IT has always been stated by others that it is necessary to use weak mixtures with air when hydrogen is used as a fuel for Otto-cycle engines. Thus Ricardo1 reported that “if an attempt were made to run with a rich hydrogen–air mixture violent pre-ignition occurred, accompanied by firing back in the carburettor, which rendered further running impossible. Even with the compression ratio lowered to 3.8 : 1, the same thing occurred”. Similar effects were obtained later by A. F. Burstall2 when using a Ricardo E.35 engine in the Engineering Laboratory, Cambridge, and still later by Egerton, Smith and Ubbelohde3, when using an ethyl knock-testing engine for experiments at Oxford.