Reas Urchin

How does a "Jake Brake" work?

A Jake Brake is a mechanism (apparently originally created by Jacobs) to allow engine braking with a Diesel engine. The Diesel engine does not throttle intake air, as typical gasoline engines do, so the braking effect when you "take your foot off the gas" with a Diesel is very limited.

The reason for this is that very nearly 100% of the energy used in intake and compression strokes is returned by the power and exhaust strokes, even when no fuel is added. This is very different from the gasoline engine, where the fully-closed throttle plate means the intake stroke is pulling a partial vacuum, doing work that is not repaid during the rest of the cycle.

For Diesel automobiles, this is not a significant problem, as engine braking is less significant -- but in a semi-trailer, lengthy braking such as long downhills can cause serious problems.

For this reason, most over-the-road trucks use "Jake Brakes" for downhill deceleration, etc. Although there are various mechanisms, most work on the same principle: opening the exhaust valves near the top of the power stroke to "waste" off the compressed air without returning the work done.

And that sudden decompression is what causes the characteristic thudding BLAT BLAT BLAT BLAT of a Jake Brake in operation. This in turn leads to the "Please, no Jake Braking" signs posted by people who live near descents used by trucks!

There are other types of exhaust brakes that actually partially block off the exhaust pipe, vastly increasing the back pressure seen by the engine, so it's doing more work; I'm not sure how well those perform, since the noisy ones are still so popular. The manufacturer of one type has some concerns about the other brands . . . .

How Stuff Works had a great page on these, but it's gone.

AnswerBag has some facts on them.

So does WikiPedia.

And The Straight Dope has a typically-excellent article.

An industrial blog on maintaining them.

posted by Tracy Logan @ 11:26 AM