Brakes are no small matter, do you really understand truck auxiliary braking?

1. What is auxiliary braking

As the name suggests, auxiliary braking is meant to support the braking system, specifically helping the main driving brakes (commonly known as 'the brakes').

It takes some pressure off the main brakes, reduces or maintains the vehicle's speed when the main brakes are not used or used sparingly, keeping them in good condition and effective in urgent situations.

Auxiliary braking can also be considered sustainable braking. Unlike driving, where continuous braking can quickly reduce braking performance, auxiliary braking provides a consistent braking force for the vehicle, which not only improves operational efficiency but, most importantly, ensures the safety of vehicle operation.

▎2. Types of Auxiliary Braking

Auxiliary braking is divided into engine braking and transmission system braking according to different working methods. Engine braking uses the engine for braking, turning it from a power source into a power absorber. Transmission system braking slows down the vehicle by reducing the transmission system's speed. The specific categories are as follows:

3. Auxiliary braking principle

Before discussing the principle of auxiliary braking, it is necessary to briefly go over how the engine works, as shown in figure (1). During normal driving, the engine does positive work on the outside (only during the expansion stroke), and its working cycle consists of intake, compression, expansion, and exhaust strokes.

1. Power Equipment Braking

(1) Engine Braking

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

With the engine no longer receiving fuel, it gradually stops running, and the car continues to move due to inertia. Since the transmission remains engaged, the car essentially drags the engine along.

At this moment, the engine keeps compressing air under the car's kinetic energy, converting the car's kinetic energy into heat. The engine, which was originally a power source, now acts as an 'air compressor' consuming the car's kinetic energy. Besides being released as heat, the car's kinetic energy is also used to overcome friction in the engine and its components, as well as losses in the transmission system components. With technological advancements, to achieve greater driving power, the engine's mechanical losses are gradually decreasing, and engine braking power will further reduce, which means the effect of engine braking is limited.

(2) Exhaust BrakeAn exhaust brake works similarly to an engine brake, the difference being that the exhaust manifold is fitted with a butterfly valve or a similar exhaust brake valve. The butterfly valve is controlled via the exhaust brake switch, usually located to the right 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 behind the exhaust valve and the sealed container inside the butterfly valve), which applies a counter force to the piston, thus boosting braking power, as shown in Figure (3).So an exhaust brake is basically like adding extra braking force on top of the engine brake.

(3) Exhaust-Type Braking

Exhaust-type braking is based on exhaust braking. It uses the back pressure created by a butterfly valve or a set of control mechanisms to keep the exhaust valve slightly open throughout its full stroke.

During the compression stroke, compressed air 'escapes' through the slightly opened exhaust valve, lowering the pressure in the cylinder and reducing the work done by the expanding compressed gas on the piston, thereby increasing braking power, as shown in Figure (4).

(4) Compression Release Brake Unlike exhaust brakes, the compression release brake only works during the compression stroke. When the piston nears the top dead centre, a hydraulic mechanism opens the exhaust valve to release high-pressure gas from the cylinder, reducing cylinder pressure and lessening the work done by the compressed gas on the piston, achieving braking. See Figure (5) for details. Compression release brakes are classified into fixed, rocker, and valve bridge types based on the structure that controls the exhaust valve opening. Since the exhaust valve is controlled by a specialised mechanism, a butterfly valve is not essential for compression release braking and mainly exists as an auxiliary braking method. For instance, the Xichai CA6DN 12.5-litre engine is equipped with both a compression release brake and a butterfly valve, whereas the Cummins ISM 11-litre engine is only fitted with a compression release brake.

2. Drive System Braking

(1) Hydraulic Retarder

The hydraulic retarder mainly slows down the vehicle by applying a counterforce to the transmission shaft torque, which is then transmitted to the wheels through the drive system. For detailed working principles, see Figure (6).

(2) Electric Eddy Current RetarderThe name itself shows that the difference of an electric eddy current retarder lies in the medium used to transmit force. A hydraulic retarder uses a liquid (hydraulic oil or water) to transmit force, while an 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 analysed using the following three physics concepts. The first is 'electricity generates magnetism', meaning a magnetic field is produced near a current-carrying conductor—for example, a coil in the stator generates a magnetic field when electrified.

The second one is 'electromagnetic induction'. When a 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 produced by the stator, a current is generated in the rotor. The third one is 'magnetic force'. A charged conductor moving in a magnetic field and cutting through the magnetic lines of force experiences a magnetic force—for instance, the rotor generating current moves in the magnetic field produced by the stator and experiences a magnetic force opposite to its direction of motion, achieving a braking effect.

▎4. Comparison of auxiliary braking characteristics

1. Braking power: The transmission system brakes can provide greater maximum braking power than the power equipment brakes. Therefore, hydraulic/electric eddy current retarders often offer higher average downhill speeds, which is beneficial for efficient logistics transport. 2. Cost: Cost is roughly proportional to braking power; the greater the braking power, the higher the cost. Hydraulic/electric eddy current retarders are the most expensive.

3. Complexity of structure: Exhaust-type/compression release brakes involve engine modifications and are usually more complex, followed by hydraulic/electric retarder, while engine brakes have the simplest structure. 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 hardest to install, followed by hydraulic/electric retarder.

5. Noise: Except for the simplest engine brake, all other types of auxiliary brakes produce some noise, with the compression-release brake being the quietest.

 6. Weight: The electric eddy current retarder is the heaviest due to the materials needed for electromagnetic induction, followed by the hydraulic retarder. Under similar braking power, the hydraulic retarder is clearly more suitable for the current lightweight commercial vehicle market. Exhaust/compression-release brakes are lighter.

▎5. Current Status and Future Outlook of Auxiliary Braking Application

1. Current Status: At present, auxiliary braking devices in domestic commercial vehicles for logistics mainly rely on power equipment brakes, with a low penetration rate of transmission system brakes, especially hydraulic retarders, which have only appeared in truck 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 reason hydraulic retarders have not been widely adopted is primarily due to the prevalence of "inferior currency"; water-cooled brakes have a deeply rooted public acceptance and a lenient legal and regulatory environment.

Firstly, water sprinkling devices have gained market favour due to their low short-term investment cost and ease of installation. Even though water-sprinkled brakes can cause brake failure and pose road safety risks, this hasn't stopped their popularity.

Secondly, there is inconsistent law enforcement, with some regulations not strictly applied. For example, in northern regions, water sprinkling is banned in winter but overlooked in other seasons, while in the southwest, sprinkling brakes is even used by traffic police to judge vehicle safety.

Next, the 'good currency' is weak. Currently, hydraulic retarders don't have significant advantages in cost or technology and can't yet meet the requirements for widespread use.

Two points roughly show the current application status of hydraulic retarders: firstly, GB7258-2017 (including GB7258-2012) states that auxiliary braking devices should 'be equipped with a retarder or other auxiliary braking device', indicating that a retarder is not mandatory.

Secondly, during the 2016 Two Sessions, Fast Chief Skills Trainer Cao Jing suggested "making retarder installation mandatory for commercial vehicles". The Ministry of Industry and Information Technology's response pointed out: efforts should be made to include retarder products with independent intellectual property rights in the "Guidance Catalogue for the Promotion and Application of Major Technical Equipment in Commercial Vehicles". Clearly, national departments attach great importance to the promotion and application of retarders, but it is also evident that large-scale promotion and application will take some time.

3. Future Prospects:

The performance characteristics of auxiliary brakes align with the future logistics market's comprehensive requirements for timeliness and safety.

A single type of auxiliary brake might slightly struggle to meet future demand, so combining various auxiliary braking methods seems like an ideal solution, making full use of the advantages of both power unit brakes and transmission system brakes to cope with different working scenarios.

However, the current penetration rate of hydraulic retarders is quite low, and multi-party cooperation is still needed to promote their application.

(1) Technological Innovation: Domestically produced hydraulic retarders have already emerged, with brands like Fast and Huasheng launching their own products on the market.

But compared with experienced imported brands, the performance of domestic products still needs to be tested by the market. Domestic OEMs and parts manufacturers should strengthen collaboration, improve vehicle compatibility, continue technological innovation, expand product range, and enhance the cost-effectiveness of hydraulic retarders to make customers willing to buy. (2) Market guidance: The national level can introduce relevant policies to guide the market. For example, there could be appropriate subsidies or policy preferences for purchasing hydraulic retarders in the southwest region, guiding user consumption, raising awareness, and expanding the influence of hydraulic retarders; the insurance industry, vehicle management offices, and other departments should acknowledge the legality of retrofitted hydraulic retarders, allowing them to enjoy normal insurance and annual inspections. (3) Laws and regulations: Accelerate relevant research and verification work, clarify auxiliary brake configuration requirements, and set unified enforcement standards for illegal devices.