INTERNAL COMBUSTION ENGINES
Mihir SenUniversity of Notre Dame
November 11, 2009
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Outline
1 Outline
2 Basics
3 Classification
4 Terminology
5 Components
6 Operation
7 Thermodynamics
8 Parameters
9 Output
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Basics
Historical
Lenoir, 1860: first auto
Otto and Langen, 1867: efficiency about 11%
Diesel, by 1892: compression ignition engine
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Basics
Combustion engines
Chemical energy in fuel converted to thermal energy bycombustion or oxidation
Heat engine converts chemical energy into mechanical
energyThermal energy raises temperature and pressure of gaseswithin engine, and gas expands against mechanicalmechanisms of engine
CombustionInternal: fuel is burned within the engine proper (includinge.g. rocket engines, jet Engines, firearms)
External: combustion is external to the engine (e.g. steam,Stirling engine, gas turbine)
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Classification
Classification of IC engines
Ignition
Number of strokes
Valve location
Design
Position and number of cylinders
Air intake
Fuel input method
Fuel used
Cooling
Application
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Classification
Ignition
Spark ignition (SI): high-voltage electrical discharge
between two electrodes ignites air-fuel mixture incombustion chamber surrounding spark plug
Compression ignition (CI): air-fuel mixture self-ignites dueto high temperature in combustion chamber caused by high
compression, Diesel engine
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Classification
Number of strokes
Four-stroke: four piston movements over two enginerevolutions for each engine cycle
Two-stroke: two piston movements over one revolution foreach engine cycle
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Classification
Valve location
Valves in head
Valves in block
One valve in head and one in block (less common)
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Classification
Design
Reciprocating
Rotary
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Classification
Reciprocating engines
Engine has one or more cylinders in which
pistons reciprocate back and forthCombustion chamber in closed end ofcylinders
Power delivered to rotating output
crankshaft by mechanical linkage withpistons
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Classification
Rotary engines
Engine made of block (stator) built around largenon-concentric rotor and crankshaft
Combustion chambers are built into the nonrotating block
http://www.youtube.com/watch?v=oGrD7FTFLJc11/ 55
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http://www.youtube.com/watch?v=oGrD7FTFLJchttp://www.youtube.com/watch?v=oGrD7FTFLJc7/30/2019 IC Engine PDF
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Classification
Position and number of cylinders
Single cylinder (e.g. lawnmowers)
In-line or straight: cylinders in straight line, one behindthe other in length of crankshaft
V: two banks of cylinders at an angle with each other along
a single crankshaft, angle typically 60-90
Flat or opposed cylinder (V with 180): two banks ofcylinders opposite each other on a single crankshaft (smallaircrafts)
W: three banks of cylinders on same crankshaft (notcommon)
Opposed piston engine: two pistons in each cylinder,combustion chamber between pistons
Radial engine: cylinders positioned radially around
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Classification
In-line V Flat
Radialhttp://en.wikipedia.org/wiki/Radial_engine
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http://en.wikipedia.org/wiki/Radial_enginehttp://en.wikipedia.org/wiki/Radial_engine7/30/2019 IC Engine PDF
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Classification
Air intake
Naturally aspirated: no air pressure boost
Supercharged: air pressure increased with compressordriven by crankshaft
Turbocharged: air pressure increased byturbine-compressor driven by exhaust gases
Crankcase compressed: two-stroke engine with crankcase asintake air compressor
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Classification
Supercharger
Supercharger on AMC V8 engine for dragstrip racing
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Classification
Turbocharger
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Classification
Fuel input method
Carbureted: air-fuel mixed atthroat
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Classification
Fuel input method
Fuel injectionMultipoint port fuel injection: one or moreinjectors at each cylinder intake
Throttle body fuel injection: injectors
upstream of intake manifold
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Classification
Fuel used
Gasoline
Diesel or fuel oil
Gas (natural gas or methane)
Liquefied petroleum gas (LPG): mainly propane,propylene, butane, and butylene
Alcohol (ethyl, methyl)
Dual fuel (e.g. methane/diesel)
Gasohol (e.g. 90% gasoline, 10% alcohol)
Biodiesel: cleaner-burning diesel fuel made from natural,renewable sources such as vegetable oils
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Cl ifi i
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Classification
Cooling
Air cooled
Water cooled
http://www.innerauto.com/Automotive_Animations/Cooling_Sys
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T i l
http://www.innerauto.com/Automotive_Animations/Cooling_Systemhttp://www.innerauto.com/Automotive_Animations/Cooling_System7/30/2019 IC Engine PDF
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Terminology
Terminology I
TDC: top dead center, piston position farthest fromcrankshaft
BDC: bottom dead center, piston position nearest tocrankshaft
Direct fuel injection: into main combustion chamber
Indirect fuel injection: into a secondary chamber
Bore: diameter of cylinder or piston face
Stroke: distance that piston movesClearance volume: volume in combustion chamber at TDC
Displacement volume: volume displaced by piston
Ignition delay: Time between start of ignition and start of
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Terminology
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Terminology
Terminology II
Air-fuel ratio: Ratio of mass flow rate of air to that of fuelSpecific fuel consumption: fuel used per unit power
Emissions: NOx, CO, HC, solids
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Components
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Components
Engine components
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Components
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Components
Block: body of engine containing cylinders
Bearing: main bearing for crankshaftCamshaft: rotating shaft used to push open valves at theproper time in engine cycle
Carburetor: Venturi flow device to draw fuel and mix with
airCatalytic converter: reduces emissions by chemical reaction
Combustion chamber: volume between cylinder head andpiston face
Connecting rod: connects piston with crankshaftCrankcase: part of engine block surrounding crankshaft
Crankshaft: rotating shaft through which engine workoutput is supplied to external systems, rotated byreciprocating pistons through connecting rods
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Components
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Components
Exhaust manifold: piping which carries exhaust gases awayfrom engine cylinders
Fan: to increase air flow through radiatorFlywheel: to smoothen engine rotation
Fuel injector: pressurized nozzle to inject fuel into air orcylinder
Fuel pump: to move fuel from tank to engineGlow plug: electrical resistance inside combustion chamberto help cold start
Head: piece which closes end of cylinders
Head gasket: sealant between engine block and headIntake manifold: piping which delivers incoming air tocylinders
Oil pan: oil reservoir on bottom of engine block, part of the
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Components
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p
Oil pump: to distribute oil from sump
Oil sump: reservoir for the oil system of the engine
Piston rings: metal rings around piston to seal gap betweenpiston and cylinder
Push rods: linkage between camshaft and valves on OHVengines
Radiator: liquid to air heat exchanger to cool engine
Rod bearing: rod connecting the piston with the rotatingcrankshaft
Spark plug: creates high-voltage discharge across an
electrode gap
Speed control-cruise control: control system
Starter: hand starter, electric motor, or small IC enginesfor large IC engines
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Components
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p
Supercharger: compressor powered from crankshaft tocompress incoming air
Throttle: butterfly valve at upstream end of intakemaniford to control air flow rate into SI engine
Turbocharger: turbine-compressor powered by exhaust flowto compress incoming air
valves; controls flow of air in and out of the cylinders
Water jacket: liquid flow passages around cylinder forcooling
Water pump: to circulate coolant
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Operation
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4-stroke SI engine operation
http://en.wikipedia.org/wiki/Four-stroke_engine
http://www.youtube.com/watch?v=L-kYu0k5lF4
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http://en.wikipedia.org/wiki/Four-stroke_enginehttp://www.youtube.com/watch?v=L-kYu0k5lF4http://www.youtube.com/watch?v=L-kYu0k5lF4http://en.wikipedia.org/wiki/Four-stroke_engine7/30/2019 IC Engine PDF
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4-stroke SI engine operation
First stroke: intake or inductionPiston travels from TDC (top dead center) to BDC(bottom dead center) with intake valve open and exhaustvalve closed
Volume increases in combustion chamber and creates
vacuumAir pushed through cylinderAs air passes through intake system, fuel is added
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Second stroke: compression
Piston reaches BDC, intake valve closes and piston travelsback to TDC with all valves closed
Air-fuel mixture compresses and temperature and pressureincreaseNear end of compression stroke, spark plug fired andcombustion initiated
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Combustion
Piston near TDC: nearly constant-volume combustion
Changes composition of gas mixture to exhaust productsand temperature and pressure increases
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Third stroke: expansion
All valves closed
High pressure pushes piston away from TDC: produceswork output of engine cycle
Piston moves from TDC to BDC: volume increases andpressure and temperature drop
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Exhaust blowdown
Late in power cycle exhaust valve is opened
pressure differential pushes hot exhaust gas out of cylinderand through exhaust system when piston is at BDC
Exhaust gas carries away high amount of enthalpy, whichlowers cycle thermal efficiency
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Fourth stroke: exhaust
When piston is at BDC cylinder is still full of exhaustgases at atmospheric pressure
Exhaust valve stays open and piston moves from BDCto TDC pushing out most of the remaining exhaust
gases into the exhaust systemNear end of exhaust stroke before TDC, intake valvestarts to open and is fully open by TDC when intakestroke starts next cycle
Near TDC the exhaust valve starts to close and isfully closed sometime after TDC
Period where both intake valve and exhaust valve areopen is called valve overlap
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Four-stroke SI operating cycle
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4-stroke CI engine operation
First stroke: intake
Second stroke: compressionCombustion
Third stroke: power
Exhaust blowdown
Fourth stroke: exhaust
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2-stroke SI engine operation
Combustion: occurs quickly with piston at TDC
First stroke: expansion powerExhaust blowdown
Intake and scavenging: simultaneous intake and exhaust
http://www.youtube.com/watch?v=LuCUmQ9FxMU
http://en.wikipedia.org/wiki/Two-stroke_engine
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http://www.youtube.com/watch?v=LuCUmQ9FxMUhttp://en.wikipedia.org/wiki/Two-stroke_enginehttp://en.wikipedia.org/wiki/Two-stroke_enginehttp://www.youtube.com/watch?v=LuCUmQ9FxMU7/30/2019 IC Engine PDF
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2-stroke CI engine operation
Differences with respect to 2-stroke SINo fuel added to incoming air; only air is compressed
Fuel injector located in cylinder
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INTERNAL COMBUSTION ENGINESThermodynamics
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Otto cycle
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Piston is essentially stationary during combustion: constantvolume
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Diesel engine
Uses heat of compression to initiate ignition and burn fuelFuel injected into the combustion chamber during finalstage of compression
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INTERNAL COMBUSTION ENGINESThermodynamics
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Combustion occurs at a constant pressure, as the piston moves
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INTERNAL COMBUSTION ENGINESThermodynamics
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Variations
Dual cycle: cross between SI and CI
Atkinson cycle
Miller cycle
Homogeneous charge compression ignition: well-mixed fueland air are compressed to auto-ignition. Ignition occurs atseveral places simultaneously.
Homogeneous charge spark ignition gasoline enginesStratified charge compression ignition diesel engine
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INTERNAL COMBUSTION ENGINESParameters
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Engine parameters
TDC
BDC
VcVd
B
S
s r
a
Stroke
S = 2a
Average piston speed
Up = 2SN
N = engine speed
Displacement for one cylinder
Vd =
4B2S
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Distance between crank axis and wrist pin axis
s = a cos +
r2 a2 sin2
Differentiating and dividing by Up
Up
Up=
2sin
1 +
a cos
r2 a2 sin2
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Clearance volume, Vc
Vc = VTDC
VBDC = Vc + Vd
Compression ratio
rc =VBDCVTDC
=Vc + Vd
Vc
High compression ratio allows engine to extract moremechanical energy from a given mass of air-fuel mixture due toits higher thermal efficiency
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Cylinder volume
V = Vc +B2
4(r a s)
Cross-sectional area of cylinder and the surface area of aflat-topped piston are given by
Ap =
4B2
Combustion chamber surface area
A = Ach + Ap + B
r + a s
Ach is the cylinder head surface area
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Work
Work is the output of any heat engineIt is generated by the gases in the combustion chamber ofthe cylinder
Force due to gas pressure on the moving piston generates
work
W =
F dx =
pAp dx
Ap dx = dV
W =
p dV
P = pressure in combustion chamberAp = area against which the pressure acts (piston face)x = distance the piston moves
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Indicator diagram
p
V
A
B
IE
TDC BDC4-stroke SII = ignition, E = exhaust opens
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Specific work w: per unit mass of air within cylinder
Brake work: actual work available in the crankshaft
wb = wi wf
wi = indicated specific work generated inside combustionchamberwf = specific work lost due to friction and parasitic loadsMechanical efficiency
m =wbwi
Modern automobile engines at high speeds m = 75% to 95%
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Engine parameters
Mean effective pressure (MEP)
MEP =w
vd
Specific displacement
vd = vBDC vTDC
Using brake work
BMEP =wb
vd
Using indicated work
IMEP =wbvd
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Torque
For one revolution
2T = Wb
=
BMEP Vd
n
so that
T =
BMEP Vd2
2-stroke
BMEP Vd4
4-stroke
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Output
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Power
Power is the rate of work of the engine
P = 2N T
=1
2nMEPApUp
=
MEPApUp/2 2-stroke
MEPApUp/4 4-stroke
n = number of revolutions per cycle, and N = engine speed
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Output
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Effect of engine speed
P
T
n4-stroke SI
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Output
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Typical values
Model Automobile Largeairplane stationary2-stroke 4-stroke 2-stroke
Bore (cm) 2.00 9.42 50.0Stroke (cm) 2.04 9.89 161Displacement/cyl (L) 0.0066 0.69 316Speed (rpm) 13,000 5,200 125Power (kW) 0.72 35 311
Average piston speed (m/s) 8.84 17.1 6.71Power/displacement (kW/L) 109 50.7 0.98BMEP (kPa) 503 1170 472
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