>> It is interesting that many people believe that the fuel actually  
>> explodes in the cylinder and drives the piston against the crankshaft.
>> That is hardly the case.  The IC engine is a heat engine just as the
>> steam engine is a heat engine.  The mileage depends on the efficiency
>> of the conversion of fuel into heat.  The fuel-air mixture burns: it
>> does not explode.

Correct.

>> When the cylinder comes up it compresses the air creating an oxygen
>> rich environment for the fuel mixture.

Movement of the piston during the compression stroke does not change
the ratio between oxygen, nitrogen, and other trace elements in air.
The cylinder just contains compressed air (plus fuel).

>> The spark causes the fuel to
>> burn and the heat from the oxidation of the fuel causes the fluid
>> molecules inside the cylinder to expand rapidly and equally in all
>> directions.  The force acting on the piston head from the expanding
>> molecules is what moves the crank shaft.

Correct.

>> The expanding forces against the side walls of the cylinder do not
>> contribute to the force on the piston head and so contribute to the
>> inefficiency.

Another use of the high pressure burning air-fuel mixture is to get
behind the rings and press them up against the cylinder walls.  This
provides an adjustable seal between the moving piston & the imperfect
cylinder wall (which has taper, out of round, etc).

>> But the optimum fuel-air mixture for the most complete
>> combustion is not used (BUT in MHO it could be).  The mixture is  
>> always on the rich side to guarantee predictable and repeatable
>> ignition without any dead cycles. The rich mixture is another
>> inefficiency because it results in less than complete combustion.
>> There are some other kinetic factors like the inertial forces and
>> migration of the mixture through the manifold, which is why polished
>> ports and manifolds yield more power.

An additional problem is keeping the air-fuel mixture exactly
the same in each cylinder.

The intake manifold runners vary in flow resistance to each cylinder.
If the fuel is inserted at the throttle butterflies, via carburetor
or throttle bore fuel injection, then the mixture can vary during its
transit to individual cylinders.  With per-port fuel injection, the
air (moslty homogenous) moves to each cylinder and the appropriate
amount of fuel is sprayed right at the intake valve thus attempting
to keep air-fuel mixture identical in each cylinder.  This allows
the controller to run the engine closer to the optimum air-fuel
mixture point for the prevailing conditions..

(In older, very high performance engines, one carburetor barrel and
a seperate intake manifold runner was dedicated to feeding each the
mixture into each cylinder, i.e. 8 Webers on a V8 engine.)

Part of the power increase of polished manifold runners is simply
the ability to insert _more_ air-fuel mixture into the cylinder;
increasing the volumetric efficiency.

>> Increasing the compression would create a richer oxygen environment
>> to produce a hotter burn (more heat) which is the whole objective of  
>> the IC engine.

Again, higher compression does not change the oxygen ratio.  Higher
compression converts more of the heat energy into "push" on the piston.
It also requires a higher octane level fuel to prevent the mixture
from exploding.  (Octane rating * is mostly the ability of a fuel
to not explode under specific engine operating points.)

>> But too much oxygen would move the process from burn
>> to explode which would damage the engine.  There is the real limit
>> on mileage and efficiency: How much heat you can get from the burn
>> of the fuel without exploding it.  In diesel engines the compression
>> ratio is much higher and it is so oxygen rich that a spark is not
>> required to initiate the burn.  But here, again, the efficiency limit
>> is imposed by the rate of oxidation; too fast and BOOM, you wrecked
>> your engine.  The reason diesel can tolerate higher oxygen ratios is
>> because there is less BTU per unit volume than for gasoline, of
>> whatever octane.

And when a diesel engine is cold, a glow plug is usually used to
initiate combustion.  When warm, the act of highly compressing the
air-fuel mixture is sufficient to ignite the charge.  Plus diesel
engines are built stronger to withstand the higher stresses.

>> So to get more mileage you need to figure out how to get more heat
>> per unit fuel, a more complete burn and keep on the safe side of  
>> explosion.

> I don't know the difference between burn fast and explode.  Does fuel
> burn fast and if the engine flies apart then the fuel was "exploding" ?

Modern engines have a knock sensor.  The computer controller leans
the air-fuel mixture as much as possible, usually based on an O2
sensor in the exhaust system.  When it senses the fuel exploding via
the knock sensor, it enrichens the mixture "enough" to stop it.

Besides mileage, most of these engine control strategies reduce the
emissions which is most of the driving force behind their design.
The government emissions limits were set prior to the fleet mileage
goals.  And emissions limits are harder limits; poor mileage just
results (in the US) an economic penalty on the manufacturer,
either directly through government penalties or loss of buyers..

						Lee Jones

* higher octane rating of aircraft engine fuels during world war 2
  allowed the allied powers to use higher compression ratios and
  produce higher engine power output for a given engine weight
  which allowed for more performance or more range...  Water
  injection was also used to prevent knock during extreme power
  operation (but the amount of water available was limited).

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