At cruising altitude, airplanes emit a steady stream of nitrogen oxides (NOx) into the atmosphere, where the chemicals can linger to produce ozone and fine particulates. Nitrogen oxides are a major source of air pollution and have been associated with asthma, respiratory disease, and cardiovascular disorders. Engineers have come up with a concept for airplane propulsion that could eliminate 95 percent of aviation's NOx emissions.
The concept is inspired by emissions control systems used in ground transportation vehicles. Many heavy-duty diesel trucks today house post-combustion emissions control systems to reduce the NOx generated by engines. The researchers now propose a similar design for aviation with an electric twist.
Today's planes are propelled by jet engines anchored beneath each wing. Each engine houses a gas turbine that powers a propeller to move the plane through the air as exhaust from the turbine flows out the back. Due to this configuration, it has not been possible to use emissions control devices, as they would interfere with the thrust produced by the engines.
In the new hybrid-electric, or turbo-electric design, a plane's source of power would still be a conventional gas turbine but it would be integrated within the plane's cargo hold. Rather than directly powering propellers or fans, the gas turbine would drive a generator, also in the hold, to produce electricity, which would then electrically power the airplane's wing-mounted, electrically driven propellers or fans. The emissions produced by the gas turbine would be fed into an emissions control system, broadly similar to those in diesel vehicles, that would clean the exhaust before ejecting it into the atmosphere.
Before airplane electrification had been seriously considered, it might have been possible to implement a concept such as this as an add-on to the back of jet engines. But this design would kill any stream of thrust that a jet engine would produce, effectively grounding the design. The new concept gets around this limitation by separating the thrust-producing propellers or fans from the power-generating gas turbine. The propellers or fans would instead be directly powered by an electric generator, which in turn would be powered by the gas turbine. The exhaust from the gas turbine would be fed into an emissions control system, which could be folded up, accordion-style, in the plane's cargo hold — completely isolated from the thrust-producing propellers.
The bulk of the hybrid-electric system — gas turbine, electric generator, and emissions control system — would fit within the belly of a plane, where there can be ample space in many commercial aircraft. If such a hybrid-electric system were implemented on a Boeing 737 or Airbus A320-like aircraft, the extra weight would require about 0.6 percent more fuel to fly the plane. This would be many times more feasible than what has been proposed for all-electric aircraft. The researchers also calculated the emissions that would be produced by a large aircraft, with and without an emissions control system, and found that the hybrid-electric design would eliminate 95 percent of NOx emissions.
The team is now working on designs for a zero-impact airplane that flies without emitting NOx and other chemicals like climate-altering carbon dioxide.