"Peter Hill" wrote in message
news
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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We do have sand drag escape roads here, although not many of them. There is
one at the bottom of a very long downgrade on a road in Devon that I used to
travel on regularly when going to carry out service work at a site that was
a customer of mine at the time. I think that I'm beginning to understand the
difference now, and I'm pretty sure that we do not have them fitted here -
in the UK at least. There are some fairly steep grades scattered around the
UK, but most of the mountainous areas are in Wales and Scotland, both of
which are fairly sparsely populated, so not too much truck traffic other
than on the main roads, which are largely flat. There are plenty of
mountainous areas in mainland Europe though, so you might have thought that
trucks operating in these areas might employ such supplemental braking ?

Arfa