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Engine braking

Engine braking is the act of using the energy-requiring compression stroke of the internal combustion engine to dissipate energy and slow down a vehicle. Compression braking is a common legal term for the same mechanism. Large trucks use a device called an exhaust brake to increase the effectiveness of engine braking.

Design

Compression of gas and vapor requires energy as described by theories in physical chemistry and thermodynamics. Compression in an engine is driven by the forward momentum of the vehicle as well as the angular momentum of the flywheel. When a driver downshifts to spin the engine at high angular velocity (or RPM) without pressing on the accelerator pedal, the engine converts energy from the vehicle's speed, which is kinetic energy, into a temperature increase in the fuel-air mixture. These hot gases are exhausted from the vehicle and heat is transferred from engine components to the air.

This energy conversion occurs because most four stroke internal combustion engines require compression of the fuel-air mixture before ignition, in order to extract useful mechanical energy from the expansion. Diesel engines are adiabatic and have no spark plugs and use energy transferred to air charge during compression to directly ignite the mixture when the fuel is injected.

Use during downhill grades

Since brakes are normally used for relatively brief periods, they have no active mechanism to eject heat energy except into the surrounding air. Lengthy brake applications, such as during extended downhill grades, may cause more heat to build up than the brakes can bleed off, causing dangerous brake overheating.

Use of engine braking will transfer heat buildup from brake components to the engine, which already has a system to dissipate excess heat energy (most notable component is the radiator).

Disadvantages

Engine braking beyond normal engine drag (i.e., down-shifting to a lower gear, without increasing throttle input) can increase the cost of owning and operating a vehicle:

It causes stopping forces to be generated within the drivetrain, contributing to greater stress on internal parts, while the brake system is implemented with consumable and readily replaced parts.
By substantially increasing engine RPM in a gasoline engine, it causes additional fuel consumption, even with a fully released throttle, in cars without overrun fuel shutoffs.
Unmuffled engine braking is extremely noisy and is prohibited by many municipalities in urban areas for this reason.

Applications

Engine braking is always active in all non-hybrid vehicles with an internal combustion engine, regardless of transmission type. Engine braking passively reduces wear on brakes and helps a driver maintain control of the vehicle. It is always 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. This is often called engine drag.

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. An additional "B" mode is also available that simulates the feel of a lower gear, and which uses the internal combustion engine to waste energy, preventing the battery from becoming overcharged.

Active use of engine braking (shifting into a lower gear) is only 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 acceleration due to gravity.

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.

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.

This is often a source of dissatisfaction to professional truck drivers, some of whom believe that municipalities are taking advantage of them due to their transient nature, and thus disregard the law, believing that they will be gone before a complaint can be lodged.