[Vwdiesel] Diesel Braking

[Dana Morphew]

> > 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.
> 
. . .

>  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.  

True...all the energy expenditure of compression is not regained, but, for 
our purpose it is close enough to.   A gasser pulling against manifold vacuum 
gets an incomplete "gulp" because of the vacuum.  Since the "gulp" is 
incomplete, it is compressing less, but, again, it  regains most of what it 
expended when comes the power stroke.  

I see now that the vacuum is, indeed, the extra bit that the diesel doesn't 
need to overcome.  Does the added compression of the diesel cause a larger 
"energy debt" after its losses are tallied on completion of the downstroke?  
I'm now thinking my perceived advantage in diesel engine braking over a 
gasser may be due to my adopting a notion that once seemed to make sense but 
actually does not hold true.

Dana
 
>  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