By Becky Stevens

What happens when engineers apply aerospace ramjet technology principles to a power-generation system? The result is the axial compression ram engine (ACRÉ), a revolutionary new power-generation system that promises to be more economical to run, significantly less polluting, and approximately one-third the cost of any existing power-generation system of its size.

The ACRÉ is based on the Brayton cycle, but supersonic propulsion is used to convert chemical energy stored in the fuel to mechanical shaft power. The primary rotor is the ACRÉ's only moving part.

The ACRÉ boasts many technical and economic advantages that should make it a major contender in the $300 billion-a-year independent-power-plant market. The engine's innovative design will produce operating efficiencies of 52 percent when equipped with a carbon/epoxy rotor, and its high performance at reduced throttle levels delivers an operating advantage of more than 63 percent over its nearest competitor in the 10- to 75-megawatt range.

Another advantage is ACRÉ's relatively small size. One 15-megawatt ACRÉ would be large enough to provide electricity for a community of 20,000 people in the United States yet still fit in a one-car garage.

That compact size could have a big impact on small-power-plant economics, particularly in developing countries. Historically, small power plants have not been economically competitive with large central-station generation facilities. Using the ACRÉ, utility planners will be able to locate electric power plants close to the need, reducing the requirement for and reliance on large transmission lines that can cost more than $1 million per mile to install.

In addition, fuel burns cleaner in the ACRÉ than in a gas turbine. When using natural gas, the ACRÉ's projected nitrogen oxide emissions are less than 5 parts per million without expensive postcombustion controls, giving it one of the "greenest" environmental tags in the power-generation industry. It will burn a variety of fuels, including natural gas, biogas, No. 2 diesel, and hydrogen. This multifuel ability could have a dramatic impact on the commercial production of hydrogen as a primary fuel. Industries associated with the conversion of biomass to methane for the generation of electricity could also be more commercially viable as a result of the ACRÉ.

ACRÉ's two main parts are the ramjet engine and a synchronous generator, which connects to the engine and locks it into a defined speed.

The benefits from the ACRÉ also show up on the bottom line. At an initial production cost of $355 per kilowatt ($4 million to $5 million per complete 15-megawatt power plant), the ACRÉ is approximately one-third the cost of competing technologies. This is due to not only the engine's design but also ACRÉ's development strategy, which is not based on production economies of scale to make it commercially viable. TRU Inc. in Bellevue, Wash., is realizing the ACRÉ by using virtual product development to its fullest.

The ACRÉ was created by Shawn Lawlor, a former aerospace engine designer and the president of TRU. It represents the culmination of more than five years and 40,000 man-hours of development. The engine technology is based on the same thermodynamic cycle as a gas turbine (the Brayton cycle), but the mechanical implementation of this cycle is different from other current combustion systems. The ACRÉ uses supersonic-propulsion (ramjet) technology, which until now was used exclusively for military and aerospace applications. It converts chemical energy stored in the fuel to usable mechanical shaft power with the only moving part being the primary rotor. The shaft output from the ACRÉ can then be used to drive any system or machine that requires mechanical shaft power as input.

To bring the ACRÉ to market, TRU is working with E.M. Power Inc. in Bellevue, a firm with expertise in setting up power plants worldwide. TRU has recruited a largely virtual, global product-development and management team.

"Virtual product development is a key premise of our business model," Lawlor said. "It has enabled us to recruit knowledgeable and experienced staff and advisers worldwide." Using CAD/CAM/CAE technology, TRU is employing both in-house staff and global suppliers to design and create a physical prototype of the ACRÉ. "This is rapidly speeding our time to market," he added, "and is a fundamental piece of our strategy to produce the engines quickly, without relying on production economies of scale."

In 1995, Lawlor joined Ken Hicks, an experienced missile/aerospace designer and seasoned CAD user, to help take the ACRÉ design from concept to reality. As senior designer and manager of systems integration, Hicks' first task was to choose a CAD/CAM/CAE system. The firm required robust, flexible three-dimensional design tools that supported industry standards—including the SAT file format for data interoperability—and had superior visualization capabilities.

ACRÉ is expected to produce operating efficiencies of 52 percent when equipped with a carbon/epoxy rotor, and its projected nitrogen oxide emissions are less than 5 parts per million when natural gas is used.

"We needed robust wire-frame-, surface-, and solid-modeling capabilities to adequately handle the ACRÉ's complex 3-D design requirements," Hicks said. Because the company had to work with outsourced suppliers extensively, adherence to industry standards and support of the ACIS-SAT file format (from Spatial Technology in Boulder, Colo.) for moving the 3-D model smoothly through various CAD/CAM/CAE products were critical issues. "Good visualization was key to creating photorealistic images that help presell the ACRÉ concept to investors and power plants, and demonstrate to our virtual outsourcers how the components will work together," he said.

With a large initial investment and bright prospects, cost was not an issue in choosing the design automation solution. Even so, TRU chose a PC-based system comprising CADKEY, FastSURF, and FastSOLID from Baystate Technologies Inc. in Marlborough, Mass., as well as TriSpectives from 3D/EYE in Atlanta for its photorealistic rendering. All the software runs on Pentium-Pro 200-megahertz PCs. The total cost of the system, including hardware, was less than $10,000 per seat.

The ACRÉ consists of two main parts: the ramjet engine and a synchronous generator, which connects to the engine and locks it into a defined speed. "We started the design process in CADKEY, generating 2-D layouts to get a visual idea of what the assembly would look like," Hicks said. "The 2-D cross section is the design bible of the ACRÉ. We went as far as we could conceptualizing the engine in a 2-D cross-section layout, then began constructing the individual components in FastSOLID. With three-dimensional solid models, we could see what the engine envelope looked like and estimated the preliminary weights." The prototype will be approximately 20 feet long and 8 feet in overall exterior diameter, and weigh approximately 40,000 pounds.

The heart of the engine is the rotor, which will spin at rim speeds up to Mach 2.5 (8,905 rpm) for a steel/titanium rotor and Mach 3.5 (12,900 rpm) for a carbon/epoxy rotor. When manufactured, the rotor will be 6 feet in diameter and weigh approximately 4,000 pounds. "The rotor was the most difficult part of the engine to create," Hicks said. "The rotor is an extremely complex shape, comprising a helix and complex curves. In fact, we were having difficulty visualizing it until we created it in FastSOLID and could manipulate the solid elements, since the rotor consists of more than 100 solid elements."

The intake fan, which drives the fuel mixture and combustion process, also posed a design challenge. The original design called for a number of small self-contained fans driven by electric motors. When these fans failed to perform satisfactorily, a 6-foot-diameter intake fan was developed. Lawlor custom-designed the fan-blade shapes, using a combination of computational fluid dynamics and simplified spreadsheet calculations. The definitional geometry was imported into CADKEY via the CADKEY advanced design language, where the blade shapes were completely developed by generating splines from the reference points.

Although models of ACRÉ's intake-fan blades had up to 30 cross sections, engineers created the whole blade contour in just one step with FastSURF software to produce a true airfoil shape.

"Some of the fan blades had up to 30 cross sections," Hicks said. "Using the GENCURV function in FastSURF, we created the whole blade contour in one step, producing a true airfoil shape," Hicks said. FastSURF was also used to perform mass-properties and center-of-gravity analyses to verify the design.

Once the engine's components were verified on the computer, the next check was to verify that they were also manufacturable. Using the 3-D models as a guide, engineers added fabrication details to the 2-D working drawings, which were sent to various manufacturers for quotations and advice on the manufacturability of the design. Since time was of the essence, TRU had to make sure the complex components could be manufactured before dedicating time to develop the solid models. "Solid models have their time and place," Hicks said.

At this point, concurrent virtual product development began. "As we finalized the design of each component, we would send the 2-D drawing to one of our manufacturing partners," Hicks added. "Based on that firm's feedback, we would optimize the 3-D model to make it manufacturable, and in many cases send the model out directly to consultants like JLR Inc. in Everett, Wash., for finite-element analysis. We made more adjustments to the model if necessary, then sent the component out for prototyping."

In this way, the engine was designed on the computer, piece by piece. Engineers verified that each new component fit into the engine envelope and met all design and manufacturing requirements.

ACIS-SAT and IGES were the two principal means by which electronic design data were transferred back and forth with outsourcers. "We found that most of our outsourcers can take a SAT file, which is our preferred method for transporting data because it preserves the model topology and associated attributes between applications," Hicks said. "This file format is also much more compact."

"The SAT file also enables us to bring 3-D shapes with multiple volumes into our ANSYS FEA program from ANSYS in Canonsburg, Pa.," said Doug Scott, an analyst with JLR. "With an IGES translation, you can only read one part at a time, and then you only get the area. With the SAT file, I can bring in several volumes at once, and they are ready to go. It's a tremendous time saver."

As the engine design came alive in the CAD system, photorealistic renderings of the ACRÉ were created to attract additional investors worldwide. "A picture really is worth a thousand words, especially if you don't speak the same language," Hicks said. "Anyone can understand a 3-D model in isometric view." TRU designers brought a SAT file of the ACRÉ into TriSpectives, then created high-quality photorealistic renderings and animations for the investors. Many of those investors wanted to see a demonstration of 2-D/3-D hybrid modeling technology, in which a combination of wire frames, solids, and surfaces is used. "It made the engine a reality to them," Hicks said, "and helped 'validate' our engineering approach and analysis."

The physical prototyping of the rotor and balance of engine is due for completion by mid 1998. The next phase calls for establishing the integrity of the ACRÉ technology by building a limited number of power projects and running the ACRÉ for one year. Full production of the ACRÉ is scheduled to begin in early 1999. The carbon/epoxy rotor is expected to be available for commercial application by late 1998. With early power projects being finalized by E.M. Power in Canada, eastern Russia, New Zealand, and the United States, a whole new dimension has been added to the concept of concurrent product development: commercialization and profits.

Becky Stevens is a consultant in Middletown, Conn., specializing in computer-aided design, manufacturing, and engineering as well as the product-development process.


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