What happens during the exhaust stroke of a gasoline engine?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

During the exhaust stroke of a four-stroke cycle gasoline engine, the piston rises within the cylinder while the exhaust valve is held open. This upward movement of the piston forces the spent products of combustion out of the cylinder and through the exhaust system.123

To optimize engine performance, the exhaust valve typically opens slightly before the piston reaches the bottom of the previous stroke and closes slightly after it reaches the top, utilizing the inertia of the gases to assist in clearing the cylinder. Once the gases exit the cylinder, they pass through the exhaust system, which often includes catalytic converters to reduce air pollution and a muffler to suppress noise by damping pressure pulsations.23