High-speed steam engine

High-speed steam engines were one of the final developments of the stationary steam engine. They ran at a high speed, of several hundred rpm, which was needed by tasks such as electricity generation.

They have two primary characteristics:

These also resulted in a number of secondary characteristics. Although these were not defining to the type, or were always the case, they were recognisably common:

High speed was not needed for electrical power generation in the largest citywide plants. As these plants were necessarily large, they could also use large-diameter dynamos with many pole pieces. This gave the necessary linear speed (in poles passed / time) for a lower rotational shaft speed.

These engines were produced with either simple or compound operating cycles. Smaller examples were usually simple, as the difficulties of achieving good regulation outweighed the efficiencies of compounding. High-speed engines did develop a reputation for profligacy. For larger engines the fuel cost savings were worthwhile and compound designs such as the Willans engine were used.

They also used a wide range of valves. Examples with either slide or piston valves were common. Multi-cylinder single-acting engines typically shared one piston valve between two cylinders, either between the cylinders or horizontally above them.

The valvegear driving these valves was usually simple, a single eccentric designed only to run at one speed, in one direction, for a fairly constant load. Although these engines were contemparaneous with sophisticated and efficient valvegears such as the Corliss, these trip valves were incapable of working quickly enough.

A key requirement for the high-speed steam engine was accurate control of a constant speed, even under a rapidly changing load. Although the control of steam engines via a centrifugal governor dates back to Watt, this control was inadequate. These early governors operated a throttle valve to control the flow of steam to the engine. This gives an inadequately responsive control for the constant speed needed for electricity generation.

The solution developed for high-speed steam engines was the "automatic" governor. Rather than controlling the flow rate of steam, it controlled the timing or 'cut-off' of the inlet valves. This governor was interspersed between the crankshaft and the eccentric driving the valve gear. It was often made as part of the engine's flywheel. A centrifugal bob weight in the governor moved out against a spring with increasing speed. This caused the eccentric's position to shift relative to the crank, changing the valve timing and causing an early cut-off. As this control acted directly at the cylinder port, rather than through a long pipe from a throttle valve, it could be very fast-acting.

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