The temperature drops below zero and the diesel engine won’t start. Gelled paraffin in the fuel might be the culprit. We took a closer look at what happens here and how drivers can best prepare for this in the winter with Florian Holub at the Schwechat refinery.
Low temperatures make different demands on diesel fuel than the warmer seasons. That is why a special diesel is sold at filling stations in the winter. The change from summer to winter diesel happens in the background, unnoticed by customers. In order to understand why there is a special winter diesel, we have to start all the way at the beginning—with the components in diesel. Diesel is made up of a mixture of hydrocarbon compounds, including paraffins. At low temperatures the paraffins start to crystalize and form solid flakes. Florian Holub, who has been working intensively with fuels and engine technologies for years, explains: “Diesel becomes cloudy when the crystals are formed. This phenomenon starts at the so-called ‘cloud point.’ If the diesel gets colder, from a certain temperature it can no longer flow through the fuel filter. The gelled paraffins lead to ignition problems, a jerky engine, or a loss of performance.” A laboratory method was developed to determine and compare the cold-resistance of diesel fuels. The objective is to define the “cold filter plugging point” (CFPP).
Diesel doesn’t like cold, because the paraffins in it crystallize at low temperatures. The diesel then loses fluidity and the engine can’t start anymore. That is why a special winter diesel is sold in the winter months that is more cold resistant.
Florian Holub, Product Development & Innovation, OMV Downstream
The diesel is mixed with special additives to prevent gelling. Voilà: diesel becomes winter diesel. “We start producing winter diesel in our refineries in September. The paraffin crystals formed at low temperatures change in shape and size with the additives which prevents the fuel filter in the vehicle from clogging.” From November to February the pumps in many European countries are only allowed to offer winter diesel with a CFPP value of at least -20°C. This is stipulated in the national diesel fuel norms in accordance with Norm EN 590.
Into the cold chamber
“We test our diesel in a cold chamber developed especially for this purpose. Here we check how the fuel performs as it gets colder and colder. We use a real fuel system from a market relevant vehicle. This way we quickly see how different additives work in a real life situation and ultimately affect the ability of the engine to start,” says Florian Holub. The admixed additives have to be developed specifically for the fuel. This way they react and can develop their positive characteristics. “We advise against adding an aftermarket fuel additive yourself. Additional additives can cause interactions which reverse the positive qualities,” explains Holub.
In addition to the classic winter diesel, OMV also offers a premium diesel at filling stations: “The OMV MaxxMotion Performance Diesel is clear at the required CFPP temperature of -20°C with no gelled paraffins. Its CFPP is a long way off, it is—in production at the Schwechat refinery—as low as -40°C. We do far better than the norm.” But that doesn’t mean that MaxxMotion is only good for the car at arctic temperatures: the diesel has a higher additive dose and a higher cetane rating all year long. The higher the cetane rating, the better and cleaner the combustion process. OMV MaxxMotion Diesel is produced in the Schwechat refinery in Austria and in Petrobrazi in Romania and is the result of many innovative cycles including numerous vehicle and engine tests. “We’ve been offering MaxxMotion Diesel at our filling stations since 2010. We are constantly developing it. In addition to the outstanding cold properties, MaxxMotion Diesel also has a cleaning effect on engine components such as the fuel injection nozzles—thanks to the additive package. This extends the life of the engine,” says Florian Holub.
Additives are oil-soluble substances that are added to mineral oil products to improve their usage and combustion properties, reduce emissions, and protect the engines burning them from wear and dirt.
Cold Filter Plugging Point (CFPP)
The CFPP is the cold resistance of diesel fuels and is determined using a normed laboratory method. Fuel is passed through a test filter with a defined mesh size while being cooled. Before reaching the CFPP the first crystals being to form at the so-called cloud point (CP), but the fuel can still pass through the filter. The temperature is gradually lowered until a certain minimum amount of fuel can no longer flow through the test filter during a specific period and the CFPP temperature is reached.