Retarder (mechanical engineering)

A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems of (usually) heavy vehicles.

Friction-based braking systems are susceptible to 'fade' when used extensively, and this can become dangerous if the braking performance drops below that required to stop a vehicle -- for instance if a truck or bus is descending a long incline. For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction-based.

Retarders are not restricted to road vehicles, but may also be used also in railway systems. The British prototype Advanced Passenger Train used hydraulic retarders to allow the high-speed train to stop in the same distance as standard lower speed trains, as a purely friction-based system was not viable.

Retarders serve to slow vehicles down, or maintain a steady speed on inclines. They are usually not capable of bringing vehicles to a standstill, as their effectiveness diminishes at low speeds. They are usually used to slow vehicles down, with the final braking being carried out by a friction brake. As the friction brake does not then need to be used so much, particularly at higher speeds, the service lifetime of friction brakes is enhanced.

The engine brake

Gasoline powered vehicles

Most petrol-driven car drivers are familiar with the use of so-called engine braking to descend inclines: put the car in low gear and do not use the accelerator. The retardation effect is not caused by friction in the engine (although that does make a contribution), but by the fact that with the throttle closed, air cannot enter the cylinder on the intake stroke of the pistons. Essentially, a partial vacuum is being created at each intake stroke, and the energy required to create this partial vacuum comes from the transmission, hence retarding the motion of the vehicle.Three words, compression and compression ratio! Engine braking is using the rotational energy from the driveline (turning wheels) to compress the air in the engine cylinders. When engine braking the engine is pretty much turned into an air compressor, instead of combusting air/fuel it is now compressing the air/fuel(which it was already doing, now without large combustion to drive the intake, compress, combust, exhaust cycle just kinetic energy). Why do you think diesel engines engine brake so much more effectively, because they have huge compression ratios. A larger compression ratio means a larger volume of air is being compressed into a smaller space, requiring more energy from the driveline. This is also why a high performance naturally aspirated car engine brakes better than your nana mobiles and turbo cars as they have low compression ratios. Also how would this vacuum that brakes the car you talk about above magically appearing. When the engine is in its intake stroke the intake valves are wide open and the throttle is NOT closed. If the throttle was to close when your foot was off the gas pedal your car wouldn’t idle. The throttle is always open slightly, never closed shut! Therefore no truck stopping vacuum cleaner engine cycles.

Diesel powered vehicles

Diesel-engined vehicles do not have a throttle, as they regulate power output purely by the volume of fuel sprayed into the cylinders, so the engine braking generated by creating partial vacuum at each intake stroke in gasoline engines does not apply to diesel engined vehicles -- they are quite 'free-running'. However Clessie M. Cummins, founder of Cummins Engine Company, realized that by opening the cylinder exhaust valves when the piston reached top dead center, rather than at the end of the power stroke the accumulated compressed air in the cylinder 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, with the energy used to compress the air coming from the transmission, hence retarding 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.

This type of retarder is known to North American heavy vehicle drivers as a Jake brake, named after such a system produced by the [http://www.jakebrake.com/ Jacobs Vehicle Systems] . A disadvantage of this system is that it is very noisy in operation, and its use is banned in some locales.

The exhaust brake

The exhaust brake is simpler in operation than an engine brake. Essentially, the exhaust pipe of the vehicle is restricted by a valve. This raises the pressure in the exhaust system, forcing the engine to work harder on the exhaust stroke of its cylinders, so again the engine is acting as an air compressor, with the power required to compress the air being taken from the transmission, and therefore retarding the vehicle. A disadvantage of this system is that the exhaust pipe has to be engineered to accommodate the high pressures generated by this method of retardation. The retarding horsepower available from this system is significantly lower than other systems. It can cause a marked increase in engine oil carry-over out the crankcase ventilation system.

The hydraulic retarder

Hydraulic retarders use the viscous drag forces between dynamic and static vanes in a fluid-filled chamber to deliver their retardation. There are several different types which can use standard transmission fluid (oil), separate oil, or water.

A simple retarder would use vanes attached to a transmission driveshaft between the clutch and roadwheels. They can also be driven separately via gears off a driveshaft. The vanes would be 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 down the vehicle. The working fluid will heat up, and will usually be 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, ofetn inaudible over the sound of a running engine, and are especially quiet in operation compared to engine brakes.

The electric retarder

The electric retarder uses electromagnetic induction to provide a retardation force. An electric retardation unit can be placed on an axle, transmission, or driveline and consists of a rotor attached to the axle, transmission, or driveline and a stator securely attached to the vehicle chassis. There are no contact surfaces between the rotor and stator, and no working fluid. When retardation is required, the electrical windings in the stator are powered up from the vehicle battery, producing magnetic fields alternating in polarity for the rotor to move in. This induces eddy currents in the rotor, which slows down the rotor, and hence the axle, transmission or driveshaft to which it is attached. The rotor is engineered to provide its own air-cooling, so no load is placed on the vehicles cooling system, and the operation of the system is extremely quiet.

A Hybrid vehicle drivetrain uses an electric retarder to assist the mechanical brakes, while recycling the energy the eddy currents produce to charge a battery. The power stored in the battery is used later to help the vehicle accelerate.

External links

* [http://www.parceng.com/newsletters/040103.html Overview of retardation systems]
* [http://fe.pennnet.com/articles/article_display.cfm?Section=ARCHI&C=Feat&ARTICLE_ID=58636&KEYWORDS=william%20peters Overview of retardation systems]
* [http://www.jakebrake.com/ A manufacturer of engine brakes]
* [http://www.jakebrake.com/products/ld-exhaust.php A manufacturer of exhaust brakes]
* [http://www.voithturbo.com/retarder_e.htm A manufacturer of oil- and water- based hydraulic retarders]
* [http://www.apt-p.com/aptbrakesystem.htm The Advanced Passenger Train 'hydro-kinetic' brakes]
* [http://www.frenelsa.com/ A manufacturer of electric retarders]
* [http://www.zelusl.com/ A manufacturer of electric retarders]
* [http://www.telmausa.com/telma_htm/default.htm A manufacturer of electric retarders]