Engine braking
Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes.

The term is often confused with several other types of braking, most notably compression-release braking or "jake braking" which uses a different mechanism entirely. Correct use of the term only applies to petrol engines and other engines that throttle air intake (As opposed to, e.g. diesel engines or electric engines).

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
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

 is closed, the air flow to the intake manifold is greatly restricted. The concept can be illustrated by the amount of effort required to blow/suck through a thin tube vs. a thicker one. It is the work the engine has to do against this restricted air flow that provides the braking effect.

Diesel engines

Diesel engines do not have engine braking in the above sense. Unlike petrol engines, diesel engines vary fuel flow to control power rather than throttling air intake and maintaining a constant fuel ratio as petrol engines do. As they do not maintain a throttle vacuum, they are not subject to the same engine braking effects.

However, some alternative mechanisms which diesel engines use that replace or simulate real engine braking include:
  • A compression release brake/"Jake-brake"
    Jake brake
    A compression release engine brake, frequently called a Jake brake or Jacobs brake, is an engine braking mechanism installed on some diesel engines...

     - This is the type of brake most confused with real engine braking; It is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, resulting in adiabatic expansion of the compressed air, so the large amount of energy stored in it is not returned to the crankshaft, but is released into the atmosphere.

Normally during the compression stroke, the compressed air would act as a spring and push back the cylinder (which is why normal diesel engines do not have any engine braking), but with a jake brake, the compressed air is released (with a loud pop). Without the compressed air to spring back the cylinder, the engine must continue to do work to pull the piston back open, so the engine loses the energy from that compressed air charge.

This type of brake is banned or restricted in many locations as it creates a sound similar to automatic gunfire, although not as loud; It is very effective however, and creates immense amounts of braking force which significantly extends friction brake life. A 565 hp diesel engine can absorb up to 600 hp.
  • An exhaust brake
    Exhaust brake
    An exhaust brake is a means of slowing a diesel engine by closing off the exhaust path from the engine, causing the exhaust gases to be compressed in the exhaust manifold, and in the cylinder. Since the exhaust is being compressed, and there is no fuel being applied, the engine works backwards,...

     - This works by causing a restriction in the exhaust, much like the intake throttle causes in a gasoline engine. In simple terms, it works by increasing the back-pressure of the exhaust. Nearly all of these brakes are butterfly valves similar to a throttle valve, mounted downstream of the turbocharger
    A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

     if there is one.

A mechanism related to the exhaust brake is back-pressure from a turbocharger. In turbodiesels 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.
Even fixed turbos, especially larger ones, will cause some back-pressure when they are below the turbo threshold (albeit not to the same extent as a variable turbo) and contribute to the braking effect.

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, causing reciprocating
Reciprocating motion
Reciprocating motion, also called reciprocation, is a repetitive up-and-down or back-and-forth motion. It is found in a wide range of mechanisms, including reciprocating engines and pumps. The two opposite motions that comprise a single reciprocation cycle are called strokes...

 parts to wear rapidly. Many old two-stroke cars (Saab, Wartburg, etc.) had a freewheel device on the transmission to make engine braking optional. Most two-stroke motorcycle engines since the 1970s have had lubrication by an oil pump
Oil pump (internal combustion engine)
The oil pump in an internal combustion engine circulates engine oil under pressure to the rotating bearings, the sliding pistons and the camshaft of the engine...

, independent of the throttle and fuel system, such as Suzuki's Posi-Force system.


As soon as the accelerator is released and the throttle closes, engine braking comes into effect as long as the wheels remain connected via the transmission to the engine. (A clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...

 or a torque converter
Torque converter
In modern usage, a torque converter is generally a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load...

 can disengage the wheels or absorb braking energy.) The braking force varies depending on the engine, but also what gear the vehicle is in (Generally, the lower the gear, the higher the braking effect as long as the wheels continue to maintain traction with the road surface).

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 overbraking, the vehicle will not regain traction until the wheels are allowed to turn more quickly; the driver must reduce engine braking (shifting back up or disengaging the clutch on a manual transmission) to regain traction.

Engine braking is intrinsically available in nonhybrid vehicles with gasoline-powered internal combustion engines, regardless of transmission type.
With diesel engines however, there is no intrinsic engine braking effect so more care must be taken. Turbo-diesel engines, on the other hand, generally have a more noticeable engine braking effect due to the turbo stalling when the accelerator is released and increasing the back-pressure in the exhaust.

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
thumb|Freewheel mechanismIn mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft...

 is not engaged. Using frequent engine braking while changing down gears will cause higher than normal wear on clutch plates as the driver needs to slip the clutch over a longer period of time to match the engine's higher rpm, if a comfortable, gradual and joltfree downshift is desired. This is in contrast to "conventional" braking where the engine's rpm is already reduced prior to the downshift.

In hybrid electric vehicles, like the Toyota Prius
Toyota Prius
The Toyota Prius is a full hybrid electric mid-size hatchback, formerly a compact sedan developed and manufactured by the Toyota Motor Corporation...

, 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.
Almost all electric and hybrid vehicles are able to convert kinetic motion into electricity, i.e. regenerative brakes, but this is not the same as engine braking.

Legal implications

Engine braking is a generally accepted practice and can help save wear on friction brakes. It's even used in some motor sports to reduce the risk of the friction brakes overheating. Additionally, most modern engines don't use any fuel while engine braking which helps reduce fuel consumption. This is known as DFCO or Deceleration Fuel Cut-Off.

Compression-release ("Jake") braking, a form of engine braking used almost exclusively on diesel engines, produces extreme amounts of noise pollution
Noise pollution
Noise pollution is excessive, displeasing human, animal or machine-created environmental noise that disrupts the activity or balance of human or animal life...

 if there is no muffler
A muffler is a device for reducing the amount of noise emitted by the exhaust of an internal combustion engine. A US Patent for an Exhaust muffler for engines was granted to Milton and Marshall Reeves in 1897....

 on the intake manifold of the engine. Anecdotally, it sounds similar to a jackhammer
A jackhammer is a pneumatic tool that combines a hammer directly with a chisel that was invented by Charles Brady King. Hand-held jackhammers are typically powered by compressed air, but some use electric motors. Larger jackhammers, such as rig mounted hammers used on construction machinery, are...

, however the loudness
Loudness is the quality of a sound that is primarily a psychological correlate of physical strength . More formally, it is defined as "that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud."Loudness, a subjective measure, is often...

 is between 10 and 20 times the sound pressure level of a jackhammer. Numerous cities, municipalities, states, and provinces have banned the use of unmuffled compression brakes, which are typically only legal in roads away from populations. In Australia, traffic enforcement cameras are currently being tested that automatically photograph heavy vehicles that use compression braking.
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