A compression release engine brake, frequently called a Jacobs brake or Jake brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves to the cylinders, right before the compression stroke ends, releasing the compressed gas trapped in the cylinders, and slowing the vehicle. The term Jake brake, which properly refers to the Jacobs brand of engine brakes, has become a genericized trademark, and is often used to refer to engine brakes or compression release engine brakes in general, especially on large vehicles or heavy equipment.
Function
The name is derived from the manufacturer, Jacobs, and was patented 1962–1965 by Clessie Cummins. When the driver releases the accelerator on a moving vehicle powered by an internal combustion engine, the vehicle's forward momentum continues to turn the engine's crankshaft. Most diesels by design do not have a throttle, so regardless of throttle setting a full charge of air is always drawn into the cylinders. Compressed air generated during the compression stroke acts as an air spring to push the piston back down. As such, even with fuel supplycut off and no power strokes taking place, virtually 100% of the energy absorbed by the compression stroke within each cylinder is returned to the crankshaft. This results in very little engine braking being applied to the vehicle. The typical compression release engine brake consists of an add-on hydraulic system using engine oil which transfers the motion of the fuel injector rocker arm to the engine exhaust valve. When activated, the exhaust valve opens very briefly near the engine's top dead center, and releases the compressed air in the cylinder so that the energy is not returned to the crankshaft. If used properly, a compression release brake can assist a vehicle to maintain or even reduce speed with minimal use of the service brakes. The power of this type can be around the same as the engine power. Contrast a gasoline engine under deceleration, where a closed throttle prevents free flow of air into the cylinders, resulting in little pressure to release at the top of the compression stroke. The closed throttle provides engine braking by forcing the engine to generate a vacuum between the throttle and the cylinders. Diesel compression release brake controls consist of an on/off switch and often a multi-position switch that controls the number of cylinders on which the brake is active. Throttle and clutch switches are integral with the system. Activation occurs when both the clutch and the throttle are released with the transmission in gear. It is the driver's job to ascertain the correct transmission gear to use, depending on, for example, the steepness of the grade and the truck's load.
Limitation
The use of compression release engine brakes may cause a vehicle to make a loud chattering or "machine gun/jackhammer-like" exhaust noise, especially vehicles having high flow mufflers, or no mufflers at all, causing many communities in the United States, Canada and Australia to prohibit compression braking within municipal limits. Drivers are notified by roadside signs with legends such as "Brake Retarders Prohibited," "Engine Braking Restricted," "Jake Brakes Prohibited," "No Jake Brakes," "Compression Braking Prohibited," "Limit Compression Braking," "Avoid Using Engine Brakes," or "Unmuffled Compression Braking Prohibited," and enforcement is typically through traffic fines. Such prohibitions have led to the development of new types of mufflers and turbochargers to better silence compression braking noise. Jacobs claims that the use of the term "Jake Brakes" on signs prohibiting engine retarding brakes violates their trademark and discriminates against Jacobs brand products. In the U.S. state of Ohio, state law allows a board of county commissioners or township trustees to prohibit the use of compression release engine brakes within unincorporated areas. These local regulations apply to all state- and locally-maintained roads except Interstate highways. The state's standard "No Engine Brake" sign is designed to avoid discriminating against the Jacobs brand name.
