Do you really understand these auxiliary braking technologies of trucks?

Unlike passenger cars, the large trucks that we truck drivers drive daily are greatly affected by factors such as vehicle weight and road conditions, which easily put traditional wheel brakes under high-intensity working conditions. Especially when encountering special conditions such as long downhill slopes or mountain roads, we truck drivers need to brake frequently. At this time, the wheel brakes accumulate high temperatures due to prolonged friction, which can easily lead to a severe decline or even failure of the vehicle's braking performance, making traffic accidents very likely to occur.

To solve such problems, auxiliary braking measures have emerged. We truck drivers all know that the main types of auxiliary braking on trucks include engine brake, exhaust brake, and hydraulic retarder. Broadly speaking, these braking methods can all be referred to as vehicle retarders. Combined with the vehicle's wheel brakes and other active and passive safety measures, they have become an important part of ensuring the daily driving safety of truck drivers like us.

Although these types of auxiliary braking methods all slow down the vehicle, many drivers cannot tell the differences between them in terms of braking principles and braking efficiency; next, the editor will explain in detail what the differences are between them.

▎ Hydraulic retarder converts kinetic energy into heat that is carried away

Retarder braking, in today's truck industry, most of the time we refer to hydraulic retarders. In fact, before the hydraulic retarder was invented, there were already retarders on the market that provided braking force through eddy currents.

With the continuous advancement of technology, hydraulic retarders can provide a large braking torque with relatively small mass, have strong continuous braking capability, are simple to maintain and low in cost. Currently, eddy current retarders are more commonly used in the bus market, whereas for truck products, hydraulic retarders are more advantageous than eddy current retarders.

For hydraulic retarders, our past articles and videos on Truck Home have provided detailed introductions, and many truck enthusiasts have learned that hydraulic retarders are divided into series retarders, parallel retarders, and independent retarders. However, some truck enthusiasts are still not very clear about the principle of hydraulic retarders.

To elaborate, the hydraulic retarder acts directly on the drive shaft. Inside the retarder’s working chamber, there is a stator with blades and a rotor with blades, and the rotor rotates together with the drive shaft. When the retarder is in operation, the ECU controls the opening of the proportional valve to inject gas into the retarder oil tank housing, and the gas presses the retarder oil into the working chamber (the higher the air pressure, the more oil flows into the working chamber).

The rotor spins at high speed, throwing the oil onto the stator, which remains stationary. The stator's blades have reverse grooves, which can redirect the hydraulic oil flung out by the rotor back onto the rotor blades, increasing the rotor's resistance. The entire process converts the vehicle's kinetic energy into thermal energy in the engine's water circulation, which is then dissipated into the atmosphere through the engine's cooling system, thereby achieving the vehicle's constant speed and deceleration effect.

Because the working principle of the retarder is to convert the vehicle's kinetic energy into heat energy in the engine's water circuit, when using the cruise gear, it is generally necessary to maintain the engine speed above 1500 rpm. The higher the engine speed, the faster the water circulation, making it less likely to overheat and allowing the retarder to play a greater role.

In general, a hydraulic retarder can reduce the frequency of braking and increase the service life of brake pads. However, retarder braking is only a type of auxiliary braking device, and in situations requiring emergency braking or stopping, our truck drivers still need to use the brake.

▎ Engine braking mainly includes the following types

In addition to the hydraulic retarder, there is another vehicle deceleration device on trucks that we truckers are also very familiar with, which is the engine brake. We usually divide engine brakes into several major categories: in-cylinder braking, exhaust braking, decompression-type engine braking, compression release engine braking, and so on.

● In-cylinder braking The magnitude of the braking force is closely related to the engine speed.

Cylinder braking refers to using the compression resistance generated during the engine's compression stroke to create a braking effect on the drive wheels through internal friction and intake and exhaust resistance.  

Taking the cylinder braking technology of a certain brand's engine as an example, during high-speed driving and downhill, the ECU central computer intelligently opens the engine exhaust valve based on the frequency and force with which the driver presses the brake, actively reducing the pressure generated by the engine fuel, thereby producing a braking effect during the compression stroke.

What our fellow drivers need to pay attention to is that the magnitude of the engine brake inside the cylinder is closely related to the engine speed: the higher the engine speed, the greater the engine braking force produced. The engine speed is also affected by the gear ratio of the transmission; the lower the transmission gear, the stronger the engine braking.

● Exhaust brake: Install a butterfly valve on the engine's exhaust manifold.

Exhaust brake is to install a butterfly valve on the engine's exhaust manifold. The exhaust brake butterfly valve is installed on the engine's exhaust manifold, and by closing the engine's exhaust passage, it allows the engine pistons to experience reverse gas pressure during the exhaust stroke, thereby slowing the engine's operating speed and producing a braking effect.

The exhaust brake is generally activated by a manual switch. When truck drivers turn on the exhaust brake, the exhaust butterfly valve will perform the corresponding closing operation (adjusting the opening according to the pressure in the exhaust manifold), causing the piston to experience gas back pressure during the exhaust stroke, preventing the engine from running and generating a braking effect, thereby controlling the vehicle speed.

● Exhaust valve brake (bleeder-type engine brake) is mainly divided into two types: passive and active.

The second type is called exhaust valve brake, also known as exhaust-type engine brake. It is mainly divided into passive and active types. Among them, the passive exhaust valve brake is similar to the exhaust brake and also installs a butterfly valve, while the active exhaust valve brake does not require installing an exhaust brake butterfly valve. It mainly works by extending the piston of the actuator inside the valve chamber to eliminate the clearance with the exhaust valve; this allows the compressed air to escape from the exhaust valve throughout the compression stroke, reducing the energy returned to the engine piston, thus slowing down the vehicle.

● Compression release engine brake: turning the engine into an air compressor

The third type is the compression release engine brake, which simply turns the engine into an air compressor. According to information from Truck Home, when the compression release engine brake control switch is turned on, the engine exhaust valve will open near the top of the compression stroke, and the high-pressure air generated during the compression stroke will be released from the engine cylinder.

In this way, most of the energy absorbed by the compressed gas is released, and only a small portion of the residual energy drives the engine piston back. As the engine operates repeatedly, the inertia of the vehicle going downhill will be dissipated by the engine's compression-release action, ultimately causing the vehicle to slow down.

Among these three types of engine braking techniques, the third type, compression release engine braking, is now widely used. Examples include the Jake Brake technology and Cummins iBrake technology, both of which fall under the category of compression release engine braking.

● Editorial Note

Whether it is a hydraulic retarder, in-cylinder brake, or engine brake, they can all help reduce the frequency of using the brake pedal to some extent when going downhill, thereby enhancing driving safety. However, these auxiliary systems are not "invincible." Every component is designed with a certain limit. If the kinetic energy generated by the vehicle far exceeds the limit that the auxiliary brake can handle, they may also become "unreliable." Therefore, we must learn to use auxiliary braking devices correctly and safely in order to ensure our driving safety.