There is no small matter when it comes to braking. Do you really understand truck auxiliary brakes?

▎1. What is Auxiliary Braking

As the name suggests, from the term 'auxiliary braking,' it can be seen that its role in the braking system is auxiliary, and its object of assistance is the driving brake (commonly known as the 'brake').

It shares the burden of the driving brake, reducing or maintaining the vehicle's speed when the driving brake is not used or used minimally, in order to keep the driving brake in good condition and function during critical emergency moments.

Auxiliary braking can also become sustainable braking. Unlike driving, where continuous braking can quickly degrade braking performance, auxiliary braking can provide sustained braking force for the vehicle. This not only improves vehicle operational efficiency but, most importantly, ensures the safety of vehicle operation.

▎ 2. Classification of Auxiliary Braking

According to different working methods, auxiliary braking is divided into: power device braking and transmission system braking. Power device braking uses the engine to brake, changing the engine from power output to power consumption.

Transmission system braking reduces vehicle speed by lowering the speed of the transmission system. The specific classifications are as follows:

▎3. Auxiliary Braking Principle

Before discussing the principle of auxiliary braking, it is necessary to briefly restate the working principle of the engine, as shown in Figure (1).

During normal vehicle operation, the engine performs positive work externally (only the expansion stroke performs work externally), and the working cycle is divided into four strokes: intake, compression, expansion, and exhaust.

1. Power Equipment Braking

(1) Engine Braking

Engine braking is one of the simplest forms of braking. You only need to release the clutch pedal and the accelerator pedal, and shift the transmission into a forward gear to enter engine braking mode.

The engine, starved of fuel, gradually stops running, and the car continues to move under its own inertia. Because the transmission is engaged, the car drives the engine in reverse.

At this time, under the action of the car's kinetic energy, the engine continuously compresses air to operate, converting the car's kinetic energy into heat energy. At this point, the engine, originally a power source, becomes an 'air compressor' that consumes the car's kinetic energy.

The car's kinetic energy, besides being dissipated as heat, is also used to overcome the friction of the engine and its auxiliary components, as well as the friction losses of various transmission components.

With technological progress, in pursuit of greater driving power, the mechanical losses of the engine are gradually decreasing, and the engine braking power will also further decrease accordingly. Therefore, the effect of engine braking is limited.

(2) Exhaust Brake

The exhaust brake is similar in principle to engine braking, with the difference being that the exhaust manifold is fitted with a butterfly valve or a similar exhaust brake valve. The butterfly valve is controlled through an exhaust brake switch (usually on the right side of the steering wheel).

During the exhaust stroke, the closed butterfly valve increases exhaust resistance, creating exhaust back pressure (the pressure difference between the exhaust manifold at the back of the exhaust valve and the sealed chamber of the butterfly valve). This exerts a reaction force on the piston, thereby increasing braking power, as shown in Figure (3).

Therefore, the exhaust brake is equivalent to adding additional braking force on the basis of engine braking.

(3) Exhaust-type braking

Exhaust-type braking is based on the principle of engine exhaust braking. It uses the backpressure generated by a butterfly valve or a set of control mechanisms to keep the exhaust valve slightly open throughout its entire stroke.

During the compression stroke, the compressed air 'leaks out' through the slightly opened exhaust valve, causing the cylinder pressure to drop, reducing the work done by the compressed gas on the piston during expansion, thereby increasing braking power, as shown in Figure (4).

(4) Compression Release Brake

Unlike the exhaust brake, the compression release brake only operates during the compression stroke. When the piston approaches the top dead center, the hydraulic mechanism is used to open the exhaust valve, allowing the high-pressure gas in the cylinder to be expelled, reducing the pressure inside the cylinder, and decreasing the work done by the compressed gas on the piston during the expansion stroke, thereby achieving a braking effect. See Figure (5) for details.

The compression release brake is divided into fixed, rocker, and valve bridge types based on the structure of the exhaust valve opening control mechanism.

Since the exhaust valve is controlled by a specialized mechanism, a butterfly valve is not a necessary component of the compression release brake, mainly serving as a parallel braking method. For example, the Xichai CA6DN 12.5-liter engine is equipped with both a compression release brake and a butterfly valve, while the Cummins ISM 11-liter engine is only equipped with a compression release brake.

2. Transmission System Braking

(1) Hydraulic Retarder

The hydraulic retarder mainly achieves deceleration by exerting a reactive torque on the transmission shaft, which is transmitted to the wheels through the transmission system. For the specific working principle, see Figure (6).

(2) Electric Eddy Current Retarder

As the name suggests, the difference of the electric eddy current retarder lies in the medium that transmits force. The hydraulic retarder transmits force using a liquid (hydraulic oil or water), whereas the electric eddy current retarder uses magnetic force. The main structure of the electric eddy current retarder is shown in Figure (7).

The principle of the electric eddy current retarder needs to be analyzed in conjunction with the following three physics concepts. The first is 'electricity generates magnetism'; a magnetic field will be produced near a conductor when current flows through it — for example, a magnetic field is generated when current flows through the coil in the stator.

The second is 'electromagnetic induction.' When part of a closed circuit conductor moves in a magnetic field, it cuts through the magnetic lines of force, generating a current in the conductor—for example, when the rotor moves in the magnetic field generated by the stator, a current is produced in the rotor.

The third is 'magnetic force.' A charged conductor moving in a magnetic field while cutting through the magnetic lines of force will be subjected to the magnetic force—for example, the rotor generating current moving in the magnetic field produced by the stator will experience a magnetic force opposite to the direction of motion, thereby achieving a braking effect.

▎4. Comparison of Auxiliary Brake Characteristics

1. Braking Power: The transmission system brake can provide greater maximum braking power than the power equipment brake. Therefore, hydraulic/electric eddy current retarders can often provide a higher average downhill speed, which is conducive to efficient logistics transportation.

2. Cost: Cost is roughly proportional to braking power; the greater the braking power, the higher the cost, with hydraulic/electric eddy current retarders being the most expensive.

3. Complexity of Structure: Exhaust-type/compression-release brakes involve modifications to the engine structure and are often more complex. Next are hydraulic/electric eddy current retarders, while the engine brake structure is the simplest.

4. Ease of Installation: The ease of installation is somewhat positively correlated with the complexity of the structure. Exhaust-type/compression-release brakes are the most difficult to install, followed by hydraulic/electric eddy current retarders.

5. Noise: Except for the simplest engine braking, other types of auxiliary brakes all produce some noise, with compression-release brakes having the best-controlled noise level.

6. Weight: The electric eddy current retarder, due to the materials required for electromagnetic induction, is the heaviest, followed by the hydraulic retarder. Under comparable braking power, hydraulic retarders are clearly more suitable for the currently lightweight-focused commercial vehicle market than electric eddy current retarders. Exhaust-type/compression-release brakes are relatively light.

▎V. Current Status and Future Prospects of Auxiliary Brake Applications

1. Current Status: At present, auxiliary braking devices for domestic commercial logistics vehicles are mainly power equipment brakes, while the prevalence of transmission system brakes is relatively low, especially hydraulic retarders, which have only appeared in trucks as optional configurations in recent years.

2. Reasons: Power equipment brakes are relatively low in cost, and based on regulations (GB7258, GB12676) regarding braking performance requirements, many trucks are already equipped with power equipment brakes as standard.

The main reason hydraulic retarders have not been widely adopted is that, firstly, "bad money drives out good." Water-cooled brakes have a deep-rooted popular base and a relaxed legal and regulatory environment.

First, water spraying devices have gained market favor due to advantages such as low short-term investment costs and ease of installation. Even though water-sprayed brakes can cause brake failures and road traffic safety hazards, this has not stopped the prevalence of water spraying devices.  

Second, there is inconsistent law enforcement, with lax implementation and differentiated enforcement. For example, in northern China, their use is prohibited in winter, while in other seasons it is often overlooked. In the southwest, brake water spraying is even one of the criteria used by traffic police to judge whether your vehicle is safe.  

Next is the 'weakness of good currency.' Currently, hydraulic retarders do not show significant advantages in cost or technology and cannot yet meet the requirements for widespread adoption.  

Two aspects of information can roughly illustrate the current application status of hydraulic retarders. First, GB7258-2017 (including GB7258-2012) requires that vehicles have 'retarders or other auxiliary braking devices,' which shows that the retarder is not a mandatory option.

Secondly, during the 2016 Two Sessions, Fast's chief skills trainer Cao Jing proposed the suggestion "to mandate the installation of retarders in commercial vehicles." In the response from the Ministry of Industry and Information Technology, it was pointed out that efforts should be made to include retarder products with independent intellectual property rights in the "Guideline Catalog for the Promotion and Application of Major Technical Equipment" for commercial vehicles. It is evident that the relevant national departments attach great importance to the promotion and application of retarders, but at the same time, it is not difficult to see that large-scale promotion and application still require time.

3. Future Outlook:

The performance characteristics of auxiliary braking align with the integrated requirements for timeliness and safety in the future logistics market.

A single auxiliary brake may be slightly insufficient to meet future demands, and the combined braking of multiple auxiliary brakes could be a relatively ideal solution, fully utilizing the advantages of power equipment brakes and transmission system brakes to handle different working scenarios.

However, the current popularity of hydraulic retarders is relatively low, and it still requires multi-party coordination to jointly promote its application.

(1) Technological Innovation: Domestic brand hydraulic retarders have already emerged, and brands such as Fast and Huasheng have launched their own products to the market.

However, compared with experienced imported brands, the performance of domestic products still needs to be tested by the market. Domestic vehicle manufacturers and component suppliers should strengthen coordination, improve vehicle compatibility, continuously innovate technology, diversify product categories, and increase the cost-effectiveness of hydraulic retarders, so that customers are willing to pay.

(2) Market guidance: The national level can introduce relevant policies to guide the market.

For example, appropriate subsidies or policy preferences can be provided for the purchase of hydraulic retarders in the southwestern region to guide consumer spending, enhance user awareness, and expand the influence of hydraulic retarders. Departments such as the insurance industry and vehicle management offices should recognize the legality of retrofitted hydraulic retarders, allowing them to enjoy standard insurance and annual inspection.

(3) Laws and regulations: Accelerate relevant research and verification work, and clarify the configuration requirements for auxiliary braking. Unified enforcement standards should be applied for illegal devices.