The IC Engine

7/29/2019 The IC Engine

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The IC Engine: Why

By: Matthew King


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Outline

A Short History

Background

Why How do we compare engines and their cycles?


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History

The internal combustion enginewas first conceived anddeveloped in the late 1800s

The man who is considered the

inventor of the modern ICengine and the founder of theindustry is pictured to theright.Nikolaus Otto (1832-1891).

Otto developed a four-strokeengine in 1876, most oftenreferred to as a Spark Ignition,since a spark is needed to ignitethe fuel air mixture.


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History

The impact on society is quite obvious, all mostall travel and transportation is powered by theIC engine: trains, automobiles, airplanes are just

a few.The IC engine largely replaced the steam engine

at the turn of the century (1900s)

Another important cycle is the Diesel cycledeveloped by Rudolph Diesel in 1897. This cycleis also known as a compression ignition engine.


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Background on IC Engines

An internal combustion is defined as an enginein which the chemical energy of the fuel isreleased inside the engine and used directly for

mechanical work, as opposed to an externalcombustion engine in which a separatecombustor is used to burn the fuel.1

IC engines can deliver power in the range from0.01 kW to 20x10^3 kW, depending on theirdisplacement.2


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Background on the Otto Cycle

The Otto Cycle has four basicsteps or strokes: 1. An intake stroke that draws a

combustible mixture of fuel andair into the cylinder

2. A compression stroke with the

valves closed which raises thetemperature of the mixture. Aspark ignites the mixture towardsthe end of this stroke.

3. An expansion or power stroke.Resulting from combustion.

4. An Exhaust stroke the pushesthe burned contents out of thecylinder.

To the right is an idealizedrepresentation of the Otto cycle ona PV diagram.

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Why

The Otto cycle IC engine has remained fundamentallyunchanged, besides slight improvements, for over 100years. Its popularity has continually increasedbecause

Relatively low cost

Favorable power to weight ratio

High Efficiency

Relative simple and robust operating characteristics Improvements are mainly lower emissions and higher

fuel efficiency


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Comparing Engines.

mep= work done per unit displacement volume

Or average pressure that results in the same amountof indicated or brake work produced by the engine

Scales out effect of engine size

Two useful types: imep and bmep

imep: indicated mean effective pressure

-the net work per unit displacement volume done by the gasduring compression and expansion

bmep: brake mean effective pressure

-the external shaft work per unit volume done by the engine

4


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BMEP

Based on torque:

dVbmep

4

Vdbmep

4

(4 stroke)

(2 stroke)

dVbmep

2


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Compare

Brake specific fuel consumption (bsfc)

Measure of engine efficiency

They are in fact inversely related, so a lower bsfcmeans a better engine

Often used over thermal efficiency because anaccepted universal definition of thermal efficiency

does not exist

N

fm

bW

fmbsfc

2

ff


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bsfc

bsfc is the fuel flow rate divided by the brakepower

We can also derive the brake thermal efficiency

if we give an energy to the fuel called heat ofcombustion or, qc

N

fm

bW

fmbsfc

2

N

fm

bW

fmbsfc

2

qcbsfcqcfm

bW

1


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Compare

Volumetric Efficiency, ev The mass of fuel and air inducted into the cylinder divided

by the mass that would occupy the displaced volume at the

densityi in the intake manifold Note its a mass ratio and for a 4 stroke engine

For a direct injection engine

NV

mme

di

fav

)(2

0fm


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Other comparisons

First law analysis- energy conservation For a system open to the transfer of enthalpy, mass,

work, and heat, the net energy crossing the control

surface is stored into or depleted from the controlvolume

Second Law Analysisentropy conservationThis approach takes into account the irreversibility

that occurs in each processAnother outcome of this analysis is the development

of the usefulness of each type of energy (exergy)


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References

1. Internal Combustion Engines, Colin R. Ferguson,John Wiley & Sons, 2001

2.Engines An Introduction, John L. Lumley,Cambridge University Press, 1999

The IC Engine

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