Eli5: Why does engine compression braking cause such a loud and deep noise?


Listening to trucks entering a downhill slope and the engine noise is really loud and I can feel it in my chest – it seems louder than when they are accelerating.?

In: 1

Yup. That seems right. When an engine is providing power, it wants to get the most energy out of the fuel, so the exhaust valve will open when the piston is at it’s lowest point (and therefore lowest pressure on that particular stroke).

When engines use engine braking, the car lets the wheels turning actually provide rotation to the engine, which closes the valves at the bottom of each stroke, compress the air that, then let it out when it’s at the highest pressure. That slows the vehicle down a little bit in the process…but is loud.

On the compression stroke, the piston moves up, compresses air, and when the piston gets to the top, the injector rocker actuates the injector, and squirts in some fuel.

But with a Jake, instead of the injector rocker being actuated, it diverts to the exhaust valves, and dumps all that compressed air out at once. It’s loud.

Basically air gets pushed out of each cylinder gradually during normal operation. But with a Jake, it gets built up, and dumped all at once.

That’s about as ELI5 as I can do it.

This is called a “jake brake”. Technically it’s “unmuffled dynamic engine breaking”. It’s illegal in many cities, because of the noise issue.