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

Shifting down means now the wheels are pushing the engine instead of the engine pushing the wheels. You haven’t hit the accelerator, so fuel is being injected — so now the engine is acting like a big air compressor driven by the wheels. Compressing air requres energy and serves to put backpressure through the drivetrain to slow the wheels.