[Vwdiesel] Diesel braking - again

[Harmon Seaver]

   So why do you think they put those expensive Jake brakes on diesel
trucks? They sure don't put them on gasser trucks.


On Tue, Jan 06, 2004 at 02:46:29AM -0800, Erik Lane wrote:
> ok, i've been thinking about this. sorry to bring this
> up again, but i just can't get it off my mind. :)
> 
> 
> i've been driving diesels for many years and have
> never noticed a problem using the engine for a brake.
> of course, since i only use it when there's a REALLY
> long hill or some such i might not use engine braking
> as much as some of you. and i don't drive gassers as
> much as i do diesels, but when i do i've never noticed
> that much of a difference. i find it hard to believe
> that would have missed it, but of course it's
> possible. you can bet next chance i get i'm gonna be
> testing it!
> 
> my understanding of the physics and all might be
> mistaken, but seems to me the higher compression is
> still a large factor, even when the air is let back
> out on the exhaust stroke. the diesel starters are
> much larger than the gas equivalent, and they spin the
> motors slower, in my experience. even with a fresh
> battery and not counting the first few turns, when
> arguably the engine hadn't had a chance for the
> compression vs exhaust to equalize, it still seems to
> take a LOT more power to turn.
> 
> and if a gasser is pulling against a vaccuum that also
> means that the air density in the cylinder when it
> goes to compression is VASTLY reduced. that's a lot
> less work.
> 
> yes, a diesel gives more fuel at higher rpms. but so
> does any engine, what does that have to do with it? if
> it's a gasser and port fuel injected then it will get
> more squirts of fuel for every intake stroke. if it's
> mass fuel injected then it will increase for the
> increase of air crossing over the air metering device.
> (cause even tho it's pulling against a vaccuum it
> should still pull more air as the engine speeds up,
> right?) and with a carb higher vaccuum will cause a
> greater differential across the metering screw and let
> more fuel in that way.
> 
> i'm not tryin to say that they are exactly the same.
> just tryin to theorize why there wouldn't be that much
> of difference, like it has been in MY experience.
> maybe some people's injection pump settings are a bit
> high at idle? but one of mine has always idled a bit
> high and it still has good engine braking if needed.
> 
> i don't want to fight anyone or argue. i'm just trying
> to understand and match what i see in the real world
> with my undertanding of HOW it all works. so if
> someone can see where what i'm saying is faulty,
> please let me know. (just tryin to be extra careful,
> cause i don't always come across well on the
> computer.)
> 
> sorry again for dragging this out again.
> 
> regards,
> erik
> 
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-- 
Harmon Seaver	
CyberShamanix
http://www.cybershamanix.com