the ic engine
TRANSCRIPT
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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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g/javappl/ottoCyc.html
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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