The International Steam Pages

Researching an Air-Steam Combined-cycle Locomotive

Harry Valentine, Transportation Researcher, writes:

As the era of the classical steam locomotive was drawing to a close, new developments occurred in combined-cycle marine propulsion. Modern advances in materials technology and fuel combustion can enable combined-cycle propulsion to be applied to railway traction. One concept may combine an external-combustion air turbine engine with steam propulsion. 

The Air Turbine:

Externally heated air turbine engines can burn fuels that would otherwise be inappropriate for use in internal combustion engines. Such fuels would include a range of low-cost solid fuels such as coal and biomass as well as the liquid low-rank coal-water fuel. Heat is transferred from the combusted fuel into the air turbine engine by means of heat exchangers. The traditional design of heat exchangers and the material from which they were made restricted the peak operating temperature and in turn restricted the engine efficiency.

Modern material such as silicon carbide and silicon nitride can operate up to 1400-degrees C (2500-degrees F) without adverse effects. This material can be used as heat exchanger elements in the rotating Ljungstrom design of heat exchanger, a design that is used as the exhaust-heat recouperator on some gas turbine engines. It may also be used on an externally heated air turbine engine that uses a pressure ratio of 4 to 1 for the compressor and 3.8 to 1 for the turbine. The rotating heat exchanger in this application will need to be equipped with several rows of ceramic heat-exchanger elements as a means to reduce the thermal shock caused by alternating hot and cool streams of air passing by the elements in opposite directions.

The hot flame that flows in one direction will only come in contact with the heat exchange elements and not the turbine blades. Air being accelerated by the compressor and flowing in the opposite direction toward the turbine will pass through the rotating heat exchanger and absorb some 90% of the heat. This heat will spin the turbine, after which this heated air will pass into the combustion chamber and support the hot flame. To ensure acceptable thermal efficiency when the engine works at part-load, a turboshaft-driven part-load fan will supply additional air directly into the combustion chamber (this air will be preheated by engine exhaust heat). 

Generating Steam:

The cooled air leaving the rotating heat exchanger will still be hot enough to convert water to saturated steam. It may leave the air turbine and be ducted through a diffuser of increasing cross-section so as to reduce air velocity and marginally raise air temperature (by up to 100-degrees F) prior to entering the tubes of a firetube boiler. Additional heat from a superheater will convert the saturated steam into superheated steam. The table below shows combustion (flame) temperature, turbine inlet and outlet temperature, turbine efficiency and the exhaust temperature:


 T.turbine in

 T.turbine out



1300-deg F

 1245-deg F

 785-deg F


 508-deg F

1400-deg F

 1341-deg F

 855-deg F


 524-deg F

1600-deg F

 1534-deg F

 996-deg F


 554-deg F

2000-deg F

 1919-deg F

 1278-deg F


 618-deg F

2200-deg F

 2112-deg F

 1418-deg F


 648-deg F

2500-deg F

 2400-deg F

 1629-deg F


 676-deg F 

The highest combustion temperature would be representative of anthracite coal while the lowest combustion temperature represents the gasified high moisture content biomass. It has a high enough turbine exhaust temperature to just barely convert water to saturate steam in a 250-psia boiler if the rate of heat transfer from turbine exhaust to water is 75%. For firetube locomotive operation, the boiler may operate between 240-psia and 300-psia pressure. A superheater could raise the steam temperature to 800-deg F prior to it being expanded in a rotary steam engine (Rand Cam, Quasiturbine or compounded Star Rotor) that would drive into the turboshaft via planetary gearing and a one-way clutch, in turn driving electrical gear.

It is expected that up to 70% of the air turbine engine exhaust heat could be transferred into the boiler and provide most (if not all) of heat required to raise saturated steam. This heat could account for about 50% of the total thermal energy needed to raise superheated steam at 300-psia at 800-deg F needed to operate a steam engine at over 20%-efficiency. If the air turbine operates at 25% engine efficiency, the combined-cycle air plus steam system efficiency could rival that of a diesel engine while burning a much lower-costing fuel (coal, biomass etc). The steam engine could deliver 250-BTU/lb of work at an exhaust pressure of 21-psia (200-BTU/lb). The thermal energy would be 1450-BTU/lb with a superheater input of 1050-BTU/lb. [(250/(1050-200)) = 29% efficiency for the steam engine and a combined engine thermal efficiency of some 40%].

A combined-cycle locomotive intended for long-haul service would need to use condensing equipment such as counter-flow and/or parallel-flow heat exchangers mounted on the sides of the locomotive (or its tender unit). The steam engine could deliver 2500-Hp while the addition of a heat pump that transfers heat from exhaust steam to the preheater may allow power to be raised to as high as 4500-Hp. A 3000-Hp air turbine engine operating at 30% thermal efficiency would transfer up to 7000-Hp of heat energy to the firetube boiler, 5000-Hp of which would generate saturated steam. Superheating to the order of 5000-Hp to 10000-Hp could allow a steam engine to deliver 2000-Hp to 3000-Hp.

Renewable Thermal Energy:

Instead of burning fuel, the engines in a combined-cycle locomotive may also be energised by stored thermal heat. Certain types of thermo-chemical reactions are renewable and allow for certain materials to be carried on-board. One such material is calcium carbonate (CaCO3) that can be heated and decomposed into calcium oxide (CaO) and carbon dioxide (CO2). The carbon dioxide may be carried in a separate tank or be combined with a low-temperature carbonate that can be decomposed be engine system exhaust heat. Calcium oxide reacts with carbon dioxide at a pressure of 5-atm and producing a temperature of 900-deg C to 1000-deg C (1650-deg F to 1830-deg F) with a heat of reaction between 168-BTU/lb to 239-BTU/lb. In some parts of the world, concentrated solar energy may become a source of energy by which railway locomotives may operate.

Internal Combustion Turbine and Steam Engine:

Ongoing research and development in solid fuel gasifiers can allow such units to operate with gas turbine engines. In such engine layouts, the ash from the gasifiers is transferred into an ash receptacle by a screw mechanism. Only the combustible gases would be burnt in the combustion system of the (single-shaft, non-reheat) gas turbine engine. The disadvantage of this turbine is that it will only deliver peak thermal efficiency at maximum power output. However, the boiler will use the turbine exhaust to raise saturated steam that may be subsequently superheated prior to expansion in a steam engine. This combined-cycle could deliver competitive thermal efficiency over a range of power that exceeds the maximum output of the turbine.


The combined-cycle system of an external-combustion air turbine and steam engine would be able to return competitive thermal efficiency over a wide range of power output. This would enhance its versatility in railway operation and exceed that of the combined-cycle of the internal-combustion turbine and steam engine.

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Rob Dickinson