Gas Turbine Engine

 ASCE Engine

Advanced Supersonic Component Engine - A Breakthrough in Military Propulsion

The gas turbine has always been the preferred alternative for marine propulsion applications, offering superior size, weight and maintenance advantages over reciprocating diesel engines, but it has been unable to compete at power levels less than 10MW, primarily due to its high fuel consumption.

At the same time, larger aero-derivative gas turbines are successfully applied at 25MW and above where the size and weight of diesel engines is prohibitive, despite the fact that the fuel consumption of the turbine is, at best, considered a compromise. Improvements in efficiency of these engines to the 40% level have been achieved through increases in both pressure ratio and firing temperature, which also results in a growth in rated engine output.

Recuperated gas turbines have been attempted for stationary power in sizes up to 5MW at 30-35% efficiency, but these are not considered reliable enough for marine propulsion applications, or efficient enough to compete with reciprocating diesel engines.

Although supersonic compression and expansion aerodynamics and high-swirl/high velocity stabilized combustion are fundamentally well established, these technologies have never been integrated into one cycle for generating shaft power. The purpose of the project is to deliver a new engine that will meet future DOD requirements for long endurance, high efficiency, and long time-between-overhaul power generating and propulsion applications.

The Defense Advanced Research Projects Agency (DARPA) awarded a “seedling” contract to Ramgen Power Systems in January 2006 to complete the notional design definition and layout of its proposed ASCE concept.

This Advanced Supersonic Component Engine (ASCE) combines many of the proven features of shock wave compression and expansion systems, commonly used in supersonic flight inlet and nozzle designs, with turbo-machinery practices employed in conventional axial flow gas turbines. The superior efficiency of the ASCE is a result of high pressure shock wave compression and supersonic expansion phenomena to produce high component efficiencies, and a unique engine configuration designed to minimize flow stream turning losses throughout the engine.

The four technologies that make up the ASCE are:

• Rotating supersonic compressors
• High velocity flow combustor
• Rotating supersonic expanders
• Direct connect to high speed PM electric generator/motor

The multi-fuel ASCE concept promises to revolutionize military and commercial land-based and sea power propulsion systems and is scaleable from 300 hp to 30,000 hp shaft power requirements. The scalability of the ASCE will make it competitive in both the diesel and gas turbine markets. RPS has proposed a 1000 hp demonstration program as both a popular rating with immediate Naval manned and unmanned combatant craft applications, and a size that would offer a manageable financial exposure.

The proposed 1000 hp engine for DARPA is a two stage counter-rotating, 30:1 pressure ratio, ~40% simple cycle efficient engine which drives a high-speed direct drive permanent magnet (PM) electric generator/motor, as shown in Figure 1, for either electric power generation or hybrid vehicle propulsion applications. The multi-fuel ASCE system promises a Specific Fuel Consumption (SFC) equal to or better than the fuel consumption of current reciprocating diesel engines in this size range, but with a 10:1 weight reduction and a 4:1 improvement in time-between-overall maintenance. This is a 2:1 increase in fuel efficiency at full power over existing gas turbines in this size range. Pressure ratios of 40:1 with ~45+% cycle efficiencies are considered feasible growth engine objectives.

The anticipated compression and expansion efficiencies, decreased footprint, and reduced part count of the RPS technologies promise revolutionary new power generation and propulsion systems with decreased heat signatures resulting from lower exhaust temperatures. These unique aerodynamic features will open up new and creative options for engine designers and package integrators that have never been realized or considered. In addition, the reduced footprint and the potential for vertical versus horizontal engine mounting will provide improvements and options to the costly intake and exhaust systems that are difficult to integrate into highly compact military vehicles and vessels.

Additional military opportunities that have been identified are Army TACOM/TARDEC hybrid vehicles, USAF airborne electric power generation systems for the directed energy weapons systems and all branches of the USSOCOM. The engine can also be applied as both a fixed base or forward deployed stationary power generator within all the branch services.

Performance & Configuration
The ASCE concept integrates supersonic shock compression and expansion systems into a high pressure ratio, compact, high efficiency Brayton simple-cycle engine. Figure 5 shows the relationship between cycle pressure ratio and resulting system efficiency for existing Brayton Cycle, land-based and flight engine systems at their design Turbine Inlet Temperature (TRIT).

The solid red line shows the theoretical Brayton Cycle efficiency limit as a function of pressure ratio at a fixed TRIT of 2000°F. Both the land-based and the flight engines have been plotted along with a Brayton Cycle efficiency estimate based upon the component efficiency assumptions indicated. These “curve fits” indicate a reasonable agreement with those assumptions.

Brayton Cycle Efficiency

This type of analysis, although popular and easily performed, does overlook the impact of engine power rating. The current recipe for increasing gas turbine efficiency has been, and likely will remain, to increase the turbine firing temperature (TRIT), which focuses much of the research and development effort on increases in high temperature materials capability. For each TRIT, however, and for a given set of component efficiencies, there is also an optimum pressure ratio to achieve either maximum thermal efficiency, or maximum power density, expressed as specific work and as shown in figure 6 below.

Efficiency and Specific Work vs. Pr and TRIT

As an example, the optimum pressure ratio to maximize thermal efficiency at a temperature ratio of 4:1 is 15:1. Temperature ratio is defined as the absolute TRIT divided by the absolute Engine Inlet Temperature, and a temperature ratio of 4:1 equates to a TRIT of 2017?F. The compressor and turbine component efficiency in this representation are 85 and 90% respectively, but without any system pressure losses included.

The important understanding from such analyses is that pressure ratio typically rises along with increases in TRIT. This does improve the efficiency, but it also increases engine power. The result of this is that there are no gas turbine engines available at the lower power levels and diesels predominate in these applications. Figure 7 is a plot of those same land-based and flight engines plotted vs. engine rated power, and as can be seen, there are no small and efficient gas turbines.

Thermal Efficiency vs. Rated Power

The ASCE engine concept is also shown on the same plot, indicating the potential for this engine to compete with small diesels. Diesel engines are not purchased on preference and not many users rush out to buy them just because they like them. Diesels are purchased because they offer the best value in these sizes, even though their emissions, size, weight and maintenance costs are considered serious negatives.

The ASCE engine offers the promise of diesel engine efficiency with the size, weight and maintenance attributes of a gas turbine, as indicated in the following graph compares the efficiencies of existing gas turbines under 5000 hp and the proposed ASCE 1000 hp engine, indicating a factor of 2x improvement in efficiency.

The two-spool simple-cycle configuration may also allow Ramgen to configure the system with two different power settings, to better match the specific mission profiles. Many of the military applications feature both a loitering power level and a maximum power level. While the lower power fuel economy is of great interest, some of these applications require that full power be available instantaneously. Ramgen is exploring concepts that would provide a unique capability to satisfy these requirements

The Military Market
The military market is difficult to project. The development process is not publicly available and the procurement process is both lengthy and political. It is possible however to make some observations to provide some insights as to the potential applications.

The applications can be broadly defined as either propulsion or power generation.

Small Craft & Littoral Hybrid Electric Propulsion – The size, weight and maintenance requirements of diesel engines now serving this application, are seen as limitations on both the system design and the operational flexibility of these smaller surface craft.

Future long endurance Unmanned Surface Vehicles (USVs) must operate semi-autonomously for long periods of time. Challenges to be overcome include: storing adequate energy for the mission assigned and efficiently converting that energy for use; reliability and affordability; self diagnostic and reconfiguration capability; and, mission effectiveness. Much as hybrid gasoline internal combustion-electric power plants are leading the way to fuel-efficient automobiles, marine hybrid power systems can meet the requirements of long-endurance USVs.

Directed Energy Weapons Support Systems – These are laser-based weapons systems that require substantial electric power to support their operation. These are typically in the form of an APU. Airborne applications have stringent space and weight constraints, whereas naval applications have somewhat more latitude. The typical airborne application is 2MW, but can be as high as 20MW. The operating envelope requires that the units be capable of these power outputs at a variety of altitudes, depending on the mission and/or the host platform. The ASCE engine is seen as completely consistent with these design objectives.

Of particular interest in the airborne APU is the potential of the Ramgen engine to respond to load changes at the speed of the weapon, eliminating heavy and expensive load following sub-systems that include lithium ion batteries, ultra-capacitors and load banks.