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When tractors were first developed, they used enormous steam engines that were notoriously unreliable and hard to maintain. These were phased out around the turn with the twentieth century and substituted for internal combustion engines which were smaller though powerful and ran on a number of fuels including kerosene, ethanol and gasoline. By the 1960s several of these engines were phased out and only more effective internal combustion engines that ran on diesel and after this, biodiesel.