An auxiliary brake is a brake used to assist the braking force of the main brake, the foot brake (the brake operated by foot). There is the natural braking force generated by the structure of the vehicle, often referred to as the engine brake, and there are auxiliary brakes that require separate additional devices. Among those using separate devices are the most common exhaust brake, the “Intarder” or “Retarder” used in automatic vehicles, the Jake Brake (produced by Jacobs Brake in the USA) offered as an option on Hyundai’s electronically controlled Powertec engine, and the COTV installed on SsangYong’s Transtar. In this long-awaited update to Lesson 540, we will take a detailed look at the auxiliary brakes installed on buses and heavy commercial vehicles.

 

2. Engine Brake

The engine brake has already been explained in a previous chapter; it is a braking method used through driving technique without attaching any additional devices. The usage method is as follows.

When a driver presses the accelerator pedal, the engine RPM increases, and the vehicle accelerates. If the driver stops pressing the accelerator pedal and releases it, the engine speed will no longer rise and will gradually decrease.

This principle is what the engine brake uses. Similarly, when going downhill, if you shift to a lower gear, the engine load increases, allowing the vehicle to slow down.

 

3. Exhaust Brake

The exhaust brake is an auxiliary brake that is standard equipment on almost all large commercial vehicles. It requires an additional device to operate.

The operating principle of the exhaust brake is as follows. After the air-fuel mixture enters the engine and goes through combustion, exhaust gases are produced and released outside. These gases pass through the exhaust manifold, travel along the piping, and exit into the atmosphere through the muffler.

If you block the exhaust gases in the middle of the pipeline, they accumulate, preventing proper combustion in the engine. This naturally reduces engine output, lowers RPM, and slows the vehicle.

The exhaust brake can be used together with the foot brake and is particularly useful on downhill runs as an alternative to the foot brake. It should be used for only about 7–8 seconds at a time; prolonged use may overburden and damage the engine. On long descents, it’s recommended to use it for about 10 seconds, release it, and then reapply.

When using the exhaust brake, a “du-du-du-du” sound is generated. Recently, to reduce this noise, an additional device similar to the exhaust brake is installed on the intake side. This operates simultaneously with the exhaust brake to prevent noise from escaping through the intake.

The exhaust brake cannot be used simultaneously with the accelerator or clutch pedal. If the exhaust brake switch is on and the accelerator is pressed or the clutch is used for shifting, it will automatically disengage. However, it can be used simultaneously with the foot brake.

 

4. Intarder and Retarder

Source: Han Sang-wol (http://home.hanmir.com/~hansw99/)

Although the names differ, “Intarder” and “Retarder” refer to the same device. The German company ZF originally developed and sold the retarder for its automatic transmissions. Later, other companies developed devices with the same function under the name “Intarder.”

There are two main types of retarders: those operating within the automatic transmission and those mounted on the propeller shaft.

First, the hydraulic retarder uses fluid dynamics to generate deceleration torque. Heat generated is dissipated through a heat exchanger. When the gears in the transmission rotate, the retarder fluid is spun in the opposite direction, creating braking force.

The braking force depends on how much fluid enters and the pressure generated. The retarder is operated by a hand lever (for example, seen on Seoul’s premium express bus route 755, model BH116) or a foot pedal-type service brake/retarder control.

One key advantage of this type of retarder is that it provides greater deceleration at higher vehicle speeds. Since the retarder’s deceleration cannot reach zero, it cannot bring the vehicle to a complete stop on its own.

Hydraulic retarders are supplied by manufacturers such as Allison, Mack (Dynatard), Voith, and Caterpillar (Brakesaver), often integrated into automatic transmissions.

Advantages include shorter travel times, extended brake lining life, reduced drivetrain wear, lower noise than engine brakes, and reduced driver fatigue.

Next is the electric retarder, which operates by inducing eddy currents in a rotating metal disc placed between electromagnets. These currents generate braking torque and heat, which is dissipated through ventilated rotors. However, excessive heat reduces magnetic strength, decreasing braking efficiency. Proper temperature control is essential.

Simply put, the electric retarder slows the propeller shaft using a magnetic field, generating heat that must be cooled to maintain performance.

 

5. Jake Brake

Diesel engines operate on a four-stroke cycle: intake – compression – power – exhaust. The Jake Brake works by opening an additional exhaust valve at the moment of combustion, allowing gases to escape, reducing power output and generating braking force.

In normal operation, the engine stores energy during compression and returns most of it to the crankshaft during the power stroke. With the Jake Brake engaged, the compressed air is released into the exhaust manifold and stack, reducing energy transferred to the crankshaft.

The system is controlled by an “on/off” switch on the dashboard. Depending on settings, it can operate on 2, 4, or 6 cylinders for different braking levels. The Jake Brake disengages automatically when the accelerator or clutch is pressed.

Compared to other systems, braking effectiveness is roughly: engine brake 45–50%, exhaust brake 60–70%, Jake Brake 90–95%, hydraulic retarder 175–200%.

 

6. C.O.T.V

The C.O.T.V auxiliary brake was installed on Mercedes-Benz engines and, in Korea, on the SsangYong Transtar. Recent Mercedes engines no longer seem to include it. The COTV unit is mounted above the engine cylinder block, similar in position to the Jake Brake. Vehicles with COTV also have exhaust brakes, and both can be used together. According to user accounts, with both switches on, pressing the accelerator disengages the exhaust brake but the COTV prevents further acceleration, maintaining a constant speed. This is particularly useful on downhill sections or roads with alternating slopes.