first practical mechanism for braking large trucks with engine compression as they descend long, steep grades at controlled speeds

The Jake Brake permits large trucks to descend long, steep grades at a controlled speed. It was the first practical mechanism for altering on demand the valve timing on a truck diesel engine, thereby converting the engine to a power absorbing machine. The modified engine can continue to power the truck in normal operation, allowing service brakes to remain cool for emergency situations. Invented by Clessie Lyle Cummins (1886-1968), this device (produced by the Jacobs manufacturing company since 1961) has contributed significantly to highway safety.