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Engine braking occurs when the retarding forces within an engine are used to slow a vehicle down, as opposed to using additional external braking mechanisms such as 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. The term only applies to petrol engines and other throttled engines (as opposed to diesel engines, electric motors, etc.)
Additionally, traffic regulations in a large number of countries require lorries to always drive with an engaged gear, which in turn provides a certain amount of engine braking (viscous losses to the engine oil and air pumped through the engine and friction losses to the cylinder walls and bearings) when no accelerator pedal is applied.
Petrol (gasoline) engines
The term 'engine braking' refers to the braking effect that occurs in petrol (gasoline) engines when the accelerator pedal is released.
When the accelerator is released, the butterfly valve that controls intake airflow (the throttle valve) is closed and the air flow through the intake is greatly restricted (but not cut off completely). This causes a high manifold vacuum which the cylinders have to work against - this saps energy and is where the majority of the engine braking force comes from. The concept can be illustrated by the amount of effort required to blow/suck through a narrow tube vs. a wider one. It is the work the engine has to do against this restricted air flow that provides the braking effect.
While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from the manifold vacuum caused by the air-flow restriction.
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 subjected to the same engine braking effects. This is partly why non-turbo diesel engined vehicles can coast in-gear for longer than an equivilent petrol engine.
The higher compression ratio in diesels means they are harder to start, but once they are running the lack of a throttle means any energy expended in compressing air is regained during the expansion stroke when the compressed air is allowed to 'spring' back, so the higher compression ratio doesn't cause any engine braking either.
Compression release brake
A compression release brake (also known as a Jacobs brake or "jake brake"), is the type of brake most commonly 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 that compressed air is not returned to the crankshaft, but is released into the atmosphere.
Normally during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down. However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel. (This sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a firearm.) Having lost the energy stored within the compressed air, there is no 'spring back' from it so the engine must expend yet more energy pulling the piston back down again
This type of brake is banned or restricted in many locations where people live because it creates a sound loud enough to disturb the peace, including waking people at night. It is very effective however, and creates immense amounts of braking force which significantly extends friction brake life - a 565 hp (421 kW) diesel engine can produce up to 600 hp (450 kW) of braking power.
An exhaust brake 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 if there is one.
Exhaust obstructions - Modern diesels are subject to many strict controls on emissions and often have many obstructions in the exhaust, which cause them to feel like they have some engine braking like a petrol engine. The main ones are:
- Turbocharger - this creates some back-pressure when it is stalled
- EGR (Exhaust Gas Recirculator) - this valve redirects exhaust gas back into the engine intake, often through a restricted/narrow pipe.
- DPF (Diesel Particulate Filter) - this is a filter designed to capture soot particles that would otherwise be released into the atmosphere; It greatly obstructs exhaust flow and can sap almost as much power as a small air conditioning pump.
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 accelerated wear. 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, 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 or a torque converter 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.
Using frequent engine braking while changing down gears may cause higher than normal wear on clutch plates if the driver uses the poor gear-changing technique of slipping the clutch to raise the engine's rpm to match the transmission speed, instead of rev-matching using the throttle.
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 RPM 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.
Engine braking is a generally accepted practice and can help save wear on friction brakes. It is even used in some motor sports to reduce the risk of the friction brakes overheating. Additionally, fuel injected engines generally don't use any fuel while engine braking. This is known as DFCO or Deceleration Fuel Cut-Off. It can be used as the coast part of Burn and Coast, though coasting in neutral is generally more efficient and engine off coasting is always more efficient.
Although no longer in production in most countries, there are still plenty of carburated engines in service, with which engine braking is counter-productive to fuel economy due to the lack of a DFCO mechanism. The cost of wasted fuel can well outweigh the gain of reduced brake wear.
Compression-release ("Jake") braking, a form of engine braking used almost exclusively on diesel engines, produces extreme amounts of noise pollution if there is no muffler on the intake manifold of the engine. Anecdotally, it sounds similar to a jackhammer, however the loudness is between 10 and 20 times the sound pressure level of a jackhammer (10 to 13 dB greater). 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.
- Certain types of braking in steam locomotives that are conceptually comparable (using compression in the steam engine cylinders for braking)