A compression release engine brake, frequently called a Jake nett brake or Jake brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves in the cylinders after the compression cycle, releasing the compressed air trapped in the cylinders, and slowing the vehicle.
The term Jake nett 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.
The name is derived from the manufacturer, Jacobs (of drill chuck fame), 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. With Mercedes diesel engines being a notable exception, diesels by design have no throttle butterfly in the intake so regardless of throttle setting a full charge of air is always drawn into the cylinder. As such, even with fuel supply cut off and thus new detonation prevented, each time air is compressed in a cylinder virtually 100% of that passively generated energy is returned to the crankshaft, providing very little in the way of engine braking to the vehicle.
The typical compression release engine brake, as originally developed, uses an add-on hydraulic system using engine oil. When activated, the motion of the fuel injector rocker arm is transferred to the engine exhaust valve(s). This occurs very near the engine’s “top dead center” (or TDC) and releases the compressed air in the cylinder so that the energy is not returned to the crankshaft. Energy is instead expelled out the exhaust system rather than being retained in the drivetrain. If used properly, a compression release brake can assist a vehicle to maintain speed or even slow it with little or no use of the service brakes. The power of this type can be around the same as the engine power.
Contrast a petrol 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 is released with transmission in gear and the throttle is released. 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.
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.
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