Date: Wed, 7 Sep 2005 00:19:06 -0400
Reply-To: Dennis Haynes <dhaynes@OPTONLINE.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: doing more brake jobs
In-Reply-To: <3479-431E6317-9734@storefull-3175.bay.webtv.net>
Content-type: text/plain; charset=us-ascii
As the owner of a Diesel powered motor home that weighs almost 25,000
pounds empty, I can answer these questions.
Diesels do not have throttles, there is very little braking action going
down hill. Compression does not matter. At speed, while a cylinder is
compressing, another is decompressing. Turning an engine by hand, you
feel what you think is only compression, but at that low speed, the air
does not stay compressed as it escapes past the rings. So when you turn
an engine by hand, you are actually feeling the resistance of one
cylinder compressing and then pulling vacuum. At speed, the effect is
more like a bounce. First there is resistance from the compression and
ten the rebound of expansion. The engine braking effect on a gasoline
engine is really created by the manifold vacuum of a closed throttle.
On large trucks, brake use is mostly avoided. Even though I have 16.5 X
7 " drums in the rear and 16.5 X 4" in front, it does not take a whole
lot of braking effort to make smelly brakes. Especially if I'm towing.
The exhaust brake works by restricting the exhaust, (another butterfly
like a throttle) actually turning the engine into an air compressor.
This is very effective. Some larger engines close the exhaust valves and
some now even use hydraulic retarders in the transmission.
As for using the brakes on downgrades, I already talked about the
heating of the brakes. Most trucks still use drums; discs simply do not
offer enough surface area and do work well with the limited air
pressure. As drum brakes heat up, the drums expand; the shoes then have
to travel further to contact the drum. Add this and some glazing and you
will get some serious brake fade.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Robert Cardo
Sent: Tuesday, September 06, 2005 11:49 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: doing more brake jobs
<< In a diesel, there's no such thing anyway,>>
Really?
Add 20 to one compression on top of 6 coffee can sized pistons and see
how much drag an oil burner has.
I wonder why they also use an accessory compression brakes too.
Must like the noise.
I wonder why trucks don't use their brakes coming down long grades.
Must be saving them for a rainy day.
Yep.
Must have a country full of idiots on the road.
Them professional driver's don't know doodly.