From: alan@lancaster.nsc.com (The Hepburn)
Newsgroups: rec.autos
Subject: Re: Engine Braking - what causes it?
Date: 10 Apr 92 15:10:54 GMT
Organization: National Semiconductor Corporation

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 :-)
|> 
|> 

This sure is a frustrating thread.  It's such a simple concept yet people insist
on making it difficult.  Let's look at the mechanics of each of the 4 strokes in
both the gas and the diesel engine:

intake:  On acceleration the gas engine has a direct path from the combustion chamber
         to the outside atmosphere.  On decceleration that path is obstructed by a
         closed throttle plate.

         On acceleration the diesel engine has a direct path from the combustion
         chamber to the outside atmosphere.  On decceleration the path is still wide
         open since there is no throttle plate to close.

compression:  On acceleration the gas engine has no path from the combustion chamber
              to the outside world.  The piston is compressing the air/fuel mixture
              in essentially a closed container.  On decceleration the gas engine is
              still closed to the outside atmosphere since the intake and exhaust
              valves are closed.  The piston is still attempting to compress the air/fuel
              charge in the cylinder, except there is not very much there since the
              engine could not pull a full charge in through the closed throttle.

              On acceleration the diesel engine has no path from the combustion chamber
              to the outside atmosphere.  The piston compresses the air (not air/fuel)
              in a closed container.  On decelleration the diesel engine is still
              closed to the outside world.  The piston is still compressing air since
              the engine was able to get a plentiful supply during the intake stroke.

ignition:  On acceleration the gas engine relies on the presence of an electrical
           spark to ignite the air/fuel mixture.  This causes the mixture to burn
           and the expanding gases push the piston down into the cylinder.  On
           decceleration the spark is still applied and a richer, though smaller,
           air/fuel charge is ignited, again pushing the piston down in the cylinder.
           
           On acceleration the diesel engine injects a charge of fuel into the 
           compressed air which ignites and pushes the piston down.  On
           decceleration there is no fuel injected into the compressed air, so no
           ignition occurs.

exhaust:  On acceleration the gas engine has a direct path from the combustion chamber
          to the outside world.  On decceleration there is also a direct path to the
          outside world.

          On acceleration a diesel engine has a direct path from the combustion 
          chamber to the outside world.  On decceleration there is still a direct
          path to the outside world.  This is where most manufacturers install the
          braking equipment.  By closing the exhaust path, resistance is created
          which causes the engine to rapidly deccelerate.

As can be seen above, the intake stroke is the only one that is substantially
different between a gas and diesel engine.  It is very difficult to pull air through
a closed valve, and that difficulty is what causes engine braking in a gas engine.
Try THIS experiment:  attempt to draw air through a common drinking straw.  Now
restrict the air flow through that straw and try again.  That's what a gas engine is
having to do.  The throttle spring is strong enough to hold the plate closed during
decceleration.  That's why it's there.  There is usually another valve installed
(at least on California cars) called a decceleration valve which holds the throttle
open during deccereration to reduce emissions from burning a richer mixture.


-- 
Alan Hepburn           "They that can give up essential liberty to obtain
National Semiconductor       a little temporary safety deserve neither
Santa Clara, Ca              liberty nor safety."
alan@berlioz.nsc.com                              Benjamin Franklin