Jake brake

A device which shuts off the exhaust valves manually so that in the exhaust stroke, the burned gasses cannot escape through the exhaust valves. Instead they press against the head of the piston and causes the piston to slow down. When the intake valve opens, some of the exhaust escapes out the intake valve and gives a distinctive loud rapping noise. Jake brakes are used in large truck engines to assist in slowing the vehicle. Many municipal bylaws prohibit the use of jake brakes because of the excessive noise.