On Sun, 24 Jun 2012 16:00:41 -0400, Phil Hobbs
wrote:


I have no idea whether there is a similar system fitted to UK trucks, but I
can't say that I've ever heard anything that sounded out of the ordinary
with a truck engine, so maybe not. I have seen signs here though at the
start of long downhill grades that tell trucks to engage a low gear, and
that was my understanding of what the term "engine braking" meant - taking
advantage of the engine's inherent compression, multiplied by the low gear
ratio, to produce an overun 'drag' to prevent the truck running away in a
manner that couldn't be readily corrected by use of the brakes, because of
brake fade, which I've also seen mentioned on IRT. Is this not the same
thing ? What is the difference / advantage of the Jake Brake over what I've
described, given that it is apparently noisy ?

Arfa

It doesn't heat up and boil the brake fluid when you're coming down a
mountain pass with a full load. (Here in NA we have real mountains.)

Neither does overrun engine braking without compression release.
Suck, squeeze, phut, blow.
The suck provides a bit of braking as it's on a shut throttle at idle
setting and the vacuum resists the piston downward motion.
The squeeze doesn't do much as it's not got much air to squeeze.
The bang usually isn't. Mixture will be so far off it misfires.
Injection systems cut the fuel.
The blow does most braking as exhaust valve opens before BDC the
cylinder will suck air in from exhaust and then expel it on the up
stroke.
It's effective on small vehicles but not trucks. To stop a truck the
engine would be revved to destruction before it did anything really
noticeable.

Release of compression.
Suck, blow, suck, blow.
The compression release valve is un-throttled so a large volume of air
is induced/expelled. Pumping losses are very high resulting in a lot
(about x4) more braking than overrun braking.

I don't think anyone in Europe uses them. That's why we have sand drag
"escape" roads on steep hills that descend more than a few 10's of
meters.
--
Peter Hill
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