Petrol-paraffin engine

Waterloo Boy tractor produced by John Deere, 1919-1920. The tractor has a small petrol tank, used for starting the engine and a larger tank (at the front) for paraffin to run it

Paraffin (kerosene) tank on Waterloo Boy tractor

A petrol-paraffin engine (United Kingdom) or gasoline-kerosene engine (North America) is an old-fashioned type of dual-fuel internal combustion engine with spark-ignition, designed to start on petrol (gasoline) and then to switch to run on paraffin (kerosene) once the engine is warm. The grade of paraffin used is known as tractor vaporising oil in the UK.

Advantages

The advantages of the petrol-paraffin engine are that (compared to petrol):

1. Paraffin can be cheaper and/or more readily available
2. Being less flammable, paraffin is safer to store
3. Being less volatile, paraffin is less likely to go "stale" in the tank.

Some of these advantages have become illusory since paraffin, once widely available as a cheap fuel, has become rarer and more expensive, particularly in developed countries. Also, while some older vessels still use marine petrol-paraffin engines, most inboard marine engines now tend to be diesels.

Equipment

A cutaway view of the intake of the original Fordson tractor (including the intake manifold, vaporizer, carburetor, and fuel lines).

A petrol-paraffin engine differs from a single-fuel petrol engine in that two independent fuel tanks containing petrol and paraffin (respectively) are required, but both fuels may be supplied through the same carburetor or fuel injection system. An example of a fuel-injected petrol-paraffin engine is the Hesselman engine.

Paraffin is less volatile than petrol, and will not normally ignite at ambient temperatures, so the petrol-paraffin engine is started using petrol, and only when the engine has attained a sufficient operating temperature will the engine be switched to paraffin. This switching can be done manually or automatically. Some engines use a vaporizer, which uses heat from the exhaust manifold to vaporize the fuel entering the intake system.

Applications

Traditional applications

Petrol-paraffin engines were traditionally found in motor boats, fishing vessels, small tractors, light railway locomotives, and stationary auxiliary engines,^[1][2][3] but not in cars or motorcycles.

Future applications

Most light aircraft have piston engines that burn gasoline - either motor grade ("mogas") or specific aviation type ("avgas"). In Europe, avgas is very expensive and is being phased out. Mogas often has an alcohol content that can cause airlocks in fuel lines; and the Civil Aviation Authority has prohibited mogas containing alcohol. Jet fuel, which has similarities to both paraffin and road diesel (DERV), is much cheaper and more widely available than avgas; and opportunities now arise to develop light aero-engines that can burn jet-fuel.

One approach is to develop Diesel aircraft engines, such as the Wilksch Airmotive WAM,^[4] the "Diesel-AIr"^[5] and the Thielert Centurions.^[6] Alternatively, one may create a petrol-paraffin aero-engine which starts on petrol before switching to jet fuel. A UK firm tried running their MidWest AE series Wankel engine on jet fuel; but tests were never completed. As Avgas becomes unobtainable, it may prove cost-effective to convert existing gasoline piston aero-engines into petrol-paraffin units.

Design

4-Stroke Cycle

Petrol-paraffin fuelling is suitable for four-stroke cycle piston engines and wankel engines. A petrol/paraffin engine tends to run hotter whilst burning paraffin, and so the cooling system must be sufficiently robust. Being slower burning, the paraffin requires the combustion longer period that a four-stroke engine can provide; so two-stroke versions are unknown. Although modern petrol engines may have compression ratios typically between 9:1 and 12:1, a petrol-paraffin engine requires a lower compression ratio of 8:1 or less, to avoid pre-ignition of the fuel-air mixture which would cause damage from engine knocking. Most existing petrol aero-engines have low compression ratios, around 8:1 or 9:1, making a dual-fuel conversions viable.

Fuel

The fuel used in petrol-paraffin engines was known as Tractor vaporising oil (TVO) in the United Kingdom and as Power kerosene in Australia. TVO was withdrawn from sale by UK suppliers in 1974 but has been re-introduced by at least one supplier.^[7]

References