Biodiesel: When winter clogs your fuel filter

Dr Garth Cambray

Cold weather can be a pain in your fuel system, especially if you use biodiesel. Here we look at how to avoid problems in cold weather. As the final long months of winter sink their teeth into Southern Africa many biodiesel users and producers in the region have discovered the irritating fact that at low temperatures biodiesel solidifies, clogging the fuel system and stopping the vehicle it was in until warmer weather thaws the fuel out. This article looks at the concepts of cloud point and plug point in biodiesel, and ways that the average person can get around these problems.

Biodiesel is a fuel made by reacting alcohol with plant or animal fats. The alcohol forms esters with the fatty acids in the fat, and this ester is what we call biodiesel. If one looks at a beef fat, it is solid at 20 degrees Celsius, whereas sunflower oil is liquid at this temperature. All plant and animal fats have structures which contain a mixture of different fatty acids, with different melting points. When converted into biodiesel alcohol esters, the melting points of these fatty acids are lowered, but in the case of the fatty acids in a fat such as beef fat, many of the biodiesel alcohol esters which form will still solidify at temperatures above 15 degrees Celsius.

In a diesel engine fuel is filtered through a fuel filter which removes all particles above a specific size. In a biodiesel mixture consisting of various fatty acid alcohol esters there will be a specific temperature at which sufficient quantities of esters have solidified to actually clog the filter. This is referred to as the plug point. In some cases the fuel will go cloudy but will still be able to pass through the filter - this is referred to as the cloud point.

To understand exactly how a filter clogs, it is important to look at the structure of a filter. Fuel filters are normally quite rough filters removing all particles above a specific size such as 30 microns in a typical prefilter and 15 microns in a typical final filter. The rate of flow through the filter is called the flux. As the filter becomes clogged, the flux will decrease - the fuel pump will also work harder to push fuel through the filter.

Figure 1 In this figure a new filter is exposed to fuel with various solids in it represented by the oblong shapes.

Figure 2 In this figure, some of the solids have been filtered out of the fuel by the fuel filter, whereas small solids have passed through the filter. The flux will be decreasing at this point.

Figure 3 In this figure, the filter has become largely clogged with solids. Liquid can still flow around the particles, and flux is maintained by increasing pressure.

Figure 4 In this figure, increased pressure applied by the pump in order to maintain flux has caused the filtered solids to compact and foul the filter completely so that no fuel can pass through it. At this point your vehicle will stop completely. Just prior to being completely fouled, the quality of fuel produced is the highest, as the fouled filter allows very few particles of any size through its clogged membranes.

Two samples of biodiesel cooled to 9°C. The sample on the left has no fuel system cleaner and the sample on the right has 2ml/l Acdelco fuel system cleaner. The clearness of the treated sample shows that the fuel system cleaner has stopped the solidification of some of the biodiesel esters removing the cloudiness seen in the untreated sample on the left.

A number of simple tricks can help avoid the irritation of having your vehicle temporarily stop due to cold weather. In the simplest case, one or two kettles of boiling water can be poured against the metal sides of the fuel filter, warming the filter and allowing the plugged fuel to liquefy and pass through into the engine. Once the engine is going it normally generates enough heat to keep the filter warm and thus stop the problem.

In patches of very cold weather it may be necessary to put an additive in the fuel tank to help the fuel system. A number of different over the counter products are available at your nearest automotive store. Products which we have used include ACDELCO, Wyns, Midas and Spanjaard Diesel fuel system cleaners. These products contain solvents which lower the plug point of your fuel, allowing easier driving at close to and sub zero temperatures. Protea specialist chemicals markets a range of pour point suppressants which are predominantly used in gear box and industrial oils, but can help in biodiesel. Another product tailored specifically for biodiesel is under development by South Africa's private biodiesel testing guru, Irene Finnegan -

In very cold weather, if these problems persist, it may be advisable to switch to using normal petroleum diesel until warmer weather arrives. Petroleum diesel also tends to gel and plug filters at low temperatures, but these temperature tend to be around 5 degrees Celsius below that at which biodiesel begins to cause problems.

Andrew Sias (left) and Godfrey Brockman (right) show a range of fuel system cleaners and filters. To extend filter life when using biodiesel, fuel system cleaners such as those shown can be very helpful, especially in winter when cold temperatures increase fuel viscosity.

It also helps to park your vehicles sensibly - park in sunny places where the vehicle will benefit from the warmth of the sun lowering the viscosity of the fuel. At night time, park the vehicle in a garage if possible, and in extreme cold, place a panel heater or heater fan near the fuel tank to keep fuel liquid. The heater can be set on a timer switch to turn on 3 hours before the vehicle is needed in the mornings.

We hope these solutions ensure that you continue enjoying biodiesel, even in winter!

More information:

  Dr Garth Cambray is a fermentation biotechnologist at Makana Meadery and consults to the biofuel industry.



August 2007