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Are the structures of ABS air brakes and hydraulic brakes the same?

Nowadays, ABS has become a standard feature on trucks. However, before this, many micro trucks and light trucks were not equipped with ABS anti-lock braking systems. When emergency braking is applied, it is easy for the wheels to lock, leading to situations such as the inability to stop, loss of steering control of the front wheels, or fishtailing of the rear wheels, which is especially dangerous on wet and slippery roads during rain or snow, posing significant safety risks.

However, in GB12676-2014 and GB7258-2017 "Technical Conditions for Motor Vehicle Operation Safety," it is required that all N2 and N1 category vehicles must be equipped with ABS anti-lock devices. According to the type of braking, there are generally air brakes and hydraulic brakes, and ABS is also divided into pneumatic ABS and hydraulic ABS. So what is the difference between them? And how do they work? What should we pay attention to in daily use?

▎ What is the principle of ABS braking and what are its advantages?

Whether it is pneumatic ABS or hydraulic ABS, their working principles are actually the same. They are based on the conventional braking system and add an electronic control system, mainly composed of sensors, brake pressure regulators, ABS warning lights, ABS electronic control unit (ECU), and other devices.

When the driver brakes in an emergency or on a slippery road, the ABS system can accurately detect the slip of the front and rear wheels. Through the ABS electronic control unit (ECU), it performs analysis, calculation, and control, timely adjusting the brake wheel cylinder (brake chamber) pressure, enhancing longitudinal and lateral adhesion during braking, preventing wheel lock-up, thereby achieving better directional stability, steering control, and shorter braking distance.

在常规制动时，虽然ABS没有介入，但整个系统随时监控轮速传感器信号，以判断是否需要启动防抱死制动。当车辆的任意车轮接近抱死时，制动压力调节器会根据ECU的指令，调节该车轮制动分泵的制动液（液压ABS）或制动气室的制动压力（气压ABS），以保持最佳压力状态，从而实现防抱死。

▎液压ABS和气压ABS的区别在哪里？

After discussing the similarities, let's take a look at their differences. The biggest difference between hydraulic ABS and pneumatic ABS lies in the braking medium. Hydraulic ABS uses brake fluid, and the system is equipped with an oil pump to circulate and control the brake fluid, preventing its loss.

The working medium of pneumatic ABS is air. Air is basically endless and does not require a separate circulation system. The compressed air after work is generally released directly. We often hear the sound of the air brake, which is the sound after the system releases the air.

Therefore, in terms of structure, air brake ABS is not only simpler than hydraulic ABS, but also has a lower failure rate. Another difference is that hydraulic ABS usually has only one brake pressure regulator, whereas air ABS requires multiple brake pressure regulators, generally installed on both the front and rear wheels.

ABS and the conventional braking system form a single integrated unit; the two cannot be separated. Once there is a problem with the conventional system, the ABS will not function properly. Therefore, during maintenance, both systems need to be serviced together. When a braking system malfunction occurs, in addition to checking the sensors, electronic controllers, and brake pressure regulators, it is also necessary to determine whether the fault lies in the conventional braking system or the ABS. Many vehicle owners tend to service them separately, which is not advisable.

▎ Emergency braking: Experience is even more important

During emergency braking, truck drivers most frequently ask about whether to stomp the brake pedal all the way or to pump it twice when using ABS. In fact, this depends on factors such as the load situation, road conditions, and the overall weight of the vehicle. In particular, trailers require real-time adjustments to brake pressure according to the situation on site. There is no standard answer here; excessive braking or poor matching between the tractor and trailer can easily lead to jackknifing or tipping during turns.

In past conversations among truck drivers, some believed that the stopping distance would be longer once ABS engaged. From a technical standpoint, when the tires lock up, they slide on the road surface, resulting in lower braking force and a longer sliding distance. With the ABS system active, it rapidly adjusts the brake force to keep the wheels between locking and rolling. This allows the brake disc to dominate braking, maximizing the reduction of stopping distance.

In many past cases, in order to avoid vehicle nose-diving during braking, many tractor trucks directly removed the drum brakes, including devices such as ABS and wheel speed sensors. Looking at such practices today, they are simply ridiculous. Nowadays, after proper calibration, the primary tractor-trailer brake gaps generally don’t have major issues. Of course, even if there are problems, they should be adjusted at a professional service station, rather than recklessly removing the front brakes, which only increases safety risks.

● Editorial Note:

In fact, hydraulic ABS and pneumatic ABS correspond to hydraulic brakes and air brakes, respectively. Regardless of which type of ABS it is, we must master the correct way to operate it. Moreover, ABS has become a mandatory regulatory requirement. A high-quality ABS system not only improves braking performance and driving safety, but also helps prevent abnormal tire wear.

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