Certain manufacturers made farm tractors specifically designed to run on heavy, low-octane fuels which were commonly called "distillate" or "tractor fuel". Other names were tractor vaporising oil (United Kingdom) and "power kerosene" (Australia). Often the tractors were referred to as "all-fuel". The most important factor in burning heavy fuels in a spark-ignition engine is proper fuel vaporization. Tractors designed to run on those fuels usually used a "hot" intake air manifold that allowed exhaust heat to warm the manifold and carburetor to aid vaporization. Given the poor vaporization at low temperatures, all-fuel tractors were started on gasoline, then switched to the heavy fuel. They were equipped with a small gasoline tank and a large fuel tank, both of which fed into a common valve supplying the fuel to the carburetor.