What are engine brakes?

Question

I see signs that say "engine brakes restricted" on some roads, what are engine brakes and why are they restricted?

Answer 1

An engine brake is a braking system used primarily on semi-trucks or other large vehicles that modifies engine valve operation to use engine compression to slow the vehicle. Also known as JAKE BRAKES. They are also known as compression release engine brakes.

A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder preventing it from returning its energy back to the piston and accordingly, the vehicle speed is reduced.

Answer 2

thats mostly for big trucks, like rigs, what uses jake brakes, going down steep grades.