主题：【原创】关于开手动车的问题 -- 淡竹

From Wikipedia, the free encyclopedia

Jump to: navigation, search

The Jake Brake or Jacob Brake is a particular brand of engine brake manufactured and sold by Jacobs Vehicle Systems, Inc. While the term Jake Brake technically only describes Jake Brake brand engine brakes, it 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.

An engine brake is a braking system used primarily on semi-trucks or other large vehicles that modifies engine valve operation to use engine compression to slow the vehicle. They are also known as compression release engine brakes.

When the accelerator is released on a truck its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses. A compression release engine brake uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. The stem of this valve telescopes during normal operation so the valve remains closed, but is locked at full length by a solenoid when the engine brake is engaged so that the valve opens as directed by the cam. This releases the compressed air in the cylinder preventing it from returning its energy back to the piston and accordingly, the vehicle.

The driver controls consist of an on/off switch and, sometimes, a multi-position switch that controls the number of cylinders which the brake is active on. When the compression release engine brake is turned on, it will activate when the driver releases the accelerator. There are also switches on the clutch and accelerator pedals that will deactivate the compression brake when the clutch is disengaged or the accelerator is pressed.

Compression release engine brakes may make a loud chattering or machine gun noise while being used, which has led many communities to ban them. There are signs on the side of some freeway offramps, or at municipal boundaries, that will say "No jake braking", "Engine brakes restricted", etc. These signs are referring to compression release engine brakes. This has led to special mufflers on trucks that also muffle the sound of the compression release engine brake. Also, some engines use a butterfly valve in the exhaust and/or stator travel beyond the normal operating range on the variable-geometry turbocharger to produce engine braking without the noise associated with a compression release engine brake.