Although the vessel can be used with a variety of fuel types, combustion involving a gaseous fuel helps assure a homogeneous mixture of air and fuel for further validation of a uniform air/fuel charge during tests. Since it's known that air/fuel mixtures (ratios) do vary within the combustion space of a running engine, the same mixture ratio may or may not be the same in a spark plug's gap from cycle to cycle, and cylinder to cylinder. So in order to remove this variable from the equation, MTU used a gaseous methane and air mixture to ensure uniform mixture homogeneity for these particular tests, again with the intent to remove yet another variable and further improve data integrity.
"We didn't want to inject liquid fuel into the vessel, run a test with one spark plug and then inject liquid fuel again for a comparative test, knowing that it would be difficult to verify that the differences we saw were attributable to variations in charge homogeneity or spark plug technology."
Actually, as you will note from the accompanying images, the results produced by MTU are particularly revealing. Since the images were recorded using the same time-based intervals, it's possible to draw direct comparisons between the flame growth rates of the two spark plugs tested. As previously mentioned, E3 had obtained data from tests conducted at other facilities, especially power data showing increases from the installation of their product alone, and wanted to scientifically verify that increases in horsepower and reductions in emissions were linked with the development of a more rapid flame kernel, utilizing their technology. Judging by the results gathered with the combustion vessel at Michigan Technological University, support for that belief is pretty clear. Installing products like this in otherwise "un-modifiable" racing (or street) engines appears to be a sensible consideration.
Ordinary Spark Plug