The International Steam Pages

The Uniflow Piston Engine in Future Steam Railway Locomotives

Harry Valentine, Transportation Researcher, writes:


Over the past 30-years,much research has been undertaken as to how to develop a competitive contemporary steam locomotive. Much of the research can directly and indirectly be attributed to one man, L.D.Porta. During two time periods over the past 30 years, oil shortages and oil price increases provided the incentive to seek ways to improve the classic steamer. Future projections are again predicting an oil shortage, in the form of an expected decline in worldwide oil production after 2010.

Fuel options:

The existing internal combustion used in the present generation of diesel powered locomotives has its limitations as far as its fuel tolerance range is concerned. This could change in the future, if a more flexible/multi-fuel internal combustion engine is developed. However, certain types of fuels would be quite unsuitable for use in an internal combustion engine .... but would work quite well in an external combustion system, such as the firebox of a steam locomotive. Most research devoted to future steam has focused around coal as the fuel of choice, though other combustible solid fuels have been offered as alternatives.

One experiment in Australia during the 1950's involved the use of powdered coal, fed into the firebox via a screw mechanism. The locomotive showed an increase of 10% efficiency over similar comparable steamers burning chunks of coal. During the 1980's in Sweden, a combustion technology was developed allowing a coal slurry (a paste of water and powdered coal) to be fed into a combustion chamber where it was burnt to boil water to raise steam in an environmentally very clean electric power station. Coal slurry can be transported via pipeline ...... and could become a fuel option for future locomotives which use contemporary burning technology. Further research may yet develop a burner system which can allow a paste or a flammable liquid (otherwise unsuitable for use in an internal combustion engine or fuel cell) to be used as a fuel.

The fluidized bed of coal is also an option which has proven itself efficient and effective. This technology is not restricted to burn only coal, many other types of solid fuel (biomass) may also be burned. Future fireboxes/combustion chambers could be built to use any of a paste, a liquid or a solid fuel to raise steam.

Expanding the steam:

Turbine:Extreme power for heavy hauling.

At the present day,  variety of turbines are readily available in the market and in a variety of power levels. Turbines yield their maximum efficiency at maximum power, but become extremely inefficient at part-load settings. One alternative around this problem is to use 4-turbines in a 1:2:4:8-power ratio, which would actually yield 15-power settings, all at maximum efficiency. The added cost of such a system could be recovered from savings in fuel consumption, as compared to a single turbine yielding equal maximum power but low efficiencies at lower power settings. The cost of a multi-turbine locomotive would require that it be an extreme-power locomotive used in extreme serve, like pulling heavy ore trains over difficult terrain, or heavy and fast freight trains requiring peak power of a 15,000-Hp steam-turbine-electric locomotive. A smaller and perhaps power different concept would be more suitable for less demanding types of railway service, in the 1,000-4,000-Hp range.

The Uniflow Steam Engine:

Rural power.

A variety of small, uniflow steam engines are in use today, mainly converted diesel engines in rural Australia. The engines were made either by Lister or Detroit Diesel, as internal combustion 2-stroke diesels. When converted to uniflow steam operation, steam enters the cylinder via the inlet valves (formerly exhaust valves) of the Detroit Diesel 3-71 engine, while one of the cylinders is near top dead centre (TDC). The steam continues to expand after cut-off (closure of inlet valves) until the piston opens the ports at the bottom of the power stroke. Much of the spent steam goes out via the ports (exhaust ports which were formerly inlet ports). During the upstroke that follows, the remaining low pressure steam is compressed and heated, maintaining heat on top of the piston. The inlet valves open as the piston reached TDC, letting in a new charge of steam and cushioning the piston. In Australian service, efficiencies going as high as 21% have been reported for this type of single-acting uniflow steam engine. That the cranks are spaced 120-degrees apart assists in starting as well as enabling smooth power flow.

The Uniflow Steam engine;

Vehicle service.

A vee-2 uniflow steam engine has been built and tested in Australia by PRSteam, for road vehicle use.The engine has delivered efficiencies in the 15%-17% range, using a flash boiler to raise steam. The inventor, Ted Pritchard, was researching a flexible fuel, low polluting alternative to the internal combustion gasoline/petrol engine .... with promising results.

The unusually high efficiency for a uniflow engine was predicted decades ago by a Professor Stumph, from Charlottenburgh in Germany, who compared the "exhaust near the bottom of the stroke" to the more conventional system where inlet and exhaust were side by side, yielding about half the efficiency for single expansion. A single piston in a uniflow expansion system has been theorized to be able to operate at the efficiency levels of a compound conventional cylinder system.

A interesting development has occurred in road vehicle engines, in that a heavy duty diesel road engine (with exhaust via ports at the bottom of the stroke) has recently come on the market. The engine is a vee-4 built by RODI in the USA .... and could be adapted into a vee-4, single-acting uniflow steam engine with steam inlet at TDC. Its size range, weight and good balance make it suitable as use in rural power generation (using steam) or in a small, narrow-gauge steam locomotive in the 50-200-Hp range. Alternatively, it may be adapted for use in a steam-powered bus or truck, operating as a future flexible or multi fuel commercial vehicle.

The Uniflow Steam Engine:

Conventional rail service.

Since 1980,new proposals for future piston-steam locomotives originated in the USA, the UK and Australia, all involving some kind of compound cylinder system. Much of the research which went into these proposals originated with L.D.Porta from Argentina. Most, however, seemed commited to the conventional cylinder system. One very unique concept came from National Steam Propulsion Company, under its president, Fred Prahl.

The NSPC concept proposed to use existing locomotive componentry, such as a diesel engine block converted to steam, existing alternators and existing trucks (bogies) and conventional electric traction motors. The proposal called for 3,500-Hp ... and locomotive builder EMD showed interest. In conventional diesel operation, the EMD locomotive uses a 2-stroke engine with (inlet) ports at the bottom dead centre and exhaust valves at TDC. By changing the EMD inlet ports to exhaust ports and the exhaust valves to inlet valves, the flow would be reversed and the engine would then be the basis of a uniflow single acting steam engine.

EMD builds its 2-stroke engines in an inline-8, vee-12, vee-16 and vee-20. Some of the cylinders in the multi-cylinder layouts could operate as high-pressure cylinders, while other "siamesed pairs" could operate as low pressure cylinders in a steam power system. The initial calculated efficiency of the NSPC single-acting piston-steamer was 18%, with an estimated potential going as high as 27%. A method of turbocharging a piston steam engine was proposed by Dr.John Sharpe (UK) in the mid 1980's, for use in a proposed Garratt locomotive, where the exhaust steam from the low-pressure cylinder would drive a turbocharger and "compress" the reheated steam entering the low pressure cylinder. Combining Dr.Sharpe's concept with the NSPC research may be able to raise the overall efficiency of either locomotive.

The EMD is by no means the only engine block which can be adapted to use as a uniflow steam piston engine for railway use. In the history of North American 2-stroke diesel locomotives, EMD had a competitor, Fairbanks-Morse. The Fairbanks-Morse was a horizontally-opposed piston system, with inlet ports at one cylinder's BDC and exhaust ports at its partner's BDC (bottom dead centre). Adapting a Fairbanks-Morse engine block to a uniflow steam cycle would mean that both sets of ports in each cylinder become exhaust ports (the easy part). Installing inlet steam valves at the TDC of each piston set would be more challenging. The added equivalent stroke of the opposed piston layout does allow for a delayed cut-off of the inlet valves, for possible higher power delivery and high efficiency levels. Using engine blocks such as the EMD or Fairbanks-Morse would result in power levels comparable to the existing range of diesel locomotives (3,500 - 4,000-Hp range).

There are types of service which have evolved over the years, which has opened a market for locomotives in the under 1500-Hp power range. An RODI engine could be adapted for use in a small shunting fireless cooker engine. A layout of 4-such engines would enable a locomotive to a haul lighter intermodal trains, such as the American "roadrailer" trains, where rail trucks/bogies are attached directly to the underside of a highway trailer. Such trains require less locomotive power output due to their lighter weight and good aerodynamics.

Some recent research into fireless cookers has focused around advances in the field of materials engineering and thermochemistry. Some early experiments in thermal energy storage involved the use of molten caustic soda (NaOH) which melts at over 400 degrees C and has over 100 BTU/lb latent heat of fusion. The corrosive nature of caustic soda and the lack of availability of suitable storage tank materials, ended the research. In more modern times, other chemical cocktails having much higher levels of latent heat of fusion (300 - 400 deg C) can be synthesised, while a variety of corrosion resistant coatings (such as teflon, kevlar and other high-temperature super-plastics and super-fibres) are available to line the insides of thermal storage tanks. With a capability of storing 500 BTU/lb in 250,000-lbs of a storage material in a fireless loco built to the maximum clearance dimensions for North American service, some 10,000 Hp-hr could be available at the drawbar (20%-efficiency) ..... allowing a power delivery of 1,500-Hp for 6-hrs ...... enough to pull an intermodal "roadrailer" train between city pairs such as Toronto-Montreal. Developing a fireless super-cooker to a capability of 20,000-Hp-hr at the drawbar could make the concept economically viable as oil prices skyrochet after the projected decline in worldwaide oil production after 2010.

The Uniflow Steam Engine:

Small Developing countries.

Developing countries often have difficulty affording the world price of oil and often lack the technical manpower skill to maintain diesel powered rail/road vehicles. A case in point is Zimbabwe, which still operates Garratt steam locomotives in their railway services. A niche may develop in the railways of developing countries for a simple, water-only fireless cooker ..... using uniflow steam piston engines as well as storage tanks able to operate at over 1,000 psig and 300-degrees C. he pistons would operate at 200 - 250 psig .... in their superheat range for 300 deg C.

Energy thermal charging would be done in stationary mode, complimentd by on board supplementary thermal charging using small, high pressure flash type boilers as used by Doble and Pritchard (PR Steam .. Australia). A dual stationary thermal recharging system would include both a heat exchanger line (when super-heated air from burning natural gas is used) as well as a perforated steam line at the bottom of the thermal tank (when water is boiled by burning biomass, or from heat from heliostats). High pressure super-heated steam would be injected into the water in the thermal tank, via the perforated tube at the tank bottom. To maximise operating range, the thermal storage system would be built to the maximum dimensions allowable on the intercity railway system of the country of operation. A locomotive needs an energy reserve, which would be provided mainly from thermal recharging from an external source. Onboard thermal charging would be used to extend the operating range or increase performance. A variety of mainly liquid or solid fuels which would otherwise be unsuitable for internal combustion engine use, would be burnt in the onboard boiler. Whereas a small low-pressure shunting cooker with standard cylinders (4% eff) would be good for 20-miles pulling a train, a high pressure uniflow cylinder(12%-15%)super-cooker having 3 -times the storage volume would be capable of pulling the same load at the same speed for over 100-miles from a stationary thermal recharge. The small onboard high-pressure boiler could extend this range to well over 200 -miles.

The Uniflow Steam Engine:

Single vs double acting expansion.

An interesting discussion has begun re the relative merits of single acting vs double acting steam expansion in uniflow steam engines. A variety of 2-stroke heavy duty diesel engines (Lister, Detroit Diesel, RODI and EMD) can or have already been converted to steam operation ..... with the valves in the cylinder heads being used as inlet valves, while the ports near BDC were used for exhaust. Such engines used in rural serve in Australia and on a high pressure flash type boiler, have reported to have delivered up to 21% efficiency. Extensive modifications have to be made to such off -the-line engines: cooling systems have to be insulated, the oil spray under the piston has to be re-directed, camshaft timing needs extensive modification ..... to enable the valve cut-off to be adjusted from a lever.

Proponents of double-acting expansion claim efficiency in size, less engine weight and lower complexity. Double acting expansion invariably requires the use of a sliding cross-head. One challenge of the double-acting system, is to protect the piston rod from cooling as it repeated leaves and re-enters the steam cylinder. This would otherwise reduce efficiency if the piston rod is repeatedly heated and cooled. One alternative to this dilemma is to either enclose the piston rod, protecting it from the surroundings, or to coat it with a ceramic compound. Double-acting expansion tends to be more severe on the crank bearings, than a comparably sized single-acting engine. Oversized roller bearings on the crankshaft would benefit both engine types as would insulation of the engine.

Harry Valentine, Transportation Researcher.

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Rob Dickinson