Well, if it was mandated nationwide, we wouldn't have any lower cetane diesel to compare it to, so it's hard to say.
But all things being equal, it should be about 2-5 cents a gallon more for 50 (or 60, for that matter) cetane than for 40 cetane.
Backing up a tad, as to what it is, the benefits, and how the fuel is made...
Both new and old engines can benefit from higher diesel fuel cetane levels. Cetane plays an important role during the combustion process and helps users get the most value from every gallon of ultra low sulfur diesel (ULSD) used. But what is cetane? Most people think of high cetane as being nothing more than something that gives you easier starts on cold mornings, but it's much more than that. It is the indicator of how quickly diesel fuel will begin the ignition process, the indicator of how readily, and completely, the fuel will burn in the combustion chamber. The sooner it starts the ignition process, the more time it has to do a complete burn in the chamber. The more complete the burn, the more power you wrestle out of every spritz from the injector, giving you better performance from the engine, and fewer harmful emissions.
The rub is, fuel with a native high cetane has prortionally lower BTUs. But there's a way around that.
The way fuel is made in the first place is through the distillation process of crude oil. Crude oil that is pumped out of the ground is a thick, gooey soup composed of thousands of different hydrocarbon compounds - compounds composed of carbon and hydrocarbon (everything from industrial grease to propane is all in that barrel of crude). The carbon atoms link together in chains of different lengths, shapes and sizes. Each individual chain length or molecular size has different properties including progressively higher boiling points, so they can be separated out by boiling point or distillation.
Distillation is the first major process at an oil refinery. As the crude oil is heated, different hydrocarbon compounds are separated by their boiling temperatures. The lightest components such as ethane and propane come off the top of the distillation tower as those have the lowest boiling points. The next components, called naphtha, are used for making gasoline. It isn't gasoline yet, although it could be used as gasoline, but it's used to manufacture gasoline.
After the naphthas, next in line are the heavier hydrocarbons used to make diesel fuel. Diesel fuel contains larger hydrocarbon molecules, with more carbon atoms than gasoline. But this simple distillation does not produce enough gasoline and diesel fuel, and it's all raw, anyway, so heavier fractions of crude oil (the still larger hydrocarbons) are broken into smaller gasoline and diesel-sized compounds by thermal or catalytic cracking (hydrocracking), to produce higher volumes of gasoline and diesel fuel.
The gasoline is then blended with gasoline from other batches to attain the consistently desired properties, as well as having the necessary petrochemicals added to achieve the desired octane and detergent levels.
The diesel, on the other hand, is mostly (in the US), what you see is what you get. Most natural diesel distillates from Light Sweet Crude (which is the oil that Texas and Oklahoma is famous for) are, not coincidentally, in the 38-40 cetane range. Viola, it magically meets the federal standards right out of the box. Amazing coincidence, that one is. Some diesel distillates are really low, like in the 30 range, and some are naturally high, like in the 60+ range. They take the low and the high and mix them together to achieve the minimum 40 cetane.
To lower the level of sulphur in the fuel, of which Light Sweet Crude is loaded, some of these heavier fractions may be hydrotreated, which is a refinery process that reacts a fraction of crude oil with hydrogen at high temperature and pressure, along with a catalyst, to reduce the sulfur content and improve color and odor. Various component streams along the entire refining process are blended to meet the required diesel fuel specifications of whatever cetane number they want. 40 is certainly the easiest.
But these various component streams can just as easily be blended differently to achieve naturally higher cetane levels, and at the same cost. It's just that there would be a fraction of a percent (like 0.1 or 0.2 percent) less diesel when its all done (but the same fraction of a percent more of something else, like butane or some kind of ether). Because naturally refined high cetane diesel has a lower BTU number, it's better to start with lower cetane/higher BTU fuel, and add the cetane improver to it.
The easiest way to get a higher cetane while keeping the high BTU level is by adding a cetane improver. The Federal diesel fuel oil spec VV-F-800D lists the following compounds as acceptable cetane improvers.
- Amyl Nitrate
- Isopropyl Nitrate
- Hexyl Nitrate
- Cyclohexyl Nitrate
- Octyl Nitrate
- 2-Ethylhexyl Nitrate
The last one is the most commonly used, and when you buy a bottle of cetane improver (like Amsoil Cetane Boost Additive, or Schaeffer's or Stanadyne ) that's what it is. You can buy a bottle of cetane improver, or you can get the cetane improver as part of another product (like winter white Power Service, which adds 4 cetane for the anti-gel formula, and 6-7 cetane for the dingy gray silver summer bottle). When you take the 40-45 cetane diesel fuel and add a cetane improver, you not only bump the cetane for a more complete burn, but it also
B100 (pure vegetable-based biodiesel) has a cetane of 46-52, and animal-based biodiesel is 56-65.
Incidentally, a gallon of diesel has 127,500 BTUs.
B20 is 125,400 BTUs.
B100 is 117,100 BTUs.
So, they can refine high cetane diesel without any additional costs, except for the BTU cost of the lighter fuel. So adding Nitrate cetane improver will add to the cost, but really only by 2-5 cents a gallon.
BP Amoco and Chevron branded diesel fuels are, with very few exceptions, going to be 50 cetane or higher. They add the cetane improver right at the truck racks (or pipeline in the case of some truck stops). Most, but not all, "premium" diesel is going to be 50 cetane or better. In Indiana, CoutryMark has their Premium Dieselex-4 that's guaranteed to be 50 cetane.
If the states or the Fed ever get around to mandating 50 cetane minumum, it will most likely be achieved the way that BP, Shell, Chevron and others achieve it, but adding concentrated cetane improver to the stocks, which is how they do it in Europe and Canada. But you can do that yourself for about 3 cents a gallon. The places where regular (40-45 cetane) and premium (47-51) is sold, the premium is usually about 10 cents per gallon more, even though it only cost them 2-3 cents to do it.
Better MPG and a cleaner burning engine would more then offset the extra couple cents...IMO