ASME-Landmark:Jacobs Engine Brake Retarder

Invented in 1957, the Jacobs Engine Brake Compression Retarder, or "Jake Brake," permits large trucks to descend long, steep grades at a controlled speed by turning a diesel engine into an air compressor.

Rather than store the energy of the pressurized air in the cylinder, created by the piston coming up on the compression stroke, the Jake Brake hydraulically opens the exhaust valve near the end of the upward piston stroke. The stored energy in the cylinder is released to the atmosphere so that when the piston descends on what would normally be the power stroke, no pressure remains in the cylinder to act on the piston. Thus, the energy transferred from the driving wheels into the compressed and highly heated air (through gravity or forward momentum of the vehicle) goes mostly out the exhaust pipe, with a portion entering the cooling water from heat transfer through the cylinder walls.

It was the first practical mechanism for altering on demand the valve timing on a truck diesel engine; the modified engine can continue to power the truck in normal operation, allowing service brakes to remain cool for emergency situations and increasing the life of the engine. Invented by Clessie Lyle Cummins (1886-1968), this device (produced by the Jacobs manufacturing company since 1961) has contributed significantly to greater control and safer operation of heavy trucks worldwide. See ASME website for more information