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A compression release engine brake, compression brake, or decompression brake is an engine braking mechanism installed on some
diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...
s. When activated, it opens exhaust valves to the cylinders, right before the compression stroke ends, releasing the compressed gas trapped in the cylinders. The compression followed by the "wasteful" release consumes great amount of energy, effectively 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. Therefore, the brakes are commonly known as Jake brakes.


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 comb ...
, the vehicle's forward
momentum In Newtonian mechanics, momentum (: momenta or momentums; 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. ...
continues to turn the engine's
crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...
. 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 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 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, 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 and Canadian 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 Autogas, liquefied petroleum gas and Common ...
under deceleration runs with a closed
throttle A throttle is a mechanism by which fluid flow is managed by construction 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'' ha ...
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 Animal communication, vocalization produced by animals as an aggression, aggressive warning but can also be found in other contexts such as playful behaviors or mating. Different animals will use growling in specific ...
", " machine gun", or " jackhammer" like exhaust noise, especially vehicles having no mufflers, which has led many communities in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
,
Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
, and
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
to prohibit compression braking within municipal limits. Drivers are notified by roadside signs with text such as "Brake Retarders Prohibited," "No Engine Brake," "No Jake Brakes," "Compression Braking Prohibited," or "Unmuffled Engine Braking Prohibited," and enforcement is typically through traffic fines. Such prohibitions have led to the development of new types of mufflers and
turbocharger In an internal combustion engine, a turbocharger (also known as a turbo or a turbosupercharger) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake air, forcing more air into th ...
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. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...
, 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, or the Eisenhower Interstate System, is a network of controlled-access highways that forms part of the National Hi ...
s. The state's standard "No Engine Brake" sign is designed to avoid discriminating against the Jacobs brand name.


See also

* Engine braking * Compression release * Exhaust brake * Retarder (mechanical engineering) * Jacobs Vehicle Systems


References

{{Reflist Vehicle braking technologies de:Motorbremse#Dekompressionsbremse