"Engine Braking"

[Gross Scruggs <GScruggs@wposmtp.nps.navy.mil>]

Greetings q-folks,  recently there was a post regarding double clutching which
referenced a neighborhood sign that read "No Engine Braking."  The mention of
the sign was folded into the double clutching discussion and was not mentioned
again.  Although I've been out of the q-loop recently, I don't recall anyone
speaking up for the actual meaning of that sign.

It doesn't refer to automobiles, it is instruction for drivers of large trucks. 
Ever notice that while slowing down large diesel trucks sometime have a very
loud exhaust noise?  That is the result of what is called the "Jacobs brake," or
in the vernacular, good buddy, the "Jake brake."  This is a device that when
certain conditions are met: Jake switch on, clutch out, fuel pedal up; the
entire engine becomes an enormous compressor.  Fuel flow to the engine is cutoff
and, I think, the valve timing is slightly altered to maximize the effect, and
the truck becomes an inertia driven compressor whose output is exhausted to
atmosphere with that characteristic "blap, blap, blap..." as the truck comes to
a stop.  

The system offers reduced wheel brake wear and an enormous amount of stopping
power.  When I worked summers in Alaska in the mid 60s at a motor freight
company in Anchorage, we had a rash of broken head bolts on the Cummins 850cuin
diesels.  Mystery was solved when it was learned that under the Jake brake the
engines developed 150% normal horsepower in braking force and the bolts weren't
up to the job.  Cummins re-specked the bolt material and problem went away.

Even back then, in less environmentally conscious times, towns along regular
truck routes were posting "no engine brake" hours within their jurisdictions in
response to the complaints of sleepless neighborhoods.

Hope this helps... a music theory teacher of mine used to say, "There is nothing
quite as bad as someone explaining something that was never misunderstood... and
doing it badly."

I'm off the list until the new computer comes in, so correspond direct as
required.

Regards,  Gross
gscruggs@wposmtp.nps.navy.mil