Reciprocating Steam Engine Cycles

Dr. W. Hallett

The classical steam engine, as exemplified by the locomotives, marine engines, and small industrial power plants of 50 years ago, is usually considered to be obsolete technology. Its main disadvantage, when compared with modern Diesel and gas turbine engines or present day steam turbine power plants, is a low thermal efficiency. However, it has some advantages which cause periodic revivals of interest:

• It can deliver torque at zero speed and produce power with reasonable efficiency over a large range of speeds. Hence, unlike an internal combustion engine, it can drive a vehicle directly without the need for a clutch or gearbox, and unlike a turbine, it is not restricted to a narrow range of optimum speeds. It can also be reversed without a gearbox.
• It is insensitive to fuel, and can be used to generate power from fuels that are unsuitable for internal combustion or gas turbine engines (eg biomass, coal, waste). In some industries these fuels are virtually free (eg wood wastes in pulp mills, straw in agriculture).
• Because combustion occurs in steady state in a boiler furnace, it can be controlled so as to produce much lower pollutant emissions than are practicable in an internal combustion engine. The State of California, the US leader in air pollution legislation, recently declared Diesel soot to be a toxic substance, a move which will revive interest in non-IC prime movers.
• It is less demanding of high tolerances and care in manufacturing and maintenance than other engines, and can be lower in capital cost. Hence it remains of interest in parts of the world where maintenance labour is unskilled.

These considerations have motivated two studies of reciprocating steam cycles undertaken as fourth year theses at the University of Ottawa, in which computer models for the boiler and for the engine cycle of classical steam locomotives were developed. The boiler model assembled standard heat transfer correlations into a program capable of predicting the heat transfer efficiency of a locomotive-type (firetube) boiler. The engine model is a detailed numerical simulation of the thermodynamic cycle - admission, cut-off, expansion, release, exhaust, and compression - and includes simple models of valve gears and valve events as well as the important effects of valve port pressure loss (so-called "wire-drawing"). Although the main focus of these projects was historical, the work provides insight into the main losses of the classical cycle, and gives a starting point for exploring the real potential of a reciprocating engine. For a locomotive, the main sources of inefficiency are:

• combustion losses - under extreme conditions, up to half of the coal fired could be elutriated from the firebed as cinder;
• boiler heat transfer losses - the efficiency of a classical locomotive boiler could be as low as 50%;
• poor control of valve events (admission,cutoff, and exhaust);
• condensation losses in the cylinder;
• back pressure in the exhaust nozzle.

All of these could be improved on substantially with modern design and controls.

Canadian Pacific SM class 4-6-0 of 1889 - one of the locomotives simulated in the engine cycle study

Fourth Year Theses

Gillard, Sandrine (INSA Lyon): Simulation/Improvement of the reciprocating steam cycle. Projet fin des études, INSA Lyon/Université d'Ottawa, 2000.

Murphy, Kathleen: Two FORTRAN Models for a single-acting reciprocating steam engine. BASc Thesis, Mechanical Engineering, University of Ottawa, 1999.

Glover, Lisa: A steam locomotive boiler analysis. BASc Thesis, Mechanical Engineering, University of Ottawa, 1995.

New York Central J1b class 4-6-4 of 1927 - simulated in the boiler and in the engine cycle studies

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Last update 21 August 2000