How does semi trucks exhaust breaking work?


ELI5, how do semi trucks exhaust break? I know it’s to do with the fact that they are diesels, but does that mean a VW Jetta could theoretically do the same?

In: 3

The exhaust valve opens at the top of the compression stroke. All the energy that went into compressing the air is lost. Normally that compressed air would help force the piston down on the power stroke. Taking this spring energy away slows down the engine.

Edit: two types, the one I mentioned is called a Jake Brake. The other type blocks off the exhaust flow as one of the other commenters mentioned.

Exhaust brakes don’t actually require a diesel engine, its just back pressure given a purpose. You could fit it to anything down to a self propelled lawn mower, its just added mass that doesn’t really provide a savings unless you have a heavy vehicle that chews through brake pads

On the exhaust stroke in an engine, the exhaust valve opens and the piston moves upwards pushing the air out of the cylinder, into the exhaust manifold, and out through the exhaust system and tailpipe

But what if the air couldn’t leave?

An exhaust brake is a valve that can close between the exhaust manifold and the rest of the exhaust system causing pressure to build up and push back on the piston which is connected to the wheels through a bunch of things, this back pressure slows the rotation of the engine and therefore the wheels and slows the vehicle down

Exhaust brakes are different from compression brakes (Jake Brakes) as those release the pressure at the top of the compression stroke so the engine wastes energy compressing air which then leaves, rather than forcing it to try to compress air that can’t leave. Exhaust brakes are also quieter than compression brakes because they’re much lower pressure releases than the full compression of a diesel cylinder.