Newsgroups: rec.autos.4x4
From: dstock@hpqmdla.sqf.hp.com (David Stockton)
Subject: Re: Diesel Land Rovers (was Solihull address...)
Date: Fri, 2 Feb 1996 16:31:21 GMT

C. Marin Faure (faurecm@halcyon.com) wrote:

: I was basing my assumption on diesel engine braking from my limited
: experience with Peterbilt semi-tractors.  The engine braking is
: tremendous, but of course, it is being assisted by the Jacobs engine brake
: ("Jake Brake", which instantly shuts off the fuel feed to the injectors
: when you reduce your foot pressure on the throttle.  With NO fuel entering
: the cylinders, the higher compression of the diesel has an extraordinary
: effect on braking.

   They don't just shut off the fuel to the injectors, the governor in an
ordinary injection pump does that anyway whenever the engine revs exceed
tickover and your foot is off the throttle. Those Engine brakes work by
obstructing the airflow somewhere.  Think of switching in a partial
obstruction in the exhaust - the engine operates as a road-driven
compressor and gets warm, the obstruction operates as an expander and
gets cold   This is rather elegant...  the braking energy is turned into
engine heat, the engine has a nice cooling system and radiator to get rid
of the heat, and it's at a time when the engine isn't creating heat by
burning fuel. You can get tremendous engine braking this way.

   Consider an engine without an obstruction, air is compressed, which
takes energy from the motion of the vehicle. The air gets hotter. Once
the compression stroke has ended, the power stroke begins. No fuel is
injected, but the air gets to expand again, restoring most of the energy
that was previously used to compress it back to the motion of the
vehicle. The air cools as it expands.   There is some inefficiency in the
process, but rather a lot of the energy that goes in on compression,
comes back out mechanically during expansion.

   Exhaust brakes are rare in this country, yet there have been several
fatal accidents involving runaway heavy trucks over the last few years.
Runaway trucks have smashed into shops and bus stops (queueing people)
near my home-town, where there are small towns in valleys with steep
roads down into them...  Maybe we brits have a few things to learn....

    Cheers
            David

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Discussion fodder - exhaust systems
Date: Fri, 20 Apr 2001 22:30:19 -0400

Mark wrote:

> >So, my question is: How does an exhaust restrictor on a gas engine work as a
> >brake?
> >
>
> This supposedly resticts the air coming out of the exhaust side of the engine
> since some air does get sucked past the throttle plate.  Double restrictions,
> double the engine braking power... that's the theory as I recall it anyway.
> Never used one so I don't know how effective they are.
>
> Mark - wondering if there's a "real" explanation somewhere.

You have it pretty much right.  The exhaust restrictor simply turns
the engine into a giant air compressor.

BTW, the common belief that diesel engines don't provide much
braking is wrong.  I've run otherwise identical engines on a
motor-dyno and learned that mechanical and pumping losses are about
the same, with the edge going to the diesel because the higher
compression deviates further from adiabatic through heat loss to the
cylinder, head and piston and because of the non-throttled intake
than a gas engine.  The difference is in the gearing.  At diesel
typically turns half the RPM for a given road speed as a gas
engine.  When the throttle is closed, that similar drag at the
crankshaft is magnified by the gas engine's higher numerical ratio
rear end into more braking force.  If you could spin the diesel up
as fast as the gas engine and then close the throttle, then it would
feel just like a gas engine.

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Discussion fodder - exhaust systems
Date: Sat, 21 Apr 2001 00:14:32 -0400

Vince Wirth wrote:

> On Fri, 20 Apr 2001 22:30:19 -0400, Neon John Wrote;
>
> > If you could spin the diesel up
> >as fast as the gas engine and then close the throttle, then it would
> >feel just like a gas engine.
>
> John,
> Why so? There is no air (butterfly) valve in the diesel so all that
> would happen is that no fuel would be introduced. In the gas
> engine when the throttle is closed, the engine becomes a vacuum
> pump in which the energy to run it comes from deceleration of the
> vehicle. What am I missing?

What energy is expended lowering a piston against a vacuum is
recovered on the upstroke by the same atmospheric pressure under the
piston.  One can see a little bit of difference between open and
closed throttle when motoring a gasoline engine but not much.  The
bulk of the losses are in the sliding friction of the piston rings
and in the valve train.  I'm afraid to rely on very rusty memory for
specific numbers but seems like about 80% of the losses are in the
rings and valve train.  That's why the low tension rings on modern
engines have such an effect on mileage.

John