From: jniewiad@csws5.ic.sunysb.edu (Jan Jozef Niewiadomski)
Newsgroups: rec.autos
Subject: Re: Engine Braking - what causes it?
Date: 9 Apr 92 01:16:37 GMT
Organization: State University of New York at Stony Brook

In article <5315@nosc.NOSC.MIL> koziarz@halibut.nosc.mil (Walter A. Koziarz) writes:
>In article <1992Apr8.004020.9998@sbcs.sunysb.edu> jniewiad@csws7.ic.sunysb.edu (Jan Jozef Niewiadomski) writes:
>>In article <5313@nosc.NOSC.MIL> koziarz@halibut.nosc.mil (Walter A. Koziarz) writes:
>
>>>predominantly, the action of trying to pull intake air past a closed throttle
>>>plate is the source of 'engine-braking'...  don't believe someone saying
>>>'compression' because *that* is *not* true -- Diesels essentially freewheel,
>>>i.e. there is far *less* engine braking with a Diesel than with a gasoline
>>      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>engine (all other factors equal).
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>>NOT TRUE
>
>>Ever take your foot off the diesel-pedal ( :-) ) while driving ?  The front
>>of the car literally DIVES....  Pulling a vaccuum in a gasoline engine results
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Forgot to mention, this is at around 3500 revs

>>in about 1 atmosphere of resistive pressure due to crankcase pressure.  The 
>>"freewheeling" diesel is compressing whatever it takes in at that particular
>>RPM into 1/23 its original volume which gives an average resistive pressure
>>of about 11.5 to 9.5 atmospheres depending on how high the revs are.
>>JJ
>
>as a matter of fact, yes, i have lifted my foot off the accelerator...  i drive
>a Cummins-powered Diesel pickup and can *assure* you and anyone else that that
>Cummins essentially freewheels compared to an equivalent displacement gasoline
>V8 (359 cdi vs. 360 cid).  i do thank you for the compression-ratio reminder,
>but i am well aware of the Diesel's higher ratio.  i have also never driven a
>'practice-Diesel' as found in small cars, but am aware that Mercedes (on early
>240D models) used pneumatic methods for engine speed control -- a butterfly
>(looks *just* like a throttle plate) valve is installed in a venturi in the
>manifold and generates a vacuum signal used by the pump...  *that* Diesel would
>produce engine braking due to my above stated mechanism.  others, yourself
>included, are free to disagree with me, but i do not consider the 'Diesel' in a
>Rabbit to be a real Diesel.  real Diesels are manufactured by (for example)
>Cummins, Caterpillar, Detroit Diesel, Perkins, etc.
>
>as i said, having never had opportunity drive a Diesel Rabbit, i cannot comment
>on that; but i have driven Diesels for several years and can comment about
>their behavior.
>
>are you certain you didn't violate my 'all else equal' qualifier?  are Rabbit
>Diesels similar/identical displacement to the gasoline counterpart?  same final
>drive ratio, etc?
>
> Walt K

  The reason I described the Rabbit diesel instead of my Detroit, is because 
it and its gas counterpart both use the same block, same stroke, only 8 ci 
difference in total engine displacement.  The gas (CIS) puts out 90 ft.lbs of
torque @ 3000 revs and the diesel puts out 97 ft.lbs @ 2800 revs.  Since the 
VW diesel can rev up to 5500 rpms without self-destructing (unlike real 
diesels), both cars use identical transaxles. Induction in the rabbit diesel 
is only obstructed by the air filter, just like many Caterpillars and the 
common Detroit diesels.  

  Since I'm not a VW diesel expert, I'd like to know what's wrong with the
following comparison for one cylinder-cycle.  ( My dad would like to know
too, he's a technical consultant for Caterpillar Repairs Dept., so he's 
ready for fresh insight anytime...) :-)


(for simplicity, let work = (average pressure)*(change in volume) and we won't
worry about non-linearity and change in temperature) 

Induction:

	Diesel:	Basically free induction. (compared with a gas engine)
	Gas:	Resistance due to closed throttle, (i.e. 1 atm. in crankcase)
		 ( approx. 2*disp if it reaches .5 atm at BDC ?)

Compression:

	Diesel:	Resistance = 11.5*disp. + early combustion due to centrifugal
		advance in injection pump.

	Gas:	Resistance = 4*disp. + early combustion due to centrif. advance

Combustion:

	The diesel might have a little more left to burn but I'm not sure.

Exhaust:

	Basically the same. ( Assuming no turbos)

Perhaps the early combustion in the previously described gasoline V8 is the 
main source of resistance ?

JJ

PS The unreal VW diesels usually get 150k+ real miles out of their injection 
  pumps unlike the Detroit pump I've had to rebuild every 60k miles. :-)

JJ
  
From: mdrayton@hpmwtd.HP.COM (Mike Drayton)
Date: Mon, 13 Apr 1992 17:28:34 GMT
Subject: Re: Oil recommendations sought.
Organization: HP Santa Rosa Site (NMD MTA MWTD). - Santa Rosa, Ca.