The effect of temperature change on diesel fuel

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The effect of temperature change on diesel fuel
Diesel fuel is a fractional distillate from petroleum. (Jupiterimages/Photos.com/Getty Images)

In 2010, sales of diesel cars outnumbered those of petrol vehicles for the first time in the UK, according to a report by the Society of Motor Manufacturers and Traders. Generally, this was due to the fact that, while diesel fuel is more expensive than petrol, it offers greater fuel efficiency and so, more miles to the gallon. However, like petrol, diesel fuel performance is affected by temperature. As such, it's worth any diesel car owner knowing how the fuel in their vehicle reacts to temperature changes.

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Technology

A diesel fuel engine does not use spark plugs to ignite fuel. Instead it concentrates heat in a small compartment through which fuel is passed. With sufficient heat, the fuel combusts as it passes through this compartment and into the engine.

Density

Fluctuations in temperature affect the density of diesel fuel. As the temperature rises the fuel expands. When the temperature falls, diesel becomes more concentrated. When diesel fuel expands, the amount of energy it contains decreases, which is why a diesel car can be hard to start on a cold day.

Effect

This effect of temperature on diesel fuel impacts upon its efficiency. A warmer fuel contains less energy, and so the vehicle will not do as many miles to the gallon as it would with a colder fuel. This is one reason why driving at a constant speed is recommended to preserve fuel economy, as the fuel temperature does not fluctuate to meet the demands of acceleration and deceleration.

Tactics

Knowing that diesel concentrates at lower temperatures can be beneficial to drivers at the pump. Diesel fuel available at garages is stored in tanks under the ground. Fill up your car either early in the morning or late in the evening when the temperature is low. Diesel pumps measure volume rather than density, so by filling up when it is cold, you get more fuel for your money.

Freezing

In freezing conditions, not only does diesel fuel expand and so lessen in energy, the fuel delivery system within the car can be compromised. At very low temperatures the water in fuel can freeze, while the paraffin in diesel can form into flakes -- a process known as gelling -- which can inhibit the passage of fuel through the filter that leads to the engine.

Solutions

In cold conditions, drivers should try to purchase diesel fuel that has additives that combat the adverse effects of weather on fuel and its delivery system. Some fuel companies add such elements as a matter of course in colder conditions. Drivers can also purchase additives that prevent gelling to add to the fuel tank. There are also mechanical devices that can be fitted to vehicles that compensate for variations in outside temperature.

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