vehicle retarders

INTRODUCTION


A vehicle retarder is an optional type of braking device that has been used developed and successfully used over the years to supplement or assist the service brakes on heavy-duty trucks. This braking device is used in case of heavy loaded commercial vehicle descending a long and steep incline. The main purpose of using this auxiliary braking system is to limit the speed of a vehicle during long descending. The braking effect is only available when the vehicle is moving. It should be noted that retarder is a devise to slow down the vehicle, not a device to stop the vehicle. It is not a substitute for the vehicles service brakes. The service brakes must be used to bring a vehicle to a complete stop.


There are basically four types of vehicle retarder in use today

1. Hydraulic retarder
2. Electric retarder
3. Engine brakes
4. Exhaust brakes

Hydraulic retarders are normally built in to the vehicles automatic transmission.
The electric retarder operating independently of the vehicles engine transmission or exhaust system is used to provide a drag force to the driving shaft. Engine brakes are the most popular type of vehicle retarder used on heavy-duty trucks. By releasing compression as each piston reaches the top of its compression stroke. The engine brake removes force that would have pushed the piston down. In effect this turns the engine in to a retarder. The exhaust brake depends on exhaust bake pressure for its retarding power.

HYDRAULIC RETARDER

Several manufacturers offer hydraulic retarder as built-in braking devices with automatic transmissions. Hydraulic retarder is similar to a fluid coupling. Typically it is mounted between the transmission torque converter and the range gear housing assembly There is only one moving part associated with a hydraulic retarder. This moving part is called a rotor, which rotates within the vaned stator housing. The rotor is installed as an integral part of the output shaft on the torque converter. The vaned stator housing is used to enclose the rotor. The fixed stator vanes are part of the housing. The use of this system affords minimal wear on the components and no service adjustments. A foot pedal or switch or a hand valve in the vehicle cabin activates the hydraulic retarder. Operation of the control pedal/switch is very similar to the operation of an air brake pedal. During operation the housing is filled with transmission oil. However when not in use no oil is present in the housing and does not interferes with the normal transfer of power to the driving wheels of the vehicle.

OPERATION:

To retard vehicle speed, the driver activates the control valve by using a hand valve or foot pedal. Depressing the pedal will open the control valve and allow transmission oil to flow in to the hydraulic retarder housing. Oil flow in to the hosing is directly proportional to pedal position, and therefore proportional to the degree of braking desired. The oil in the hydraulic retarder housing is under pressure and works against the vanes on the rotor. As the rotor turns with the torque converter out put shaft the vanes on the rotor force the transmission oil in to the gaps or opening between the fixed stator vanes. This action tends to work against the rotor, slowing it down and providing braking power to the vehicles driving wheels. The pumping action of the rotor circulates transmission oil between the hydraulic retarder and the transmission heat exchanger to provide the necessary cooling. When the hydraulic retarder is engaged and operated the engine throttle will be in idle position, thereby reducing the load on the vehicles cooling system. Once this driving load is reduced, the capacity of the cooling system can now readily absorb the heat generated by the hydraulic retarder.






ELECTRONIC HYDRAULIC RETARDER

This type of retarder basically consist of the

1. Retarder unit contained with the heat exchanger
2. Retarder hand switch
3. Electronic control unit (ECU)

To operate the system properly also requires the following inputs from the host truck;
Air pressure source, electrical power source, and cooling source
Compressed air from the truck supply reservoir is used to control the retarder. The (ECU) controls the pressure regulator in accordance with the setting of the retarder hand lever at the steering column. The trucks existing electrical system readily supply the small amount of electrical power required by the ECU. The electric current required is on the order of 1.6 amps

Heat generated by the retarder in operation is dissipated through the trucks existing cooling system. Maximum use of the trucks cooling capacity is utilized and the use of a thermostat provides the necessary over heating protection for the trucks cooling system. The retarder basically consists of various connections for air, coolant and oil, an internal rotor and stator configuration, and an integral heat exchanger. The retarder is mounted after the gearbox so that braking torque is not affected by gear changes. The impeller or rotor is mounted on the propeller shaft and turbine wheel, or stator is fixed firmly to the retarder housing. Energy is transferred wear-free from rotor to stator by the oil circulating between them. The kinetic energy of the truck slowed down by the retarder is converted in to heat in the operating oil with in the retarder. This heat is transferred to the truck cooling system by oil /water heat exchanger and dissipated continuously in the trucks radiator.

The retarder control system uses an ECU and a hand leer or the trucks service brake pedal or a combination of both. The ECU is a microprocessor with preprogrammed data that is ready to be accessed with in a fraction of a second. With the hand lever the driver can select six different braking steps for the truck. A more precise control of the retarder braking action is provided by a push button on the lever. When this button is pressed the ECU stores the actual driving speed of the truck and inhibits the truck from exceeding that speed. The ECU has the capability to monitor itself, showing automatically any trouble in the system by activating a warning light in the cabin.

OPERATION

The rotor and the stator in the retarder face each other, and oil is fed in to the chambers between the blades. The rotor is connected to the truck propeller shaft and rotates with it, while the stator is fixed to the retarder housing. When the propeller shafts the rotor, the oil takes up or follows this same rotation. The mechanical energy of the rotor is thereby converted in to kinetic energy in the fluid. The centrifugal force due to the rotation of the rotor passes the oil in to the stator chamber and tries to turn or rotate it. However the stator is fixed thereby casing the kinetic energy of the oil tube dissipated back through the rotor. This retardation effect of the rotor reacts back on to the propeller shaft, causing truck to slow down. To assist the flow of oil in to the chambers between the rotor and the stator, the ECU and a solenoid valve on the retarder introduce air pressure in to the housing in a controlled manner.

To operate the retarder, the driver moves the hand lever on the steering wheel or depressed the brake pedal. Compressed air is then routed to the retarder. The higher the retarding torque required the greater the air control pressure. Depending up on the speed the greater or lesser quantity of oil is fed from the oil sump in to the retarder to obtain the desired braking torque. Once the retarder is disengaged, the oil in the operating circuit returns to the oil sump. The retarder is controlled electro pneumatically. When the hand-operated lever is activated an electric signal is send to the ECU. The signal is combined with other data stored in the memory circuit of the ECU and is transformed in to a control signal for the pressure regulator unit. The pressure regulator then transmits the relative air pressure command to the retarder to act up on the circulating oil. With this kind of system the retarder braking torque can be varied in stages simply by adjusting the hand lever. A temperature sensor fitted on the retarder heat exchanger monitors the water temperature and provides a balance between the heat tubes dissipated via the engines cooling system and the heat generated by the retarder.
Temperature data is routed from the sensor to the ECU. The intelligence service unit in ECU is used to locate any fault.

ELECTRIC RETARDER

The electric retarder has see more wide spread use I America. The electric retarder has been used on trucks, uses and various on-and off-high way vehicles. including vehicles equipped with diesel or gasoline engines. The electric retarder offers noise-free raking with or without the use of series rakes. This type of retarder works independently of the engine, transmission, exhaust system, or hydraulic system. The vehicles 12 or 24 electrical system provides the power for the electric retarder. The retarder was offered in various sizes and capacities.

DESCRIPTION

The electric retarder is mounted between the side frames of the vehicle. The major components of the retarder include a cast steel frame for the components, core assembly, coil assembly, and two rotors equipped with vanes for cooling purpose. A master control switch and a multi position switch are used to activate and control the retarder. These switches are located in the vehicles cabin.

OPERATION

Approximately sixteen coils or electromagnets mounted with in the cast steel frame of the retarder. A typical arrangement includes eight electromagnets on each side of the frame. Energizing the retarder will cause the electromagnets to exert a strong dragging force on the two rotors in the frame. Because the electric retarder is mechanically in line with the drive shaft, the drag force is transmitted directly to the drive shaft, providing the required retarding action to slow down the vehicle. This operation is an application of the eddy current principle.

To understand the eddy current principle one must consider the electric retarder as a large generator. In this case however no power is drawn out or outputted y external cabling. The transmission drives the retarder rotor assembly. As the rotor rotates with in the confines of the electromagnetic fields, small currents (or eddy currents) are generated within the rotor somewhat similar to a power generator. However the developed eddy currents have no place to flow from the rotor, so they circulate via paths not influenced y the electromagnets. These eddy currents infract create a magnetic reactive force that tends to stop or retard rotor rotation, hence a engine raking system. The electrical energy generated in this design cannot e taken away or outputted to a typical load and, therefore must dissipate itself in the form of heat. The vanes that are cast on the rotors create airflow to dissipate this heat.

The cabin master control switch should e set in position when the vehicle in use. Activation of the electromagnets is progressively controlled by the multi position switch or lever mounted on the steering column or the dash. Operation of the control switch varies the degree of braking desired y the driver/ operation for the existing road conditions. The multi position control switch also has the advantage of an automatic time delay that will gradually increase the deceleration rate with out driver engaging each switch position until the desired braking level is reached. A low speed detector can e utilized with the electric retarder whenever vehicle speed is below 5 miles/hr.

MAINTENANCE

Maintenance of an electric retarder involves the removal and replacement of the retarder from the chassis or frame rail. The frame of the retarder is bolted to the rails and the rotors are connected in line with the vehicles drive shaft. The electric retarder can be rebuilt in designed shops in much the same manner as electric motors or alternators are rebuilt. This could involve rewinding the coil assembly, replacements of rotor assembly. Electrical wiring between the system switches and the retarder can also cause problem but can readily e repaired.







ENGINE BRAKES

Over the years any manufactures attempted to design an engine rake that would utilize the high compression air on a diesel engine as a form of braking power. The design concept was to convert the diesel engine in to an air compressor when engine power was not needed but engine braking was. Engine brake is the most popular type of vehicle retarder used on heavy-duty trucks.

DESCRIPTION OF COMPONENTS

Atypical engine brake system consists of the following components;

1. On/off dash switch
2. Selector switch
3. Clutch switch
4. Fuel pump switch
5. Brake Housing

The first four items make up the electrical control system for a basic engine brake installation while the final item in the actual engine breaks mechanism. The on/off switch and the multi position selector switch are used to activate the engine brake and control the amount of braking desired. The clutch switch is installed so that the switch actuator arms in contact with the clutch pedal or other clutch members. The clutch switch allows the engine brake system to be activated only when the clutch is in the engaged position. The fuel pump switch is installed on the fuel pump housing and interfaces witch the throttle shat. The fuel pump switch allows the engine brake system to be activated only when the throttle shat (accelerator) is in the idle position. The brake housings are installed on the diesel engine and vary in number depending on the type of engine. The brake unit consists of a frame casting that houses a solenoid valve, control valve, master position and slave position. The number of brake units to be installed on an engine depends on engine and cylinder head design. It should be noted that the engine brake can be used on vehicles with manual transmission as well as automatic transmission. Installation of an engine brake on a vehicle with an automatic transmission requires the use of a pressure sensitive brake control switch and elimination of the clutch switch.

THEORY OF OPERATION

The principle behind engine brake is rather simple. It’s a device that converts the vehicles diesel engine in to a power absorbing retarding mechanism. To understand how the engine brake operates, we will compare the engines combustion cycles with and with out an engine brake.






With an engine break
Without an engine brake
1) Intake valves opens; air fills the cylinder 1) Intake valves opens; air fills the cylinder
2) Air in cylinder is compressed, with the corresponding increase in cylinder pressure and temperature

Near TDC, the engine brake releases the compressed air mass through the exhaust system. No combustion occurs because the engine brake only works when the engine is in the no fuel mode. 2) Air in cylinder is compressed between 500 to 1000 psi; temperature rises to about 10000 F. Fuel is injected into the cylinder and combustion takes place. This causes a pressure rise to 1500 psi.
3) No positive power is generated because the compressed air mass was vented through the exhaust system during the modified combustion phase. The momentum of the vehicle is used to return the position to its bottom position. It is this two-step process – venting the compressed air and using the vehicles momentum to move the piston – that develops the engine breaks retarding power. 3) High pressure resulting from the combustion process forces the piston down, imparting power to the drive train.
4) Any residual air is forced out of the cylinder by the upward motion of the system. 4) Exhaust gases are forced out of the cylinder by the upward motion of the piston.




ELECTRICAL CONTROL SYSTEM

An electrical control system is used to activate or energize the solenoid valve in the engine brake unit when engine braking is desired. In a typical engine braking system the on/off switch is powered from the vehicle’s electrical system through ignition power. A circuit breaker is used protect the system. The clutch switch and fuel pump switch is also wired in series with the fuel pump and clutch switches. These 4 switches make power available to the brake control valve solenoid.
The fuel pump switch is in position, which allows the throttle linkage to contact the switch’s push button when the operator releases the throttle to slow the vehicle. The switch senses when the engine is in the closed throttle mode of operation. At this time, the engine brake should activate to slow drive wheels. As the throttle is appl9ied the engine goes in to fueling or positive power mode of operation. When the throttle is applied the brake must deactivate immediately. This is accomplished as the throttle linkage releases the fuel pump switch push button. As the switch push button is released, the electrical contacts open, interrupting power to the brake control valve solenoid. It should be noted that the engine can be stalled unless the operator applies the clutch, turns off the on/off switch or applies the throttle once the brake is on and activated by releasing the throttle. The use of an optional foot switch with the system provides added driver convenience and control. Adding the foot switch to the circuit provides fully manual control of the engine brake. Another option that can be provided with the engine brake is a low speed shut off switch. This option provides a fully automatic engine brake control system that senses engine speed and electrically de energizes the engine brake at speeds below 800 to 900 rpm. This prevents the possibility of engine stalling.

HYDRAULIC OPERATION

Energizing the engine brake unit will convert the vehicle’s diesel engine into a power absorbing air compressor. The movement of a master slave piston arrangement within the brake unit accomplishes this. Near the top of the engine’s normal compression stroke the brake unit causes the cylinder exhaust valves to open thereby releasing the compressed cylinder charge into the exhaust system. This action prevents the engine from producing any positive power. A typical operational sequence for engine brake unit is as follows:

1) The control valve and the solenoid valve regulate the flow of engine oil through out the braking unit. When the engine brake is not in use, the solenoid valve is closed preventing engine oil from entering the unit. The engine oil is allowed to drain back to the sump.
2) When the driver energizes the engine brake system, the completed electrical circuit energizes the solenoid valve4. For this to happen all circuit switches must be closed. This means that the driver must have his foot of the clutch pedal and the accelerator. When the solenoid valve is energized, the valve opens and the oil will fill the passageway to the control valve. This exerts sufficient force to raise the control valve and unseats its check ball. The engine oil will then flow out through the cross port on the control valve, and fill the passage way between the slav3e and master pistons. The presences of oil in this passage way will force the master piston down against the injector rocker lever. This action basically connects the engine cam shaft motion to the engine brakes timing.
3) When the master piston contacts the rocker lever, oil pressure will increase cause the check ball in the control valve to be seated. This will close the hydraulic system between the master and slave pistons. As the oil pressure tends to increase in this closed loop system, the slave piston will be forced down against the exhaust valve cross head. The cylinder exhaust valve will then open just before the engine piston reaches TDC, releasing the compresses air charge from the engine cylinder.
4) Removing electrical power de-energizes the solenoid valve. The engine oil is now prevented from entering the engine brake unit. The inner control valve spring forces the control valve downward allowing the trapped engine oil from the master, slave pistons to escape from under the control valve cover. This ends the engine brake cycle, the process being repeated if the system is again energized and activated.



EXHAUST BRAKES

The exhaust brake is a simple and relatively inexpensive alternative to an engine brake. The exhaust brake typically gets only a maximum of 60 percent of engine horse power in full retardation mode. Often it is much less. Nevertheless while the exhaust brake develops somewhat less retarding strength than the engine brake, it remains a desirable auxiliary braking system within low initial cost and low cost easy installation.
The exhaust brake is basically a valve, either a slide action gate valve or a butterfly valve, that is installed in the exhaust pipe line between the manifold and the muffler side. It restrict the exhaust flow, which causes exhaust back pressure to build up on the engine cylinders. The exhaust brake actually transforms the engine into a low pressure air compressor driven by the wheels. As the piston attempt to force air in the cylinders back against the restricted exhaust system, a retarding force is actually developed in the engine and delivered through the drive line to the drive wheels. With each succeeding exhaust stroke, the exhaust back pressure increases in the manifold to oppose the normal piston movement up the cylinder. A build up of between 30 to 60 psi is developed in the exhaust manifold, the actual amount depend the following characteristics.
Engine speed, engine size, maximum back pressure approved by the engine manufacturer, vehicle gearing ratios, location of the exhaust brake in relation to the exhaust manifold and turbo charger.
The brake activating controls are electric over air. The system is turned on by a control switch on the dash. When the dash switch is on, a micro switch on the throttle operates the system automatically applying the brake whenever the accelerator is in the idle position and releasing the brake whenever the accelerator is depressed past the idle position. The state of the micro switch determines whether the solenoid valve that controls the air supply to the exhaust brake is energized or de-energized. The recommended range of air pressure used to control the exhaust brake operation is usually between 85 to 150 psi. When pressure is applied it compresses the internal return spring within the exhaust brake cylinder. The compression of the spring moves the sliding gate valve into position. Exhaust brake can be actuated manually by the driver when preferred.
ADVANTAGES

1) Less wear on service brakes: the typical vehicle retarder starts to slow the vehicle when the driver removes his foot off the throttle. The vehicle retarder will then reduce the vehicle speed without having touched the service brakes. This leaves the service brakes cool, in good working order, and ready for any emergency. The less service brakes are used, the longer they will last.
2) Less wear on tyres: every operation of the service brakes leaves a deposit of tyre rubber on the highway. The vehicle retarder however will slow down the vehicle in an even smooth process, preventing tyre flat spots and wheel hop.
3) Longer clutch life: there will be less clutch slippage when using a vehicle retarder especially when attempting high speed down shift.
4) Better fuel economy
5) Less cargo damage( through gradual braking)
6) Safety: a vehicle retarder coupled with efficient service brakes and in some cases with ABS, provides greater control over vehicle speed, thereby contributing to high highway safety standards.

CONCLUSION


Vehicle retarders improve safety. Reliability and durability of service brakes are increased, and maintenance dollars are saved by increasing the life of break linings and tyres. Anyone who has driven a motorhome equipped with an accessory brake is not likely to give up this option willingly. Even on flat lands or city streets, the device improves control by increasing the available braking ability. Nowadays trucks are equipped with a vehicle retarder to supplement or assist service brakes. Therefore proper steps should be taken to increase the use of vehicle retarders on passenger buses and other automobiles in order to improve the safety.



BIBLIOGRAPHY


• Diesel equipment - Erich J. Schulz
• Heavy duty truck systems - Ian Andrew Norman
• Automotive technology - Jack Erjavec
• Automotive braking systems - Thomas W. Birch
• Light and heavy duty vehicles – Nunny
• Automobile engineering – R. B. Gupta
• Fleet watch online journal