Listening to trucks entering a downhill slope and the engine noise is really loud and I can feel it in my chest – it seems louder than when they are accelerating.?
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Yup. That seems right. When an engine is providing power, it wants to get the most energy out of the fuel, so the exhaust valve will open when the piston is at it’s lowest point (and therefore lowest pressure on that particular stroke).
When engines use engine braking, the car lets the wheels turning actually provide rotation to the engine, which closes the valves at the bottom of each stroke, compress the air that, then let it out when it’s at the highest pressure. That slows the vehicle down a little bit in the process…but is loud.
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