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Why do two-stroke gasoline engines often create more noise and air pollution?

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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.

Two-stroke gasoline engines are often associated with higher levels of noise and air pollution primarily due to their design and the process of loop scavenging.1

In a two-stroke engine, the intake and exhaust processes occur simultaneously near the end of the power stroke and the beginning of the compression stroke. Because the exhaust ports remain open while the fresh fuel mixture is being introduced, some of the unburned fuel mixture can escape directly through the exhaust port before combustion occurs. This loss of fresh charge not only reduces fuel economy but also contributes to increased emissions.1

Furthermore, the simplified design of many small two-stroke engines—which often rely on crankcase compression—lacks the more precise, positive clearing of exhaust gases found in the four-stroke cycle. This less efficient scavenging process, combined with the frequent power strokes, results in higher noise levels and significant carbon emissions, leading some regions to restrict or ban their use in certain devices.12