page_title: High-speed steam engine
section_title: Defining characteristics
hierarchical_section_title: High-speed steam engine / Defining characteristics
caption_attribution_description: Racine high-speed steam engine (New Catechism of the Steam Engine, 1904).jpg
context_page_description: 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.
context_section_description: They have two primary characteristics:
  High speed.
  This is sufficient to drive a small dynamo directly, rather than needing a step-up drive by belts.
  Accurate speed regulation.
  Generation by dynamo requires a stable rotation speed for a stable output voltage, even when the load changes. When an alternator was being driven, the output frequency also depended upon a stable rotation speed.
  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:
  Improved lubrication, as required by their high speed.
  This often used an enclosed crankcase with an oil sump and lubrication by 'splash' or by ring oilers. Some went so far as to have engine-driven oil pumps and a circulation system.
  Small, rigid cast iron frames, so that they did not require the large stationary engine's usual complex masonry foundations.
  Single-acting cylinders.
  As the crankcase became more important for lubrication, the layout was simpler if steam pressure was only applied to one side of the cylinder. This has a second advantage: as the steam's force is now only on one side of the piston, the force on the bearings will still vary through the stroke, but it no longer reverses its direction. This reduces the effects of any bearing slop at high speed.
  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 contemporaneous with sophisticated and efficient valvegears such as the Corliss, these trip valves were incapable of working quickly enough.
caption: Typical horizontal engine, showing the compact frame
exif: {}