> On Jun 27, 2007, at 11:34 PM, Rich wrote:
>
> 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.   
> When the
> cylinder comes up it compresses the air creating an oxygen rich  
> environment
> for the fuel mixture.  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.
>     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.  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.
>     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.  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.
>     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.
>     Now here is an interesting point.  Some people talk about  
> alternative
> energy on one side of the page and better efficiency on the other  
> side.  So
> they came up with ethanol.  Ethanol has less BTU per unit volume than
> Gasoline.  To save energy they advocate mixing ethanol with the  
> gasoline and
> at some pumps it is already premixed.  Question!  How is mixing  
> something
> with less energy with something with more energy going to result in
> increased efficiency, or better mileage, and so on.  The heat  
> produced by
> the burn will actually decrease, resulting on lower power.  The  
> problem of
> repeatable ignition cycles requiring a richer mixture remains.  Is  
> it an
> oxymoron?  Or is it simply a moron idea? I am baffled by it.

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" ?

Here are some excerpts from the internet:
1)  Piston failure in two-stroke engines.(SHOP TALK)
In my opinion, overheating is the most common cause of engine  
failure. ... The carburetor is adjusted too lean. Such failures are  
common with operators who ...
2)  Rough engine. Misfire. Vibration in Lycoming or Continental ...
This causes overheating of the points and erosion of point  
surfaces. ... Excessive lean mixtures at low engine speed will result  
in rough and erratic idle ...
3)  Engine Seizures
If the high speed circuit is lean enough to cause piston seizure, it  
will also tend to ... If an engine is operating on the ragged edge of  
overheating, ...

Cedric

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