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Understand in three minutes: What experts mean by 'air-top-up fuel'
Air brakes and hydraulic brakes are not unfamiliar to us, and there is also a type of brake system called air-over-oil, which was commonly found on Japanese trucks years ago. Air-over-oil brakes, also known as air-assisted hydraulic brakes or pneumatic-hydraulic braking systems, utilize both air pressure and hydraulic mechanisms.
There are three types of conventional power braking systems: pneumatic braking systems, pneumatic-hydraulic braking systems, and fully hydraulic power braking systems. The pneumatic-hydraulic braking system uses a combination of air pressure and hydraulic pressure, and can also be seen as a braking system where air brakes and oil brakes are connected in series.
The Mitsubishi T850 uses an air-over-hydraulic braking system.
Similar to air brakes, the front part of the air-over-hydraulic system, which serves as the power source, also uses an air pressure system. However, after the brake air chamber, a hydraulic system is added (this hydraulic system is also similar to a hydraulic brake system) to act as the brake actuator.
The brake chamber and hydraulic master cylinder are connected in series. By converting pneumatic energy into hydraulic energy, the brake fluid is delivered to each wheel cylinder, thereby producing a braking effect. Compared with pure air brakes, the brake chamber in a pneumatic-hydraulic (air-over-hydraulic) system is smaller in size and more compact in arrangement. Additionally, the air-over-hydraulic brake system has the following advantages:
1. The pneumatic system has shorter pipeline lengths, which can reduce lag time;
2. The hydraulic wheel cylinders as actuators are lightweight;
3. When towing a trailer, the trailer can use either pneumatic brakes or hydraulic brakes.
According to forum users' experiences, air-over-oil brakes do have good braking power and respond quickly. They are not as abruptly strong as pure air brakes, but they also have drawbacks. High temperatures on long downhill slopes can reduce the performance of the brake fluid, and the seals in the hydraulic wheel cylinders (slave cylinders) can melt or even fall off, causing brake fluid to leak and splash, making it feel like the braking power suddenly disappears (ultimately, the quality of these rubber seals is indeed a major issue). In addition, long-term maintenance and disassembling the brake drums are particularly troublesome, so in terms of reliability, they are ultimately inferior to pure air brakes with a relatively simple structure.
● Editor's Note:
Nowadays, aside from vehicles like loaders and dump trucks that require hydraulic systems, it is increasingly rare to see air-over-hydraulic brake systems. In addition, a certain type of clutch assist on trucks is often referred to as 'air-over-oil,' but this term is actually inaccurate. The clutch system uses a slave cylinder that is assisted by air pressure, which operates on a principle completely different from air-over-hydraulic brakes. Next, the author will also introduce this type of clutch assist pump.
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