[Vwdiesel] Diesel Braking

[Val Christian]

> I've always been happy with the "braking" of my diesel when I'm off the 
> pedal.  It "compression brakes", if that is the phrase, well because my idle 
> rpm is set at a smooth 700 (love the balanced rod/piston/pin sets).  It also 
> "brakes" well because each piston pulls in an unhindered gulp of air before 
> each compression stroke and the Total Seal rings seem to be seated.   It is a 
> 1.9 which will, all things being equal, give more braking effect than a 
> smaller diesel.

	I haven't quantatively studied this, but playing in the mountains
	going down long interstate grades, I've come to the conclusion that
	the NA VW diesel (experience with 1.5 and 1.6 mostly) has very little
	braking compared to a gasser.  If it did brake allot, then fuel 
	economy would be less.

	My unnumerically substantiated testing suggested that the AC 
	compressor induced more than 50% additional drag in 5th gear.
	More drag resulted opening all windows at 60 MPH.  At 30 MPH 
	the window drag exceeded the AC drag, in 4th gear.

	I agree with the commentators that the synchros in the gearbox
	are a wear factor, and I can point to cars I've worn out in
	my downshift days, and say that they tended to have more worn
	synchros in 2nd and 3rd.  In recent years (the last 10 or so),
	I've changed my ways and remain in a higher gear until the 
	RPMs drop to idle point, and haven't worn out downshift synchros.

	My TDI is different, and in particular, the lower RPMs 
	probably result in lower drag.

	Bottom line, a VW with a diesel coasting down a hill rolls MUCH
	better than a gasser similarily configured.

> A gas engine must contend with manifold vacuum when attempting to fill its 
> cylinders.  This vacuum, (@ 25 lbs. or a bit less than 2 atm.), causes some 
> resistance to engine rotation.  If the cylinders were able to get all the air 
> their capacities allow, they only have 1/3 to 1/2 the numeric compression my 
> diesel has.

	Given my limited knowledge of physics, the drag on a gasser is:
		
		1. Pumping air against a vacuum.
		2. Engine and transmission frictive losses.
		3. Engine accessories.

	The compression ratio differential on a diesel is not much of a 
	factor because the compressed air is released after pushing the
	piston to the exhaust valve release point.  

	A notable exception to this is a "jake brake" which releases the
	compressed cylinder contents, prior to the piston extracting the
	energy from the compressed air.  This results in allot of noise
	and substantial engine drag.  I'm sure that someone has made one,
	but jake brakes are not on VW diesels that I've seen.


Val