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

To understand what happens you need to understand how an engine works:

* The piston moves down, sucking in air and fuel.

* The piston moves up, compressing the air and fuel to a very high pressure. This obviously requires quite a bit of force to compress the air that much.

* The compressed air and fuel are ignited, resulting in heat and gases from burning that have an even higher pressure than what was previously compressed. This pushes the piston down, with a very high force.

* Valves open to let the smoke out and the piston moves back up to push all the smoke out.

Then the cycle repeats.

So basically what you have in an engine is multiple cylinders, with one piston in the process of the explosion that creates so much force, and the other cylinders in the process of using some of that force to compress the air and fuel that they have in the cylinder.

If you cut the fuel to the engine, there will be no explosions and no force being generated, but all of the compression of the air still happens, so bottom line you have to manually put a lot of force into trying to turn an engine by hand, because of all the compression that happens inside.

So semi trucks use that for braking. They keep the transmission “locked” into a lower gear so that the wheels are forced to keep turning the engine, and the huge resistance from the compression phase in all the cylinders can act as a huge brake and slow down the truck.