Typically they injected a small amount of the usual liquid diesel fuel,
say 5% of the total, which functioned as an igniter to start the natural
gas burning, much like a spark plug. The natural gas was mixed with the
intake air.
They were called dual-fuel or oil-field engines, because they could
http://www.cat.com/cda/layout?m=37508x=7
From reading the caterpillar website more, it actually appears that
their natural gas engines are spark ignition, not compression
ignition. They use the same block and such as the diesel engine, but
different heads and fuel system apparently. It also
