Why do 2-cylinder steam locos puff louder on every 4th puff?

Two-cylinder steam locomotives often produce a louder puff of steam on every fourth puff due to the timing of their pistons. In these engines, one cylinder typically exhausts steam at a different interval than the other, leading to a periodic buildup of pressure that results in a more forceful release of steam. This phenomenon, known as "puffing," occurs because the exhaust from one cylinder coincides with the timing of the other, creating a more pronounced sound every fourth cycle. This rhythmic pattern is characteristic of the locomotive's design and contributes to its distinct auditory signature.