Compared to spark-ignition fuel system car operation, where lack of adherence to seasonal gasoline quality requirement changes causes very rare operability issues, Diesel fuel propelled vehicles are at the very opposite end of the scale. While for a gasoline car the only seasonal, easy-to-avoid threat is usage of high vapor pressure winter grades of gasoline in really hot summer that can impair the fuel delivery from a tank to an engine due to fuel vapor cushion creation, in case of Diesel fuel with poor low temperature properties in a truck tank in cold winter surely prevents from any further voyage.
Compression ignition engines require fuel with high so called cetane number that minimizes the delay between fuel injection into cylinders and its ignition and thus provides smooth engine run. However, the high cetane number part of Diesel fuel is formed by long and straight hydrocarbons, paraffins, tends to create solid crystals at temperatures that are common even during mild winters. While the temperature goes down, the crystals grow in size, agglomerate and form large lattices. The most susceptible part of a vehicle is the low-pressure fuel distribution system and especially fuel filters that are, if they are not heated by a separate electric circuit and the fuel quality does not comply with the weather conditions, often blocked with excessive amount of solidified wax.
There are several methods how to describe the low temperature behaviour of the Diesel fuel. The most common way is to run a laboratory test ‘Cold Filter Plugging Point’ (‘CFPP’) and it is the minimum temperature, which the fuel is able to pass through a mesh sieve at. We can call it a filterability test and it gives good indication how the fuel behaves in a fuel filter, but in any case, we should not take for granted, that the fuel might not cause operational issues above this temperature.
The topic of Diesel fuel low temperature behaviour begins to be quite prominent each year in autumn when fuel suppliers have to start delivering so called transition or directly winter grades of Diesel fuel. The exact situation in terms of specific quality requirements and time periods naturally depends on the geographic location of the country where the fuel distributor operates. In Central Europe, the transition from summer grade (CFPP max. 0°C) starts at the beginning of October (CFPP max. -10°C), while the fully compliant winter grade Diesel fuel (CFPP max. -20°C) must be available by the middle of November. One would think that this should not be any problem, but this is not true in every case.
Although there are clear deadlines for the winter quality to be present on the filling stations and everybody should be in line, non-conformities in this area being found by specialized laboratories are one of the most frequent (usually on 2nd place). The complete and successful transition from summer to winter grade must be done at every single level of the fuel distribution chain. It starts in refineries, where a part of heavier component in Diesel fuel, gas oil, has to be replaced with lighter fraction, kerosine, the fuel propelling airplanes. Kerosine is not only lighter, but also more expensive, and therefore refineries cannot make the shift too prematurely due to economic reasons. At the opposite end of the chain, a filling station has to quickly sell out the outstanding summer quality and stock the material compliant with the upcoming period. This might be quite tricky for small stations or those which sell lower turnaround fuel types (e.g. premium Diesel fuel). For a well-established and large company, however, it is just a more demanding period of the year to cover the transition logistically.
In practice, depending on the vehicle’s fuel system sensitivity to poor low temperature properties of Diesel fuel, there are cases that engines are unable to run up at temperatures up to 5°C higher than the CFPP value (e.g., problems may occur at -15°C, even though the fuel meets the winter spec of CFPP -20°C). This is since paraffin crystals begin to precipitate much sooner (this point is called ‘cloud point’ and can be even more than 10°C higher than the CFPP value), but their quantity is high enough to block 3 – 5 micron pores of the standard fuel filters in vehicles compared to the mesh sieve of 45-micron porosity in CFPP apparatus. It is therefore obvious that violation of the standards in terms of cold flow properties can bring many difficulties when it starts freezing outside.
The first instance is the choice of reliable fuel supplier. Such a supplier buys the fuel from proven fuel suppliers (refineries and distributors) and keeps an eye on the incoming fuel quality or, even better, regularly checks the quality by an independent third party (laboratory). Eurowag selects top quality products for our customers because we know that mileage, smooth engine performance, and power matter. Moreover, renowned sources do not run at the edge of the qualitative standards and provide certain buffer. We can thus see, for example, CFPPs of -25°C in winter period, although only -20°C is required.
Along with the decision on the fuel supplier, we can prevent the above-described issues with proper vehicle maintenance. Fuel filter clogging is being promoted with increasing water content in the fuel and especially within the fuel filter itself. It is highly recommended to check or even replace the fuel filter before winter comes. With increasing usage of biofuels (biodiesel, FAME) as inherent component in the Diesel fuel, this procedure’s importance is rising because a mixture of wax, water and biodiesel (including its degradation products) tends to form quite stable matter blocking the filters.
What can be done more while awaiting severe weather change or going abroad to locations with very low temperatures? In certain cases, fuel suppliers expecting huge ambient temperature slip beyond normal occurrences start adding arctic fuel grade into the standard Diesel fuel to avoid any mass problems. The truck operator himself can also use the same approach. Much more elegant way is to add aftermarket Diesel additives for winter, depressants, that can suppress the paraffin aggregation. However, commonly available fuel already contains significant level of those additives and additional improvements are up to 9°C and it cannot be predicted without testing. And beware, the treatment must be timely. Adding the additive to already crystalized fuel does not help at all!