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How to adjust the air pressure in the air tank? There are specific considerations for high and low truck air pressure
On the chassis of medium and heavy trucks, besides the engine, transmission, and axle, one of the most commonly seen components is a large number of pipelines, solenoid valves, and air tanks. For most truck drivers, air tanks are very familiar and simple components, but the functions they are involved in are far from trivial. These functions include the braking system, trailer brake system, clutch booster pump, air horn, airbag adjustment, and many other air-driven configurations.
It is clear that the importance of air tanks themselves is self-evident. For truck drivers, besides regularly checking and replacing the dryer, if one modifies certain configurations or encounters issues such as slow or overly sensitive brake system response, or slow inflation of airbag seats, how should one manually adjust the air pressure inside the air tank? Today, let's take a closer look at this.
The air source used by a typical truck is supplied by an air compressor that draws in external air, passes it through a drying tank, and delivers it to the APU air handling unit. The APU air handling unit then reduces the pressure through a four-circuit protection valve and distributes the air to various parts of the vehicle. The four-circuit protection valve has one inlet and four outlets, each supplying air to different areas of the truck that require it.
Taking the WABCO four-circuit protection valve on the Sinotruk T7H model as an example, the thickest pipeline without a labeled size is the air inlet from the air dryer, responsible for the total output pressure of the vehicle's air tanks. Port 21 is the outlet, responsible for supplying brake air pressure to the rear axle of the tractor, while port 22 is the outlet responsible for supplying brake air pressure to the front axle steering shaft. Ports 21 and 22 each have their own set of diaphragms, valves, adjustment springs, and other components. Additionally, the thicker pipelines ensure more reliable output air pressure.
The No. 23 air outlet is the air supply port for components such as the trailer and handbrake, while the No. 24 air outlet is for auxiliary air pressure, responsible for delivering air pressure to vehicle components other than the brakes, such as the air horn, clutch booster pump, transmission shift cylinder, exhaust butterfly valve, and the four-point suspension airbags of the cab. Finally, the air source is delivered through different pipelines to various air tanks for storage and supplied to different components.
After understanding the basic principles, we can know that, except for some special vehicle models, the APU air handling unit in most trucks serves as the 'commander' for the truck's air system. It is responsible for regulating the total intake pressure of the air tanks and distributing it to different pipelines, making its importance comparable to that of the dryer and air tanks. Therefore, we need to pay attention to the following issues when making adjustments.
On the APU air handling unit of the Sinotruk Howo T7H, we can clearly see the air inlet and four air outlets. The adjustment method is fairly simple; you can use a hex key to adjust the tightness of the connections between the pipes and the valve body. However, at the current stage when the vehicle is leaving the factory, and before any modifications are made by the truck owners, this is generally calibrated by the factory engineers. If there are no abnormal issues, no adjustment is needed.
Assuming our truck is in normal condition, blindly lowering the total pressure can lead to unimaginable consequences, such as the brake air pressure dropping very quickly, feeling weak brakes after just a couple of presses, the pressure in the air tank decreasing sharply, which worsens the braking effect, and a series of components controlled by the auxiliary air pressure also experiencing various operational issues.
Blindly increasing the total pressure can make the truck's brakes overly sensitive and cause the pressure values inside the air tanks and air circuits to spike. It may also lead to the failure of solenoid valves and pipelines, preventing the truck from shifting gears, and causing components like the clutch booster pump and differential lock to malfunction. Therefore, under normal conditions, there is no need to adjust the air tank pressure on a truck independently; such adjustments should only be carried out under the guidance of a service station. Blind adjustments can create serious safety hazards.
Of course, if any truck drivers experience low brake air pressure, poor braking performance, or encounter low internal pressure in the brake air reservoir during normal driving, and don't have time to go to a service station, the brake air pressure can be appropriately increased. The adjustment method is also very simple. Locate the vehicle's APU air handling unit, then use an Allen wrench to insert into the intake, and turn clockwise one or two turns to increase the pressure by one or two units.
At the same time, some vehicle models' APU air handling units have intake ports with adjustment screws, and tightening them one turn clockwise can also increase the pressure by 1 bar. It should be noted that after increasing the overall vehicle pressure, we need to reduce the pressure at auxiliary air outlets; otherwise, excessively high total pressure entering the auxiliary air tanks can also cause the aforementioned issues. Simply increasing the pressure blindly will not solve all problems.
Poor braking performance is not necessarily entirely caused by air pressure issues. Components of the brake system, such as the wheel cylinders, master cylinder, valve body, as well as brake drums and brake shoes, can all affect the braking performance of a truck. In cases of overloaded transport, even if the air pressure is stable and all configurations are normal, excessive vehicle weight can still lead to reduced braking efficiency.
● Editor's Note
From our understanding, many high-end domestic tractor trucks nowadays have already implemented electronically controlled intelligent APU air processing units. For instance, the APU air processing unit assemblies used in the Jiefang J7 and Eagle series allow the vehicle's VCU to monitor air tank pressure in real time and, through the APU control valve, control whether the air pump works or stops, achieving air filling or not. This not only saves energy and reduces noise, but also eliminates the need for drivers to manually adjust intake pressure.
At the same time, truck owners who modify or add certain components need to constantly monitor the condition of the air system, air tanks, and air compressor. Increasing the air pressure will also increase the stress and working pressure on these components. Any modifications should be done under the premise of safety, and adjustments to air systems and pressure should be performed with professional assistance. Blind operation must be avoided.
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