Retarder Auxiliary Brake Systems Explained

This truck system is what provides that alternate braking capacity when needed. While coming down a mountain grade, extended application of light braking, results in overtemp of the brake drums and shoes, which reduces the braking capacity, and this situation is referred to as brake fade. Extreme brake fade can result in a run-away rig. Retarder brake systems really shine in this situation, since they can apply extended light braking action with no brake fade, thereby controlling the vehicle's downgrade speed, which saves the vehicle service brakes until they are needed.

A large percentage of brake applications use less than 20% of the vehicle's braking capability. Retarders can effect this light braking action when needed, which will reduce the wear on the vehicle's service brakes. The retarders are designed with no servicable parts which wear out, therefore extensive use of retarder systems has no cost.

Engine Compression Retarders

The engine compression retarder uses the engine's compression stroke to absorb energy from the vehicle's motion. Normally the piston compresses the cylinder air for the next power stroke, and this compression of the cylinder air takes energy. That energy is normally provided by another cylinder which is in its power stroke.

The engine compression retarder uses up vehicle motion energy during the compression stroke of each piston because the other cylinders are not producing power during their power stroke time interval. No (or insignificant) engine power is generated, because the throttle must be at the idle position for the retarder to operate.

During the retarder mode of operation, when the piston approaches the top of the compression stroke, the retarder system opens the exhaust valves for that piston, which vents the energy which was absorbed from the vehicle's motion. The retarder works by absorbing energy during the compression stroke, and throwing away this energy through the exhaust valves as the piston completes its compression stroke.

The most popular engine compression retarder is the Jake Brake. This retarder system is manufactured by the Jacobs Manufacturing Company, and their product is available for Caterpillar, Cummins, Detroit Diesel, and Mack engines, just to name a few.

Refer to the diagram above. The electric signal activates the solenoid valve. The active solenoid valve applies engine oil pressure to the control valve. The engine oil presure force moves the ported piston of the control valve up until the piston port aligns with the high pressure oil passage to the slave piston. The check ball in the control valve allows the engine oil pressure to fill the slave piston and the master piston voids via the high pressure oil passage. As the cylinder completes its compression stroke, the cylinder injector pushrod rises and lifts the master piston. This forces the oil out from above the master piston into the slave piston. Remember, the check ball in the control valve prevents the high oil pressure from bleeding back into the lower pressure engine oil supply.

As the injector pushrod moves upwards, the slave piston pushes down on the exhaust crosshead which opens the exhaust valves. This releases the compressed air which provided the retarder braking action. When the retarder is deactivated, the solenoid valve vents the engine oil pressure, which allows the control valve spring to lower it's ported piston, which seals off the high pressure oil passage.

To activate the Jake Brake, four requirements must be met, as shown in the diagram above. The dash switch must be on, the clutch must be engaged, the throttle must be at idle, and the driver and/or cruise control must have activated the retarder. Engine RPM must be kept high for the most retarder effect.

Engine Exhaust Retarders

The engine exhaust retarders absorb vehicle motion energy by not letting the engine exhaust flow out freely. By restricting the engine exhaust flow, the exhaust cycle of a piston (which should require very little energy) now becomes a compression cycle for that piston. During the exhaust cycle of the piston, the intake valves are closed and the piston is rising. Restriction of the exhaust outlet (back pressure) opposes the piston's travel upward, and absorbs vehicle motion energy in the process.

The Williams Exhaust Brake is the most popular exhaust brake in use. It comes in two flavors, either the sliding gate version, or the butterfly version. Both restriction devices are placed after the turbo charger exhaust outlet, and both devices restrict exhaust flow when activated. These restrictors normally use the vehicle's system air pressure to activate. The air control valve is controlled by an electrical circuit.

To activate the exhaust retarder, five requirements must be met. The ignition switch must be on, the clutch must be engaged, the throttle must be at idle, system air pressure must be adequate, and the driver and/or cruise control must have activated the retarder. Engine RPM must be kept high for the most retarder effect.

Engine Hydraulic Retarders

The hydraulic brake retarders are a whole different animal. Engine hydraulic retarders are mounted on the engine. The Caterpillar Brake Saver is the most popular retarder in this class. This retarder operates on the torque converter principle.

In a normal torque converter, vanes on the input housing (which is attached to the engine crankshaft) transmit power through the fluid coupling to the output shaft vanes of the torque converter, which is connected to the input shaft of the transmission. The hydraulic retarder transmits power from the rotor vanes (which are attached to the engine crankshaft) to the retarder housing vanes (which are attached to the engine block or bellhousing). This produces a hydraulic drag which absorbs vehicle motion energy while activated.

To activate the hydraulic retarder, oil is pumped into the housing, and the housing vanes exert drag due to the swirling oil, upon the rotor vanes. To stop the retarder action, simply drain the chamber, which uncouples the two vanes.

The retarder oil pressure supply is delivered from a secondary section of the engine oil pump. The oil used for the retarder comes from the oil sump of the engine, which must therefore have a larger capacity. This section of the oil pump is entirely separate from the engine lubrication section of the oil pump. This oil pump pressure can be modulated before it reaches the retarder housing. More oil pressure in the retarder housing means more retarding action. Less oil pressure results in less retarding action. While the oil is pressurized within the retarder housing, it is constantly being circulated through the oil cooler and back into the oil sump. This is required to remove the heat generated by the retarder friction.

This retarder has a BrakeSaver control valve which regulates the retarder action once it is activated. This control valve in conjunction with a driver control handle, will set a specific retarder action. As the engine slows in RPM due to the retarding action, the BrakeSaver control valve automatically increases the oil pressure in the retarder housing, which keeps the braking action constant as the pumping action of the retarder vanes decreases.

The retarder control valve also provides a proportioning valve function. The driver control handle will set the retarder force desired, when in manual mode, but while in auto mode, the driver's release of both the throttle and the clutch pedal, will apply maximum retarder action automatically. The proportioning valve uses the larger of these two pressures to control the retarding action.

As required with the above retarders, engine RPM must be kept high for the most retarder effect.

Stand-alone Hydraulic Retarders

The stand-alone hydraulic retarder can be easily built into automatic transmissions, just behind the torque converter. It is placed here so that the retarding force can be applied to the transmission input shaft. This retarder can also be mounted behind transmissions when required.

The stand-alone hydraulic retarder offers advantages over the engine hydraulic retarder. The weight of this unit can be distributed away from the engine for better scaling of the axle weight. This retarder is normally connected to a drive line component or the transmission output shaft, which means that maintaining high engine RPMs is not necessary. This also means that gear shifts may not interrupt retarder action. This retarder normally uses transmission fluid which is thicker than engine oil, which effects more retarder capability.

This retarder works just about the same as the engine hydraulic retarder described above. It also employes its own oil pump, oil cooler, regulated retarder action, and it also uses a driver controlled lever or foot pedal. It can also be controlled by a cruise control unit.

Electric Retarders

The electric retarder has been popular in Europe for several years. It is quiet in operation, is placed within the drivetrain, and provides adjustable retarder action control for the driver. It does require DC control power from the vehicle battery charging circuit, in the order of 10 amps.

In operation, the electric retarder is a generator whose load is itself. All power generated by the generator is dissipated back into the generator as heat. The retarder has cooling turbines which remove the heat as it is generated. The vehicle's motion energy is converted into heat by the generator.

The electric retarders do not place additional requirements upon the vehicle cooling system, nor do they impact the vehicle's engine lubrication system. It doesn't restrict the engine's compression or exhaust cycles in any way. The electric retarder is an easy bolt-on, and independant option.

The electric retarder consists of 16 electromagnet coils which surround a rotor which is connected to the vehicle drive shaft. Gear shifts do not impact the operation of this retarder, and engine RPMs also have no impact upon the operation of this retarder.

The retarder effect is controlled by enabling one through 16 of the electromagnet coils. The more coils that are activated, the more retarder force that is generated. There is typically a driver switch which can select the number of active coils. Recently, electronics has provided a progressive switch position which enables more coils as the length of application time increases. There is also a manual driver control lever option, and a foot brake pedal option. The foot brake pedal option activates the electric retarder before the service brakes are applied.

There is also a low speed sensor which disables the retarder when the vehicle speed falls below 5 MPH (8KMH).

Conclusion & Jake Brake Double-Clutch

There are many retarder options available today. The retarder will normally make your rig a safer rig on the road and will extend your service brake life. There is a lot of arguing amongst drivers about whether or not to use the jake brake during shifting.

During the shifting double clutch, the conditions are correct for jake brake activation. If your gear shifts are close together, then the jake brake will fight you on the engine RPM match while shifting. However, if the gear shift result in large RPM drops, or if the shift is on a steep grade which results in rapid truck speed loss, then use of the jake brake may very well help you to grab that next gear quicker. In this situation, the truck speed is falling rapidly, and the engine RPMs are falling normally, so during the double clutch, the engine RPMs are yanked lower by the jake brake. This might be helpful to get that next gear in place during a slow and steep grade climb.

One concern that we have is the potential for retarder drive wheel slippage on a slippery road surface. The drive wheel slippage would normally hold the tractor straight, but with a heavy trailer behind, I would be concerned that the drive wheel slippage coupled with the trailer pushing forces could result in a jackknife under extreme conditions. You may want to keep this in mind. Remember, most trucks do not have anti-lock retarder computer controls!

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