Retarders are supplementary braking devices used on large trucks, buses and heavy vehicles. By providing additional braking power, retarders reduce wear on the normal service brakes and allow drivers to maintain control on steep descents. There are 4 main types of retarders used on commercial vehicles today.
What is the Purpose of Retarders?
Retarders give drivers an extra tool to slow their vehicle, especially when traveling downhill. They significantly reduce brake fade, overheating and wear on the standard friction brakes. Retarders allow small adjustments to speed, helping maintain a consistent descent. They also provide braking even if the main brakes fail. Many experienced drivers consider retarders essential safety devices for mountain driving or hauling heavy loads.
The 4 Main Types of Retarders
There are 4 common types of retarders used on trucks and buses:
- Exhaust Brake Retarders – Also called "Jake Brakes", they use engine compression
- Engine Compression Brakes – Use the engine itself to provide braking
- Hydraulic Retarders – Slow drive shaft using viscous fluid
- Electric Retarders – Use electromagnetic induction to create resistance
Below we look at how each type of retarder works and key specs.
Exhaust Brake Retarders
Exhaust retarders, or Jake Brakes, are the most popular type of retarder. They use a mechanism to alter the normal exhaust flow out of the engine. By blocking exhaust passageways, they force the engine to work harder to expel each cylinder‘s exhaust gas during the exhaust stroke. This creates powerful engine braking.
How They Work: Exhaust retarders allow the engine to compress intake air as normal on the compression stroke. But on the exhaust stroke, they force the engine to compress the exhaust gases inside the cylinder before expelling them. Some designs use a butterfly valve in the exhaust outlet, while others alter the exhaust valves timing and lift.
Benefits: Very effective at all engine speeds. Simple, rugged design with few moving parts. Low maintenance.
Drawbacks: Can be noisy, which limits where they can be used. Requires high engine back pressure which reduces fuel economy.
Key Brands: Jacobs Vehicle Systems, Pacbrake
Engine Compression Brakes
Engine compression brakes work by altering the normal operation of the engine‘s exhaust valves. By staying closed longer, the valves make the pistons work harder to compress gases on both the compression and exhaust strokes. This provides continuous engine braking.
How They Work: During braking, the exhaust valves are not allowed to open at the normal point. Instead they stay closed through the compression stroke. This forces the engine to compress intake and exhaust gases, causing resistance. The valves briefly open at the end of the exhaust stroke to release the gases.
Benefits: Very effective, especially at higher rpm. Lower noise than exhaust brakes.
Drawbacks: Works best with larger diesel engines. Requires advanced valve actuation technology.
Key Brands: Jacobs Vehicle Systems, PAC Brake
Hydraulic Retarders
Hydraulic transmission retarders use fluid viscosity to slow the drive shaft. An impeller or rotor attached to the drive line spins inside a static set of stator blades, forcing viscous silicone fluid through the small gaps. This creates continuous drag that slows the transmission output shaft.
How They Work: The spinning rotor churns the viscous silicone fluid, creating friction and heat. The static stator blades convert the rotational energy into heat, slowing the drive shaft.
Benefits: Smooth automatic variable braking. Completely wear-free. Low noise.
Drawbacks: Depend on hot fluid – limited cold braking. Require fluid cooling system. More complex maintenance.
Key Brands: Telma, Voith, ZF
Electric Retarders
Electric retarders generate magnetic eddy currents in a rotor mounted on the drive shaft. The magnetism creates resistance which slows the rotor and drive shaft. An electric power source is required.
How They Work: Electromagnets mounted on the stationary stator interact with conductive fins on the spinning rotor. This induces eddy currents in the rotor which create magnetic resistance.
Benefits: Very smooth automatic variable braking. Totally wear-free. Quiet operation.
Drawbacks: Require large power supply. Sophisticated controls. Limited cold braking ability. High component cost.
Key Brands: Telma, Al-Ko
Comparison of Retarder Types
| Type | Pros | Cons | Notes |
|---|---|---|---|
| Exhaust Brake | Effective, simple, low-maintenance | Noisy, high backpressure | Works best at mid to high rpm |
| Compression Brake | Powerful, lower noise | Complex controls, for large diesel only | Works across engine speed range |
| Hydraulic | Smooth, wear-free, automatic | Limited cold braking, maintenance | Continuous friction braking |
| Electric | Wear-free, smooth, quiet | Expensive, requires large power supply | Sophisticated controls, limited cold braking |
How Do Drivers Operate and Control Retarders?
Retarders are relatively easy for drivers to operate. Most types can be turned on or off using a dashboard switch or button. The braking power or effect is often adjustable, allowing drivers to maintain just the right speed.
- Exhaust brakes usually have on/off control only.
- Compression brakes may have 2-3 stages of braking power.
- Hydraulic and electric retarders adjust braking smoothly and automatically but often have a shutdown override.
The retarder controls are independent from the normal service brakes. Drivers manually apply retarders when they want supplemental braking. The retarder should be shut off before coming to a complete stop to avoid overheating.
How Do Retarders Interact With the Brake System?
Retarders are integrated into the brake system hydraulically and pneumatically. This allows some automatic control of the retarder in emergency situations.
If service brake pressure drops below a certain level, the retarder will automatically disengage. This is a safety feature in case of brake failure. The retarder may also automatically shut off if its temperature gets too high to prevent overheating.
Some retarder systems apply the retarder when the brake pedal is pressed, increasing overall stopping power beyond just the service brakes. This is called a modulated retarder.
Retarder Maintenance Tips
Most retarders are designed to be low maintenance. But they do have some routine inspection and service needs:
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Check fluid levels in hydraulic retarders and top up with approved fluid as required. Change fluid per manufacturer interval.
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Inspect electric retarder rotors and stators for dirt buildup or damage. Clean and replace brushes as required.
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Ensure proper exhaust backpressure for exhaust brakes. Check for exhaust leaks.
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Follow manufacturer guidelines for any valve adjustments, cleaning or part replacement intervals.
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Check retarder piping and hoses for leaks or kinks. Replace worn components.
Key Safety Tips When Using Retarders
It‘s important to properly train drivers on retarder operation and limitations:
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Don‘t ride the retarder downhill. Apply intermittently to prevent overheating.
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The retarder may not work well when cold – don‘t depend on it before fully warmed up.
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Be aware the retarder may disengage automatically if the service brakes are activated or temperature gets too high.
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Avoid sliding tires by synchronizing retarder braking with engine rpm.
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Know the different retarder control locations between vehicles – don‘t rely on muscle memory.
Conclusion – Retarders Are Essential Safety Tools for Truckers
Adding a supplemental retarder system is a smart investment for any heavy truck or bus. Retarders take wear off the standard friction brakes while providing drivers with much better speed control. They are especially valuable in mountainous driving. With 4 main types available, most operators can find a retarder ideally suited to their vehicle and budget. When specified correctly and maintained properly, retarders will provide years of reliable braking performance.
