Ic 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 mechanicalenergy

Thermal energy raises temperature and pressure of gaseswithin engine, and gas expands against mechanicalmechanisms of engine

Combustion

Internal: 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 dischargebetween 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 highcompression, 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 whichpistons reciprocate back and forth

Combustion chamber in closed end ofcylinders

Power delivered to rotating outputcrankshaft 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

http://www.youtube.com/watch?v=oGrD7FTFLJc


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 alonga 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 aroundcrankshaft

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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_engine


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 injection

Multipoint port fuel injection: one or moreinjectors at each cylinder intake

Throttle body fuel injection: injectorsupstream 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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Classification

Cooling

Air cooled

Water cooled

http://www.innerauto.com/Automotive_Animations/Cooling_System

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http://www.innerauto.com/Automotive_Animations/Cooling_System


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 moves

Clearance volume: volume in combustion chamber at TDC

Displacement volume: volume displaced by piston

Ignition delay: Time between start of ignition and start ofcombustion

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Terminology

Terminology II

Air-fuel ratio: Ratio of mass flow rate of air to that of fuel

Specific fuel consumption: fuel used per unit power

Emissions: NOx, CO, HC, solids

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Components

Engine components

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Components

Block: body of engine containing cylinders

Bearing: main bearing for crankshaft

Camshaft: rotating shaft used to push open valves at theproper time in engine cycle

Carburetor: Venturi flow device to draw fuel and mix withair

Catalytic converter: reduces emissions by chemical reaction

Combustion chamber: volume between cylinder head andpiston face

Connecting rod: connects piston with crankshaft

Crankcase: 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

Exhaust manifold: piping which carries exhaust gases awayfrom engine cylinders

Fan: to increase air flow through radiator

Flywheel: to smoothen engine rotation

Fuel injector: pressurized nozzle to inject fuel into air orcylinder

Fuel pump: to move fuel from tank to engine

Glow plug: electrical resistance inside combustion chamberto help cold start

Head: piece which closes end of cylinders

Head gasket: sealant between engine block and head

Intake manifold: piping which delivers incoming air tocylinders

Oil pan: oil reservoir on bottom of engine block, part of thecrankcase

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Components

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 anelectrode gap

Speed control-cruise control: control system

Starter: hand starter, electric motor, or small IC enginesfor large IC engines

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Components

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

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_engine

http://www.youtube.com/watch?v=L-kYu0k5lF4


Operation


First stroke: intake or induction

Piston travels from TDC (top dead center) to BDC(bottom dead center) with intake valve open and exhaustvalve closed

Volume increases in combustion chamber and createsvacuumAir pushed through cylinderAs air passes through intake system, fuel is added

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Operation

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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Operation

Combustion

Piston near TDC: nearly constant-volume combustion

Changes composition of gas mixture to exhaust productsand temperature and pressure increases

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Operation

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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Operation

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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Operation

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 exhaustgases into the exhaust system

Near 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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Operation

Four-stroke SI operating cycle

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Operation

4-stroke CI engine operation

First stroke: intake

Second stroke: compression

Combustion

Third stroke: power

Exhaust blowdown

Fourth stroke: exhaust

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Operation


Combustion: occurs quickly with piston at TDC

First stroke: expansion power

Exhaust 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=LuCUmQ9FxMU

http://en.wikipedia.org/wiki/Two-stroke_engine


Operation

2-stroke CI engine operation

Differences with respect to 2-stroke SI

No fuel added to incoming air; only air is compressed

Fuel injector located in cylinder

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Thermodynamics

Otto cycle

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Thermodynamics

Piston is essentially stationary during combustion: constantvolume

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Thermodynamics

Diesel engine

Uses heat of compression to initiate ignition and burn fuel

Fuel injected into the combustion chamber during finalstage of compression

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Thermodynamics

Combustion occurs at a constant pressure, as the piston moves

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Thermodynamics

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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Parameters

Engine parameters

TDC

BDC

Vc

Vd

BS

s r

aθ

Stroke

S = 2a

Average piston speed

Up = 2SN

N = engine speed

Displacement for one cylinder

Vd =π

4B2S

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Parameters

Distance between crank axis and wrist pin axis

s = a cos θ +√

r2− a2 sin2 θ

Differentiating and dividing by Up

Up

Up

=π

2sin θ

(

1 +a cos θ

√

r2− a2 sin2 θ

)

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Parameters

Clearance volume, Vc

Vc = VTDC

VBDC = Vc + Vd

Compression ratio

rc =VBDC

VTDC

=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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Parameters

Cylinder volume

V = Vc +πB2

4(r − a − s)

Cross-sectional area of cylinder and the surface area of aflat-topped piston are given by

Ap =π

4B2

Combustion chamber surface area

A = Ach + Ap + πB(

r + a − s)

Ach is the cylinder head surface area

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Output

Work

Work is the output of any heat engine

It is generated by the gases in the combustion chamber ofthe cylinder

Force due to gas pressure on the moving piston generateswork

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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Output

Indicator diagram

p

V

A

B

IE

TDC BDC4-stroke SI

I = ignition, E = exhaust opens

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Output

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

ηm =wb

wi

Modern automobile engines at high speeds ηm = 75% to 95%

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Output

Engine parameters

Mean effective pressure (MEP)

MEP =w

vd

Specific displacement

vd = vBDC − vTDC

Using brake work

BMEP =wb

vd

Using indicated work

IMEP =wb

vd

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Output

Torque

For one revolution

2πT = Wb

=BMEP Vd

n

so that

T =

BMEP Vd

2π2-stroke

BMEP Vd

4π4-stroke

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Output

Power

Power is the rate of work of the engine

P = 2πNT

=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

Effect of engine speed

P

T

n4-stroke SI

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Output

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 311Average 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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