Cylinders in a normal two cylinder engine are double acting, that is steam is admitted alternately on each side of the piston and the connecting and coupling rods are set at ninety degrees to each other so that one cylinder is exerting its maximum thrust when the other is at the end of it stroke. This results in four puffs per minute. It also results in the engine being unbalanced with pushes and pulls on opposite sides of the engine alternately. A three cylinder engine has the cranks set at one hundred and twenty degrees, so it has six puffs per revolution and is also better balanced. A four cylinder engine would normally have the cranks at ninety degrees so that the engine is completely balanced and it would have four puffs per revolution but there have been engines where the cranks have been set so that there are eight puffs per revolution. Presumably the idea there being to give a much more even turning force with a thrust from the cylinders every forty five degrees.

Bob Milne.

To complete what I said about the number of chuffs per rotation of the driving wheel. In the case of two cylinder compound locomotives, for example the numerous Austrian engines designed by Golsdorf before the First World War, or a three cylinder compound with a single low pressure cylinder, there will only be two chuffs per revolution as that would be the only cylinder feeding the exhaust through the chimney.

Bob Milne.