ASCE Engine

 A Breakthrough
 The Market

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.

Ramgen Power Systems, Inc. (RPS) is proposing to develop a simple-cycle gas turbine engine that has the efficiency of a diesel, but with the size, weight and maintenance attributes of a gas turbine. This advanced supersonic component engine “ASCE” would combine 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 projected 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 would 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. Ramgen is proposing a 1,000 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.

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 proposed engine could also be applied as both a fixed base or forward deployed stationary power generator within all the branch services.

The proposed 1,000 hp engine for The Defense Advanced Research Projects Agency “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 electric generator/motor, as shown in the above figure, for either electric power generation or hybrid vehicle propulsion applications. The multi-fuel ASCE system promises a Specific Fuel Consumption 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 Ramgen’s 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.

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.

DARPA awarded a contract to Ramgen in January 2006 to complete the notional design definition and layout of its proposed ASCE concept.

Performance & Configuration

The ASCE concept integrates supersonic shock compression and expansion systems into a high pressure ratio, compact, high efficiency Brayton simple-cycle engine. The following figure 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 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 the figure below.

Figure 6 – 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. The following figure 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 is Unique

The reason why conventional axial gas turbines cannot be simply scaled down to these power levels is not obvious, and a function of aerodynamic limits related to Reynolds Numbers, combined with tip clearance effects.

Tip clearance effects are relatively straight forward. Rotor dimensions scale with power, but tip clearances do not scale proportionately. Mechanical design constraints moderate the scaling dimensions such that the effect of clearance is more pronounced in smaller sizes, and this shows up in the form of reduced small engine efficiency.

Less obvious is the effect of the Reynolds Number. The Reynolds Number is an aerodynamic scaling parameter used to characterize the effect of aerodynamic boundary layer build-up and subsequent blockage in flow passages. Reynolds Number effects result in efficiency decrements in smaller sizes which, as a matter of practice, limits the minimum size of equipment offerings.

The National Aeronautical & Space Administration “NASA” has fully characterized the extent of these performance degradations in several representative aerodynamic designs and has published those effects, which are represented in the figure below.

The GE-90/NASA E3 core compressor, which, at 94-96%, is arguably includes the most efficient axial compressor available today, is show at its rated 43,000 horsepower. This engine is then also shown in a hypothetical version, scaled to a nominal 1,500 hp rating, and equivalent to an ASCE engine at that size. The scaled version has been subjected to the NASA’s scaling parameters for the combined effect of tip clearance and Reynolds Number, resulting 78-80% compressor efficiency at this size. The corresponding ASCE engine maintains its compressor efficiency because the Reynolds Number, for its unique configuration, is substantially higher than this corresponding scaled axial design and the loss mechanisms are not as pronounced.

As a result, the ASCE engine design offers the promise of diesel engine efficiency with the size, weight and maintenance attributes of a gas turbine, as indicated in the figure below compares the efficiencies of existing gas turbines under 5,000 hp and the proposed ASCE 1,000 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 figure above illustrates the use of these two spools and its impact on overall engine performance. The cross-over point can be tailored to specific applications by altering the respective pressure ratio split between the spools.