Engine braking

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, for example friction brakes or magnetic brakes.

Petrol (gasoline) engines

The term engine braking usually refers to the braking effect caused by throttle position induced vacuum in petrol (gasoline) 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 do not have an intake throttle. The fuel itself is the throttle, and thus diesel engines are not subject to the same engine braking effects as gasoline engines are.

However, there are several other mechanisms which diesel engines use that could also technically fall under the term engine braking:

An engine retarder/engine brake

A compression release brake, which is used mainly in large diesel trucks. It works by opening the exhaust valves at the top of the compression stroke, resulting in adiabatic expansion of the compressed air so that the large amount of energy which is stored in it is not returned to the crankshaft but is released into the atmosphere. This creates a sound similar to automatic gunfire, although not as loud; consequently some areas have laws banning the use of unmuffled engine brakes. The vacuum that is subsequently created when the piston moves down again also adds to the immense amounts of braking force. A 565 hp (421 kW) diesel engine can produce up to 600 hp (450 kW) of braking power.

An exhaust brake

This works by causing a restriction in the exhaust much like the intake throttle causes in a gasoline engine. Nearly all of these brakes are butterfly valves mounted downstream of the turbocharger.

A mechanism related 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 braking effect similar to an exhaust brake.

Two-stroke engines

Engine braking in a premix 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 wear rapidly. Many old two-stroke cars (Saab, Wartburg etc.) had a freewheel device on the transmission to make engine braking impossible. Most two-stroke motorcycle engines since the 1970s have had lubrication by an oil pump, independent of the throttle and fuel system, such as Suzuki's Posi-Force 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.

Legal implications

Compression braking, a form of engine braking, produces extreme amounts of noise pollution if there is no muffler on the intake manifold of the engine. Use of an exhaust brake produces similar effects, due to release of compressed gasses, but the mechanism is distinct from regular car engine braking. Anecdotally, it sounds similar to a jackhammer, however the loudness is between 10-20 times the sound pressure level of a jackhammer. Numerous cities, municipalities, states, and provinces have banned the use of unmuffled compression brakes. In Australia, traffic enforcement cameras are currently being tested that automatically photograph heavy vehicles that use compression braking.

See also

• Air brake
• Exhaust brake
• Jake brake
• Retarder