Diesel Fuel

By C.J. Baker

Ask people what they know about gasoline and most folks will tell you they know the current price and that the local station sells three grades. Ask if they know the difference between the three grades and, if you’re lucky, they’ll tell you it’s the octane rating. That’s about where the common knowledge stops, although you might get a few misunderstandings thrown in, like premium fuel is good for a car even though it requires only regular. Now, if you’ve managed to keep the conversation going this far, ask people what they know about diesel fuel. If you get any response at all, other than “Nothing”, chances are it’ll be incorrect.

Depending on the crowd of people you’re quizzing, you might get responses like these: “It costs less than gasoline.” (maybe, depending on market conditions); “It has less energy than gasoline.” (incorrect); “It has a peculiar smell.” (correct, but so does gasoline); “It won’t burn if it is spilled, such as in an accident.” (incorrect); “It’s oily.” (correct);“It’ll freeze if it gets cold enough.” (not exactly); “It pollutes the environment.” (correct yesterday, but incorrect tomorrow); “It makes smoky exhaust.” (only if the vehicle is out of adjustment); “There are two diesel fuels: No. 1 diesel is low octane, and No. 2 diesel is high octane.” (beware the self-proclaimed expert!).

The point is, diesel fuel is different from gasoline in many ways, and because you can bet that diesels are going to play a much bigger role in our future transportation, maybe it is worth a little discussion.

Just like gasoline, ordinary diesel fuel is refined from crude oil. During the refining process, different hydrocarbon compounds are distilled from the crude; light gases such as methane and propane at one end of the spectrum, and heavy tar and asphalt at the other end. In between are naphtha (a cleaning solvent), gasoline, kerosene (jet fuel), diesel fuel, heating oil, and lubricants (motor oil, grease), just to name a few. How the crude is refined can vary the percentages of each of these categories of distillates somewhat. For example, if you decrease the amount of gasoline, you can increase the amount of diesel by roughly the same amount, within limits. What you get also depends on the composition of the crude oil you’re refining. For example, crude oil that contains a high percentage of sulfur is called “sour crude” as opposed to low sulfur “sweet crude”. We won’t turn this into a petro-chemistry discussion, but the sulfur content is important, especially as it relates to diesel fuel. A high sulfur content is undesirable in fuels, such as gasoline and diesel, because it forms sulfur dioxide and sulfates during the combustion process, which contribute to pollution and “acid rain”. In case you’ve ever wondered, the acid we’re talking about is sulfuric acid — not the stuff you want to rain on your parade. Sulfur has another drawback; it “poisons” catalytic converters intended to reduce NOx (oxides of nitrogen), another polluting by-product of combustion. Unfortunately, NOx is the prominent polluting gas emitted by diesels.

To meet future clean air emission standards, diesels will have to use special NOx catalytic converters in the exhaust system. This means there must also be a switch to “ultra low-sulfur diesel” (ULSD) fuel. That has already been mandated by the Environmental Protection Agency by 2006. When the conversion to ULSD is complete, modern diesel engines will be incredibly clean in terms of polluting gases, particulates (smoke), and noxious odors. Today’s modern diesels have already made great strides toward eliminating smoke, pollutants, odors, and even noise. Diesels in 2006 will be nearly indistinguishable from gasoline engines, except the diesels will have more torque, get much better fuel economy and last longer, but that’s another story (see "Diesel Evolution" elsewhere on this site).

Unfortunately, progress comes at a price. Removing the sulfur from today’s sour crude is complicated and expensive. As a result, sweet crude commands a higher price than sour crude, and to add insult to injury, most sweet crude comes from the Middle East. Further reducing sulfur to the ULSD levels required by 2006 will cost even more – guestimates are about 5-8¢ a gallon.

So where will that leave the price of diesel fuel compared to gasoline? Currently, diesel is approximately the same price as regular gasoline, depending on where you buy it and the state taxes that apply. World oil supplies also impact the variance between the price of regular gasoline and diesel. If nothing else changes, ULSD will cost about 6-12¢ a gallon more than today's diesel fuel, however, much of Europe has offered fuel tax and licensing tax incentives to get people to switch to diesel, and in fact, one out of every three cars in western Europe is now a diesel. Perhaps we’ll see similar incentives in this country by the end of the decade. Such incentives could make diesel much less expensive than regular gasoline.

Regardless of whether we see tax incentives or not, diesels operate more efficiently than gasoline engines. Diesels frequently get fuel mileage as much as 40 percent better than comparable weight vehicles with similarly sized gasoline engines, and some experts say fuel economy up to 60 percent better than gasoline is within reach. That alone will make diesels more economical. Moreover, because diesels emit far lower levels of carbon dioxide, carbon monoxide, and hydrocarbons, the environment will love diesels too, but more on that later. Plus turbo-diesels (all modern automotive diesels are turbocharged) will be faster and more fun to drive, but again, we digress.

Part of the reason diesels get better fuel economy and make more power than gasoline engines is that diesel fuel has approximately 11 percent more energy per gallon than gasoline. A typical gasoline has 124,800 Btu per gallon, while typical diesel has 138,700 Btu per gallon. Diesel fuel does have an oily feeling if you get some on your fingers, and it doesn’t evaporate as quickly as gasoline either. It is the low volatility of diesel fuel that makes it less likely to ignite if spilled, but given an ignition source, it will ignite. It is, after all, fuel. So much for that myth.

Another myth is that diesel fuel freezes if the temperature gets too low. While diesel won’t “freeze” at temperatures most of us are likely to encounter, it does have peculiarities related to low temperature. We’ve already mentioned diesel’s low volatility. That contributes to hard starting at cold temperatures in older diesels, but today’s modern diesels with common-rail fuel injection and ultra high fuel injection pressure have little problem, especially if the diesel has “pilot injection” (see ”Diesel Evolution” elsewhere on this site). Diesel fuel also has another characteristic called “cloud point”. This is the temperature at which tiny wax crystals begin to form in diesel fuel. If sufficient wax crystals form, it can affect the ability of the fuel to flow through fuel lines and fuel filters. This may be what some people refer to as “freezing”. Many modern diesels have fuel heaters to prevent wax formation at cold temperatures. Or there could be water in the fuel.

Water is a real problem in diesel fuel since it can cause injector pump damage and fuel injector corrosion. Water also promotes the growth of microorganisms in the fuel that can plug fuel filters. Water gets into the fuel from condensation in storage tanks and vehicle tanks, or through unshielded tank vents. Diesels have water filters in the fuel system to remove minor water contamination, but if the problem gets too bad, water deposits, which will seek low points in the fuel line or tank, can actually freeze during low temperatures, but that’s not technically the fuel that’s freezing. Diesel fuel must also be kept very clean to prevent damaging the close-tolerance fuel injection pump or plugging the fuel injection nozzles. Dirt or particles in the fuel are the result of mishandling of the fuel or storage in dirty containers or tanks.

The refining companies are well aware of the starting problems associated with diesels in cold weather, and they usually “winterize” their diesel fuel by mixing 15-20 percent No. 1 diesel with the conventional No. 2 diesel in cold climates. This improves the volatility of the fuel and reduces the cloud point to slightly lower temperatures. At this point, we should discuss the differences between the two grades of diesel fuel. Diesel No.1 and No.2 are the two common grades of diesel fuel. No. 2 diesel is by far the most widely used since it provides the most energy per gallon for improved power and mileage, and the highest lubricity for the fuel injection pump. Since it is a heavier distillate than No. 1 diesel, No.2 diesel is usually a few cents per gallon cheaper too. Some diesel engines are designed to operate specifically on No. 1 diesel, especially if the engine will be subjected to frequent starting and stopping, but No.1 diesel has several disadvantages. First, it provides less lubricity than No. 2 diesel, which can prove troublesome for some fuel injection pumps, and second, it contains roughly 95 percent of the heat energy of No. 2 diesel. And as we just mentioned, it’s a little more expensive too.

Unlike gasoline grades, the two grades of diesel fuel do not reflect different octane numbers. In fact, octane is not a measurement associated with diesel fuel at all. Octane is a quality of a fuel to resist self-ignition when subjected to heat and pressure. That’s important in gasoline engines since the fuel is already mixed with the air when the compression stroke occurs. It is important that the mixture doesn’t self-ignite in a gasoline engine until the timed spark ignites it for maximum power and efficiency. Pre-ignition could seriously damage a gas engine. Diesel engines, by comparison, rely on self-ignition of the fuel when it is injected directly into the cylinder at the top of the compression stroke. The ease with which a fuel is ignited when exposed to heat and pressure is measured by its cetane number. This is what is important in diesel fuel. Fuels with a high cetane number have good low temperature startability, and smooth, even combustion. Think of cetane as the fuel’s ignition quality. Most diesel fuel sold in this country has a cetane number of at least 40. While higher cetane numbers would help cold starting, there’s a trade-off. The higher the cetane number, the higher the cloud point of the fuel, which causes other problems at low temperatures, as previously discussed. In warm climates, fuel clouding is not a problem and the cetane number is frequently higher. European diesel fuel commonly has a cetane rating of 50 or higher, and the ultra low-sulfur diesel now in limited use in this country has a cetane of 55 to 60.

We talked a little about pollution earlier, but let’s take a closer look. First, diesel fuel emits far less evaporative emissions because of its lower volatility than does gasoline. Other pollutants relate to engine operation and the by-products of combustion more than to the attributes of the fuel. For example, an overly rich air/fuel ratio will produce black smoke from both diesel and gasoline engines, so that is mostly a tuning factor, although diesels do tend to produce more particulates than gasoline engines if the combustion is incomplete. Computerized fuel management has eliminated most particulate emissions from diesels, and self-cleaning particulate traps in the exhaust system will handle the rest. Because of the higher combustion temperatures of diesels, which are a function of compression ignition, diesels do tend to generate higher levels of NOx than do gasoline engines. Interestingly, recent studies have shown that over the lifetime of diesel and gasoline engines, diesels actually emit less total NOx than gasoline engines. NOx levels are controllable in diesels by retarding the fuel injection timing, adding cooled exhaust gas recirculation, and by “shaping” the rate of fuel injection by using multiple injection pulses to keep peak temperatures down during each combustion cycle. That sounds complicated, and it is, but modern diesels are already doing these things. What little NOx remains can be eliminated with catalytic converters, as mentioned above, when ultra low sulfur diesel fuel becomes the norm.

While NOx is a major contributor to the generation of smog, it is hydrocarbon emissions and the “greenhouse” gases of carbon dioxide and carbon monoxide that concern environmentalists. Although many people are misinformed about diesels, emissions of hydrocarbons and greenhouse gases from diesels are very low. That, and fuel economy, is why the Europeans have enthusiastically embraced diesel technology (see ”Turbo-Diesel Fact & Fiction” elsewhere on this site).

We’ve covered the major differences between gasoline and diesel fuel, hopefully dispelling some commonly held misinformation. Is diesel the fuel of the future? In the very near future, we are certain to see the percentage of diesel-fueled vehicles grow in this country. The economics and the environmental benefits of diesel are simply too strong to ignore. Diesel will not totally replace gasoline-fueled vehicles, and in fact, it is doubtful if diesels will exceed more than 15-20 percent of the automobile and light truck (including SUVs) vehicle population in the next 10 years. However, there is a growing movement toward fuel conservation, and other technologies such as electric, fuel cell, compressed natural gas, and even hybrids are not yet economically viable. Clean diesel is doable now, especially with ultra low sulfur diesel fuel. Even if diesel eventually loses out to one of these other technologies, it will certainly fill the gap until something better is available.