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What is the Freezing Point of Diesel Fuel ?
How cold does it have to get to cause DERV to freeze?

Just about everyone knows that water freezes at 0 degrees C (32 degrees F), but What is the Freezing Point of Diesel Fuel ? At what temperature does diesel fuel freeze? You may ask, and I'll try to answer.

Usually when people as "What is the Freezing Point of Diesel Fuel ?" they don't mean "Scientifically, what is the temperature of the point of total solidification of pure diesel in a laboratory?", and what they really mean is "How cold does the weather have to get before I am going to have trouble getting my truck/van/car to start?!". These are different questions, but let's start with the important one, the one which is going to help you with your engine getting started: Diesel Fuel (DERV) in its basic form starts to have trouble at -9.5 degrees C. If it's minus 10 Celsius all night, you might have a bit of trouble getting the engine to start, but then again, maybe not, depending on what TYPE of diesel fuel you've got.

(Petrol/Gasoline car owners should not worry, as all this stuff about your fuel freezing doesn't apply to petrol/gasoline. It can be frozen in a laboratory, but it's not likely to freeze outdoors even in the coldest weather).

When you look around on the Internet you'll see some people who believe that pure diesel freezes at -80 degrees C. This may be a scientific fact in a lab, but it's no comfort when you are stuck because the freezing cold weather has "frozen" or "waxed" your fuel, and your engine won't start. There is however a scientific reason why the matter of freezing diesel fuel doesn't have a perfect answer: It's because the stuff that you have sloshing about in your vehicle fuel tank is not a single substance, but is more of a cocktail of different stuff. It would be like saying "what's the freezing point of gin & tonic?". If you put it in a freezer the tonic would freeze at about zero, but the alcohol in the gin would act a bit like antifreeze, before finally going solid at a much lower temperature than the tonic. (Freezing point of alcohol/ethanol = -114.1 degrees C).

Similarly, actual diesel fuel that you buy from filling stations is a mixture of various hydrocarbons. Some are heavy and behave somewhat like candle-wax, and some are more light and volatile. The resulting combustible fuel cocktail is a mixture, and as the temperature drops, some of the ingredients start to turn to wax sooner than others. Also, some diesel fuel has special additives in so that it can cope with lower temperatures.

At cold temperatures, you can still have liquid in your fuel tank, and yet the engine won't start. The failure mode occurs because of the heavy waxy components of the mix going solid and forming cloud or wax in the fuel. This then clogs up the filter. It's the clogging of the fuel filter that stops the engine, and that's what happens if it's cold enough. If you are stranded, try warming up your fuel filter before warming up the fuel pipes and fuel tank.

Method of "warming" components of vehicles vary. The most commonly quoted method involves a side-effect function of human biology, and it is generally regarded as easier for the male than for the female!

Warming a fuel tank is even more questionable, and I have heard stories of truckers lighting small fires under their fuel tanks! The health and safety aspects of such things are serious, so if you try such a thing BE CAREFUL. ("Health and Safety" fanatics typically ignore the fact that the alternative might be freezing to death).

A better long-term solution to the problem of diesel freezing or "waxing up" in cold weather is to have either fuel-additives or to have fuel heaters, or both. Fuel companies can be contacted and asked "What temperature does Your diesel freeze at?" and they'll boast of different cold-resistances, some as cold as minus 40. There's also a do-it-yourself approach, where you can add a small amount of petrol to the diesel. This is inadvisable for various reasons, but that's not to say it doesn't work.

Fuel heaters are also practical. An inline fuel pre-heater (not to be confused with preheaters) is good. This only requires a few watts and heats the fuel as it goes along the pipe. Additionally, a fuel tank heater is a very low power version of a kettle element, and it's a bit like those frost-prevention heaters in greenhouses, with just enough heat to take the chill off. In Alaska, and in other cold climates, all of this was solved a while ago, and the technology is there to be found. See Trucks USA

Once the engine has started, the fuel should not freeze, as diesel engines have a flow and return system, which means that some fuel is warmed by the engine and then returned to the fuel tank. It's a function of the way diesel injectors work.

This page is one of the many pages at Zyra's website which is well worth bookmarking and having a browse around, as there is a wealth of helpful information like this, about a wide variety of diverse subjects!

Diesel engine related pages include firing order and how to fit manual preheat control (useful for more efficient starting of the engine).