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The primary sources of the characteristic sound of an internal combustion engine are:
First of all combustion in an internal combustion engine is not explosions; it is rapid controlled-rate combustion. So actual combustion noise is not one of the top sources of engine sound.
1) The primary sound of an engine is actually the sound of the sudden pressure release of the the exhaust valves when each exhaust valve opens releasing the residual pressure in each cylinder. The residual pressure is the pressure left in the cylinder just before the exhaust valve opens (after the power stroke). When an exhaust valve opens, it opens very rapidly and sends a pressure wave down the exhaust port and through the exhaust manifold and eventually out the exhaust system.
2) The other major source of engine sound is the mechanical sound of each valve closing. When valves close, they hit the valve seat at a significant velocity, and this impact speed rises with engine speed. The valves closing is the only place in an engine where 2 pieces of unlubricated metal actually collide with each other.
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