Using straight vegetable oil (SVO) – crude oilseed rape oil, for example – as a diesel substitute is not a new phenomenon. Every time there is a fuel shortage, which happens pretty frequently, there is renewed interest in its use.

It happened during the 1930s and 1940s and again in the 1970s and early 1980s, the last of these dates marking the point when the level of scientific interest in the use of vegetable oil was probably at its highest.

Since the 1980s, though, with falling fuel prices and the realisation that biodiesel (a processed version of SVO) could be used without having to make any serious modifications to engines, research into using straight vegetable oil to power engines almost disappeared.

But now there is pressure on engine manufacturers to produce designs that can run on SVO as well as biodiesel. Not only that, but growing your own tractor fuel has an undeniable attraction for many farmers, especially in the light of hefty rises in red diesel costs.

However there is growing concern that HM Revenue and Customs will be anxious to retain its slice of fuel duty when SVO-powered vehicles are used on the road. It argues that vegetable oil does not meet the official definition of biodiesel and is instead a fuel substitute which is taxable at the normal diesel rate. This view is challenged by the UK Pure Plant Oil Association.

Economics aside, many vegetable oils, including oilseed rape oil, have similar fuel properties to diesel fuel. The main differences are their higher viscosity (they’re thicker) and lower oxidative stability (they need to be stored correctly).

To use them in a tractor or car, the viscosity must be lowered to allow the oil to atomise properly. With diesel engine compression ratios producing pressures of more than 600psi and temperatures of 540C, trying to spray SVO in raw form into the cylinders would give incomplete combustion and carbon build-up and eventually wreck the engine.

The simplest way to lower viscosity is to pre-heat the oil before it is used by using a separate supply of diesel fuel to start the engine and generate the required heat. Several manufacturers do this.

Equally, there are engines which can run on SVO from cold by using a fuel heater to pre-heat the fuel. These modified engines – usually those with indirect injection – are said to run successfully at temperatures as low as -10C, though more frequent oil changes are usually required and engine maintenance takes on a new importance.

But the current trend is for a two-tank system, with one tank for diesel fuel and another larger one for the vegetable oil.

The system has been used to power various types of vehicle in the past but it is only recently that it has been used in diesel engines to power tractors.

Both Fendt and Deutz-Fahr now offer a tractor powered by SVO, but using diesel fuel for engine start and warm-up, and again just prior to shut-down to clean out the rape oil from the injection system and allow the engine to be started on diesel once more.

How it works

The main tank on tractors with Deutz-Fahr’s Natural Power engine holds the crude rapeseed oil and accounts for 90% of the total fuel volume. The diesel tank is much smaller and holds 10% of fuel volume.

On start-up, the diesel leaves its tank and passes through a solenoid-activated valve. Because the diesel is being used to start the engine, this valve allows the fuel to continue first through a pre-filter, then the pump, then the main fuel filter and finally to the common rail injector system – a pretty conventional diesel system.

While this is happening, the cooling water which circulates the engine is diverted through a heat exchange system to warm up the vegetable oil. When sensors report that the engine and the vegetable oil is warm enough to reduce its viscosity to acceptable levels, the solenoid valve on the diesel flow is automatically closed and a similar valve on the vegetable oil feed is opened to allow the engine to continue running on oil.

The return system for unused fuel is controlled by a valve system which ensures both fuels return to their respective tanks.

When the engine is idling and the temperature of the vegetable oil is in danger of dropping below set limits, the diesel system can be set to cut in. And, just before engine shut down, the operator switches to diesel once more to flush the rape oil from the injector system.

Deutz-Fahr says there is no loss of power and as a mark of confidence in the system, the company also offers a two year warranty.