From: rat@erg.sri.com (Ray Trent)
Subject: Re: Engine Braking - what causes it?
Reply-To: rat@erg.sri.com (Ray Trent)
Organization: SRI International, Menlo Park, CA
Date: Tue, 14 Apr 92 18:39:06 GMT

In the referenced article, rjs@hpfcso.FC.HP.COM (Bob Schneider) writes:
>>perhaps I'll rephrase my assertion to read '... any Diesel of significant
>>output will provide negligable engine braking effect ...'  that will

Wait: I think I may have thought through the answer, see the end of
this article.

I don't really know enough to say anything about Diesel engines except
that many of the following should apply to them as much as they do to
gasoline engines (I'll mark the ones I think are significantly
different with a *...Dieselheads let me know if I'm missing anything).

Forces that act to keep a vehicle in motion:
Inertia
Any fuel that is ignited during the power stroke for whatever reason
  (Diesels may have either more or less of this than typical gas
   engines during "engine braking", I don't know, does a Diesel
   produce more or less power during idle than a gas engine...I've
   heard that it's more, but...).

Forces that act to provide "braking" to a moving vehicle:
Wind resistance
Rolling resistance
Brakes (but that's not important right now :-)
Pulling air in through the throttle plate during intake (*)
Pushing air out through the exhaust back pressure (*?)
Compression energy escaping in the form of heat (shouldn't be that 
  significant, but...)
Accelerating the pistons up and down
Engine friction (more or less for Diesels?)
Any compression lost during the compression and power strokes will
  tend to take energy (again, I don't know whether Diesels might be
  better or worse at this...they operate at higher compression ratios,
  but I would tend to think they would have to be better sealed for
  the same reason)

Note: many of these factors tend to come into play non-linearly with
engine RPMs. Diesels are not exactly known for having a wide RPM range
(this statement, however, it not based on any real knowledge other
than "common knowledge" which is notoriously inaccurate). It may be
that Diesels in actual use don't show much engine braking due to their
owners not being stupid enough to run them above their rated RPMs.
Gasoline engines don't exactly provide much braking at low RPMs
either.

The thing I'm not sure I buy about the argument that throttle plate
vacuum is what causes "engine braking" is the same thing that is wrong
with the argument that compression causes engine braking. The same
"air spring" action should work (except in reverse). Any energy
expended during intake to pull air in through the throttle plate
should, in principle, after the compression and power stroke, leave an
equivilent partial vacuum in the cylinder that should gain the same
energy back during the early parts of the exhaust stroke. Hmmm, I
wonder if a turbulence issue could be coming into play here? The
partial vacuum in the cylinder should suck some air in through the
exhaust port during the beginning of the exhaust stroke. This
accelerates some air in the inwards direction which then has to be
turned around and pushed out of the cylinder. Hmmm, that might
actually end up increasing the exhaust backpressure rather than
decreasing it because of the turbulence...hmmm.
	
Wait: at zero "throttle" (or whatever), a Diesel must still be letting
in enough fuel to maintain idle (there may be a few smart ones out
there that don't do this, just like there are a few smart gasoline
fuel injection systems that turn off fuel flow during heavy engine
braking...though gasoline fuel injection systems usually aren't
mechanical). 

My guess is that when you overrun a Diesel, it still produces the same
amount of (idle) power per stroke regardless of the engine speed:
a little more than is required to overcome the engine resistance.

In a gasoline engine, at higher the RPMs the intake vacuum is stronger
and less fuel air mixture gets into the cylinder compared to the
amount necessary for engine operation, which implies that less than
the idle power is produced on each stroke. A gasoline fuel injection
system typically only injects enough gasoline to burn in the amount of
air that is actually coming in. A carbuerated gasoline engine
typically lets in an amount of fuel that is (roughly) governed only by
the throttle setting (i.e. it doesn't inject the same amount fuel on
each stroke but rather allows in the same amount of gasoline in a
given time...roughly, give or take slight vacuum effects). In either
case, less air is pulled in per stroke at high RPMs than at low with
the same throttle plate setting, as demonstrated by the fact that the
vacuum is higher.  Regardless of how much fuel actually makes it in,
only enough to completely consume the air that got in will be burned.
(ick, unburned hydrocarbons, no wonder fuel injection with active
feedback is the rule rather than the exception these days).

Thus, my conclusion is that Diesels at idle "throttle" produce exactly
as much power as is required to maintain engine speed, whatever that
is. Gasoline engines, on the other hand, produce less power than would
strictly be needed to maintain engine speed when operated at high RPMs
but idle throttle thereby sucking power out of the vehicle`s inertia.

Of course, none of this explains to me why trucks are required to use
low gear on strong downhill grades...what good does it do if there`s
minimal engine braking?
-- 
"When you're down, it's a long way up
 When you're up, it's a long way down
 It's all the same thing
 And it's no new tale to tell"                      ../ray\..