The International Steam Pages

A Combined Cycle Locomotive?

Harry Valentine, Transportation Researcher. writes (16th April 2001). (Harry adds, 24th September 2001 "There is a modification which can be made to the combined cycle steam locomotive concept. A US company called Ramgen builds a ramjet which turns a shaft, which in turn drives electrical generating gear."

There seems to be some modern opportunity to re-introduce the combined cycle locomotive (gas turbine plus steam) to modern service. As you may be aware, there is a new gas-turbine project underway with the Railpower group http://www.railpower. com

A combined cycle locomotive using steam could pave the way for a return of steam traction, but burning bio-fuel instead of coal.


Combined cycle power systems are exist in power stations, in the form of bottom-cycle engines which operate off the waste heat of another power generation system. The CHP (combined heat and power) power station can qualify as a combined system, since its waste heat goes to another use. In navy operation, combined power cycles usually saw the use of diesel and gas turbine, or diesel and steam turbine powering the same vessel, where waste heat from one system was used to supplement the energy input into another system. Combined power cycle was used in railway operations, in the UK in 1927, in which the Still locomotive combined steam and diesel power. The Still company was a small company which went bankrupt during the economic upheavals of the late 1920's, yet their locomotive achieved an in-service efficiency of 27-percent. The waste heat from the diesel engine (22-23-percent efficient) preheated the water prior to entering the boiler, thus raising the overall locomotive drawbar efficiency above that on any pre-1930 steam or diesel locomotive.

Periodically, the idea of using the waste heat of diesel locomotives has resurfaced, to no avail. Railway companies had developed an aversion to steam locomotives following the switch to mainline diesel power. The concept of a combined diesel and steam system on rails was not something to be mentioned to top railway management. During the 1960s, railroad management were all to aware of the high maintenance and low availability of steam traction. A successful water recirculation system had not yet been developed (oil from the pistons would get into the exhaust steam), on-board water purification was also essentially unavailable, fluidised bed combustion was still in the experimental stages, steam turbine was out of the question and direct current generators were still in widespread use on diesel locomotices. Besides, in countries where mainline diesel traction had become the predominant motive source, diesel fuel was still selling at bargain prices. When diesel fuel prices started rising after the mid-1980's, discussions revolved around electric traction in some nations.

The Contemporary Situation:

Rising fuel prices as well as environmental concerns have focused attention on alternative energy sources for the transportation industry, including railways. The price of high-horsepower fuel-cell locomotives is still quite prohibitive (US$2, 000-US$3, 000 per Kilowatt). The cost of long distance railway electrification in countries like the USA (cost of US$6-million/mile) is also prohibitive ..... the USA is also amidst a shortage of electrical generating capacity, not to mention the clearance problems of overhigh US rail cars such as triple-stacked shipping containers and tri-level car-carriers going under bridges and through tunnels.

One alternative fuel being tested for rail power in the USA is natural gas. It can be combusted in a reciprocating engine (a converted diesel engine), it can be processed by solid-oxide fuel cells or be combusted in gas turbine powerplants. Contemporary turbines such as the Rolls Royce Trent and the Mercury 50 by Solar Turbines operate as high as 38-40-percent efficient on natural gas, while a piston engine delivers less power as well as lower thermal efficiency. A locomotive experiment is getting underway in the USA to use a natural gas fueled 5500-Hp Mercury 50 regenerative gas turbine powerplant driving electrical generating equipment. This locomotive is intended for heavy freight use, while another gas turbine high-speed locomotive experiment is also underway.

The price of natural gas has risen with market demand, but is still lower that diesel fuel. The classic part-load efficiency loss still exists for new generation, high-efficiency regenerative turbines ..... which typically will deliver 80-percent of maximum efficiency at 50-percent power output. Despite this part-load efficiency loss, the cost of the natural gas turbine is still very competitive against modern diesel locomotives. The Mercury 50 Solar Turbine can be developed to an output of 10, 000-Hp on natural gas. Savings are not only realised from the lower cost of fuel, there is also a substantial cost savings in lubricating oil and engine coolant. In operation, the contemporary gas turbine locomotive is expected to be cost competitive against its diesel counterparts, in heavy-haul freight service. It is also expected to be far more environmentally friendly, emitting only a miniscule amount of the pollutants typically emitted by a modern diesel locomotive.

The Potential of a Combined Steam Locomotive:

A modern steam locomotive, using all the improvements that have been developed or researched thus far, could operate in a combined cycle mode with the natural gas Solar Turbine. The Mercuty 50 Solar Turbine engine operates at 39-percent efficiency at 5500-Hp .... and rejects some 8500-Hp (21, 000, 000-Btu's/hr at some 300-degrees centigrade) from its exhaust. This is energy that could be used to boil water, in either a firetube or water-tube boiler. If the gas turbine locomotive is developed to a level of 10, 000-Hp at 40-percent efficiency, it would reject some 15, 000-Hp(38, 000, 000-Btu's/hr at 300-degrees centigrade) via the exhaust. If 20-percent of that reject heat could be delivered to the driving wheels, that would be 0. 2 x 15, 000 = 3, 000-Hp from the steam section.

In American heavy-haul freight operation, slug units are at times used to provide added tractive effort, but source their energy from a companion locomotive. A combined cylce locomotive could at various times operate as a slug unit allowing added tractive effort, it could provide additional horsepower to help pull heavy trains and it could be used to enable higher fuel efficiencies. A pound of natural gas holds 20, 500-Btu's of energy. At 39% efficiency, the 5500-hp turbine processes35, 900, 000-Btu's/hr or 1750 lbs of natural gas (3. 14-Hp/lb of fuel). The waste heat from the turbine is sufficient to raise superheated steam at 600-psia. If higher steam pressures are desired, or higher superheat temperatures needed, small amounts of natural gas could be burned to these ends (5-Hp/lb of fuel). A triple-turbine in a 1-2-4 power ratio(250-Hp, 500-Hp, 1000-Hp) could enable maximum efficiency at 7-power settings. If the gas turbine is rated for 10, 000-Hp, the steam section could use a quad turbine in a 1-2-4-8 power ratio (250-Hp, 500-Hp, 1, 000-Hp and 2, 000-Hp), yielding 15-power settings at maximum efficiency.

At power levels below 33-percent of maximum gas turbine power, the part-load efficiency loss is sufficiently drastic that the steam section running on its own, directly burning the natural gas, would offer a more efficient and cost effective alternative. Such operations may be desirable on occasions where the combined cycle locomotive is being used to pull a lightly loaded trains between major centres, so that it may be transferred to a much heavier train at the destination. On other occasions, the combined cycle locomotive may be pulling a very heavy heavy train through a speed restricted zone, requiring low overall power (gas turbine off, steam in service) and high tractive effort.

The steam section of the combined-cycle locomotive would benefit from the work of people who have been researching modern and future steam. Concepts like the onboard water purifier and chemical cleaning have reduced boiler wash-downs to twice a year. The modern boiler insulation techniques that reduce heat loss and the concept of using a recirculating water heater to reduce steam generation times, will also enhance the steam combined cycle locomotive. The research at ACE in the area of re-using the steam and using a cooled, multiple-expansion valve to convert exhaust steam to hot water, to be cooled in a radiator, will extend the operating range of the combined cycle locomotive. The research of maintaining heat on cylinder walls can be applied to steam turbines ... a large section of the non-insulated turbine outer casing can be located inside the "firebox", so that high heat will be maintained on the steam turbine stators (or nozzles), raising efficiency. The turbine bearings will be located outside the "firebox", on a driveshaft and away from heat.

Overall Efficiencies:

The gas-turbine concept, fueled by natural gas, has competitive potential over piston engines. The efficiency of modern regenerative gas turbines is almost on par with modern diesel engines ...... with savings in cost of lubricating oils, engine coolants and longer in-service duration between powerplant tear-downs and rebuilds. The gas turbine is also more readily adaptable to combined cycle operation, due to the energy volumes and temperatures in the exhaust, which can raise steam ...... something not easily achieved with a diesel piston engine. The steam side of the combined power cycle can also be used with natural gas powered solid-oxide fuel cells (SOFCs) which operate at temperatures well excess of 500-degrees centigrade. Whereas the overall efficiency of a combined cycle gas turbine and steam (COGAS) can approach 50-percent, a combined cycle of SOFC and steam could operate in the 60-70-percent range .... on natural gas.

Future Steam Railway Traction:

The steam locomotive could return to mainline railway service, as part of a combined cycle locomotive. The advances that have occurred in steam technology, including advances which have occurred in naval service as well as power stations, will inevitably be incorporated into the steam section of a combined cycle locomotive. The potential return of steam technology to railway service does open a door for expanded steam operation, in a non-combined cycle system. Some precedents in the UK, such as Thetford Power Station, indicate a possible future scenario as to what types of fuels a non-combined cycle steam locomotive would use. The environmental revolt against coal makes coal an unlike future locomotive fuel.

Renewable solid fuels are gaining popularity ..... eg, UK's Yorkshire Power Station's Willows to Watts program. Biofuels such as wood pellets (or other bio pellets), alfalfa stems, compacted farm livestock bio-solids, compacted municipal treatment centre bio-solids all feature prominently as potential future locomotive fuels. There is a considerable amount of support amongst environmental groups, various green groups and even in numerous political circles to use domestic bio-fuels. Very powerful farm lobbies exist in many nations. There is a growing revolt against using bio-solids from municipal treatment centres as fertilizer, plus increasingly more stringent regulations pertaining to the disposal of bio-solids from livestock farms. Burning it as locomotive fuel has large-scale support ....... and there is a large availability of such fuel across countries like the USA. First, various railway companies around the world would have to become comfortable with operating combined cycle locomotives which have a steam power section. Once such locomotives gain acceptance, the next step would be the introduction of bio-solid fuel burning steam locomotives ..... ones which use the pioneering work of Andre Chapelon and Livio Dante Porta.

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Rob Dickinson