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. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...

-based braking systems, usually on heavy

vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), ...

s. Retarders serve to slow vehicles, or maintain a steady speed while traveling down a hill, and help prevent the vehicle from "running away" by accelerating down the hill. They are not usually capable of bringing vehicles to a standstill, as their effectiveness diminishes as vehicle speed lowers. They are usually used as an additional "assistance" to slow vehicles, with the final braking done by a conventional friction braking system. As the friction brake will be used less, particularly at higher speeds, their service life is increased, and since in those vehicles the brakes are air-actuated helps to conserve air pressure too. 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 cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame constructi ...

or bus is descending a long decline. 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 linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types o ...

motor vehicle A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on rails (such as trains or trams) and is used for the transportation of people or cargo. The ...

s, but may also be used in

railway Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a p ...

systems. The British

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 programming. A prototype is generally used to ...

Advanced Passenger Train The Advanced Passenger Train (APT) was a tilting high speed train developed by British Rail during the 1970s and early 1980s, for use on the West Coast Main Line (WCML). The WCML contained many curves, and the APT pioneered the concept of acti ...

(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 or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...

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

Engine brake

Diesel-powered vehicles

Diesel engines regulate

power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...

output purely by the volume and timing of fuel injected into the

combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Intern ...

s. The engine braking 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 In a reciprocating engine, the dead centre is the position of a piston in which it is either farthest from, or nearest to, the crankshaft. The former is known as Top Dead Centre (TDC) while the latter is known as Bottom Dead Centre (BDC). ...

, rather than at the end of the power stroke, the accumulated compressed air in the

cylinder A cylinder (from ) 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 ...

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 pneumatic device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces m ...

, with the energy coming from the

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual 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 (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...

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.

Exhaust brake

Exhaust brakes 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 ...

are simpler in operation than an engine brake. 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 system ...

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 slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...

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

cylinders A cylinder (from ) 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 in ...

, 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 The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the in ...

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), 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

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 and torque and rotation, usually used to connec ...

between the

clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...

and roadwheels. They can also be driven separately via

gear A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic ...

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 (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1831, and James Cle ...

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 rotating 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 the former case, beari ...

, or

driveline A drivetrain (also frequently spelled as drive train or sometimes drive-train) is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components o ...

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. Energy flows through a stator to or from the rotating component of the system. In an electric m ...

securely attached to the vehicle

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

. 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 is a vector 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 to its own velocity and to ...

through which the rotor moves. This induces

eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnet ...

s 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. Hybrids come in many configurations. For example, a hybrid may receive its energy by burning gasoline, but s ...

uses electrical retardation to assist the mechanical brakes, while recycling the energy. The electric traction motor acts as a generator to charge the

battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...

. 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 into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction mo ...

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 that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

or diesel-electric

railroad Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a prep ...

locomotive A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, motor coach, railcar or power car; the ...

s, means that the

electric motor An electric motor is an electrical 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 wire winding to generate f ...

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 into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction mo ...

, 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, current, and frequency to power the load. As ...

(i.e.

overhead catenary An overhead line or overhead wire is an electrical cable that is used to transmit electrical energy to electric locomotives, trolleybuses or trams. It is known variously as: * Overhead catenary * Overhead contact system (OCS) * Overhead equipm ...

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 railway ...

), 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), motor, a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc) into energy ...

(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