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Engine braking is the act of using the retarding forces within an engine to slow a vehicle down, as opposed to using an external braking mechanism, e.g. friction brakes or magnetic brakes.
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Petrol engines
The term engine braking usually refers to the braking effect caused by throttle position induced vacuum in petrol engines. While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from vacuum.
When the throttle is lifted less air is allowed to pass through the intake manifold, and the engine works against this vacuum. It is the deceleration of the engine against this vacuum which provides the braking effect.
Diesel engines
Diesel engines do not maintain a throttle vacuum as they always use a full cylinder of air, and thus are not subject to the same engine braking effects.
A common misconception with diesels is that they experience a braking force from the air being compressed in the cylinders - This is not the case, as that same air is not released (Unless a Jake brake is in use) and actually helps the cylinder to spring back open after the compression stroke.
However, there are several other mechanisms which diesel engines use that could also technically fall under the term engine braking:
A Jake brake - Also known as Compression Braking, this is used mainly in large diesel trucks. It works by opening the exhaust valves at the top of the compression stroke, releasing the compressed air in a pop so it can't push the cylinder back out. Its use is restricted as the multiple sudden pressure releases make an active jake brake sound a lot like a pneumatic drill or machine gun.
An Exhaust brake - This works by closing off the exhaust manifold to create back-pressure in the engine, similar to the back pressure caused by heavy turbos. This provides an engine braking effect very similar to that in a petrol car, but instead of the braking effect coming from the vacuum in the intake manifold, it comes from the engine working against the back pressure in the exhaust manifold.
A related mechanism to the Exhaust brake is back-pressure from a turbocharger. In turbo diesels with variable-vane turbos, the vanes will close when the accelerator is released which creates a back pressure breaking effect similar to an exhaust brake.
Two-stroke engines
Engine braking in a two-stroke engine can be extremely harmful to the engine, because cylinder and piston lubricant is delivered to each cylinder mixed with fuel. Consequently, during engine braking, the engine starves not only of fuel but also lubricant, while parts reciprocate rapidly. Four-stroke engines typically have lubrication by an oil pump, independent of the throttle and fuel system.
Applications
Engine braking passively reduces wear on brakes and helps a driver maintain control of the vehicle. Active use of engine braking (shifting into a lower gear) is advantageous when it is necessary to control speed while driving down very steep and long slopes. It should be applied before regular disk or drum brakes have been used, leaving the brakes available to make emergency stops. The desired speed is maintained by using engine braking to counteract the gravitational acceleration.
Improper engine braking technique can cause the wheels to skid (also called shift-locking), especially on slippery surfaces such as ice or snow, as a result of too much deceleration. As in a skid caused by over-braking, the vehicle will not regain traction until the wheels are allowed to turn more quickly; the driver must reduce engine braking (shifting back up) to regain traction.
Engine braking is intrinsically available in non-hybrid vehicles with gasoline-powered internal combustion engines, regardless of transmission type. In almost all cases, it is active when the foot is lifted off the accelerator, the transmission is not in neutral, the clutch is engaged and a freewheel is not engaged.
In hybrid electric vehicles like the Toyota Prius, engine braking is simulated by the computer software to match the feel of a traditional automatic transmission. For long downhill runs, the "B" mode acts like a lower gear, using higher RPMs in the internal combustion engine to waste energy, preventing the battery from becoming overcharged.
See also
References
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