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The IC Engine

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The IC Engine Powered By Docstoc
					The IC Engine: Why…
By: Matthew King

Outline
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A Short History Background Why… How do we compare engines and their cycles?

History
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The internal combustion engine was first conceived and developed in the late 1800’s The man who is considered the inventor of the modern IC engine and the founder of the industry is pictured to the right….Nikolaus Otto (18321891). Otto developed a four-stroke engine in 1876, most often referred to as a Spark Ignition, since a spark is needed to ignite the fuel air mixture.

History
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The impact on society is quite obvious, all most all travel and transportation is powered by the IC engine: trains, automobiles, airplanes are just a few. The IC engine largely replaced the steam engine at the turn of the century (1900’s) Another important cycle is the Diesel cycle developed by Rudolph Diesel in 1897. This cycle is also known as a compression ignition engine.

Background on IC Engines
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“An internal combustion is defined as an engine in which the chemical energy of the fuel is released inside the engine and used directly for mechanical work, as opposed to an external combustion engine in which a separate combustor is used to burn the fuel.”1 “IC engines can deliver power in the range from 0.01 kW to 20x10^3 kW, depending on their displacement.”2

Background on the Otto Cycle
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The Otto Cycle has four basic steps or strokes:
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1. An intake stroke that draws a combustible mixture of fuel and air into the cylinder 2. A compression stroke with the valves closed which raises the temperature of the mixture. A spark ignites the mixture towards the end of this stroke. 3. An expansion or power stroke. Resulting from combustion. 4. An Exhaust stroke the pushes the burned contents out of the cylinder.

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To the right is an idealized representation of the Otto cycle on a PV diagram. http://www.rawbw.com/~xmwan g/javappl/ottoCyc.html

Why…
The Otto cycle IC engine has remained fundamentally unchanged, besides slight improvements, for over 100 years. Its’ popularity has continually increased because…  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

Comparing Engines….
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mep= work done per unit displacement volume
Or average pressure that results in the same amount of indicated or brake work produced by the engine  Scales out effect of engine size  Two useful types: imep and bmep
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imep: indicated mean effective pressure
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-the net work per unit displacement volume done by the gas during compression and expansion -the external shaft work per unit volume done by the engine

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bmep: brake mean effective pressure
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bmep 

4    Vd

BMEP
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Based on torque:

4    bmep  Vd

(4 stroke)

2    bmep  Vd

(2 stroke)

Compare…
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Brake specific fuel consumption (bsfc)
Measure of engine efficiency  They are in fact inversely related, so a lower bsfc means a better engine  Often used over thermal efficiency because an accepted universal definition of thermal efficiency does not exist
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  mf mf bsfc    Wb 2     N

bsfc 

  mf mf   Wb 2     N

bsfc
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bsfc is the fuel flow rate divided by the brake power

  mf mf bsfc    Wb 2     N

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We can also derive the brake thermal efficiency if we give an energy to the fuel called heat of combustion or, qc  Wb 1  
 mf  qc bsfc  qc

Compare…
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Volumetric Efficiency, ev
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The mass of fuel and air inducted into the cylinder divided by the mass that would occupy the displaced volume at the density ρi in the intake manifold Note it’s a mass ratio and for a 4 stroke engine

  2(ma  mf ) ev  iVdN
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For a direct injection engine

 mf

0

Other comparisons…
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First law analysis- energy conservation
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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 control volume

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Second Law Analysis – entropy conservation
This approach takes into account the irreversibility that occurs in each process  Another 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


				
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