Lean and Clean Propulsion Systems

NASA’s efforts to improve engine design stand out as the Agency’s great­est breakthroughs in "lean and green” engine development because of their continuing relevance today. Engineers are constantly seeking to increase efficiency to make their engines more attractive to commer­cial airlines: with increased efficiency comes reduced fuel costs and increased performance in terms of speed, range, or payload.[1396] Emissions have also remained a concern for commercial aviation. The International Civil Aviation Organization (ICAO) has released increasingly strict stan­dards for NOx emissions since 1981.[1397] The Environmental Protection

Agency has adopted emissions standards to match those of ICAO and also has issued emissions standards for aircraft and aircraft engines under the Clean Air Act.[1398]

Подпись: 12NASA’s most important contribution to fuel-efficient aircraft technol­ogy to date has arguably been E Cubed, a program focused on improv­ing propulsion systems mainly to increase fuel efficiency. The end goal was not to produce a production-ready fuel-efficient engine, but rather to develop technologies that could—and did—result in propulsion effi­ciency breakthroughs at major U. S. engine companies. These break­throughs included advances in thermal and propulsive efficiency, as well as improvements in the design of component engine parts. Today, General Electric and Pratt & Whitney (P&W) continue to produce engines and evaluate propulsion system designs based on research conducted under the E Cubed program.

The U. S. Government’s high expectations for E Cubed were reflected in the program’s budget, which stood at about $250 million, in 1979 dol­lars.[1399] The money was divided between P&W and GE, which each used the funding to sweep its most cutting-edge technology into a demonstra­tor engine that would showcase the latest technology for conserving fuel, reducing emissions, and mitigating noise. Lawmakers funded E Cubed with the expectation that it would lead to a dramatic 12-percent reduc­tion in specific fuel consumption (SFC), a term to describe the mass of fuel needed to provide a certain amount of thrust for a given period.[1400] Other E Cubed goals included a 5-percent reduction in direct operat­ing costs, a 50-percent reduction in the rate of performance deteriora­tion, and further reductions in noise and emissions levels compared to other turbofan engines at the time.[1401]

The investment paid off in spades. What began as a proposal on Capitol Hill in 1975 to improve aircraft engine efficiency ended in 1983[1402] with GE and P&W testing engine demonstrators that improved SFC between 14 and 15 percent, exceeding the 12-percent goal. The dem­
onstrators were also able to achieve a reduction in emissions. A NASA report from 1984 hailed E Cubed for helping to "keep American engine technology at the forefront of the world market.”[1403] Engineers involved in E Cubed at both GE and P&W said the technology advances were game changing for the aircraft propulsion industry.

Подпись: 12"The E Cubed program is probably the single biggest impact that NASA has ever had on aircraft propulsion,” GE’s John Baughman said. "The improvements in fuel efficiency and noise and emissions that have evolved from the E Cubed program are going to be with us for years to come.”[1404] Ed Crow, former Senior Vice President of Engineering at P&W, agreed that E Cubed marked the pinnacle of NASA’s involvement in improving aircraft fuel efficiency. "This was a huge program,” he said. "It was NASA and the Government’s attempt to make a huge step forward.”[1405]

E Cubed spurred propulsion research that led to improved fuel effi­ciency in three fundamental ways:

First, E Cubed allowed both GE and P&W to improve the thermal efficiency of their engine designs. Company engineers were able to sig­nificantly increase the engine-pressure ratio, which means the pressure inside the combustor becomes much higher than atmospheric pres­sure. They were able to achieve the higher pressure ratio by improv­ing the efficiency of the engine’s compressor, which condenses air and forces it into the combustor.

In fact, one of the most significant outcomes of the E Cubed pro­gram was GE’s development of a new "E Cubed compressor” that dra­matically increased the pressure ratio while significantly reducing the number of compression stages. If there are too many stages, the engine can become big, heavy, and long; what is gained in fuel efficiency may be lost in the weight and cost of the engine. GE’s answer to that prob­lem was to develop a compressor that had only 10 stages and produced a pressure ratio of about 23 to 1, compared to the company’s previous compressors, which had 14 stages and produced a pressure ratio of 14 to 1.[1406] That compressor is still in use today in GE’s latest engines, including the GE-90.[1407]

P&W’s E Cubed demonstrator had a bigger, 14-stage compressor, but the company was able to increase the pressure ratio by modify­ing the compressor blades to allow for increased loading per stage. P&W’s engines prior to E Cubed had pressure ratios around 20 to 1; P&W’s E Cubed demonstrator took pressure ratios to about 33 to 1, according to Crow.[1408]

Подпись: 12The second major improvement enabled by E Cubed research was a substantial increase in propulsive efficiency. Air moves most efficiently through an engine when its velocity doesn’t change much. The way to ensure that the velocity remains relatively constant is to maximize the engine’s bypass ratio: in other words, a relatively large mass of air must bypass the engine core—where air is mixed with fuel—and go straight out the back of the engine at a relatively low exhaust speed. Both GE and P&W employed more efficient turbines and improved aerodynam­ics on the fan blades to increase the bypass ratio to about 7 to 1 (com­pared with about 4 to 1 on P&W’s older engines).[1409]

Finally, E Cubed enabled major improvements in engine com­ponent parts. This was critical, because other efficiencies can’t be maximized unless the engine parts are lightweight, durable, and aero­dynamic. Increasing the pressure ratio, for example, leads to very high temperatures that can stress the engine. Both P&W and GE devel­oped materials and cooling systems to ensure that engine components did not become too hot.

In addition to efforts to improve fuel efficiency, E Cubed gave both GE and P&W opportunities to build combustors that would reduce emissions. E Cubed emissions goals were based on the Environmental Protection Agency’s 1981 guidelines and called for reductions in car­bon monoxide, hydrocarbons, NOx, and smoke. Both companies devel­oped their emissions-curbing combustor technology under NASA’s Experimental Clean Combustor program, which ran from 1972 to 1976. Their main efforts were focused on controlling where and in what pro­portions air and fuel were mixed inside the combustor. Managing the fuel/air mix inside the combustor is critical to maximize combustion efficiency (and reduce carbon dioxide emissions as a natural byprod­uct) and to ensure that temperatures do not get so high that NOx is generated. GE tackled the mixing issue by developing a dual annular
combustor, while P&W went with a two-stage combustor that had two in-line combustor zones to control emissions.[1410]

Подпись: 12Ultimately, E Cubed provided the financial backing required for both GE & P&W to pursue propulsion technology that has fed into their biggest engine lines. GE’s E Cubed compressor technology is used to power three types of GE engines, including the GE90-115B, which powers the Boeing 777-300ER and holds the world record for thrust.[1411] Other GE engines incorporating the E Cubed compressor include the GP-7200, which recently went into service on the Airbus A380, and the GE-NX, which is about to enter service on the Boeing 787.[1412] P&W also got some mileage out of the technologies developed under E Cubed. The company’s E Cubed demonstrator engine served as the inspiration for the PW2037, which fed into other engine designs that today power the Boeing 757 commercial airliner (the engine is designated PW2000) and the U. S. military’s C-17 cargo aircraft (the engine is designated F117).[1413]