Ever wondered why it is that big rigs sometimes make a lawnmower like sound as they slow down? That is the Jacobs Brake, named after its inventor, often called a Jake Brake. It is a system that uses the engine to brake the vehicle instead of using friction at the wheels. This reverses the usual role of the engine, which is to accelerate or maintain constant speed. The Jake Brake works by using the cylinders in the engine to compress air. This compressed air is then vented to the atmosphere. There is no power stroke, and since it takes energy to compress air, this energy is bled out of the kinetic energy of the vehicle. The rotating drive train and wheels are the source of the energy which is used to compress that air. The distinctive sound is the result of venting that compressed air to the outside atmosphere.