A retarder is a device used to augment or replace some of the functions of primary

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

-based braking systems, usually on heavy

vehicle A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...

s. 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. Friction-based braking systems are susceptible to brake fade when used extensively for continuous periods, which can be dangerous if braking performance drops below what is required to stop the vehicle: for instance, if a

truck A truck or lorry is a motor vehicle designed to transport freight, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construct ...

bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a motor vehicle that carries significantly more passengers than an average car or van, but fewer than the average rail transport. It is most commonly used ...

is descending a long decline, and would otherwise require something such as a

runaway truck ramp A runaway truck ramp, runaway truck lane, escape lane, safety ramp, emergency escape ramp, or truck arrester bed is a traffic device that enables vehicles which are having braking problems to stop safely. It is typically a long, sand- or gravel- ...

to stop safely. For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction-based. Retarders are not restricted to

road A road is a thoroughfare used primarily for movement of traffic. Roads differ from streets, whose primary use is local access. They also differ from stroads, which combine the features of streets and roads. Most modern roads are paved. Th ...

motor vehicle A motor vehicle, also known as a motorized vehicle, automotive vehicle, automobile, or road vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on railway track, rails (such as trains or trams), does not fly (such ...

s, but may also be used in

railway Rail transport (also known as train transport) is a means of transport using wheeled vehicles running in railway track, tracks, which usually consist of two parallel steel railway track, rails. Rail transport is one of the two primary means of ...

systems. The

British British may refer to: Peoples, culture, and language * British people, nationals or natives of the United Kingdom, British Overseas Territories and Crown Dependencies. * British national identity, the characteristics of British people and culture ...

prototype A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototype ...

Advanced Passenger Train (APT) used hydraulic retarders to allow the high-speed train to stop in the same

distance Distance is a numerical or occasionally qualitative measurement of how far apart objects, points, people, or ideas are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two co ...

as standard lower speed trains, as a pure friction-based system was not viable.

Engine brake

Diesel-powered vehicles

Diesel engines regulate power output purely by the volume and timing of

fuel injected Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of a fuel injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All co ...

into the

combustion chamber A combustion chamber is part of an internal combustion engine in which the air–fuel ratio, fuel/air mix is burned. For steam engines, the term has also been used for an extension of the Firebox (steam engine), firebox which is used to allow a mo ...

s. The

engine braking 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. The term is ofte ...

generated by creating partial vacuum with a closed throttle at each intake stroke in petrol/gasoline engines does not apply to vehicles equipped with diesel engines, as such engines are quite "free-running". However Clessie L. Cummins, founder of Cummins Engine Company, realized that by opening the cylinder exhaust valves when the piston reached top dead centre, rather than at the end of the power stroke, the accumulated compressed air in the

cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

could be vented before it could act as a "spring" to drive the piston back down again. By doing this, the engine acts as an

air compressor An air compressor is a machine that takes ambient air from the surroundings and discharges it at a higher pressure. It is an application of a gas compressor and a Pneumatics, pneumatic device that energy conversion, converts mechanical power (from ...

, with the energy coming from the transmission used to compress the air, hence slowing the vehicle. The amount of power extracted from the transmission can be up to 90% of the rated power of the engine for certain engines. In a compression release engine braking system for a turbocharged internal combustion engine, excessive stress associated with opening the exhaust valves of the engine near top dead center of engine compression strokes when the engine is turning at high speed is prevented by reducing the intake manifold pressure from what it otherwise would be at that high speed. This is done by retarding the

turbocharger In an internal combustion engine, a turbocharger (also known as a turbo or a turbosupercharger) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake air, forcing more air into th ...

so that its speed is less than it otherwise would be at high engine speed. This type of retarder is known as compression release engine brake or "Jake brake". A disadvantage of this system is that it becomes very noisy in operation, particularly if the exhaust muffler is faulty; its use is, therefore, banned in some locales. Type 2A test is required to certify the Engine brake efficiency.

Exhaust brake

Exhaust brakes are simpler in operation than an

engine brake 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. The term is often ...

. Essentially, the

exhaust pipe An exhaust system is used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall syste ...

of the vehicle is restricted by a

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

. This raises the pressure in the exhaust system, forcing the engine to work harder on the exhaust stroke of its

cylinders A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

, so again the engine is acting as an air compressor, with the power required to compress the air being withheld from the exhaust pipe, retarding the vehicle. Turbocharger retarders that restrict the flow of exhaust gas can also help in increasing the exhaust pressure to achieve the same objective.

Hydraulic retarder

Hydraulic retarders use the

viscous Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

drag forces between dynamic and static vanes in a fluid-filled chamber to achieve retardation. There are several different types which can use standard transmission fluid (

gear oil Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It has high viscosity and usually contains organosulfur compounds. Some modern automatic transaxles ...

), a separate oil supply, water, or a mix of oil and magnetic retardation. Magnetic retarders are similar to the electric retarder discussed below. A simple retarder uses vanes attached to a transmission

driveshaft A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power, torque, and rotation, usually used to connect ...

between the

clutch A clutch is a mechanical device that allows an output shaft to be disconnected from a rotating input shaft. The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does th ...

and roadwheels. They can also be driven separately via

gear A gear or gearwheel is a rotating machine part typically used to transmit rotational motion and/or torque by means of a series of teeth that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or ...

s off a driveshaft. The vanes are enclosed in a static chamber with small clearances to the chamber's walls (which will also be vaned), as in an automatic transmission. When retardation is required, fluid (oil or water) is pumped into the chamber, and the viscous drag induced will slow the vehicle. The working fluid will heat, and is usually circulated through a cooling system. The degree of retardation can be varied by adjusting the fill level of the chamber. Hydraulic retarders are extremely quiet, often inaudible over the sound of a running engine, and are especially quiet in operation compared to engine brakes.

Electric retarder

Electric retarders use

electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force, electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1 ...

to provide a retardation force. An electric retardation unit can be placed on an

axle An axle or axletree is a central shaft for a rotation, rotating wheel and axle, wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In ...

, transmission, or driveline and consists of a rotor attached to the axle, transmission, or driveline—and a

stator The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors, or biological rotors (such as bacterial flagella or ATP synthase). Energy flows through a stator to or from the rotat ...

securely attached to the vehicle

chassis A chassis (, ; plural ''chassis'' from French châssis ) is the load-bearing framework of a manufactured object, which structurally supports the object in its construction and function. An example of a chassis is a vehicle frame, the underpart ...

. There are no contact surfaces between the rotor and stator, and no working fluid. When retardation is required, the electrical windings in the stator receive power from the vehicle battery, producing a

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

through which the rotor moves. This induces eddy currents in the rotor, which produces an opposing magnetic field to the stator. The opposing magnetic fields slows the rotor, and hence the axle, transmission or driveshaft to which it is attached. The rotor incorporates internal vanes (like a ventilated brake disk) to provide its own

air cooling Air cooling is a method of dissipating heat. It works by expanding the surface area or increasing the flow of air over the object to be cooled, or both. An example of the former is to add cooling fins to the surface of the object, either by maki ...

, so no load is placed on the vehicle's engine cooling system. The operation of the system is extremely quiet. A

hybrid vehicle drivetrain Hybrid vehicle drivetrains transmit power to the driving wheels for hybrid vehicles. A hybrid vehicle has multiple forms of motive power, and can come in many configurations. For example, a hybrid may receive its energy by burning gasoline, but swi ...

uses electrical retardation to assist the mechanical brakes, while recycling the energy. The electric traction motor acts as a generator to charge the battery. The power stored in the battery is available to help the vehicle accelerate.

Regenerative braking Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy or potential energy into a form that can be either used immediately or stored until needed. Typically, regenerativ ...

and eddy current braking are separate types of electric braking. Regenerative braking might not be classified as a retarder as it uses no extra physical hardware in addition to the existing rotor/stator pair of the motor. It effectuates braking by using the electric field created by the rotational inertia in the rotor/stator that is delivered into the rotor by the momentum of the vehicle(wheels). Additional circuitry in the controller is used to manage this current flow from the stator windings into the battery, some of which dissipates as heat within the circuitry of the controller. In contrast, eddy current retarder brakes comprise a distinct and purpose-built static armature and rotor that are explicitly made and added to a vehicle for braking and dissipation of heat and not for motive power; it is a purpose-built system distinct from the motor. Finally, "dynamic" braking is the complex use of controller braking where the controller can be used either for regenerative braking or by switching the circuit to feed the current to resistors. In this latter way "rheostatic" braking can be achieved. Whereas an eddy brake relies on eddy currents to create magnetic resistance some of which is incidentally dissipated as heat, rheostatic braking relies on controller circuitry resistors which directly dissipate current-borne electric energy as heat. Some dynamic braking vehicles describe the rheostatic braking as "plug" braking. In particular, forklift dynamic braking has been developed to take advantage of combining this type of braking with controllers specialized to quickly reverse vehicle direction. Dynamic and regenerative braking, when used on

electric Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

or diesel-electric

railroad Rail transport (also known as train transport) is a means of transport using wheeled vehicles running in railway track, tracks, which usually consist of two parallel steel railway track, rails. Rail transport is one of the two primary means of ...

locomotive A locomotive is a rail transport, rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, Push–pull train, push–pull operation has become common, and in the pursuit for ...

s, means that the

electric motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...

s which usually are used to drive the road wheels are instead used as generators being driven by the wheels on a downslope. In

regenerative braking Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy or potential energy into a form that can be either used immediately or stored until needed. Typically, regenerativ ...

, the electric current created is typically fed back into the

power supply A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, electric current, current, and frequency to power ...

(i.e.

overhead catenary An overhead line or overhead wire is an electrical cable that is used to transmit electrical energy to electric locomotives, Electric multiple unit, electric multiple units, trolleybuses or trams. The generic term used by the International Union ...

third rail A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a track (r ...

), and can be used by other locomotives or stored for later use. In this way a locomotive will receive current while on level ground or traveling uphill, but acts as a current supply when braking, transforming the kinetic energy created from traveling downhill (or less often, converting forward momentum from travel on level ground) into electricity. In a diesel-electric, rather than being generated remotely and collected from a power source, the power supply is generated directly by the onboard

prime mover Prime mover may refer to: Philosophy *Unmoved mover, a concept in Aristotle's writings Engineering * Prime mover (engine or motor), a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc.) into ener ...

(engine) and transmitted to the motors; there is presently rarely any way of storing electricity up for later use, so instead, the motors are used as generators, retarding the wheel rotation, and the generated power is routed through resistors mounted on the roof of the locomotive, where it is transformed into heat energy (much like an electric heating element) and dissipated into the atmosphere with large fans. While this has the drawback of not re-utilizing the energy created while traveling downhill, it does create a powerful and safe retarding system that is not prone to brake fade or wearing out like mechanical brakes.

References