Eli5: What is the purpose of downshifting for a semi-truck and how does it work?

Question

I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?

In: 547

Answer 1

The loud noise is called “engine braking” and, sometimes, “Jake brake” (as in, “using the Jake brake”).

Simply put, with diesel engines, you can, essentially, open the engine to reduce the compression, slowing the vehicle down. This lets out a lot of the angry engine noises.

This is the what. The why is to save on the normal wheel brakes. Trucks are heavy and often heavier still because of the loads they carry. Relying on wheel brakes alone can cause them to overheat and fail. This is bad for crashy reasons. A mix of braking techniques increases safety.