- 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 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 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" - 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 (421 kW) diesel engine can absorb up to 600 hp (450 kW).
- An exhaust brake - 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 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.
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 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, 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.
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 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, 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.
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 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. 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.
- Air brake
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