A steam piston engine, also steam engine, is a machine which is a part of a device for realization of steam cycle or it can be used in reducting pressure unit for reduction of steam pressure. Its work is usually outputted in form of shaft torque. The steam piston engine belongs to class of engines with external heat transfer (the heat is fed through a steam boiler or a parogenerator to the cycle and the heat is removed through a condenser or the steam expands to surroundings). In the same class is the Stirling engine, the steam turbine, the gas turbines with closed cycle etc. This property of the steam piston engine theoretically allows use many sources of the heat for its feed.
Principle of the steam piston engine is simple and easily understandable similar like its fundamental of construction. Through these properties was able its mass use at the epoch when the machinery industry was at a very low level.
The History of the steam piston engine is very long and in a way this is parallely a history of machinery industry. Currently, steam engines to 100 kW power input are being developed by several companies, because their thermodynamic efficiency is bigger at low power than steam turbines as seen in the chapter Difference between piston engine and turbomachine [11.]. In the Czech republic the steam engine are being developed by companies PolyComp, a.s.  and Tenza, a.s. *.
The steam engine with power outputs up to 10 kW are used for various purposes and also for domestic cogeneration:
|3.id825 Household cogeneration unit with steam piston engine.|
Outer dimension of the unit: 83x62x126 cm; weight 195 kg; electric power output 0,3 to 2 kW; heat power output 3 to 19 kW; efficiency of production electric and heat power is 89% (in relation to the heating value of an fuel). The unit contains a uniflow steam engine with a linear electric generator. The manufacturer: OTAG Vertriebs GmbH & Co.KG. Photo: .
Fundamental parts of the steam piston engine are the pistons with piston rod inside cylinder and steam distribution unit that drive the entering and the exhousting of steam from the cylinder. This force makes work through move of the piston. The work is distributed as a torque on the crankshaft:
The work cycle of the steam piston engine has four events:
The start respectively the end of a cycle event is called distribution point. A scatter of the distribution points in work cycle has a major impact on work of an engine and steam consumption, because they defines a duration of the events. More detailed description about the distribution points is shown the article 29. Thermodynamic design of steam piston engine and the article 30. Calculation of move and dimensions of slide valve.
The steam piston engine is usually constructed as double-acting. In this case the cycle on other side of the piston is shifted by half duration of cycle:
|5.id78 Simple cross section of double-acting steam piston engine.|
The piston is on the position where exhaust is being run above the piston and admission under the piston.
Other type of the steam piston engine is the uniflow steam engine. In this case the distribution unit only controls the admission. The exhaust is controlled through the piston which is open/close of the exhaust ports during its move. The steam is flowing only one direction and the steam exhaust is not cooling entering passages:
|6.id252 Simple schema uniflow steam engine.|
1 exhaust ports inside cylinder; 2 exhaust. The piston is on the position where the exhaust is being run above the piston and the admission under the piston.
The expansion of steam is can divided between two or more cylinders arranged in a row if it is useful. This solving is used at higher admission steam pressure, because in these cases is difucult reached require steam pressure on the end expansion through only one cylinders (piston would have to be a large stroke):
|7.id488 A double expansion steam piston engine.|
The exhaust from the first cylinder is the admission of the second cylinder.
Inside the distribution units are usually used Slide valves or valves. The slide valve moves on the surface with port of channels to the piston. These ports are alternately opened/closed by move of the slide valve. One slide valve can manage of distribution of steam simultaneously above and under piston for case double-acting engine. One cylinder can contain up to four slide valves-the more the better regulation of scatter of the distribution points. The distribution unit with valve has discontinuous movement and the valves are arranged on the ends of the channels to the piston. All working volume of the cylinder must contain one admission and one exhaust valve, because steam flows only one direction through the ports. For double-acting engine be can used four valves (with an exception of a uniflow steam engine, where are only two valves):
|8.id489 Distribution units of steam piston engine.|
(a) example of unit with slide valve-piston valve; (b) example of unit with valves. 1 enter of steam; 2 exhaust of steam.
The slide valves and valves are connected through mechanical parts with the crank shaft. These mechanical parts can be used for power control of engine (the regulation drives these mechanical parts and thus point of the events). The power control of the steam piston engines can be done also by changing of admission and exhaust steam parameters (e.g. throttling).
|9.id490 Prototype of Tenza PPM-054-10 steam piston engine.|
One cylinder double-acting engine with one slide piston. The engine feeds an generator about up to 25 kWe at its displacement 1,113 l, max steam pressure 1,6 MPa, nominal speed 1500 min-1. 1 steam entery chamber; 2 slide valve (the slide is hollow and through its center flows exhaust steam from upper volume of cylinder); 3 exhaust. The power control is done through changing of steam parameters. The prototype was built for experimental purposes focused on research use possibillities use the steam piston engine at power generation from low temperaturu heat.
An efficiency of energy transformation inside the steam piston engine is descripted by the internal tehermodynamic efficiency, that is defined as ratio between indicated work inside cylinder and ideal internal work of engine (work without any losses). This efficiency, for very good engines, is 80 to 85% (exceptionally) and it is function on type of distribution unit, speed-time of thermodynamic process, state of steam, type of mechanical parts and other losses:
|10.id889 Aproximately energy flows through small steam piston engine with one slide valve.10.id889 Approximate energy flows through small steam piston engine with one slide valve.|
Δiiz [%] enthalpy difference for case isentropic thermodynamic processes inside engine; zvlh [%] losses at condensation of steam on internal surface of engine and losses of steam through leaks; ak [%] compression work; ztd [%] internal thermodynamic losses (deviations between ideal p-V diagram and indicated p-V diagram); zm [%] mechanical losses; ai [%] internal work of engine (indicated of work in p-V diagram); am [%] indicated of work on shaft.
This document is English version of the original in Czech language: ŠKORPÍK, Jiří. Pístový parní motor (Parní stroj), Transformační technologie, 2010-06, [last updated 2012-04]. Brno: Jiří Škorpík, [on-line] pokračující zdroj, ISSN 1804-8293. Dostupné z http://www.transformacni-technologie.cz/pistovy-parni-motor-parni-stroj.html. English version: Steam piston engine. Web: http://www.transformacni-technologie.cz/en_pistovy-parni-motor-parni-stroj.html.