The Possibility of a Stirling-Hydraulic 


A Stirling cycle power plant using an air-over-oil system, driving a hydraulic Quasiturbine, may actually be able to be developed into a workable and efficient future railway locomotive propulsion system. Pressurized air would exert force on a large piston (sides coated with teflon), which would drive a hydraulic plunger in an oil cylinder and generating very high oil pressure. The high oil pressure flowing through a converter such as a Quasiturbine, could generate the desired levels of torque to directly drive the rail wheels. Alternatively, a direct air-over-oil system may be used, involving modified hydraulic accumulators able to operate at peak pressures up to 100-atmospheres or more.


Energy forecasts are predicting that worldwide oil production will peak between the years 2004 and 2010. After that, a long-term decline in supply and related price rises may be expected. In railway operation, several energy options are available. Electrification of high-density mainlines is possible and at a cost exceeding US$5-million per mile, however, electrical power generation in both the USA and Canada is near market-demand capacity. Any attempt at large-scale railway electrification will involve the high cost of building new power stations.

Another alternative would be to borrow a proven piece of technology from a bygone era and modernize it. The steam locomotive and its problems of thermal inefficiency and labour intensity are solvable. Successful efforts in Argentina, South Africa, and Cuba and by DLM in Western Europe have been able to up rate the efficiency of steam locomotives. Their exhaust emissions have been reduced by using fluidized and gasified combustion, which cleans the fireboxes, and smoke boxes. On-board water purifiers and chemical water treatment now require two to four annual boiler wash downs. The downtime as well as their labour intensive maintenance and staffing requirements have been effectively reduced as well. Modern and future steam powered locomotives would incorporate many of the technical advantages which have appeared in the fields of thermodynamics, heat transfer, and modern insulation technology. Such modernized steam locomotives could use a wide variety of solid, liquid and gaseous fuels that could not otherwise not be used in normal internal combustion engines, without causing such problems as engine fouling or even engine damage. For high power requirements (over 5000-Kw at the rail), technological advances in thermodynamics, thermo-chemistry and general engineering can enable modernized steam locomotives to become extremely cost competitive and efficient.

Moderate Rail Power Requirments:

In the area of low to moderate rail power requirements, it may be possible to use the same fuels and combustion system advances as a modernized, high-powered steam locomotive, but in an efficient steam-less variant. A useable, efficient and competitive Stirling cycle locomotive, using hydraulic technology could be developed. Lower powered locomotives would see service in inter-modal operations, pulling trains such as road-railers, which are highway semi-trailers with the rail wheels being directly attached. The Stirling cycle may be well suited for service in dry climates and in lower-powered rail operations, including shunting and also in passenger service. Its efficiency levels would be competitive with internal combustion engines (20% to 30%) and be marginally better than what may be achieved in a modernized steam locomotive. During the early/mid 1980’s, the Government of Canada’s Transport Department funded some research into a modern coal burning locomotive, using a Stirling cycle power plant. Many technological advances have since appeared since that research, one of them being the Quasiturbine, which can operate either on pressurized gas or as a hydraulic engine which could yield higher levels of efficiency than conventional hydraulic turbine or piston systems. The Quasiturbine technology enables an air-over-oil approach to be used in large, locomotive-sized Stirling cycle applications, while solving some of the problems of large Stirling power-plants (such as sealing and low gas pressures). See for further information on the Quasiturbine concept.

The Stirling Cycle:

Stirling engines are external combustion engines. They can operate from solar energy (stationary installations), they can operate from the top of a wood-stove (to drive a fan or to generate low levels of electric power), and they can operate from a heat storage system or even a direct flame. The typical Stirling engine has a hot side and a cold side, often on opposite ends of the same cylinder and with a piston in between. The heating and cooling of a gas such as helium pushes the piston back and forward, often without the complexity of inlet or exhaust valves. The major problem of the Stirling engine is sealing, resulting in a loss of the helium. A rotary engine design called a Quasiturbine can operate as a Stirling Cycle power plant, using a gas such as helium as its working fluid, provided the sealing problems of a 1.5-ft wide by 3.5-ft-diameter rotor can be solved. A direct-drive low speed Stirling cycle engine could directly drive locomotive wheels (through side rods or gears).

The Quasiturbine has operated on compressed nitrogen gas and can be adapted to operate on hydraulic (liquid) pressure, using water or oil. For low to medium power mobile applications, the use of hydraulic fluid enables two Quasiturbines, set at 45-degrees out of phase from each other on the same driveshaft, to deliver power characteristics and efficiency levels which are suitable for freight locomotive use. Large low-speed (400RPM) or small, compact high-speed (1500RPM) Quasiturbines may be used. The lower viscosity and higher density of the hydraulic fluid (compare to helium) minimizes the sealing problem of a Stirling engine. The hydraulic fluid could also act as a lubricant, prolonging the Quasiturbine’s service life. Prolonged running at low speeds under heavy load can overheat electrical drive systems of diesel-electric locomotive. This overheating problem could be avoided by using hydraulic Quasiturbines. Using hydraulic operation can save the high cost typically associated with the electrical running gear of diesel-electric locomotives, which can account for 40% to 60% of the total cost.

Air-over-hydraulic-piston Concept:

In a large locomotive sized Stirling power plant, an air-over-oil approach could be used together with helium driven displacer pistons inside large cylinders. The hot and cold cylinders would contain the pistons connected to small-diameter hydraulic rams. During the power stroke, the hot cylinder would push the hydraulic ram, forcing oil under high pressure into either the Quasiturbine, or a hydraulic accumulator connected to it. As well, the hot piston would compress a return-spring, which would push the piston upward after each power stroke. Helium gas lines would connect to the top openings of the hot and cold cylinders, which would be of equal volume but different cross sections. Heat would be applied to heating coils connected to the hot cylinder, while the cold side chambers would be connected to a heat sink (cooling coils) to remove heat from the system. One-way air-flaps, similar to those used on the intakes of two-stroke engines, would be included in the helium gas lines to improve efficiency. A regenerator or heat exchanger would envelope two gas lines connecting the hot and the cold cylinders, enabling some of the rejected heat to be re-used to enhance engine efficiency. One gas line would go directly from the hot side, through the regenerator, through the cooling coil, then into the cold side cylinder.

The return gas line would go directly from the cold side, through the regenerator, into the heating coil and then into the hot side cylinder. During the hot cylinder up-stroke, the cooled gas leaving the hot side would bypass the heating coil, flowing directly through the regenerator, then through the cooling coil before entering the cold cylinder (which would be in the down-stroke or maximum-volume position). Hydraulic pressure acting on the cold-side hydraulic ram would force the cold-side piston upward, to its minimum volume position. Cold gas would be pushed directly through the regenerator (a one-way flap would ensure that the gas flow can by-pass the cooling coil) and into the heating coil. The gas would be heated, raising its pressure while both hot and cold side pistons are at their minimum volume positions (the cold piston held there by a valve in the hydraulic system).

The increased pressure caused by heating the gas would force the hot-piston downward, forcing oil to flow at high-pressure into either the Quasiturbines, or a hydraulic accumulator connected to them. When the hot-side piston reaches maximum volume, the hydraulic valve keeping the cold side piston at its minimum volume position would release. A compressed spring could then push the cold-side piston toward its maximum volume position, which a similar compressed spring returns the hot-side piston to its minimum-volume position. Oil levels in the hydraulic chambers below each piston, may be managed by pressure-sensitive valves. Computer control of the hydraulic system is also an option.

Heating and Cooling Coils:

It is a known fact that for a small-volume object, the ratio of interior volume compared to external surface area is less than the same ratio for a scaled up version of the same object. A small body loses heat faster than a larger body of the same shape profile. When heat is applied to an external surface, the interior of a small body heats up faster than a large body. In Stirling cycle engines, heat may be applied directly to the cylinder walls of a small-bore variant, which would heat up the gas inside the cylinder, enabling it to do work and operate to rotational speeds up to 650-RPM’s. In a large bore Stirling cycle powerplant, heat applied externally to the cylinder head and cylinder walls would involve a very long time delay in heating up the gas inside. This would result in very low expansion rates and very low heat transfer efficiencies. The use of external heating and cooling coils increases the heat transfer to/from the working gas. The increased heat transfer efficiency enables higher gas temperatures entering the hot power cylinder, increasing gas pressure and increasing the power delivery on each expansion power stroke. The increased rate of heat transfer to/from the gas also enables more power cycles to be delivered within a given time period, from a very large bore cylinder. Further increases in efficiency may be realised by pre-heating the air that would be used to combust the fuel. A portion of the outside air being used to transfer heat from the cooling coils may be used as intake air to support fuel combustion. This air may then be further pre-heated from the exhaust combustion gases, leaving the heating coils.

Piston Diameter:

While each of the cylinders would able to hold equal volumes of liquid, their internal diameters would differ. The cylinder connected to the cold helium chamber, i.e. the cold reservoir would have less piston diameter and greater height than the hot cylinder, which would be wider and lower in height. Further research would be needed to establish optimal piston diameters for given volumes of oil and helium, both of which would be constant within the system. Large, hollow pistons could have a spherical profile, to enable lighter pistons to be used in a high-pressure helium environment. If the hot cylinders are positioned vertically, some oil may be use on the topside of the contact area between piston and cylinder wall (where a piston ring may be located), to ensure reliable sealing of the helium gas above the piston.

The typical pressure-volume (P-V) diagram of a Stirling cycle indicates a fairly rapid decline in gas pressure as the piston moves from minimum volume to maximum volume in the power/expansion stroke. To maintain a 500-psi average hydraulic pressure throughout the power stroke, a peak pressure exceeding 1,000-psi would be needed while the displacer power piston is at minimum volume. If the hydraulic plunger were 12-inch diameter (, the displacer would need to generate an average down-force of 113,100-lbs during its expansion stroke. For a helium gas pressure differential of 110-psi, an area of 1000-sq.ins. would be needed, or a power piston diameter of 36-inches. If 7-cubic feet Per second of hydraulic oil were delivered to the Quasiturbine at 1,000-psi (or 15-cu.ft at 500-psi) and at an overall conversion efficiency of 93%, there would be 1700-horsepower available at the driving wheels. Weight wise, Stirling cycle power-plants have weighed 12 to 25 - pounds for every 1-horsepower output, meaning that 50,000-lbs of power-plant weight in a 1700-Hp locomotive would be within an expected range.

Variable Stroke:

The absence of a crankshaft or rotary cam enables the hydraulic Stirling engine to operate with variable length power strokes. Variable stroke operation enables the volume of oil flow and its pressure to be varied, from low-volume flow at high pressure, too high volume flow at lower oil pressures. Short stroke, high-pressure low-volume oil flow forced into an accumulator would enable the Quasiturbines to deliver high starting torque at zero-RPM as well high-torque as at low rotational speeds. Oil pressure levels in the accumulator may be increased to higher levels by using a hydraulic transformers, which can similtaneously reduce oil flow rate while increasing oil pressure. One variation would use a large-diameter hydraulic piston, receiving oil-flow directly from the Stirling power cylinders, driving a smaller-diameter hydraulic piston which would feed a low-volume flow rate of oil into the accumulator at high-pressure.

A second method of raising oil-pressure in the accumulator could result from using a large diameter Quasiturbine driving a much smaller diameter Quasiturbine at the same rotational speed. The larger Quasiturbine would receive oil directly from the Stirling power cylinders at 800-psi to 1,200-psi, while the smaller Quasiturbine would deliver a lower volume of oil to the accumulator at higher pressures (1,600-psi to 2,400-psi). At low rail speeds, short-stroke operation would deliver the high oil pressures to enable a locomotive to accelerate a heavy train from a standstill. At higher rail speeds, full power stroke operation from the Stirling power cylinders would enable larger volumes of oil to be delivered to the Quasiturbines at lower pressures. If the oil flow rate would allow an intermodal freight train using two Quasiturbines (2500-Hp) and directly driving the axles, to travel at 60-miles per hour (95-kilometers per hour), a single Quasiturbine could be operated to attain higher rail speeds in passenger train service. Direct-drive would provide the momentum to carry a single Quasiturbine through its zero-torque region, at high train speeds.

Hydraulic Control:

When the Stirling cycle is at rest and the temperature equal in both cylinders, the oil levels and piston locations within the system would be at an equilibrium level. During the start-up phase, paired hot and cold side pistons would be pressure driven (by an electric starting motor) to their minimum volume positions, after which heat would be applied to the the heating coils. In locomotive operation, multiple pairs of cylinders would be required to ensure steady, high-pressure fluid flow through the Quasiturbines. Oil levels in the system, including all accumulators, oil cylinders and oil storage chambers, would be managed during different phases of the operating cycle. A series of pressure sensitive valves, one-way valves (hydraulic diodes and hydraulic thyristors), oil flow rates from hydraulic pumps (driven by the main Quasiturbines) and solenoid operation would be subject to computer control.

Operating Cycle:

During operation, the heated and pressurised gas would exert force on the hot side piston, which would force oil under high pressure, through a one-way valve, into the hydraulic accumulator (at low speeds). High-pressure oil would flowing from the accumulator would drive the Quasiturbines, which would drive both the drive-wheels as well as a hydraulic pump. When the hot cylinder is at maximum expansion, the hydraulic pump would pump oil from the cold side cylinder, through a hydraulic thryristor through a second one-way valve (on the hot cylinder) and to the hot side cylinder. A hydraulic thyristor is a pressure-controlled one-way hydraulic valve on the cold cylinder, activated by an electrical solenoid. Alternatively, an oil pressure-line from the hydraulic pump may also trigger the thyristor, allowing oil to flow from the cold cylinder to the power cylinder.

The hot side piston would then move to its minimum volume position, while the cold side piston would be at its maximum volume position. This piston relocation would push cooled gas from the hot cylinder and through the heat exchanger and cooling coil, into the cold cylinder. The gas pressure and temperature would be lowered. Once the cool cylinder reaches its maximum volume, the hydraulic pump would pump oil from the oil resevoir, through a one-way valve, into the cool side cylinder. This action would force the cold piston back to its minimum volume position (and keep it there). As a result, the working gas would be forced through the heat exchanger (regenerator) and through the heating coil, raising the gas temperature and pressure. The heated and pressurised gas would then exert force on the hot side piston, forcing it to deliver a high-pressure oil supply to the accumulator, as it travels downward to its maximum volume position.

This is the basic operating cycle for each pair of hot and cold air-over-hydraulic cylinders. For locomotive operation, four pairs of cylinders should ensure steady power flow. At high speeds, the oil flow from the hot power cylinders may be sent directly to the Quasiturbines (bypassing the accumulator). A high-speeds, a single Quasiturbine may be used instead of both.

Power Maximization:

The use of one-way valves in the air pipes enables a high level of thermal efficiency to be realised. The cooled gas leaving the power cylinder travels directly to the regenerator and cooling coil, bypassing the heating coil, due to one-way valve operation. The valves have no connection to the exterior, enabling more reliable sealing of the working gas inside the system. The one-way inlet-valve allows hot gas to enter the power cylinder and prevents cooled gas from the power cylinder from re-entering the heating coil. The one-way outlet valve allows cooled gas leaving the power cylinder, to flow toward the regenerator and cooling coil, while preventing a back-flow of cool gas from re-entering the hot power cylinder.

To maximize power, a self-contained internal valve system would need to be included in the hot side cylinder head, supplementing the one-way valves located near the hot side inlet and exhaust. The pressurised hot gas entering the power cylinder would otherwise push the one-way exhaust valve open, causing hot gas to flow into the regenerator and into the cooling coil, causing a loss of thermal energy. A set of interconnected inlet and outlet valves would be desirable. The in-flowing hot gas entering the hot cylinder would push open a one-way flap, which would then activate a second valve that would temporarily seal off the opening to the outlet. The combination of a sealed outlet inside the power cylinder and a one-way valve at the entry to the heating coil would enable higher gas pressures and temperatures to be generated inside the power cylinder.

After reaching maximum volume, the pressure on both sides of the one-way inlet valve would stabilize. This pressure stabilization would neutralize the force keeping the one-way valve at the hot cylinder inlet, open. A small pressure-release valve conneted to the outlet and activated by an electric solenoid, would open and release the residual pressure from within the power cylinder. A counterweight on the interconnected valves would cause the one-way flap at the inlet to close, while opening the valve covering the outlet. Alternatively, the solenoid could open the pressure release valve as well as release the flat covering the outlet. There are many different valve designs that can enable this type of operation. With the outlet uncovered, cooled gas at residual pressure would flow out of the power cylinder and into the regenerator, then through the cooling coil and cold cylinder. This happens as the power cylinder returns to its minimum volume setting, in preparation for its next power-delivery cycle.

Electrical solenoids are used to drive fuel injectors in commercial diesel engines and more recently, are being developed to operate the engine valves. They are well proven in the hot environment in commercial engine comparments.

Smooth Power flow:

The arrangement of a single hot cylinder and single cold cylinder driving a single Quasiturbine would result in interruptions in the hydraulic flow and encounter a small zero-torque region. It would come to a stop during a short zero power area. To overcome this problem, two Quasiturbines mounted on the same driveshaft and at 45-degrees phase difference from each other, would ensure continuous power flow. Having the Quasiturbines being connected to three pairs of hot and cold cylinders would further enhance performance. The hot cylinders would expand helium in a sequence, one after the other, something that could be achieved by various hydraulic valve arrangements. This is analogous to an electronic circuit that can energize an array of lights in sequence. The hydraulic accumulators, which will be incorporated into the system, will reduce fluctuations in oil flow rates and oil pressures entering the Quasiturbines. When 3-pairs of hot and cold cylinders are used, one hot cylinder would be going through its expansion stroke, a second would be evacuating cooled gas following a power stroke, while the third would be at top dead center ready to receive heated gas to begin its power stroke.

The heating coils serving each cylinder may be served by individual heat sources or by a common heat source. The cold cylinders and their cooling coils would be served by the same heat sink, that is, direc air cooling from the atmosphere. An on-board sink may be used. A pair of transversely mounted Quasiturbines (set 45-degrees out of phase) could directly drive 60"-diameter drive wheels through side rods and bearings (336 RPM at 60-miles per hour). Using a 1:1 drive ratio each Quasiturbine rotor would be 21-inches diameter by 8-inches wide and together capable of delivering 1,600-horsepower at 360RPM. The system would operate on 500-psi average hydraulic pressure (1,200-psi maximum, 200-psi minimum, with pressure-sensitive control valves regulating the minimum oil volumes in the power cylinders).

Modern 42-inch diameter drive-wheels (480 RPM at 60-miles/hour) driven through a 2.25:1 gear ratio would require a smaller set of Quasiturbines operating at 1,000-RPM (freight service) to 1500-RPM (passenger service). The hydraulic power system allows several large hot cylinders to expand slowly and operate in sequence to enable higher rotational speeds and smooth power delivery on the Quasiturbine. Two Quasiturbines each 45-degrees out of phase and using rotors 10-inch diameter by 4-inch wide and operating on 1,000-psi average hydraulic pressure, could deliver 1400-horsepower at 1200-RPM or 1750-Hp at 1500-RPM on a common driveshaft. Increasing the width of the Quasiturbine would enable higher oil flow rates to be processed at lower speeds, improving low-speed performance characteristics.

For freewheel operation or coasting, a bypass valve and oil pipe will need to be included in the Quasiturbine hydraulic circuitry. This is because the Quasiturbine can also operate as a hydraulic pump, a feature that does lend it self to enhanced safety and energy conservation during locomotive operation.

Heat Exchanger:

Heat exhanger layout and operation for the triple pairs of cylinders would differ from conventional Stirling cycle practice, in that heat will transferred directly from warm to cold air lines. The use of separate inlet and outlet lines at each cylinder, as well as the expansion sequence of the hot cylinders, enables this. The line carrying cooled gas from a hot cylinder would exchange heat with the line carrying cold gas from a cold cylinder to the heating coil of a different hot cylinder. This reduces the need to store heat in the regenerator and still enables thermally efficient operation of the Stirling-hydraulic system.

Pumping Oil:

The hydraulic pump feature of the Quasiturbine can be very useful in railway service, in braking as well as in acceleration. The air-over-oil hydraulic accumulators plis extra oil storage reservoirs would be included in a Stirling-Quasiturbine locomotive. During deceleration, the Quasiturbine would operate its pump circuit (which would be activated from the locomotive cab) enabling kinetic energy to be converted into potential energy to be stored in the accumulators. Safety is improved while braking system wear is reduced. If the accumulators are energized to capacity, any extra oil being pumped under pressure from the Quasiturbine could be forced through a hydraulic retarder, which could convert the excess kinetic energy into heat. This excess heat energy may be stored in a thermal storage tank connected to the regenerators, or may be dissipated through radiators.

Energy Storage:

During acceleration, the stored energy in the hydraulic accumulators would assist in starting the locomotive from standstill, a feature that would be extremely useful in suburban commuter rail service. As the accumulators become exhausted of energy, any extra thermal energy that may be stored onboard, could be added to the regenerators to preheat the helium flowing to the heating coils, improving energy efficiency. A Stirling-Quasiturbine locomotive could operate with either stored on-board thermal energy (zero combustion), or carry solid or liquid fuel that would be combusted in a firebox or gasifier-combustion system. A gasifier system forces the solid or liquid fuel to release combustible gases, which are then combusted with very low exhaust emission levels.

Direct Air-over-Oil System:

Direct air-over-oil systems have been used in hydraulic accumulators as well as in vehicular suspension systems. Companies such as Citroen developed such a suspension system during the 1950’s, as did the defunct British Leyland Motor Company. In heavy vehicle applications, the American Caterpillar Company developed hydraulic suspensions for off-road mining trucks. In vehicular operation, work is performed on the trapped air within these suspension systems, causing the gas temperature and pressure to increase. Various types of separators have been used to keep the gas and oil apart, to prevent foaming of the oil. These separators operated in high temperature regimes and appear to provide long and reliable service lives. In suspension and accumulator operation, oil pressures up to 5,000-psi (345-bar) were often encountered. The suspension hydraulic cylinders were typically less than 12-inches (30-cms) diameter. In a locomotive direct-air-over-oil system, maximum oil pressures of between 500-psi (35-bar) and 1,000-psi (70-bar) would be required, while the hot side piston is at its minimum-volume position, to enable the Quasiturbines to develop sufficient zero-RPM starting torque to begin moving a train. A maximum pressure of 1,000-psi to 1,200-psi at minimum hot cylinder volume (top dead center) and a minimum pressure of 300-psi to 400-psi at the maximum cylinder volume (bottom dead center) would be able to produce the desired starting-torque levels.

Up to the present, most Stirling cycle engines have been of small cylinder bore and have operated at between 10 and 20-atm (145 to 290-psi) maximum pressures. The most likely reason would be to manage the gas-sealing problem caused by the piston rings sliding inside the cylinders. If pistons and piston rings, as well as mechanical drive componentry were discarded in favour of an all-hydraulic system, the sealing problems associated with piston rings would be reduced. This would allow for higher gas pressures, provided acceptable long-life, high temperature air/oil separators would be available. Flexible separators made from neoprene have operated in the high-temperature environments of automotive hydraulic suspension systems, in which oil has reached its boiling temperature. Flexible separators could use material such fiberglass matting as a base for a flexible high-temperature plastic made from fluorine-carbon bonds. The fiberglass has high tensile strength as well as high temperature abilities. Oil typically has a boiling point around 375-degrees Celsius (700-degrees F), which has restricted the allowable cylinder temperatures in internal combustion as well as in steam piston engines. One alternative to managing high cylinder head temperatures at the top of a hydraulic power cylinder would be to use a ball-shaped hollow piston floating on top of the oil. This would prevent super-heated gas from coming into direct contact with the oil.

The desired pressurized gas sealed in the direct-air-over-oil Stirling cycle engine would be helium, since it does not adversely affect solid materials like metals and glass to the extent as compressed hydrogen. Other gases such as carbon dioxide or nitrogen may also be considered as the working gases. The valve operation described for the piston-over hydraulic variant may be used for a direct-air-over-oil layout, to enable precise timing of the hydraulic flow between hot and cold cylinders. Oil-coolers may be used on the hot side, in conjunction with the thermal regenerators used on modern Stirling engines. Heat from the hot-side oil may be moved to the regenerator to simultaneously improve system efficiency while extend oil life.

Further research will be needed to explore the relative merits of the two variants (the direct air-over-oil vs. the piston-over-hydraulic system). The direct system could enable higher gas pressures with minimal (near zero) gas leakage, while the piston system may need to operate at lower gas pressures to control possible gas leakage. A floating piston on top of the oil in both cylinders may be a third option, if there is the threat of the working gas becoming soluble with the oil and either mixing with or dissolving in the oil. An extra Quasiturbine, driven by the main hydraulic Quasiturbine, would need to be used as an oil pump to enable oil levels to be managed in the hot and cold cylinders during the various phases of the operating cycle. Air-over-oil systems typically have a maximum oil flow rate of 4-feet/second. At oil pressures of up to 500-psi, oil typically can flow at 10-ft/sec to 15-ft/sec, while at higher pressures (500-psi to 3,000-psi) oil flow rates of 15-ft/sec t0 20-ft/sec are possible. Using a direct air-over-oil Stirling cycle approach, a hot cylinder measuring 2-ft diameter and having a 4-ft stroke, could theoretically discharge 12.5-cubic-feet of oil in 1-second, at an average pressure of 500-psi. This yields a theoretical average of 1,636-hydraulic-horsepower, indicating that it may be possible to design a Stirling -hydraulic locomotive capable of delivering 1,000-Hp to 2,000-Hp at the drawbar. A locomotive developing such power levels should be able to operate in various types of inter-modal freight service, as well as certain types of passenger-train services, including commuter train operation. It may be possible to develop a direct air-over-oil Stirling locomotive to deliver higher horsepower output levels. For extreme power output hauling heavy freight trains on renewable solid fuel, modernized steam locomotives may still be able to deliver 3-times the horsepower of a Stirling locomotive which uses the same type of fuel.  

Fuel Tender:

A Stirling-Quasiturbine locomotive would operate with a fuel tender unit, however, unlike steam locomotives, would not need to carry on-board water or water-condensing systems. Solid fuel pellets already exist in the market and can be used in a locomotive. Such fuel pellets are made from weeds, agricultural waste and even various animal manures like pig and poultry. The Thetford (thermal) Power station near London, UK, uses poultry droppings as fuel. With modern solid-fuel, gasifier combustion systems, exhaust emission levels can be kept low and ash is continually removed from the combustion area (by auger), into an ash storage area. The ash has been shown to have good fertilizer properties and can periodically be removed from locomotive ash-storage compartments for agricultural purposes. There do exist liquid fuels that burn better in external combustion systems than in internal combustion systems, due to the problem of possible engine damage. Such fuels include solvents and various coal-water fuels which hold about the same amount of thermal energy, on a per weight unit basis, as do the solid pellet fuels. Swirl combustion systems enable such liquid fuels to be combusted cleanly.

Despite the compact and efficient operating characteristics of the Quasiturbine, a large volume of space would be required to house the hot and cold cylinders operating in a Stirling power cycle. The volume would equal to the volume inside existing railway carbodies, including certain types of locomotives and rail passenger car bodies. Several types of light passenger and intermodal freight operations in the USA and Canada are well suited to locomotives of up to 2,000-horsepower, levels which may expected of Stirling powered locomotives.

Zero Exhaust Option:

A zero-exhaust Stirling-Quasiturbine railway option is possible, using on-board hot thermal (and very well insulated) storage tanks. The Ranotor Company of Sweden has developed a Steam Buffer Technology into which high-pressure superheated steam may be piped (rapid system re-charge) and which stores the thermal energy in a porous ceramic (sponge metal). An alternate system would involve the use of storage tanks lined with a shell of carbon-fibre or a high-temperature fluoro-plastic (e.g.: replacing the hydrogen atoms with fluorine atoms in the chemical structure of Kevlar or Nomex). Molten metal compounds would be contained in the thermal tanks, with the heat stored in the latent heat of fusion.

Possible candidates include some of the aluminum-oxygen polymers, which are presently being researched by companies like Alcan. (Polymers are super molecules in which tens, hundreds, even thousands of atomic elements are bonded together chemically, forming a single molecule... like carbon and hydrogen forming into plastics such as nylon or polypropylene). The latent heats of aluminum-oxygen polymers are high (300-400 Btu/lb) and the equivalent of up to 45,000-Horsepower-hours could be stored as thermal energy in an on-board rail storage system. If 25% of this can be delivered to the locomotive drawbar in a Stirling-Quasiturbine system, an intermodal roadrailer train could travel for up to 6-hours at speeds of 50-60 Miles per hour, covering distances of 300 to 400-miles. Energy to recharge such a locomotive could come from heliostats, ultra-clean garbage incineration, off-peak thermal energy from a nuclear power station (in the future, from a hydrogen fusion power station).

Thermal energy storage is well proven in rail shunting technology. Thermal energy storage systems have many times the life expectancy of rechargeable electric storage batteries, including the nickel-metal-hydride variety. Capital costs of both storage system types would be competitive initially, however, after 10-years of intensive service, the original thermal storage system would still be functional after hundreds of deep-drain cycles. Fuel cells also need replacement after prolonged period of deep-cycle intensive service, during which they may be operated at maximum output and maximum efficiency. In long-term operation, the thermal option would be cost competitive.


As oil production declines in the future, prices will rise in accordance to demand- supply relations. With oil, a 2% decline in availability can result in price rises of 25% and even 50%. Other fuel sources will become competitive for railway traction in the future. The system most favoured by governments, is the fuel cell. Some fuel cells need to use hydrogen, or require a reformulator in order to extract hydrogen from natural gas. Natural gas prices may be expected to increase in the future, in response to an increased use of natural gas in thermal power stations. Most of the methanol in the US/Canada market is made from natural gas and an increase in natural gas demand raises in the future (beyond 2010), oil prices will climb rapidly in

In future fuel cell locomotives, the solid-oxide fuel cell (SOFC) is a competitor. It can use natural gas directly and its high operating temperature allows steam technology and Stirling cycle technology to operate from the SOFC’s reject heat. The high cost of SOFC’s and the expected high cost of natural gas could make non-fuel cell locomotives more cost competitive, in terms of fuel cost and capital cost. The proton exchange membrane (PEM) fuel cell will have to operate with a methane reformulator in locomotives and in terms of efficiency, capital cost and fuel cost, locomotives like a Stirling-Quasiturbine concept could become a serious competitor in a market that is free from political interference.

The use of bio-fuels such as soy-diesel would enable diesel locomotives to contune to operate. The solvent properties of combusted soy-diesel could reduce engine life. During drought conditions, production of soy-diesel fuel could decline and its price could rapidly escalate. A locomotive using external combustion would have greater flexibility in the range of fuel options it could use, especially under adverse operating condition.

In terms of efficiency, Livio Dante Porta achieved 18% on a modified coal burning (using low-grade coal) steam locomotive on Argentina’s Rio Turbio Railway. Future steam concepts could equal present day diesel-electric locomotives in terms of drawbar efficiency; something quite possible with a low-grade (8,000 Btu/lb) fueled Stirling/Quasiturbine locomotive. In view of the high cost of railway electrification, system-wide electrification may not be an option for US/Canada freight railway operation. During the late 1980’s, the London School of Economics undertook a study comparing modernized steam locomotive operating costs against diesel and electric overall costs. The study concluded that on the basis of cost, modern steam power would be more cost efficient as well as more cost effective.


Worldwide, research has been undertaken in ultra-clean combustion of renewable solid and liquid fuels. Such technology is transferable into both steam and Stirling locomotives, along with other evolving technologies and concepts. Political support for using fuel cells in railway propulsion will undoubtedly hinder development for modern steam and Stirling locomotives in US/Canada. In terms of capital cost, a prototype Stirling cycle locomotive could cost the same as a production fuel cell locomotive. It would be more flexible in the fuels it may use, including operating using stored solar thermal energy (collected using solar-thermal reflector technology), which is less costly and more energy-efficient than photo-voltaic power generation required to re-energize a hydrogen-fueled PEM fuel-cell locomotive. A production Stirling-hydraulic locomotive should be cost competitive with a diesel-electric unit of similar power output (1000-Hp to 2000-Hp), while its fuel costs would be lower.

Further research will need to be undertaken in order to develop an optimal hydraulic Stirling cycle engine. The gas sealing problems would need to be managed, the gas would need to be prevented from mixing with or dissolving in the oil and a control sequence for the hydraulic valves would need to be developed. Much of the technology needed to build a hydraulic Stirling powerplant already exists and would involve modifying componentry already being used in the hydraulics industry.

Harry Valentine,
Transportation Researcher
January 14, 2002

Further information about Quasiturbine can be found at:

Further information about Stirling Engine operation can be found at:

Background information on proposed steam turbine locomotives can be seen at: