Researchers in the Oregon State University (OSU) College of Engineering have learned the mechanisms behind a means of improved ignition. Their work may lead to better performance in combustion systems ranging from car engines to jet propulsion.

They say their findings are important because reliable ignition underlies the safe and efficient operation of every type of combustion system.

OSU assistant professor of mechanical engineering David Blunck and doctoral student Jonathan Bonebrake traveled to Dayton, Ohio, to work with a research group headed by Tim Ombrello of the U.S. Air Force Research Laboratory.

The researchers used a high-speed infrared camera to record the effects of nanosecond pulsed high-frequency discharges on ignition kernels, the balls of hot gas that form just after ignition but before the development of a freely propagating flame. The fuel in the study was a methane and air mixture.

The team says it already was known that the nanosecond pulsed high-frequency discharges — sparks occurring with a frequency in excess of 10 kilohertz, or more than 10,000 times per second — led to improved ignition. The research question sought to answer why there was enhanced ignition.

Bonebrake and Blunck developed an "inverse deconvolution technique" to determine the changing temperature and size of the ignition kernels based on the infrared images. They tested the technique successfully against experiments conducted with a McKenna burner, a tool used in combustion studies.

"With our camera, we could measure the radiation and then back out the temperatures of those balls of hot gas as those flame kernels were growing," Blunck said. "We could quantify the differences in the temperatures of the kernels."

As the researchers increased the frequency of the spark discharges, more energy and more ignitability occurred. If the sparks were too far apart, they couldn't interact, but when the frequency was higher, energy was deposited more efficiently, resulting in higher temperatures and better ignition.

Not only did increased frequencies lead to an increase in kernel temperatures, but also an increase in how quickly the kernel temperatures grew. The researches say the rate of kernel growth can be crucial to ignition success and combustion completion. That's because the developing kernel can be affected by whatever fluid motion is taking place within the combustion system.

Supported by the Office of Naval Research and the Air Force Office of Scientific Research, the study was built around trying to identify why an improved ignition approach increased the probability of ignition of scramjets, a high-propulsion system that evolved from the ramjet technology of the 1960s.

Both ramjets and scramjets use their vehicles' supersonic airspeed and the shape of the inlet valve to compress air prior to the adding of fuel and ignition in a combustion chamber.