From: floyd@eng.umd.edu (Mike Hughes)
Subject: Re: Engine Braking - what causes it?
Date: Tue, 14 Apr 92 19:19:41 GMT
Organization: University of Maryland
Distribution: na

In article <5337@nosc.NOSC.MIL> koziarz@halibut.nosc.mil (Walter A. Koziarz) writes:

>In article <1992Apr13.043351.4651@posms.cactus.org> rick@posms.cactus.org (Rick Kirchhof) writes:

>>I have followed this topic with interest, and would like to suggest an
>>angle not mentioned.  While it is true that spark ignition engines have a
>>throttle valve and it's angle greatly effects the amount of pumping
>>restriction (and engine braking there is on decel, compression ignition 
>>engines use much higher compression ratios.  This means that if you try 
>
>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...
>

[I had a rude flame here, but erased it when I reread Walt's passage] 
[Your statement above is correct..I mis-read it as saying that you thought the 
force to pull a piston down at closed throttle deceleration was equal to the
force required to push a piston up and compress a charge.  I now think that
engine braking is caused mainly by ring friction, and MAYBE a little bit by
pulling vacuum (though I can't feel the difference vacuum makes).]

*MORE*
energy is required to compress the mixture in the cylinder than is required 
to pull a vacuum in that same cylinder.  The reason behind this is that when
a mixture is compressed in an engine, there is no limit to how much pressure
can be created (except for sealing and structural factors).  When a vacuum is
pulled, there *IS* a limit.  That limit is 0 lbs/in^2 (absolute pressure).
Givin atmospheric pressure is ~14.7 lbs/in^2 (at sea level), we can calculate
exactly how many lbs of force would be required to hold a 4" dia. piston at a
piont in a cylinder with 0 apsi on one side, and 14.7 apsi on the other.
                           ^^^^--Absolute Pressure (in lbs) per Square Inch

Area = pi * (2 in^2) 
Area = 12.57 in^2
Force = Pressure * Area
Force = (14.7 lbs/in^2) * (12.57 in^2)
Force = 184.78 lbs

Now, on our last compression check, we found that the engine was making about
170 lbs/in^2.  Our compression guage reads in "GUAGE PRESSURE", so to convert
to absolute pressure, we add 14.7.  We now have a 4" dia. piston being held 
with 14.7 lbs/in^2 on one side (the crankcase), and 184.7 psi on the other.
Subtract the two to get the force. (Of course, I could have skipped the last
step, but since this demonstration involves pressure AND vacuum, I chose not
to.)

Area = 12.57 in^2
Force = (170 lbs/in^2) * (12.57 in^2)
Force = 2167.5 lbs

Well, that proves that the amount of force required to compress a gas is MUCH
greater then the force required to pull a vacuum (at a ratio of 11.73:1 in this
case).  I still believe that the braking effect felt during engine deceleration
is due to other factors than a closed throttle plate (like ring friction).-Mike

:Suicidal Tendencies=====> He's a carrier of death a stork in reverse,        :
: =8^) Mike Hughes  =8^( : he blesses you with sickness cause love is a curse.:
:Mail---floyd@eng.umd.edu: The arsenic in your Koolaid, the bomb in your mail,:
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