In fact, splitting the steam expansion into three stages avoids the wide change of temperature in cylinders, and the consequent heat loss, that occurs with single expansion. As a further advantage it avoids complete expansion in a single cylinder, which is difficult to obtain. Notice that compound expansion complicates the engine operation and starting the engine may become difficult. In fact, since steam is admitted into the engine through the high pressure cylinder only, steam cannot pass to the other cylinders until the first expansion in the high pressure cylinder is completed. Thus, the very first movement is attained by the high pressure cylinder only.
Further problems arise if the crank is close to the dead point.
If this is the case, fresh steam must be directly introduced into the medium and low pressure cylinders. This practice was mandatory in steam locomotives that adopted compound expansion because, unlike boat, train service requires full power on starting. The wheels derive from a model introduced in England by William Morgan in 1830. They have floats that can turn on their axis. Each float has a crank arm that is driven by a rod connected to a ring that turns with the paddle wheel but is not concentric with it.