From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Engine Braking
Date: Fri, 21 Jan 2005 23:07:32 -0500
Message-ID: <5mj3v0hql8dbh6mdbq6ehjr8vf37lk7mia@4ax.com>

On Fri, 21 Jan 2005 19:35:24 -0800, Lloyd Sumpter <lsumpter@dccnet.com>
wrote:

>On Fri, 21 Jan 2005 07:37:28 -0800, DC1999 wrote:
>
>> I thought I needed a Jake Brake on my HRC Navigator towing a Jeep
>> Cherokee(now a Liberty),
>
>Just curious: Can someone explain to me exactly how Jake-brakes (and/or
>"exhaust brakes") work? I used to listen to the logging trucks coming down
>the mountain behind our farm, and I can make a pretty good "18-wheeler on
>the Jake" sound, but I don't know how they work, except that they use
>compression somehow...?

The Jake Brake consists of an extra cam and linkage installed in the head.
It is designed to open the exhaust valve at about TDC on the compression
stroke.  Thus, all the work that went in to compressing the air is
dissipated out the exhaust instead of being recovered on the expansion
stroke.  The engine is forced to act like a big, heavily loaded air
compressor.  That loud noise is the highly compressed air in the cylinder
suddenly being released.

An exhaust brake is simply a valve located in or shortly downstream of the
exhaust manifold that mostly closes the exhaust.  This causes high
pressure air to build up behind the valve, again forcing the engine to be
a highly loaded air compressor.

An exhaust brake can be added to most any engine while a Jake Brake has to
be designed in the engine.  Even though I have a factory-installed exhaust
brake on my GMC/Izuzu truck and use it as necessary, I have concerns about
the whole exhaust manifold, turbo housing and all the gaskets being
subjected to potentially several hundred PSI of pressure.  If given an
option, I'd prefer a Jake.  The Jake doesn't subject any part of the
engine other than the exhaust valve to higher than normal pressure.

The effectiveness of a compression brake is limited by how much pressure
the engine maker and/or the brake maker think to be safe.  The one on my
truck is set fairly conservatively.  I can feel it engage and it will hold
back the truck on a hill but it doesn't generate much of a deceleration
feeling like a Jake does.  A Jake at full power is like applying the
service brakes fairly hard.

If I had my 'druthers on a heavy vehicle I'd have an eddy current
retarder.  This thing goes aft of the engine and transmission in the axle
and brakes by dissipating heat in an eddy current brake.  It makes no
noise, has no effect on the engine and can be set to generate just about
any amount of braking force.  The disadvantage is the initial cost.  The
advantage is, the retarder can be moved from the old vehicle to the new
when you trade.

A quick google will dig up info on all three types.

John