Tractor vaporising oil (or TVO) is a fuel for internal combustion engines, produced from paraffin (kerosene). In the United Kingdom and Australia, after the Second World War, it was commonly used for tractors until diesel engines became commonplace. In Australian English it was known as power kerosene.
The post war Ferguson TE20 tractor, a carefully researched and near-ideal tractor for use on British farms, was designed around a petrol (gasoline) engine. Although there was a campaign for the reintroduction of agricultural Road Duty (tax)-free petrol, which had been curtailed during the war, this was not forthcoming. Perkins Engines supplied some conversions into diesel engines which could use un-taxed red diesel.
As a substitute for petrol, TVO was developed. Paraffin (kerosene) was commonly used as a domestic heating fuel and was un-taxed. Paraffin has an octane rating of zero and would damage an engine built for petrol. The manufacture of paraffin involves the removal of aromatic hydrocarbons from what is now sold as heating oil. These aromatics have an octane rating, so adding some of that otherwise waste product material back in a controlled manner into paraffin gave TVO. The resulting octane rating of TVO was somewhere between 55 and 70.
In practice TVO had most of the properties of paraffin, including the need for heating to encourage vapourisation. As a result the exhaust and inlet manifolds were adapted so that more heat from the former warmed the latter. To get the tractor to start from cold, a small second fuel tank was added that contained petrol. The tractor was started on the expensive petrol, then – once the engine was warm – the fuel supply switched over to TVO. So long as the engine was working hard, like when ploughing or pulling a load, the TVO would burn well. Under light conditions, such as travelling unloaded on the highway, the engine was better on petrol.
Some tractor designs included a radiator "blind" that would restrict the flow of air over the radiator which led to the engine running hotter, which could help with starting. If the radiator blind was left shut, though, there was a risk of engine damage, especially in warm weather.
The phrase petrol-paraffin is often used to describe engines that use TVO. This can be interpreted either as
- the use of the two fuels, starting on petrol then switching to the paraffin-based TVO
- the use of a mixture of petrol and paraffin as a substitute for the proper TVO
TVO was withdrawn from sale by UK suppliers in 1974. An approximation to the correct specification can be made from a blend of petrol and heating oil (burning oil).
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