2. RETARDERS
• A retarder is a device used to boost or replace some of the
functions of primary friction-based braking systems, usually
on heavy vehicles.
• Friction-based braking systems are subject 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 or bus is downhill for long distance.
• For this reason, such heavy vehicles are frequently fitted
with a supplementary system that is not friction-based.
3. • Retarders are not restricted to road motor vehicles, but may also
be used in railway systems.
• The British prototype Advanced Passenger Train (APT) used
hydraulic retarders to allow the high-speed train to stop in the
same distance as standard lower speed trains.
• Retarders serve to slow vehicles, or maintain a steady speed, and
help prevent the vehicle from "running away" by accelerating.
• They are not usually capable of bringing vehicles to a idle, 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.
4. Contents
1 Engine brake
– Petrol and Diesel powered vehicles
– Exhaust brake
2 Hydraulic retarder
3 Electric retarder
6. Petrol and Diesel powered vehicles
• The engine braking generated by creating partial vacuum with a
closed throttle at each intake stroke in petrol/gasoline engines.
• Diesel engine vehicles do not have a throttle.
• Diesel engines regulate power output purely by the volume and
timing of fuel injected into the combustion chambers.
• This does not apply to diesel engine vehicles. Diesel engines are
quite "free-running".
• 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.
7. • 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 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.
• This type of retarder is known as a compression release brake.
• A disadvantage of this system is that it becomes very noisy in
operation if the exhaust muffler is faulty; its use is therefore
banned in some locales.
11. Hydraulic retarder
• Hydraulic retarders use the viscous 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.
• Hydraulic retarders are extremely quiet, often inaudible
over the sound of a running engine, and are especially
quiet in operation compared to engine brakes.
14. Electric retarder
1. Electric retarders use electromagnetic induction to provide a retardation
force.
2. 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.
3. There are no contact surfaces between the rotor and stator, and no
working fluid.
4. When retardation is required, the electrical windings in the stator receive
power from the vehicle battery, producing a magnetic field through
which the rotor moves.
5. This induces eddy currents in the rotor, which produces an opposing
magnetic field to the stator.
Cond..
15. 6. The opposing magnetic fields slows the rotor, and
hence the axle, transmission or driveshaft to which it
is attached.
7. The rotor incorporates internal vanes (like a ventilated
brake disk) to provide its own air cooling, so no load is
placed on the vehicle's engine cooling system.
8. The operation of the system is extremely quiet.
