From: rick@posms.cactus.org (Rick Kirchhof)
Subject: Re: Engine Braking - what causes it?
Organization: Department of Redundancy Dept., Austin
Distribution: na
Date: Tue, 14 Apr 1992 06:42:21 GMT

>ah, but you make the same error that *everyone* else in this discussion, except
>the Diesel-drivers make...  while it is true that a fair amount of work is
>performed in *compressing* the the cylinder contents, *any* downward travel of
>that cylinder's piston will return virtually *all* of the work (to a first
>approximation) -- EVEN WITHOUT ANY FUEL BEING BURNED, hence the only
>restriction is internal friction in the engine in a Diesel and the attempt to
>pump air past a closed throttle plus internal friction in a gasoline engine...
>
You are correct in assuming that some force is reapplied due to the same
air expanded as was compressed.  This does not begin to address ring and
bearing friction which is dramatically higher in the diesel due to the
extra compression ratio.  If you were to place a closed throttle valve 
in a diesel engine in the normal location, the engine would loose much of 
the torque required to turn it on deceleration.  In both the normal 
diesel and the diesel with the throttle valve, air would be trapped 
compressed and expanded again as the engines go through compression and
power strokes.  This does not mean each engine would have the same passive 
torque requirement since each engine would have different compression 
pressures and therefore different friction loads.

Piston rings work by differential pressure.  Contrary to common belief
the springlike expansion of the ring matters little except to keep it in
place and ready for action.  The real activity begins when the pressure
above the piston moves behind the ring into the ring land.  This forces 
the ring away from the piston and against the cylinder wall or liner.  
Higher compression ratios cause higher compression pressures causing much 
greater drag between the ring and cylinder wall.


>hmmmm, still doesn't *really* account for the case of an empty trailer or a
>tractor alone...  1000+ cubic inches probably scales quite well to the
>'passenger-car' case...  I tend to agree with Alan Hepburn that VW and Mercedes
>do *something else* to instill an artificial sensation of engine braking in
>their Diesels because there is no physical mechanism that accounts for this in
>normal Diesel operation...  a Diesel freewheels, period...
>
>Walt K.

These engines are very conventional in their design.  While all parts are 
thicker, heaver, stronger and more expensive, there are no extra parts in 
the valve train until you get into optional engine brakes as used by BIG 
trucks.  This leaves only the above normal operating forces to account for 
the gasoline vs diesel difference.  Since diesels *always* compress all
of the air that they can intake, the rings are always working hard and 
there is never a period of decreased friction.
-- 
Rick Kirchhof   Austin, Texas                   | Experience is what you
Domain: rick@posms.cactus.org                   | get when you don't
Bang path: ...!cs.utexas.edu!peyote!posms!rick  | get what you want.            
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