A brake retarder uses pressure created within the engine to lessen the speed of a vehicle. While most commonly used in commercial semi-tractor trailer rigs, it can also be used in railroad applications. The brake retarder systems are sometimes referred to collectively as "Jake Brakes" in reference to the original device, the Jacobs Engine Brake. According to the web site of the American Society of Mechanical Engineers, brake retarders allow truckers to slow vehicles without causing additional wear to the conventional brake system.
Air is drawn into the cylinder of the diesel engine when the intake valve is open. The air enters the engine at atmospheric pressure in a normal diesel engine or under pressure in a turbocharged diesel.
On the upstroke of the piston, the air in the engine cylinder is compressed to approximately 227kg. per square inch. The energy for this compression is supplied by the drive train of the truck and amounts to an additional drag on the rotating wheels.
Near the top of the piston's movement within the cylinder, the exhaust valve opens, venting the pressure in the cylinder. This is the popping sound often associated with brake retarders. This releases the energy of the compressed air held in the cylinder.
The exhaust valve closes before the cylinder begins its down stroke. The piston is working against a vacuum during the down stroke, absorbing more energy from the drive train, which continues to slow the truck.
The process is repeated as long as the brake retarder is engaged. Once the brake retarder is disengaged, the normal functions of the diesel engine return.