All it takes is the right piston, a little tweaking of the cylinder and some trick fuel injection timing. That’s the general idea behind something called Sonex Controlled Auto Ignition, which can best be described as residing somewhere between conventional spark ignition and true diesel cycles, according to Sonex’s Andrew Pouring, who recently sent us a white paper on the technology. SCAI doesn’t exactly mean you could convert your IO-550 to burn kerosene, but a purpose-built SCAI aircraft engine could take the same form factor and, more important, would be nearly as light as a gasoline engine. How do they do that? By controlling the combustion event and keeping cylinder pressures under 1000 PSI, just as in gasoline engines.
The cycle is a cross between constant volume Otto cycle engines and constant pressure true diesel cycle technology. The piston has a complex combustion bowl that allows time for the incoming, direct-injected fuel charge to break down into chemical radicals during the compression process. Small inlets in the bowl, called microchambers, take in a portion of the fuel charge and pay it out during the power stroke, improving efficiency and, along with precise injection timing, smoothing out the spikey pressure pulses of the typical compression ignition engine. Pouring and his lab developed the technology in a research project extending back to the late 1970s. A derivative of it is used in the ScanEagle UAV built by Boeing. Increasingly, UAVs need the fuel economy of diesels but they can’t take the weight hit, so so-called radical ignition technology looks attractive because it can burn any fuel from gasoline to palm oil and perform similarly. Pouring would like to see the technology commercialized.
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Podcast interview with Dr. Andrew Pouring
