Engine braking

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Engine braking occurs when the retarding forces within an internal combustion engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes.

Contents

• Engine braking in different engine types
• Gasoline engines
• Diesel engines
• Two-stroke engines
• Electric motors
• Applications
• Limitations
• See also
• References

The term is often confused with several other types of braking, most notably compression-release braking or "jake braking" which uses a different mechanism.

Traffic regulations in many countries require trucks 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.

Engine braking in different engine types

Diesel engines

Diesel engines in personal cars provide little engine braking as they are not equipped with a throttle body and thus cannot draw a vacuum in the intake manifold.

In heavy vehicles the engine is often made to provide extra braking power to take some strain off the vehicle's regular brake system and to help avoid overheating the brakes. In its simplest form this consists of a butterfly valve that restricts the exhaust flow. This is referred to as an exhaust brake and mostly found on older trucks. It has a limited effect, and more advanced systems as described below are near universal on newer heavy vehicles.

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, so the large amount of energy stored in that compressed air is not returned to the crankshaft but is released into the atmosphere. It is very effective method of braking, creating large 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 at 2,100 RPM. ^[3]

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" so the engine must expend yet more energy pulling the piston back down again.

This type of brake produces extreme amounts of noise pollution if there is no muffler on the intake manifold of the engine, loud enough to disturb the surrounding area. 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 on roads away from populations. In Australia, traffic enforcement cameras are currently being tested that automatically photograph heavy vehicles that use compression braking. ^[4]

Exhaust brake

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 gasoline engine. The main ones are:

• Turbocharger creates some back-pressure when it is stalled
• Exhaust gas recirculator (EGR) valve redirects exhaust gas back into the engine intake, often through a restricted/narrow pipe.
• Diesel particulate filter (DPF) is 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 compressor.

Applications

As soon as the accelerator is released enough to slow the engine, engine braking comes into effect as long as the wheels remain connected via the transmission to the engine. A slipping or disengaged clutch, or a torque converter, would disengage the wheels or absorb braking energy. The braking force varies depending on the engine, and the gear the transmission is in. The lower the gear, the higher the braking effect due to higher rpm and the torque transferred through the transmission (higher torque is delivered from the engine in lower gears).

Engine braking avoids wear on brakes, and can help a driver maintain control of the vehicle. Active use of engine braking by shifting into a lower gear can help control speed while driving down very steep and long slopes, saving the brakes from overheating or excessive wear. If it is applied before the brakes have been used, it can leave the brakes available to make emergency stops. The desired speed is maintained by using engine braking to counteract gravitational acceleration. Potential transmission wear caused by engine braking can be mitigated by certain techniques. Slipping the clutch to complete a downshift wears the clutch plate as it slows the vehicle, doing the job of the brake pads or shoes. A well-executed rev-match in the same setting minimizes stresses on transmission components, so engine braking does the work of slowing the vehicle.

Improper engine braking technique can cause the wheels to skid (also called shift-locking), especially on slippery surfaces, 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. If the driver reduces engine braking by shifting back up, or disengaging the clutch on a manual transmission, traction can be regained.

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 the higher RPM of the internal combustion engine to waste energy, preventing the battery from being overcharged. ^[5] Almost all electric and hybrid vehicles are able to convert kinetic motion into electricity, i.e. regenerative brakes, but since the internal combustion engine is not used to slow the vehicle when using regenerative braking, it is not the same as engine braking.

Limitations

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 injection engines generally do not use fuel while engine braking. This is known as deceleration fuel cut-off (DFCO).

Although no longer in production in most countries, there are still plenty of carbureted 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.

See also

• Dynamic braking, similar effect with electric engines
• Retarder
• Certain types of braking in steam locomotives that are conceptually comparable (using compression in the steam engine cylinders for braking)
  • Countersteam brake
  • Counter-pressure brake

Related Research Articles

In engineering, the Miller cycle is a thermodynamic cycle used in a type of internal combustion engine. The Miller cycle was patented by Ralph Miller, an American engineer, U.S. patent 2,817,322 dated Dec 24, 1957. The engine may be two- or four-stroke and may be run on diesel fuel, gases, or dual fuel. It uses a supercharger or a turbocharger to offset the performance loss of the Atkinson cycle.

In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NO_x) emissions reduction technique used in petrol/gasoline, diesel engines and some hydrogen engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. The exhaust gas displaces atmospheric air and reduces O₂ in the combustion chamber. Reducing the amount of oxygen reduces the amount of fuel that can burn in the cylinder thereby reducing peak in-cylinder temperatures. The actual amount of recirculated exhaust gas varies with the engine operating parameters.

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum in the cylinder through its downward motion.
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A compression release engine brake, compression brake, or decompression brake is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves to the cylinders, right before the compression stroke ends, releasing the compressed gas trapped in the cylinders, and slowing the vehicle.

An inlet manifold or intake manifold is the part of an internal combustion engine that supplies the fuel/air mixture to the cylinders. The word manifold comes from the Old English word manigfeald and refers to the multiplying of one (pipe) into many.

Dieseling or engine run-on is a condition that can occur in spark-plug-ignited, gasoline-powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, drawing fuel through the carburetor, into the engine and igniting it without a spark.

Manifold vacuum, or engine vacuum in a petrol engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

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 slows down the vehicle. The amount of negative torque generated is usually directly proportional to the back pressure of the engine.

A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems, usually on heavy vehicles. Retarders serve to slow vehicles, or maintain a steady speed while traveling down a hill, and help prevent the vehicle from unintentional or uncontrolled acceleration when travelling on a road surface with an uneven grade. They are not usually capable of bringing vehicles to a standstill, as their effectiveness diminishes as a vehicle's speed lowers. Instead, they are typically used as an additional aid to slow vehicles, with the final braking done by a conventional friction braking system. An additional benefit retarders are capable of providing is an increase in the service life of the friction brake, as it is subsequently used less frequently, particularly at higher speeds. Additionally, air actuated brakes serve a dual role in conserving air pressure.

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Diesel engine runaway is an occurrence in diesel engines, in which the engine draws extra fuel from an unintended source and overspeeds at higher and higher RPM, producing up to ten times the engine's rated output until destroyed by mechanical failure or bearing seizure due to a lack of lubrication. Hot-bulb engines and jet engines can also run away via the same process.

Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.

Jacobs Vehicle Systems, Inc. is an American company that engineers, develops and manufacturers commercial vehicle retarding and valve actuation technologies. The company produces light-duty, medium-duty, and heavy-duty engine brakes, recreational vehicle exhaust brakes, aftermarket parts and tune-up kits to heavy-duty diesel engine manufacturers in its domestic market in America, as well as in Asia and Europe.

An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance. This process transforms chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to.

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References