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Diesel - Why no compression backpressure braking?

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This is a question that I've wondered about for years and I've asked a great many people - including drivers of (articulating lorries) semi-trucks but have never gotten a valid answer.  Why doesn't engine compression slow down the vehicle equipped with a diesel engine (unless it's equipped with a "Jake Brake"?)

The first time I came across this problem was when as a teenager,  I was helping a neighbor on his farm baling hay.  He asked me to turn the hay over to dry it with his diesel, David Clark tractor pulling a hay tedder.  All went well until I started down a hill and the tractor took off freewheeling down the hill to my complete surprise.  Our gasoline tractors, (Allis-Chalmers & IH Case) would prevent this 'run-away' condition by merely reducing the throttle.  But with the diesels - there is absolutely no discernible back pressure to help you decelerate.  (WHY?)


Knowing that a diesel engine has somewhere around twice the compression ratio of a gasoline engine, I'd expect it to slow me down quite effectively when going down a hill but it seemingly has no effect at all on deceleration.  Of course, the truck drivers are quick to point out that you can close the Jake Brake to help you slow down - but the question remains...  Why does it NEED one?



Edited by Xecho

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