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28. Steam piston engine

Author: Jiří Škorpík twitter, skorpik@fme.vutbr.cz : updated 2012-04

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.

Cross section of Spilling steam piston engine. Spilling steam piston engine - two-cylinder design. Spilling steam piston engine - two-cylinder design.
1.822 Spilling steam piston engine.
left cross section of Spilling steam piston engine [5, p. 1010]; right Spilling steam piston engine - two-cylinder design* [photo: author's archive]. The Spilling steam piston engine is designed as modular machine. The fundamental modul of this engine is one-cylinder double-acting engine with single cylinder slide valve. The fundamental moduls be can connected through common shaft and also through steam flow (steam expansion can be distributed until three cylinders with different diametrs). Through a connections of the fundamental modules (maximum 6 cylinders) be can built an engine about 100 to 1500 kW at pressure admission steam from 0,6 to 6 MPa.
*Remark
This steam piston engine powers an electric generator with 1000 rpm at Slavonice sawmill (Czech republic), where was installed at end of 20th century. The steam expansion is distributed between two cylinders. The conection between these cylinders is good visible (the pipe nearer to the electric generator). The admision steam for first cylinder is distributed through outer surface of the slide valve, at case of second cylinder is distributed through internal surface of the slide valve. On the photo on right are good visible drain valves for up and down volumes of the cylinders.

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.

Using of steam piston engine

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. [2] and Tenza, a.s. [3]*.

*Remark
Primary use this engine is for a realization of a combined cycle with a combustion engine. It means the engine use heat from exhaust of the combustion engine. The realization this cycle is similar as CCGT (combine cycle gas turbine), where is a combination combustion turbine and steam turbine.
Steam engine PM-VS.
2.823 Steam engine PM-VS.
The engine is built on the block of a diesel engine, where the pistons are used as a crosshead bearings of the piston rods of the steam section. The steam cylinders are fixed instead of the original engine head. The flow of steam is controlled by rotary valves. The manufacturer offers three class of these engine about power from 10 to 75 kW. The manufacturer PolyComp, a.s. Photo [2].

The steam engine with power outputs up to 10 kW are used for various purposes and also for domestic cogeneration:

Household cogeneration unit with steam piston engine. 3.825 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: [4].
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Description and principle of operation

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:

Work cycle of the steam piston engine.
4.477 Work cycle of the steam piston engine.
R distribution unit of steam; V cylinder; P piston and a piston rod. 1 enter of steam into distribution unit; 2 enter of steam into the cylinder from distribution unit; 3 exhaust of steam from distribution unit. (a) admission; (b) expansion; (c) exhaust; (d) compression.

The work cycle of the steam piston engine has four events:

The admission
High pressure steam flows through the distribution unit into the cylinder, Figure 4a (the enter of steam into the cylinder 2 is open, the exhaust of steam 3 from the distribution unit is closed.
The expansion
Steam expands inside the cylinder (decreasing of pressure and increasing of volume), Figure 4b (the enter of steam into the cylinder 2 is closed).
The exhaust
Low pressure steam flows through the distribution unit from the cylinder to the exhaust, Figure 4c (the enter of steam in the distribution unit 1 is closed).
The compression
Compression of Steam inside the cylinder, Figure 4d (the exhaust from the cylinder 2 is closed).

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.

Types of steam piston engines

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:

Simple cross section of double-acting steam piston engine. 5.78 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:

Simple schema uniflow steam engine. 6.252 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):

Double expansion steam piston engine. 7.488 A double expansion steam piston engine.
The exhaust from the first cylinder is the admission of the second cylinder.

Fundamentals of design

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):

Distribution units of steam piston engine. 8.489 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).

Prototype of Tenza PPM-054-10 steam piston engine. 9.490 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.

Energy flows

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:

Aproximately energy flows through small steam piston engine with one slide valve. 10.889 Aproximately energy flows through small steam piston engine with one slide valve.10.889 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.

References

  1. Spilling Energie Systeme GmbH, 2012. Výrobce a dodavatel pístových parních motorů. Adresa: Werftstraße 5, 20457 Hamburg, Deutschland, http://www.spilling.de.
  2. PolyComp, a.s., 2012. Výrobce a dodavatel pístových parních motorů. Adresa: Na Hrázce 22, 290 01 Poděbrady VIII, Česká republika, http://www.polycomp.cz.
  3. Tenza, a.s., 2012. Vývoj a výroba prototypů pístových parních motorů. Adresa: Svatopetrská 7, 617 00, Brno, Česká republika, http://www.tenza.cz.
  4. OTAG Vertriebs GmbH&Co.KG, 2012. Výrobce kogenerační jednotky do domácnosti lion – powerblock. Adresa: Zur Hammerbrücke 9, D-59939 Olsberg, Deutschland, http://www.otag.de.
  5. MILLER, Rudolf, HOCHRAINER, A., LÖHNER, K., PETERMANN, H. Energietechnik und Kraftmaschinen, 1972. Hamburg: Rowohlt taschenbuch verlag GmbH, ISBN 3-499-19042-7.

Citation this article

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/28.html. English version: Steam piston engine. Web: http://www.transformacni-technologie.cz/en_28.html.

©Jiří Škorpík, LICENCE
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