Question: What fuel did the earliest four-stroke internal combustion engine use?

1876, Otto, a German, made the first four-stroke reciprocating piston internal combustion engine (single cylinder, horizontal, gas as fuel, power about 2.2 1KW, 180r/min). On this engine, Otto added a flywheel to make it run smoothly, lengthened the air intake and improved the cylinder head to make the mixture fully formed. This is a very successful engine, and its thermal efficiency is twice that of the steam engine at that time. Otto integrated the three key technical ideas of internal combustion, compressed gas and four stroke, which made this internal combustion engine have a series of advantages such as high efficiency, small size, light weight and high power. At the 1878 Paris World Expo, it was praised as "the greatest achievement of power machine since Watt". The four-stroke cycle of constant volume combustion is realized by Otto cycle, also known as Otto cycle.