The combination of advances in engine management technology, in the design of heat exchange technology and the chemistry of high-temperature ceramic materials and compounds, provide the basis of enhancing the efficiency and market competitiveness of open-cycle and closed-cycle gas turbine engines, including for future railway propulsion.
Introduction
Traditional open-cycle gas turbine engines flow atmospheric air through a compressor, combustion chamber and power turbine. Peak fuel efficiency only occurs when the engine operates at maximum power output, with the turbine spinning at maximum RPM with maximum turbine inlet temperature. Engine efficiency decreases drastically as power output decreases. In railway operation, Union Pacific at one time had a small fleet of GTELs (gas turbine-electric locomotives) running at maximum output and peak efficiency to pull heavy freight trains up long gradients. Running at reduced output on flat territory resulted in high fuel consumption and high fuel costs compared to diesel-electric locomotives.
Small gas turbine engines were installed into short, higher-speed passenger trains that operated in the Northeastern and Central U.S. and central Canada. They were the Turboliner and United Aircraft Turbo Train. Running the trains at higher speed in limited-stop service allowed the turbine engines to operate near maximum output and near peak engine efficiency. However, both Amtrak and VIA Rail Canada entually replaced the turbine powered passenger trains with diesel-electric locomotive-hauled equipment. Amtrak expanded electric propulsion on the Northeast Corridor from New Haven, Conn. to Boston, with diesel locomotives assigned to passenger trains elsewhere. But now, development of new materials and computer-controlled engine operation allows for development of a new generation of gas turbine engines that would be suitable for railway propulsion.
Promising Earlier Engines
Engine developers who recognized the fuel efficiency problem of gas turbine engines developed alternative solutions that date back to the 1960s. One concept was externally heated closed-cycle gas turbine engines that continuously recirculate the same gas through compressors and turbine. While such engines can deliver high efficiency at 25% of power output, the materials of which heat exchange units were made at time incurred temperature restrictions that curtailed peak efficiency and peak power output. Lack of development of suitable heat exchange material resulted in stagnation in further development of closed-cycle gas turbine engines.
The complex-cycle gas turbine engine was an open-cycle, triple-shaft engine that combined two compressors with three turbines, two combustion chambers and two heat exchange units. The high-pressure and power turbines spun on separate shafts, with their own combustion chamber. Inability to accurately and continually control air/fuel ratios manually for each combustion chamber resulted in less-than-optimal engine performance. As a result, in real world operation, the complex-cycle gas turbine rarely delivered peak efficiency over a range of power output. Despite having shown great promise, further development of the complex-cycle gas turbine went stagnant.
Modern Advances
Modern reciprocating internal combustion engines operate with advanced technology. Mass-flow rate sensors, air/fuel ratio sensors, air temperature sensors and computer managed fuel injection have greatly improved fuel efficiency. There is theoretical potential to adapt such technology to the old classical complex-cycle gas turbine engine, along with the possibility of introducing new-generation heat exchange technology that operates at much higher temperature and higher effectiveness than earlier generation technology. Modern annular counter-flow heat exchange technology developed by Ed Proeschel offers heat transfer effectiveness of more than 90% compared to 80% for earlier-generation counter-flow heat exchange units.
The annular counter-flow heat exchange unit can be adapted for operation in both open-cycle and closed-cycle gas turbine engines. Modern turbine blades are made from ceramic material that retain mechanical properties at 1,400-degrees C (2,550 degrees F), allowing combustion temperatures of 1,200 degrees C without the need to cool turbine blades. While earlier generation heat exchange units were made from stainless steel, there is an evolving possibility of making heat exchange units from materials such as aluminum-nitride and high-purity boron-arsenide that offer higher coefficients of thermal conductivity than that of steel, at much higher temperatures.
Upgraded Complex-Cycle Engine
An upgraded complex-cycle gas turbine engine would include annular counter-flow heat exchange units installed downstream of both the low-pressure and high-pressure compressors, with computer-controlled fuel injection into combustion chambers placed upstream of both the high-pressure as well as power output turbine. The high-pressure turbine would operate on a computer-controlled ultra-lean-burn air/fuel ratio to assure sufficient oxygen in its exhaust gas to assure additional combustion to sustain operation of the power output turbine. Sensors connected to the computer would continuously monitor properties of gases flowing from the high-pressure turbine, to assure optimal overall engine performance.
The power output of a computer-monitored and -controlled complex-cycle gas turbine could rival that of the latest locomotive diesel engines rated at more than 4,000 hp and near 50% thermal efficiency. While the complex-cycle gas turbine would be quite compact, the heat exchange units that enhance its performance would occupy a considerable volumetric space within the locomotive carbody. With the potential to develop the turbine engine to output levels of 6,000 to 8,000 hp, railway freight operation would require that a slug unit be coupled to each gas turbine powered locomotive.
Closed-Cycle Engine
Externally heated closed-cycle cycle gas turbine engines continuously recirculate the working gas by cooling it after leaving the power turbine. It replaces the combustion chambers of open-cycle engines with high-temperature heat exchange units, with low-pressure and high-pressure compressors and turbines rotating on a common shaft that drives either an electrical generator or reduction gearbox. Closed-cycle engines can be designed to operate with wide variation in the mass of gas that recirculates within the closed system. Such operation maintains high efficiency over a range of power output as turbines spin at maximum design RPM, with gas entering the turbine at maximum temperature.
A closed-cycle engine that delivers 5,000 hp with mean-average internal system pressure at five times atmospheric pressure (5-ATM), would deliver 1,000 hp at comparable thermal efficiency with mean system pressure reduced to 1-ATM. During operation, some exhaust heat will be reintroduced into the engine downstream of the high-pressure compressor. The combination of heat rejected from the low-pressure compressor intercooler and residual exhaust heat would partially sustain the operation of a bottom-cycle steam engine. Future development of high-temperature heat exchange units made from compounds such as highly purified boron-arsenide with high thermal conductivity promise to assure engine efficiency.
Fuel Flexibility
While locomotive diesel piston engines require liquid fuel with very specific properties, internal combustion gas turbine engines have a much wider fuel tolerance and can operate on a much wider range of liquid and gaseous fuel, including low-cost fuel with high solvent properties. Externally heated, closed-cycle gas turbine engines can operate on a much wider range of liquid, gaseous and even solid fuels and without incurring any internal damage to the engine. Over the long term, gas turbine engines can incur savings in fuel cost and engine lubricant cost while offering extended service life, compared to diesel engines.
Bottom-Cycle Steam Engine
The complex-cycle gas turbine engine has two sources of reject heat to assist in the operation of a steam engine, preheating water flowing from the water pump to the boiler. Reject heat from the inter-cooler for the low-pressure compressor would provide primary preheating while turbine engine exhaust gas would provide a secondary source of heat that would further raise water temperature. Combustion of liquid or gaseous fuel would convert the preheated water to steam to operate a steam turbine engine at the equivalent of an elevated level of efficiency.
If a gas turbine engine of 6,000 hp operates at near 50% thermal efficiency, its intercooler and exhaust would release the equivalent of 6,000 hp of thermal energy to preheat water to operate a bottom-cycle steam engine. A combined-cycle engine could operate at the equivalent of between 55% and 60% overall thermal efficiency on either liquid or gaseous fuel. The open-cycle turbine engine offers greater flexibility than a reciprocating engine, in the variety of competitively priced fuel that could sustain its operation. Higher overall thermal efficiency combined with lower fuel cost enhances the engine’s marketability.
Solid-State Bottom-Cycle Engine
Solid-state technology involving specially chemically treated shapes of silicon that are heated on one side, with coolant applied to an opposite side, can directly convert heat into electric power, at 5% conversion efficiency. Future research into such solid-state conversion of heat into electric power would require involvement of artificial intelligence technology, to raise conversion efficiency when converting rejected engine (exhaust) heat to electric power, to assist in locomotive propulsion. The ideal long-term objective of AI research would involve matching the efficiency of a diesel engine in converting heat into electric power.
Conclusions
A three-shaft open-cycle gas turbine engine using modern electronic engine management technology installed in duplicate or even triplicate would operate efficiently and reliably over a wide range of power output, even for extended duration cycles. Likewise, a single-shaft closed-cycle gas turbine engine using modern and evolving ceramic-based, annular configuration heat-exchange technology could operate at competitive levels of efficiency using a wide range of fuels, including stored thermal energy. The exhaust heat of both types of engines would sustain the operation of a bottom-cycle steam engine, allowing the combined-cycle engine to deliver very competitive levels of thermal efficiency.
While locomotive diesel engines require massive volumes of lubricating oil, turbine engines require a fraction of the amount of lubricant to assure proper operation of engine bearings. Turbine engines also avoid the problem of internal friction and engine wear caused by piston rings sliding on cylinder walls, thereby extending usable service life. While turbine engines are compact, the heat exchange units attached to them will occupy a substantial amount of volume, which would likely be available inside a locomotive carbody. The combination of a gas turbine engine with a steam bottom-cycle engine represents a future propulsion option for railway operation.
Harry Valentine holds a mechanical engineering from Carleton University, Ottawa, Canada, where he undertook post-graduate research in transportation. He has worked in engineering and research capacities for Cummins Engines Company, Langson Energy (turbine engine builder), Quasiturbine Engines Company of Montreal, and in rail vehicle redesign at a rebuilding shop in Montreal. He is widely published in the energy and transportation industries.
