From: elson@otc.otca.oz (Elson Markwick)
Newsgroups: rec.autos
Subject: Re: Engine Braking - what causes it?
Date: 14 Apr 92 13:05:50 GMT
Reply-To: elson@otc.research.otca.oz.au (Elson Markwick)
Organization: R&D, OTC Sydney, Australia

In article <5342@nosc.NOSC.MIL> koziarz@halibut.nosc.mil (Walter A. Koziarz) writes:
>
>>In article <5411@otc.otca.oz>, elson@otc.otca.oz.au (Elson Markwick) writes:
>
>>|> I can't imagine that pulling air past a closed throttle plate would be the cause
>>|> of engine braking...  After all, the throttle plate would have to be able to
>>|> resist quite a vacuum, which would make it pretty tough to open when you wanted
>>|> to give it a boot-full.  Try an experiment - prop open the throttle butterfly
>>|> with something (take the spring off if you have to), and try to turn the engine
>>|> by hand.  I'll bet you it's just as tough with the butterfly open as with it 
>>|> closed. (BTW, if you're going to try this, make *sure* your ignition is off :-)
> 
>Here's a 'better' experiment -- descend a grade and switch ignition off,
>observe rate of descent with throttle closed and then repeat with throttle
>open...  You *will* find that engine-braking *vanishes* or decreases
>*significantly* with throttle open vs closed...
>
>I say this is better because it addresses the point in question -- cranking has
>little to do with running as far as  operating conditions in the cylinders that
>affect crank-rotation are concerned...
>
Ok - I concede that your experiment is more relevant to real operating
conditions.  So I tried it.  In fact I tried it several ways.  I tried it
sitting in my garage, and bringing the engine revs to about 3000rpm (the rev
limit is 4500) which is plenty for engine braking in my car, then killing the
ignition.  As I killed the ignition, I tried both opening and closing the
throttle butterfly.  Guess what - there was *no* *difference* in the speed that
the engine stopped.  So I decided to try it on a (quiet) slope I know, so that
the engine could continue being turned.  Start descending the slope, 3000 rpm,
2nd gear.  Switch off ignition (Wow - glad I was ready for that lurch ;-), and
stomp my foot to the floorboards.  About halfway down, check speedo - still
doing about the same speed I was at the top, so I lift my foot right off the
throttle. Guess what - I'm still maintaining the same speed - i.e. no change in
decelaration.  Just to prove that the slope doesn't get steeper halfway down, I
do it the other way - i.e. lift my foot off when I kill the ignition, then
plant it about halfway down.  Guess what - (yeah, you guessed it :-), no change
in rate of deceleration.  So I put it to you - a closed throttle plate has no
discernable effect on engine braking (unless you see a fatal flaw in my
experimental method).  Try it for yourself if you don't beleive me.

I still say that if pulling air past a closed throttle plate was the cause of
engine braking, you'd go through a hell of a lot of butterfly bushes, not to
mention spindles.  The bushes in my carb (a 2 barrel Carter) are just brass
bushes, and the spindle is just a light alloy.  Here's a thought experiment for
you.  If the closed throttle butterfly is doing all the work, then all the
force required to slow the vehicle is being placed on the spindle and the
bushes in the carb (Newton's whatever law about equal and opposite reactions).
Therefore, it would be like having a free rolling mass (equal to the mass of
the vehicle in question) connected by say a cable to something that is
dissipating the energy (say another vehicle with it's brakes slightly on), by
a throttle butterfly spindle.  You get the picture?  Rolling mass connected to
cable connected to throttle spindle connected to braking object.  You cannot
make me beleive that a 1/4 inch throttle spindle can stand having the force
required to slow a vehicle transmitted through it...
> 
>There is a piece of aftermarket equipment called a 'Jacob Engine Brake' which
>is installed on some large Diesels which *actually* allows
>'compression-braking'.  This devices opens the exhaust valve at TDC such that
>essentially *none* of the work expended in compression is returned on the
>stroke from TDC to BDC.  An engine so-equipped is the only case anything that
>can be refered to as compression-braking is occurring.
>
>This is in addition to, and distinctly different from the exhaust brake already
>mentioned (butterfly in the exhaust system)...

This butterfly in the exhaust system - lets think for a moment how it's
operating.  On the exhaust stroke, the exhaust valve is open, however the
butterfly is presumably still closed (am I correct in this assumption?)
Therefore, the rising piston is pushing against compressing air, just like the
intake stroke, except there's a higher volume of gas due to the recently fired
idle fuel charge.  Ok - then the exhaust valve closes, but there is still a
compressed gas volume in the top of the cylinder.  So how is this different to
the intake stroke?  The only difference I see is that with an exhaust butterfly
blocking the exhaust path, you are getting the rising piston fighting with
compression on both the exhaust stroke and the intake stroke - i.e. double the
effect.

>Walt K.

Cheers, Elson


-- 
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