Tractor vaporising oil (TVO) is a fuel for petrol-paraffin engines. It is seldom made or used today. In the United Kingdom and Australia, after the Second World War, it was commonly used for tractors until diesel engines became commonplace, especially from the 1960s onward. In Australian English it was known as power kerosene.
History
TVO existed for at least fifteen years before it became widely used. A 1920 publication mentions it as a product of British Petroleum. But it was not until the late 1930s that it first became widely used. 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, the Standard inline-four. 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 untaxed red diesel.
On the early Fordson model N, the tap which changed over from petrol to TVO was marked G for gasoline and K for kerosene, reflecting that these tractors had their design origin in the USA. In the UK tractor vaporising oil was usually called TVO.
Octane rating
As a substitute for petrol, TVO was developed. Paraffin (kerosene) was commonly used as a domestic heating fuel and was untaxed. Paraffin has a low octane rating 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.
{| class="wikitable"
|+Octane ratings of various fuels
|-
! Fuel
! Octane rating
! Notes
|-
| Petrol
| 98
| -
|-
| Kerosene
| 15-20
| -
|-
| Paraffin
| 0
| -
|-
| Diesel fuel
| 0
| -
|}
{| class="wikitable"
|+Octane ratings of petrol/heating oil mixtures
|-
! Petrol<br/>(pbv)
! Heating oil<br/>(pbv)
! Octane rating
|-
| 1
| 0
| 98
|-
| 2
| 1
| 72
|-
| 1
| 1
| 59
|-
| 1
| 2
| 46
|-
| 0
| 1
| 20
|}
- pbv = parts by volume
The words paraffin and kerosene are often used interchangeably but the tables suggest that this is incorrect because they have different octane ratings. However, kerosene and heating oil have similar octane ratings. Paraffin, kerosene and petrol (gasoline) are all rather loosely defined. For example, gasoline may have an octane rating between 88 and 102.
Engine modifications
Compression ratio
Because TVO has a lower octane rating than petrol, the engine needs a lower compression ratio. On the TVO version of the Ferguson TE20 tractor, the cylinder head was re-designed to reduce the compression ratio to 4.5:1. This reduced the power output, so the cylinder bore was increased to 85 mm to restore the power. In the UK there is an exception that permits the use of rebated kerosene and fuel oils in vintage vehicles.
North American distillate fuel
In North America a similar product, called distillate, was produced. Of lower quality than TVO, its octane rating varied between 33 and 45. Manufacture of tractors using distillate ended by 1956, when gasoline and diesel-engined tractors had captured the North American farming equipment market.