Engine braking

[shark]

Originally Posted by icehog3

Too funny. I saw a "No Engine Braking" sign on the interstate in Wisconsin yesterday, and I was trying to think of what it sounds like. Is it a noise that sounds like hissing?

That occasional hissing sound you hear when you're by a truck or bus is from the pneumatic (air) system for the brakes. They use compressed air stored in air reservoirs that's generated by a compressor. They have a safety system that lets off some air sometimes to keep the pressure within limits.