What is the freezing point of petrol and diesel?

The freezing point, also known as the pour point, is the temperature at which a liquid begins to solidify and can no longer flow. Knowing the freezing point of fuels like petrol (gasoline) and diesel is important, especially in colder climates where temperatures frequently drop below freezing.

What is the freezing point of petrol?

The freezing point of petrol ranges from -40°C to -60°C (-40°F to -76°F). However, most petrol freezes around -57°C (-70°F).

Petrol is a complex mixture of hydrocarbons derived from the refining of crude oil. The exact composition can vary, but typical petrol contains hydrocarbons with 4 to 12 carbon atoms per molecule.

The freezing point of petrol depends on its composition. Lighter components like butane and pentane have lower freezing points than heavier components like decane and dodecane. Refineries carefully blend petrol to achieve the optimal balance of volatility and freeze resistance.

Additives called pour point depressants are also added to improve flow at low temperatures. These help prevent the formation of wax crystals that can block fuel lines and filters.

Typical freezing points of petrol components:

• Butane: -138°C (-216°F)
• Pentane: -130°C (-202°F)
• Hexane: -95°C (-139°F)
• Heptane: -90°C (-130°F)
• Octane: -56°C (-69°F)
• Nonane: -53°C (-63°F)
• Decane: -30°C (-22°F)
• Dodecane: -10°C (14°F)

As you can see, lighter hydrocarbons like butane and pentane have very low freezing points, while heavier ones like decane and dodecane freeze at warmer temperatures. Adjusting the proportions of these hydrocarbons allows refiners to fine-tune the freezing point of the final petrol product.

What is the freezing point of diesel?

The freezing point of diesel ranges from -15°C to -40°C (5°F to -40°F). However, most diesel freezes around -27°C (-16°F).

Like petrol, diesel is produced by refining crude oil. It contains hydrocarbons in the range of 8 to 21 carbon atoms per molecule, making it heavier overall than petrol.

Heavier hydrocarbons have higher freezing points, so diesel turns solid at warmer temperatures than petrol. The actual freezing point depends on the particular hydrocarbon composition.

Pour point depressants are added to improve the cold flow properties of diesel. These help prevent wax settling out of the fuel that can block filters.

Typical freezing points of diesel components:

• Octane: -56°C (-69°F)
• Decane: -30°C (-22°F)
• Dodecane: -10°C (14°F)
• Tetradecane: 5°C (41°F)
• Hexadecane: 18°C (64°F)
• Octadecane: 28°C (82°F)

As the hydrocarbon chains get longer, the freezing point increases. Refiners balance the amount of shorter and longer hydrocarbons to produce diesel with the desired cold flow properties.

How are the freezing points of petrol and diesel tested?

Oil companies carefully test the low temperature flow characteristics of their fuels. Some standard test methods include:

• ASTM D97 – Measures the pour point, the temperature where movement of the fuel stops.
• ASTM D5950 – Determines the temperature where wax crystals first start to form.
• ASTM D4539 – Evaluates the low-temperature flow of diesel using a calibrated funnel.

The freezing point is usually below the pour point, as some movement is still possible before complete solidification. However, both measures are indicators of cold weather operability.

Laboratories cool a sample of the fuel at a controlled rate and observe its flow characteristics. The temperature where flow problems first appear is noted.

In addition to standardized lab tests, manufacturers will field test fuels in cold climates. This provides real-world data on performance when used in vehicles and equipment.

What factors influence the freezing point of fuels?

There are several key factors that determine the freezing point of petrol and diesel:

Chemical composition

As discussed earlier, fuels with a higher proportion of smaller, lighter hydrocarbons like butane and pentane will have lower freezing points. The chemical makeup of the fuel is a primary factor controlling freeze resistance.

Additives

Pour point depressants and other additives improve low temperature flow. These work by inhibiting the formation and growth of wax crystals that block flow in cold conditions.

Contamination

Contamination from water or particulates can alter fuel chemistry and cause it to freeze at higher temperatures. Even small amounts of water mixed in fuel can raise the freezing point significantly.

Biofuel content

The use of biofuels like ethanol and biodiesel is increasing worldwide. Biofuels have different freeze resistance than conventional fuels. Ethanol in petrol lowers the freezing point slightly, while biodiesel in diesel raises it.

Altitude

Higher altitudes have lower atmospheric pressures. This effectively lowers the boiling point of fuel, making vapor lock issues more likely. To compensate, fuels intended for high altitude use may have a slightly higher freezing point.

Does the freezing point of fuels change over time?

The freeze resistance of petrol and diesel can deteriorate over an extended period of storage. This is due to a few reasons:

• Evaporation of lighter, more volatile components over time.
• Oxidation reactions producing heavier hydrocarbons and sediments.
• Possible mixing with water that increases freezing point.

For these reasons, very old petrol or diesel stored for years may freeze at warmer temperatures than fresh fuel. Periodic fuel testing can detect these changes so that remediation can be done if needed.

Best practices for handling fuel in cold climates

Here are some tips for dealing with petrol, diesel, and other fuels in cold weather conditions:

• Use winter blend fuels where available – These have improved low temperature operability.
• Consider fuel additives – Pour point depressants and anti-gel additives can lower the freezing point.
• Keep storage tanks full – This minimizes air space where condensation and freezing can occur.
• Insulate fuel lines and filters – Prevents wax buildup that can plug flow.
• Drain water separators frequently – Even small amounts of water mixed in fuel can freeze and cause problems.
• Store fuel indoors or underground – This protects it from wide temperature swings.

Paying close attention to fuel condition and storage in cold weather is key to avoiding frozen fuel lines, filters, and tanks. Vehicles and equipment may fail to operate if the fuel supply is interrupted by freezing.

Conclusion

The freezing point of petrol ranges from -40°C to -60°C, while diesel freezes between -15°C and -40°C. Freezing points depend on the hydrocarbon composition of the fuels. Lighter components like butane and pentane lower the freezing point, while heavier ones like decane and dodecane raise it. Fuel additives, contamination, biofuel content, and altitude also influence the temperature where freezing occurs. Careful fuel blending and handling practices are required to prevent flow problems in cold climates.