[Vwdiesel] Diesel Braking

[Roger Brown]

Dana Morphew wrote:
> 
> > > 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.

The braking in a gas engine is mainly due to the restriction of the throttle plate.  You are having the engine pulling air through the closed throttle, which creates the pressure drop and vacuum in the intake.  

As noted, in a diesel there is normally no intake restriction.  Apparently some engine designs do make use of an intake butterfly (Mercedes?).  I would imagine this would require that fuel be cut to the engine since you may not be able to build sufficient
cylinder pressure to burn the small amount of fuel injected.

The Jake brake is a system that alters the valve timing, turning the engine into an air compressor, you get a normal intake and compression stroke, but then the exhaust valve opens at TDC and releases the pressure in the cylinder into the exhaust (thus
explaining the loud noise when the Jake brake is applied).  

Another option, common on the smaller engines like the Dodge Ram, etc., is an exhaust brake, where you add a butterfly valve to the exhaust.  This raises back bressure and makes the engine work to force the exhaust gas out.  Doing this requies that
extra-stiff exhaust valve springs be used to keep them from being pushed open by the increased back pressure.  This would probably be the easiest system to set up, since you don't need to alter the engine or fuel delivery as much.  Not sure if stiffer
springs are available for the VW diesel engines.

-- 
    Roger