Winter operability

Consumers expect their vehicles to start and run no matter how cold it gets. However, in recent years a significant number of operability issues have been reported during some cold winter months. This has particularly been the case in Germany, where a significant number of vehicle breakdowns have occurred.

Sometimes vehicles have had difficulty starting or they have stalled soon after start up – issues that are attributed to fuel starvation resulting from the build-up of wax crystals on the fuel filter. OEMs, refiners and additive companies are working together to improve protection in the marketplace.

Wax forms in diesel fuels at low temperatures

Diesel fuel is a complex mixture of hydrocarbon molecules containing 15-30% n-alkanes, straight long chain molecules with carbon chain lengths from C8 to C36. The absolute proportion of these depends on the crude oil source and the degree of refinery processing.

Thin, flat wax crystals of up to 5mm in diameter can form in untreated diesel fuels at low temperatures

When distillate fuels are subjected to low temperatures, some of the higher n-alkanes separate from the fuel as wax crystals. These crystals grow as thin, flat, rhomboid plates, and as the temperature falls they grow quickly, reaching up to 5 mm across. Because the wax crystals are flat, only a relatively small number are required to cover the fuel filter surface with a thin, almost invisible wax cake. This can lead to fuel starvation, loss of power and eventually stalling of the engine.

Wax crystal modification with cold flow additives

Cold flow additives are used to modify the shape of the wax crystals so that fuels keep flowing at lower temperatures. They typically contain ashless copolymers of ethylene and vinyl acetate that are absorbed onto the fast growing sides of the paraffinic wax crystals. By blocking the preferred growth sites they force growth out of the crystal plane and slow down the growth rates. The result is a higher number of smaller, needle-shaped crystals, which form a porous filter cake. This can build to a relatively thick layer before fuel flow through the filter is restricted.

Cold flow additives modify the wax crystals to compact needle shapes, which allow the fuel to pass through the wax layer

Because the fuel is still able to flow through the filter cake, the engine has time to warm up. This means that the hot fuel returned from the fuel injection equipment (FIE), to either the tank or in some cases directly to the filter, is sufficient to melt the wax before problems occur.

Increasingly complex fuels

In recent years diesel fuels have become more complex as fatty acid methyl ester (FAME), hydrogenated vegetable oils (HVO), gas-to-liquid (GTL) etc. have been increasingly introduced into diesel blends. The use of biofuels can significantly affect the cold flow performance of fuels. For FAMEs, the extent of their impact depends on the type of feed stock used, in particular its fatty acid distribution and percentage of saturates. Like n-alkanes, saturated FAMEs crystallise upon cooling and form part of the filter cake on fuel filters. Even rapeseed methyl ester, which has the lowest proportion of saturates, can affect cold flow performance, particularly in difficult to treat fuels.

Another effect of FAME use is precipitation of impurities at temperatures above the cloud point, which can cause further filterability issues. It is widely accepted that FAME with a high content of saturated mono-glyceride and/or sterol glycosides can cause filter blocking. Infineum supports industry activities aimed at developing performance based tests rather than relying solely on the limitation of selected impurities.

Vehicle operability

Cold weather vehicle operability is defined as the minimum temperature at which a vehicle can perform satisfactorily. However, vehicle operability is highly dependent on a variety of variables including: vehicle type, vehicle low pressure system architecture, FIE technology, fuel used, whether the fuel system includes a heater and the type and size of filter fitted. This means that different vehicles can have widely different cold temperature operability limits when running on the same fuel.

Consumers expect their vehicles to start and run no matter how cold it gets

To ensure that market representative fuels operate robustly in the vast majority of vehicles, operability tests are run in vehicles with ‘severe’ fuel system designs. Cold flow additives are essential to ensure satisfactory low temperature operation.

Cold flow performance tests

Natural phenomena, related to the wax content of the fuel, can be used to control cold flow performance. These include ‘cloud point’, which is the temperature at which the first wax crystals appear and ‘pour point’, which is the temperature below which the fuel will not flow – with vehicle operability lying somewhere between these two parameters.

Cold filtration tests have been developed to assess the amount and the effect of the wax formed at low temperatures. The tests used are Cold Filter Plugging Point (CFPP), Simulated Filter Plugging Point (SFPP), and the Low Temperature Flow Test (LT FT). In addition, a wax settling test gives an idea of the rate of sedimentation of the wax crystals. A slow rate of sedimentation results in a more homogeneous fuel when stored below its cloud point, and is also related to good wax crystal modification and small crystal size.

Cold flow additives are essential to ensure satisfactory low temperature operation

Globally CFPP is the most widely used filtration test. It was developed based on a set of field test data, adjusting the filtration conditions to achieve a good correlation to the operability of a representative set of vehicles available at that time.

The CFPP alone gives a good projection of vehicle operability in fuels where the difference between cloud point and CFPP is within 10°C. However, differences between cloud point and CFPP of more than 10°C are outside of the original experiment design envelope and can cause CFPP to become less reliable. But, even outside of these original parameters, the CFPP test, when combined with wax settling and operability tests, remains a reasonable balance between the practicalities associated with a performance specification test and the real world operability of the vehicle population.

Continually working to protect vehicles

In Germany fuel is expected to operate at temperatures of -20°C or below, depending on vehicle severity. This is also the national standard requirement for CFPP in winter. Fuel cloud points are typically blended to between -6 and -10°C leading to a delta of up to 14°C between cloud point and expected operability. In practice, fuels with -22 to -30°C CFPP with wax anti-settling performance are marketed to ensure the required operability is achieved.

The Infineum Worldwide Winter Diesel Fuel Quality Survey shows that a significant percentage of the commercial fuels in Germany show poor performance in the short sediment test. This trend, combined with changes in vehicle designs and very low temperatures over longer periods, may have contributed to the recent field problems observed.

As vehicle technology continues to evolve and diesel fuels become more complex and difficult to treat, defining true vehicle operability is becoming increasingly difficult.

Infineum continues to work with other industry stakeholders including OEMs and refiners to better understand the field issues related to hardware and fuel changes.

Infineum is also actively participating in industry efforts to ensure that diesel fuel operability tests continue to provide the level of protection required by today’s and tomorrow’s vehicles, in a broad spectrum of fuels, in all conditions. These programmes will help to ensure changing market needs are anticipated and advanced additive technologies are commercialised to improve protection in the marketplace.

Infineum has been at the cutting edge of cold flow fuel additive technology developments for seven decades, and we are continuing to invest in technology to develop advanced additive solutions for the future.

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