eli5 Why do trucks and busses do the “tsshhh” thing when they stop?
In: Engineering
1. When the driver presses the brake pedal, compressed air is supplied to the brakes which is what makes the truck stop. When the driver releases the pedal after stopping (even if they just release it partially while still holding it down a bit) the air gets released under the truck making a “tsshhh” sound.
2. If the driver going to be stopped for a while they might want to apply the park brake. The park brake works the opposite way, it uses air pressure to release and springs to apply. So normally when the truck is driving, there is air pressure in the park brake system holding it off.
When the driver applies the park brake, it releases the air from the park brake system. This makes an even bigger “tsshhh” noise than just releasing the brake pedal.
3. Then there is air dryer purge. Trucks have an air compressor driven by the engine. When the air pressure in the tanks is too low the compressor turns on, when the pressure reaches the maximum amount it turns off.
But to prevent all the moisture in the air from filling the air tanks with water, most trucks have an air dryer fitted which absorbs moisture from the air as it passes through after the compressor but before the tanks. Each time the compressor finishes topping the air tanks up, a valve opens underneath the air dryer and high pressure air is blown though it, spraying moist air under the truck and drying the air dryer ready for the next cycle.
This makes a big “tsshhh” and can happen any time the engine is running. You might hear it when the truck is sitting still or when it is driving.
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