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

[Paul]

On Tue, 06 Jan 2004 17:47:15 -0800
Roger Brown <r.c.brown at ieee.org> wrote:

> I initially thought you could rig up a switch to shut off the fuel cut-off solenoid on the pump, but I guess that wouldn't be a good idea since I assume the pump is lubricated/cooled by the fuel and it would run dry pretty fast with the input feed cut
> off.

Well roger.. You got me thinking too..  Maybe applying a pwm signal from a 555 timer fed to a FET to drive the solenoid could do the trick.. But alas.. there is a problem..  A quick look at the most spiffy ve pump diagrams on matrin's westy page shows it.

http://www.cs.rochester.edu/u/jag/vw/engine/fi/injpump.html
specifically: http://www.cs.rochester.edu/u/jag/vw/engine/fi/YANDET2.JPG

If you restrict the fuel flow into the plunger chamber, you risk causing a vaccume of sorts.. I don't know the physical charecteristics of diesel.. But considering the positive pressures that the pump generates, it is concievable that it could generate a pretty good vacuume in the chamber also.  If the vapor point is low enough.. the fuel might boil, and the bubbles would probibly get caught in the injectors.  But maybe not.. you guys tell me..

well.. this has all been speculation.. but I like my pump working just fine and don't want to hurt it doind something crazy..

Thinking in retrospect.. I should be possible to put a solenoid in place of the idle screw(not the fueling screw inside the pump). This would give a quick way to reduce the fuel to the engine without hurting the pump.. that are other issues with this to.. but anyways, would this provide a significan amount of added braking? donno.. 

Just as an aside.. my rabbit brakes awesome with the engine.. my new golf doesn't at all.  Had a bad front caliper on the rabbit last winter.  It was too cold to change it.. so, I pried it loose and drove around for 3 weeks using only the engine and very rarly the hand brake.. I would never be able to pull this off with the tdi.. that engine doesn't seem to settle down very fast.


paul