1 [translation: in the name of allah, the most merciful, the most kind.]

1[Translation: In the name of Allah, the Most Merciful, the Most Kind.]

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REG NO: BME: 01083004 SECTION: BDEPARTMENT: MECHANICAL

THE UNIVERSITY OF LAHORE

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The Sparking Ignition engine.

The term spark-ignition engine is normally used torefer to internal combustion engines where the fuel-airmixture is ignited with a spark. The term contrasts with

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compression-ignition engines, where the heat fromcompression alone ignites the mixture. Spark-ignitionengines can be either two-stroke or four-stroke, and arecommonly referred to as "gasoline engines" in Americaand "petrol engines" in Britain. However, these terms arenot preferred, since spark-ignition engines can (andincreasingly are) run on fuels other than gasoline, such asauto gas (LPG), methanol, ethanol, compressed naturalgas (CNG), hydrogen, and (in drag racing) nitromethane.A four-stroke spark-ignition engine is an Otto cycleengine.

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Until recently, a major distinction between spark ignitionand compression-ignition engines has beenwhere the fuel is mixed - spark-ignition engines mix fueloutside the cylinders and compression-ignition enginesmix fuel inside the cylinders. However, both two-strokeand four-stroke spark-ignition engines are increasinglybeing designed with gasoline direct injection (GDi),eliminating this distinction between the two systems.

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Pressure - Volume Diagrams

• Graph of pressure as a function of cylinder volume above the piston

Bottom Dead Center - volume is maximum

Top Dead Center - volume is minimum

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Cylinder Volume

• Change in volume as piston goes up and down

Volume goingTDC

Volume going

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Change in VolumeV

olu

me

BDC TDC BDC

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P-V Diagram - Gas Engine

Bottom Dead CenterStart of compression stroke - volume above piston filled with fuel/air mixture

BDCTDC

Volume

Pre

ssure

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P-V Diagram - Gas Engine

Piston travels up, fuel/air compressed and pressure rises

BDCTDC

Volume

Pre

ssure

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P-V Diagram - Gas Engine

Top Dead CenterEnd of compression stroke - volume in cylinder at a minimum

BDCTDC

Volume

Pre

ssure

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P-V Diagram - Gas Engine

Then have an instantaneous introduction of heat, which increases pressure again

COMBUSTION BDCTDC

VolumePre

ssure

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P-V Diagram - Gas Engine

BDCTDC

Volume

Pre

ssure

Pressure forces piston down, creating torque on crank shaft

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P-V Diagram - Gas Engine

BDCTDC

Volume

Pre

ssure

Piston reaches bottom dead center again, exhaust valve opens, burned fuel/air expelled

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Gas Engine Efficiency

• Remember, efficiency is work out divided by heat in

• P-V diagram tells us both things

Heat in: temperature change from burning fuel

BDCTDC

Volume

Pre

ssure

Work out: area between curves

And, make a few assumptions, can calculate efficiency

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Gas Engine Efficiency

• Define compression ratio as:

• Then efficiency can be shown to be:

• Higher efficiency means higher compression ratio.

rVBDCVTDC

BDCTDC

Volume

Pre

ssure

e11

r0.4

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(sparking ignition engine)

Advantages for Spark Ignition Engines:- Increase efficiency at part load operating conditions byoperating at a higher effective compression ratio- Successfully avoid spark knock at full load conditions,even during rapid transients

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Following is the explanation of the Four-Stroke and Two-Stroke SI Engines and their P-V Diagrams.

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Four Stroke SI Engine

Stroke 1: Fuel-air mixture introduced into cylinder through intake valve Stroke 2: Fuel-air mixture compressedStroke 3: Combustion (~constant volume) occurs and product gases expand doing workStroke 4: Product gases pushed out of the cylinder through the exhaust valve

CompressionStroke

PowerStroke

ExhaustStroke

A I R

CombustionProducts

Ignition

IntakeStroke

FUEL

Fuel/AirMixture

Crank shaft

90o

180o

BC

TC0o

270o

Spark plug for SI engineFuel injector for CI engine

Top Center(TC)

BottomCenter(BC)

Valves

Clearancevolume

Cylinder wall

Piston

Stroke

CylinderComponents

IVO - intake valve open, IVC – intake valve closeEVO – exhaust valve open, EVC – exhaust valve closeXb – burned gas mole fraction

Four-Stroke SI Engine

Exhaust gas residual

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Two Stroke SI Engine

The two-stroke engine is simpler mechanically thanthe four-stroke engine. The two-stroke engine deliversone power stroke every two strokes instead of one everyfour; thus it develops more power with the samedisplacement, or can be lighter and yet deliver the samepower. For this reason it is used in lawn mowers, chainsaws, small automobiles, motorcycles, and outboardmarine engines.

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Two Stroke SI Engine

Intake (“Scavenging”)

Compression Ignition

ExhaustExpansion

Fuel-air-oilmixture

Fuel-air-oilmixture compressed

Crankshaft

Checkvalve

Exhaustport

EPO – exhaust port open EPC – exhaust port closedIPO – intake port openIPC – intake port closed

Exhaust area

Intake area

Two-Stroke SI Engine

scavenging

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Achievements:• Three prototype pistons were manufactured: twopistons for a spark ignition engine and one pistonfor a compression ignition engine. All three weretested experimentally in single cylinder engines.Models of the piston operation were also developedto investigate the piston concept at differentoperating conditions.• Spark Ignition Engine Test Result SummaryThe spark ignition PRP prototypes demonstrated theutility of the PRP concept when applied to a sparkignition engine. At low loads, the piston behavedlike the elevated compression ratio, yielding a brakespecific fuel consumption (BSFC) improvement of3.5-10% during low load conditions. At high loadconditions, the PRP limited the peak cylinderpressure to that of the baseline (lower compressionratio) piston.

Cylinder Arrangement

Single-cylinder engine gives one power stroke per crank revolution (2 stroke) or two revolutions (4 stroke). The torque pulses are widely spaced, and engine vibration and smoothness are significant problems.Used in small engine applications where engine size is more important Multi-cylinder engines spread out the displacement volume amongst multiple smaller cylinders. Increased frequency of power strokes produces smoother torque characteristics. Engine balance (inertia forcesassociated with accelerating and decelerating piston) better than singlecylinder.

Most common cylinder arrangements:- In-line 4-cylinder- In-line 6-cylinder- V-6 and V-8

Power Regulation (Throttling)

An IC engine is basically an air engine, the more air you get into thecylinder, the more fuel you can burn, the more power you get out.The initial pressure in the cylinder is roughly equal to the pressurein the intake manifold.

Pressure in the intake manifold is varied by opening and closing thethrottle plate to change the pressure drop. Maximum air flow (andpower) achieved at wide-open-throttle (WOT). Minimum air flow at idle

PatmPint < Patm

Intake manifold

Fuel

WOT

Idle

Basic Carburetor Design

Venturi

Throttle

Air Flow

Mixture to manifold

Fuel

Fuel Injection System

Throttle

Fuel tank

Air intakemanifold

During start-up the components are cold so fuel evaporation is very slow, as a resultadditional fuel is added through a second injecting valve

Superchargers and Turbochargers increase the power of an IC engineby raising the intake pressure and thus allowing more fuel to be burnedper cycle. Knock or autoignition phenomenon limit precompression.

Superchargers are compressors that are mechanically driven by theengine crankshaft and thus represent a parasitic load.

Compressor

Patm

Pint > Patm

Turbochargers couple a compressor with a turbine driven by the exhaust gas. The compressor pressure is proportional to the engine speed

The peak pressure in the exhaust system is only slightly greater than atmospheric – small P across turbine.

In order to produce enough power to run compressor the turbine speed must be very fast (100k-200k rev/min) – long term reliability an issue.

It takes time for turbine to get up to speed so when the throttle is opened suddenly there is a delay in achieving peak power - Turbo lag.

Waste gate valve controls the exhaust gas flow rate to the turbine.It is controlled by the intake manifold pressure

INTAKEAIR

EXHAUSTFLOW