A compression release engine brake, compression brake, or decompression brake, frequently called a Jacobs brake or Jake Brake, is an

engine braking Engine braking occurs when the retarding forces within an engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes. The term is often confused with severa ...

mechanism installed on some

diesel engine The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-ca ...

s. 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. Clessie Cummins was granted a patent for the engine compression brake in 1965, and the first company to manufacture them was Jacobs Vehicle Systems. The terms ''Jacobs brake'' and ''Jake Brake'' properly refer to compression brakes manufactured by Jacobs, though the terms have now become

generic trademark A generic trademark, also known as a genericized trademark or proprietary eponym, is a trademark or brand name that, because of its popularity or significance, has become the generic term for, or synonymous with, a general class of products ...

s, and are often used to refer to compression brakes in general, especially on large vehicles and heavy equipment.

Function

When the driver releases the accelerator on a moving vehicle powered by an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

, the vehicle's forward

momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass ...

continues to turn the engine's crankshaft. Most diesel engines do not have a throttle body, so regardless of the throttle setting, air is always drawn into the cylinders (excluding the valve fitted to certain diesels, such as fire appliances and generators on oil and gas platforms, to prevent diesel engine runaway). The fuel-free air only mix that is compressed on the compression stroke now starts to act as an air spring. After the piston reaches maximum compression, the compressed air mixture returns its energy to the piston by pushing the piston back down. The end result is that even if the fuel supply to the cylinder is stopped, some energy absorbed during the compression stroke is still returned to the crankshaft. Because of this returned energy, there is very little engine braking applied to the vehicle. The typical compression brake consists of a hydraulic system using engine oil which transfers the motion of the fuel injector rocker arm to the engine's exhaust valve(s). When activated, the exhaust valve opens very briefly near the engine's

top dead center In a reciprocating engine, the dead centre is the position of a piston in which it is either farthest from, or nearest to, the crankshaft. The former is known as Top Dead Centre (TDC) while the latter is known as Bottom Dead Centre (BDC). ...

, and releases the compressed air in the cylinder so that the air compression energy is not returned to the crankshaft. A compression release engine brake can assist a vehicle to maintain or even reduce speed with minimal use of friction brakes. The power of this type can be around the same as the engine power. In contrast, a

gasoline engine A petrol engine (gasoline engine in American English) is an internal combustion engine designed to run on petrol (gasoline). Petrol engines can often be adapted to also run on fuels such as liquefied petroleum gas and ethanol blends (such as '' ...

under deceleration runs with a closed

throttle A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''throttle'' ...

that 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. Typically, the controls for a compression brake consist of an on/off switch and often a selector 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 factors such as the steepness of the grade and the vehicle's load.

Limitations

The use of engine compression brakes may cause a vehicle to make a loud "

growling Growling is a low, guttural vocalization produced by animals as an aggressive warning but can also be found in other contexts such as playful behaviors or mating. Different animals will use growling in specific contexts as a form of communicat ...

", "

machine gun A machine gun is a fully automatic, rifled autoloading firearm designed for sustained direct fire with rifle cartridges. Other automatic firearms such as automatic shotguns and automatic rifles (including assault rifles and battle rifl ...

", or "

jackhammer A jackhammer (pneumatic drill or demolition hammer in British English) is a pneumatic or electro-mechanical tool that combines a hammer directly with a chisel. It was invented by William Mcreavy, who then sold the patent to Charles Brady K ...

" like exhaust noise, especially vehicles having no mufflers, which has led many communities in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tota ...

, and Australia to prohibit compression braking within municipal limits. Drivers are notified by roadside signs with legends such as "Brake Retarders Prohibited," "No Engine Brake," "No Jake Brakes," "Compression Braking Prohibited," "Please No Engine Brake," "Avoid Using Engine Brakes," or "Unmuffled Engine Braking Prohibited," and enforcement is typically through traffic fines. Such prohibitions have led to the development of new types of

muffler A muffler (North American and Australian English) or silencer (British English) is a device for reducing the noise emitted by the exhaust of an internal combustion engine—especially a noise-deadening device forming part of the exhaust sy ...

s and

turbocharger In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pr ...

s to better silence braking noise. Jacobs claims that the use of the term "Jake Brake" on signs prohibiting engine retarding brakes violates their trademark and discriminates against Jacobs brand products. In the U.S. state of

Ohio Ohio () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Of the List of states and territories of the United States, fifty U.S. states, it is the List of U.S. states and territories by area, 34th-l ...

, state law allows a board of county commissioners or township trustees to prohibit the use of compression brakes within unincorporated areas. These local regulations apply to all state- and locally-maintained roads except

Interstate highway The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly known as the Interstate Highway System, is a network of controlled-access highways that forms part of the National Highway System in the United States. T ...

s. The state's standard "No Engine Brake" sign is designed to avoid discriminating against the Jacobs brand name.

References

{{Reflist Vehicle braking technologies de:Motorbremse#Dekompressionsbremse

Function

Limitations

See also

References