Diesel gel temperature: when does diesel gel?

Ordinary No. 2 diesel starts to gel at its cloud point, the temperature at which wax crystals first form, and Colonial Pipeline, which carries Gulf Coast diesel up the East Coast, caps that at 15.8°F (-9°C) for shipments from August 1 through March 14 and 19.4°F (-7°C) the rest of the year. The wax plugs a fuel filter a few degrees colder, and the fuel sets into a gel that will not pour near its pour point, which the same spec caps at 0°F (-18°C) in the cold months. No. 1 diesel, a kerosene-type fuel, clouds at -40°F or colder, so the winter diesel sold in the North is No. 2 cut with No. 1, treated with additives, or both.

Three temperatures, three tests

Diesel is a blend of many hydrocarbons, and the long straight-chain paraffins in it come out of solution as wax when the fuel cools. Three standard tests mark how far that has gone.

• Cloud point (ASTM D2500, or D5773, the most common automated method): the temperature at which a haze of wax crystals first shows as the sample is cooled. The Coordinating Research Council (CRC), the research body the petroleum and automotive industries support jointly, calls it the most conservative estimate of how cold a vehicle can run, and it is the figure most specs and fuel contracts use.
• Cold filter plugging point (ASTM D6371): the fuel is cooled quickly and drawn through a 45-micron screen at each 1°C step until it fails to pass within 60 seconds. The test was built in Europe in 1965 around light-duty vehicles. North America's heavy-truck counterpart, the low-temperature flow test (ASTM D4539), cools a larger sample at 1°C an hour through a 17-micron screen, closer to the way a truck's tank cools overnight.
• Pour point (ASTM D97): reported 3°C above the temperature at which the cooled sample stops moving, the point where it has set into a gel. CRC treats it as a storage and handling figure, not a guide to whether a vehicle will run.

In a truck, the filter is the weak point: CRC calls filters the most sensitive part of the fuel system in the cold. Fuel lines and filters hold little fuel and cool faster than the tank, so fuel can gel in the lines or the filter while it still flows in the tank. Station tanks are buried and sit at about 41°F to 55°F all year, which is why cold-weather trouble tends to start on the vehicle rather than at the pump.

Typical temperatures by fuel

No. 2 diesel. Colonial's caps are the most a shipment may read, not typical readings. CHS, the farm cooperative behind the Cenex fuel brand, puts the cloud point of No. 2 at about 14°F. CRC's 2016 operability guide lists lab results for ten US ultra-low sulfur diesel samples; setting aside the two No. 1-D fuels and the three biodiesel blends, the other five clouded between -3°F and 21°F, failed the filter-plugging test between 0°F and 21°F and stopped pouring between -27°F and 16°F. NREL's Biodiesel Handling and Use Guide puts diesel fuels as a whole at cloud points of -31°F to 23°F, with some outside that range.

No. 1 diesel and winter blends. No. 1 diesel and kerosene cloud at -40°F to -60°F, per NREL. The pipeline delivers No. 2 at its cap, and CRC notes the deeper protection a northern winter needs is blended in at the refinery or the terminal for the region where the fuel is sold. Buckeye Partners' receipt spec for its marine terminals, which include South Portland, Maine, accepts diesel blended with No. 1 for Maine from December 1 through March 14 only at a cloud point of -16°F (-27°C) or lower. CHS's rule of thumb for blenders is about 3°F off the cloud point for every 10 percent of No. 1 added.

Additives. Cold-flow improvers usually leave the cloud point where it is and lower the filter-plugging temperature, per NREL. They change the size and shape of the wax crystals, CRC explains, so the wax builds a porous cake a filter can still pass fuel through. That is why a treated fuel's cold rating is quoted as a CFPP or flow-test result, and CRC found CFPP readings more than 18°F (10°C) below the cloud point a weaker guide to how light-duty vehicles actually run.

Biodiesel. Neat biodiesel (B100) clouds at about 27°F to 61°F depending on the feedstock, per NREL, and blending it in raises a diesel's cloud point.

How cold a fuel has to handle

ASTM D975, the US diesel specification, sets no cold-flow limit. Supplier and buyer agree on a cloud point, CFPP or flow-test figure for the place and month, and the spec's appendix supplies tenth-percentile minimum temperatures by state for October through March: the daily low falls below that figure on about 3 days of a 30-day month, on average. NREL notes that many buyers write the cloud point into the contract at or below it. D975 takes each figure from the coldest location in the state or region (Minnesota's comes from International Falls, per CRC), so it runs colder than most of the state.

Worked example: what the cold protection costs

No. 1 costs more than No. 2 and carries less energy. EIA's heat-content factors put kerosene at 135,000 Btu a gallon, about 2 percent below the roughly 137,500 Btu EIA estimates for US distillate fuel. When EIA last published refiner prices by product, a series that ended with March 2022, refiners sold No. 1 distillate for resale at a median 30.3 cents a gallon over No. 2 diesel, across the 146 months from January 2010 with both prices reported.

Take No. 2 with the 14°F cloud point CHS cites, bound for a market that wants, say, 0°F. At about 3°F per 10 percent, that takes a No. 1 cut of roughly 47 percent (14 ÷ 3 × 10). At the 30.3-cent median, the No. 1 in each gallon of blend adds about 14 cents (0.47 × 30.3) before any additive, and the blend gives up a little under 1 percent of its energy. Additives are the other lever: by lowering the filter-plugging temperature, they cut how much No. 1 a blender needs.

Where the season shows in the pump prices

EIA's weekly survey, the data this site tracks, prices the diesel each region actually sells, so its winter numbers already carry whatever blend the local market burns. Each line below is the region's price minus the US average from the same Monday survey, averaged over the surveys since EIA's June 2022 redesign:

• New England (PADD 1A): the gap has averaged +43.0 cents a gallon in December through February and +27.3 cents in June through August, so on average New England diesel has sat 15.7 cents higher against the US average in winter than in summer.
• Midwest (PADD 2): the gap has averaged -9.2 cents a gallon in December through February and -5.0 cents in June through August, so on average Midwest diesel has sat 4.1 cents lower against the US average in winter than in summer.
• Rocky Mountain (PADD 4): the gap has averaged -6.3 cents a gallon in December through February and +0.2 cents in June through August, so on average Rocky Mountain diesel has sat 6.5 cents lower against the US average in winter than in summer.
• Gulf Coast (PADD 3): the gap has averaged -30.4 cents a gallon in December through February and -31.4 cents in June through August, so it has held about the same through the year.

Those gaps carry the cost of winter-grade fuel together with everything else the season does to a region's price against the national number. On the East Coast that includes heating-oil demand, which peaks in the same months and draws on the same distillate barrel, and a winter rise there lifts the national average every other region is measured against. So the gaps show what winter does to each region's pump price relative to the country, not what blending alone costs; the arithmetic above is the closer measure of that. Winter diesel: blends, gelling and the seasonal premium covers the blending season and the basic, standard and premium tiers at the pump, and the Rocky Mountain, Midwest and New England pages carry each region's full weekly record.

Monday Diesel reports these figures as their sources publish them. It does not recommend a fuel, a blend or an additive.