I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?
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Under normal operation you let fuel into the cylinders which burns pushing the cylinders down and eventually using that energy to turn the wheels.
When you are off throttle no fuel should be going into the engine thus it is providing no energy. Instead the energy the truck has from moving is used to move the cylinder up and down against friction and also compressing any air in their providing a braking effect. The faster the cylinder is turning the more firction and the more braking their is.
Big trucks can take this even further with a J brake which lets the compressed air out at max compression making the noise you hear, this stops the air acting as a spring and returning some of the energy once compressed.
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