[Vwdiesel] Brakes --- engine brakes -- ( The physics of it ).

[Val Christian]

> Physics :   Many years of playing around with air compressors
> single stage 300 to 400 PSI tells me the incredible heatloss involved.
> Red hot outlet pipe.  In the VW the energy comes from where the rubber
> hits the road.

The exhaust gets red because you are dumping energy...just like the jake 
brake.  But in a NA diesel engine, without a jake, you are just using that
compressed gas like a spring.  You get nearly as much energy out of the 
"burn" stroke as you put into the "compression" stroke.  This IS where the
big difference is.  

Yes, a SMALL level of energy is derived from cooling the compressed air at
TDC.  Please remember that TDC doesn't last for long, and that air is a
relatively good insulator (good thing, or our cars would never start),
so only a small fraction of the energy which could be taken out, will be.
My guess is that systemic friction will introduce more losses than TDC
air load cooling.

When a NA VW diesel is coasting, the exhaust will cool down, the engine 
will start to cool.  That's because there is little energy dissappated.
You could start a model of that, using the cranking power.  Frictive 
losses will go up about the square of the rotational velocity.  Energy
is also introduced through the "idle injection" fuel, reducing the
potential braking effect.

If just one person asks, I'll bore everyone with why a TDI seems to brake 
less than a NA VW diesel.  There are two primary factors, and several
secondary ones...oops, I'll wait.

Just remember that the losses from engine braking will be roughly the square 
of the rotational velocity.  So if you REALLY need to stop, downshift first.
(And if you're driving any VW diesel, don't expect to stop quickly.)

Val