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