The Distillate Tug-of-War: How Refiners Choose Between Diesel and Jet

Walk a crude distillation unit and you will find the two most fought-over fuels on the planet stacked one on top of the other in the same tower. Kerosene, which becomes jet, draws off a tray or two above the gasoil that becomes diesel. There is no hard wall between them, only a temperature. Move the cut point a few degrees and molecules that would have flown in a turbine instead go into a truck. That single adjustment, repeated across hundreds of refineries, is how the global middle-distillate market clears itself week to week.

Traders talk about jet and diesel as separate commodities with separate cracks. Refiners know them as one pool with a movable seam. When the jet crack pays better, the seam slides down and more barrels go to the aviation cut. When diesel pays, the seam slides up. Understanding that mechanism explains most of what looks confusing about distillate pricing, including why jet can trade at a discount to diesel one quarter and a premium the next.

One cut, two products

A crude barrel fractionates by boiling range. The kerosene fraction and the light gasoil fraction sit adjacent in the tower, and their boiling ranges overlap at the edges. That overlap is the refiner's lever. The end of the kerosene draw and the front end of the gasoil are not cleanly separated by nature; the operator decides where one ends and the other begins by setting the cut point on the side-draw trays.

Pull the kerosene cut point higher and you drag heavier molecules into the jet stream, lengthening the jet pool and shortening diesel. Pull it lower and those same molecules fall into gasoil and lengthen the diesel pool. In a typical configuration the kerosene/jet cut runs somewhere in the 10 to 15 percent range of the barrel and the distillate/gasoil cut runs closer to 30 to 35 percent, but those are starting points, not fixed outputs. The whole point of the design is that the boundary moves.

Because the two products share a precursor, the switch is fast. Refiners can shift yield between jet and diesel within days to a couple of weeks, subject to the treating and hydroprocessing steps each stream still has to pass. That responsiveness is why the distillate complex is one of the principal shock absorbers in the refined-products market.

Following the crack

The signal a refiner watches is the crack spread, the gap between crude cost and product value. A crack spread is simply the difference between the price of crude and the price of the refined product it yields, and it is the quick-and-dirty gauge of whether a given barrel is worth making. When the jet crack sits well above the diesel crack, the economics say maximize jet. When diesel leads, yield swings back the other way.

This is not a theoretical exercise. It is the day-to-day optimization that runs a refinery's linear program. The plant is not loyal to aviation or to trucking; it is loyal to margin. If jet is worth more per barrel than the equivalent diesel, and the freeze-point and smoke-point specs can still be met, the operator lifts the kerosene cut and sells the incremental barrel as jet. The elegance is that the same hardware serves both markets, so the response to a price signal is a valve adjustment rather than a capital project.

What 2020 did to the pool

The pandemic ran this machine to its limit in the opposite direction. Aviation stopped almost overnight. U.S. jet fuel supplied fell to roughly 597,000 barrels a day in May 2020, the lowest reading since 1968, and the IEA warned that global jet demand could crater by more than a quarter across the year. There was suddenly nowhere to sell the aviation cut.

Refiners did the only thing the plumbing allowed: they stopped making jet and shoved the kerosene into the diesel pool. The industry took as much of the distillate pool as it could and made as little jet fuel and as much diesel as possible. Because kerosene is a lighter, cleaner distillate, a large share of what would have been jet can be blended down into the diesel pool instead of sold as aviation fuel. That release valve kept the tower running when the jet market vanished.

The consequence showed up in tanks. Distillate demand recovery stalled while supply kept coming, and U.S. distillate stocks climbed to their highest level in decades. Diesel yields hit ten-year highs in Japan and record highs in Europe as everyone made the same move at once. The lesson stuck: jet is the swing product, and when its demand disappears, the barrels do not vanish, they migrate.

Where the specs bite

The migration is not free, and that is the part traders often miss. You cannot pour unlimited jet into diesel, and you cannot make unlimited jet out of diesel-range material either. Each product carries hard specifications that cap how far the cut point can move.

• Freeze point. Jet A must freeze at minus 40 degrees Celsius or colder; Jet A-1, the international grade, at minus 47 or colder. Freeze point is set by the heaviest, waxiest molecules in the cut. Push the kerosene cut point too high to grab more jet volume and you drag in heavy paraffins that raise the freeze point past spec. That ceiling is the single biggest constraint on maximizing jet yield.
• Smoke point and aromatics. Jet has to burn clean in a turbine. Smoke point falls as aromatic content rises, so a cut that is too aromatic fails the smoke-point test even if everything else checks out. Diesel has no such worry; it actually tolerates more aromatics.
• Flash point and sulfur. Moving jet into the diesel pool runs into the reverse problem. Diesel carries a higher flash-point requirement and, in the U.S. and EU, a 15 ppm ultra-low-sulfur cap, while jet can run far higher on sulfur. Blend jet down and you may have to re-treat it to meet the diesel flash and sulfur specs. On-road diesel also has to make cetane, which the lighter kerosene molecules do not deliver as well.

So the swing is bounded on both ends. Freeze point and smoke point cap how much jet you can pull; flash point, sulfur, and cetane cap how much jet you can dump into diesel without extra processing. Cold weather tightens the freeze-point limit further, which is why winter can pinch jet availability just as heating-oil demand competes for the same gasoil.

The takeaway for the crack

Put it together and the price behavior stops looking mysterious. Jet and diesel cracks move together because they draw from one pool, and they diverge only as far as the specs and the seasonal demand let the cut point travel. When you see the jet-diesel differential blow out, the question to ask is not which fuel is winning but which constraint is binding: is freeze point capping jet supply, or has weak aviation demand pushed kerosene back into the diesel pool the way it did in 2020?

The refiner does not care about the answer in principle. It cares about the valve. Every week the linear program reads the cracks, checks the specs, and picks a cut point. That quiet decision, made a barrel at a time across the global fleet, is the tug-of-war that sets what you pay to fly and what you pay to haul.