The efficiency of the gas turbine engines that power today’s commercial and military aircraft is approaching the highest level possible with current turbofans. But a totally new technology being pioneered by GE Aviation and researchers worldwide promises far simpler, more efficient engines that will extend aircraft range, cut fuel costs and reduce emissions.
For more than 60 years, the technology of turbine engines in which air is sucked into the engine by a fan, compressed, burned and expanded through a turbine to produce thrust has been a highly successful way of powering both civil and military aircraft. Modern turbine engines are incredibly reliable, spending thousands of hours on-wing, burning fuel with ever more efficiency.
That is unlikely to change in the near future. But GE and other engine researchers are convinced a “game changing technology” may lie in the pulse detonation engine (PDE), which they believe could potentially reduce fuel burn in aircraft engines by as much as 10 percent. Most admit there is a long way to go before the technology is sufficiently mature, however.
“We think it will be at least a decade before we have a flightworthy engine,” explained GE’s PDE program leader Narendra Joshi. “But we think the technology is sufficiently promising to be included as one of six major strategic corporate initiatives now under way at GE. It is one of the big bets we’re making on the future.”
The secret of the PDE (see below) is in the way the fuel actually burns. In conventional engines combustion takes place after fuel and air are mixed in a combustor, the resulting pressure increase being used to drive a turbine. The PDE changes all that by harnessing the immense power resulting from detonating fuel in what is essentially a series of controlled explosions.
GE’s current work dates back six years, when it conducted a program studying the fundamentals of PDE detonation technology with the U.S. government’s DARPA defense research agency. Since then the company has been working both alone and with NASA to push the discipline forward. Two years ago, at GE’s Global Research Center in Niskayuna, New York, the two ran a major test of an eight-tube PDE using an ethylene gas and air mixture in which the exhaust gases drove a supercharger turbine, producing around 1,000 hp.
“We ran for a total of 144 minutes,” said Joshi. “Each test ran for five or six minutes–enough to get steady-state conditions. There were no major show-stoppers. The engine ran just fine.”
GE has also tested a combustor that can detonate commercially viable fuels such as jet-A, JP8, JP10 and diesel. “We also have a much better understanding of the thermodynamics of the engine cycle and learned a great deal from screening tests in which we have combined PDE combustion tubes with GE’s turbines,” said a company spokeman.
There is still a long way to go, however, before a flightworthy PDE engine takes to the air. One of the major issues is the noise resulting from the supersonic detonation of fuel. A lot more understanding is also needed about the technology of pulse detonation. “We’re just scratching the surface on how the PDE works,” said Joshi.
The first application of PDE technology is already in the field in coal-fired power stations, where the extreme sound pressure generated by the detonations is used to remove coal dust particles from the cooling towers, greatly improving efficiency. “But aviation is the Holy Grail,” added Joshi. “If we can save five percent of the $100 billion a year spent on fuel that will be huge.”
Initial applications for the technology will be ground-based, according to Joshi, who predicts a need for between five and 10 years’ more research before useful work is generated by a PDE in a generator or powerplant application. And after that? “Once reliability, durability and other issues are settled, we’ll get into the air,” he told AIN.
How Does a PDE Work?
As its name implies, pulse detonation engines (PDEs) generate extremely powerful, supersonic pulses of energy in a series of shock tubes to which a fuel and air mixture is introduced and detonated. The key is to generate a series of controlled fuel detonations at between 20 and 100 cycles per second and take advantage of the huge pressure rise that occurs, as opposed to simply burning the fuel in what is called a degradational way in a conventional combustor.
In GE’s hybrid engine concept, the resulting supersonic pulses would be used to drive a turbine. Because the series of tubes would replace the conventional high- and low-pressure compressors and combustor, the resulting engine would feature considerably reduced core complexity, bringing benefits in maintenance and reliability.
Research is concentrating on several key areas, including detonation initiation, extracting the resulting energy, optimizing engine configuration and developing advanced computer visualization techniques to model the flow through the engine.