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Can we reduce jet engine pollution while improving fuel efficiency?

A new type of combustion chamber presents a potential win-win in the alternative fuel technologies space.

The combustion chamber inside a jet engine funnels a hurricane of heat into the thrust that pushes planes through the atmosphere. | iStock/Dexion5

Inside the combustion chamber of a jet engine, fuel and air burn with super-heated intensity to provide the thrust to push the aircraft quickly, smoothly and safely through the atmosphere.

“It’s like a high-temperature hurricane in there,” said Matthias Ihme, an associate professor of mechanical engineering at Stanford.

But is there a way to make that hurricane more efficient and environmentally sustainable? Ihme is among a handful of researchers working with NASA on alternative fuel technologies. His lab’s first goal is to curb emissions of NOx compounds, or nitrogen oxides, which attack the ozone layer that shields Earth against harmful radiation, and to reduce NOx emissions in a way that also ends up improving the energy efficiency of jet engines.

Jet engines are already pretty efficient, but Ihme and team believe they have found a way to more precisely control the ratio of air to fuel inside the combustion chamber — the critical variable that governs emissions, efficiency and safety. Sadaf Sobhani, a graduate student in Ihme’s lab, has designed a ceramic matrix that is carefully perforated to allow the fuel-air mixture to flow through lengthwise. The structure contains innumerable tiny cavities along the air passageways where fuel sprayed into the matrix can burn without being extinguished by the hurricane-like forces. Essentially, the ceramic foam will heat up and immediately warm the air rushing into the burn space. (Preheating the air makes it easier to burn.) That means less fuel must be squirted into the chamber to support the same amount of thrust exiting the engine.

Ihme estimates that the new design will improve engine efficiency by more than 10 percent, thereby getting more thrust from less fuel. Burning less fuel will also reduce the engine’s emissions of carbon monoxide and NOx compounds, though more study is needed to make reliable estimates of by just how much.

Right now, the project is at the stage of small-scale prototype research. The experimental ceramic matrix Sobhani designed fits roughly in the palm of her hand. To design and make these heat-resistant matrix foams, the Stanford team used the most up-to-date techniques in additive manufacturing, more commonly called 3D printing. They are now using these tiny prototypes to explore the fundamentals of how a flame behaves inside the voids of the ceramic chamber, specifically by measuring the internal temperature distribution.

Here, they had to develop a new way to measure temperatures inside the ceramic matrix. Typically, researchers would use optical systems to study the flame. But since combustion occurs inside cavities within the ceramic matrix, optics won’t work. So Ihme’s group is shooting X-rays through the matrix, and adding heavy krypton gas to the air-fuel mixture inside their prototype combustion chamber. The heavy gas absorbs the incoming X-rays and acts as an in situ temperature probe for making the combustion process visible. Using this diagnostic technique, they are able to map the internal temperature distribution of a flame embedded inside the cavities of any opaque material.

Ihme’s group recently worked with NASA to run a small-scale test of how liquid fuel burned inside their ceramic chamber under the sort of high-heat, high-pressure conditions that prevail inside a jet engine during takeoff. The researchers are using the data from these combustion chamber experiments to develop simulations that will enable them to predict the durability and performance of a jet-sized version of their technology. “No one is going to go to the expense of building a test engine on this design unless they have data to suggest it’s worth the effort,” Ihme said.