Engine braking

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Engine braking is the act of using the energy-requiring compression phase of a heat 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.

It is most commonly considered in the context of internal combustion engines but it is used on other types of engine as well; for example, the Snowdon Mountain Railway implements compression braking on steam locomotives.

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 lifts off the throttle pedal while the vehicle is in motion, 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 a vehicle's friction brakes are normally only used for relatively brief periods, they have limited ability to reject heat energy except into the surrounding air. Air ducting and vented disks are attempts to improve this.

Lengthy brake applications, such as during extended downhill grades or with excess loads, may cause more heat to build up than the brakes can dissipate before overheating. Brake failure occurs when heat from the brakes is sufficient to cause either the brake pads or brake hydraulic fluid to overheat. Overheated brake pads suffer a reduced coefficient of friction owing to surface outgassing. Overheated brake fluid may boil, particularly if it is contaminated with water. As a hydraulic system relies on the incompressible nature of liquids, replacing some of this with a compressible boiled vapour reduces the force that they can transmit and thus the braking force they can apply.

Use of engine braking will reduce heat buildup in the brake components by transferring this work instead to the engine. Most of the work done here will be as air compression, producing heated air that is merely vented through the exhaust pipe. Any extra heating of the engine can also be dissipated through the existing cooling system and 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:

1. Braking forces are applied to the engine and driveline components, although forces are generally less than those created when running the motor near peak power.
2. By substantially increasing engine RPM in a older gasoline engine with a carburettor, it may waste fuel owing to the large airflow, even with a fully released throttle. Later carburettors were sometimes fitted with overrun fuel shutoff valves.
3. Unmuffled engine braking can be extremely noisy and is prohibited by many municipalities in urban areas for this reason.

Applications

Engine braking is always active in all modern 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 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.

Legal implications

Compression braking, a form of engine braking, produces 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.^[1]

This is often a source of disagreement between professional truck drivers, and law makers. Many truck drivers^[who?] 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.^{[citation needed]} Some truck drivers^[who?] also claim the prohibition of engine braking places themselves and the public at a safety risk by removing their ability to use it as an alternative should their normal braking system become degraded or fail completely. In actuality though, almost all engine braking prohibitions still allow its use in emergencies.

See also

• Air brake
• Exhaust brake
• Jake brake
• Retarder

References