You always hear gasoline guys talking about their fuel’s octane rating, while diesel engine owners don’t care about octane at all. Why is that? Well, it turns out gasoline’s octane rating (which is basically just a measure of the fuel’s resistance to auto-ignition) is meaningless for diesels, because our engines don’t inject fuel into the combustion chamber until we want it to ignite. If our fuel can’t begin to burn until it’s injected—there’s no reason to reduce its tendency to want to burn. Right? In fact, in nearly every case we can think of, we want our diesel fuel to ignite as soon as it comes out of the injector. We measure our fuel’s tendency to want to burn by giving it a cetane rating.
Cetane, which is also sometimes referred to as hexadecane or isocetane, is a colorless gas with the chemical formula C16H34. Cetane ignites easily when exposed to a small amount of heat and was therefore given a perfect cetane rating of 100. The easier it is for diesel fuel to ignite, the higher we say its relative cetane rating is. Generally, the faster the fuel in a diesel ignites, the better the engine will run. However, the higher a fuel’s cetane rating is, the less energy dense that fuel tends to be.
Diesel fuel’s cetane number (or rating) is calculated based on laboratory tests. The benchmark standard we traditionally refer to was created by the American Society For Testing Material (ASTM). There are four basic ways to assign a fuel a cetane number rating.
1. Cetane Engine: This machine is the gold standard for setting ratings. It can cost more than $500,000. Fuel is injected into a pre-combustion chamber, and then precise measurements are taken to analyze when significant combustion first occurs.
2. Ignition Quality Tester: This machine is cheaper and easier to use. It has high accuracy and injects a measured amount of fuel into a constant-volume chamber. The data gets fed into a formula to get the rating.
3. Near Infra-Red Analyzer: This device is used to obtain the absorption spectrum of the diesel fuel. The data is then fed into a chemometric model to estimate the cetane rating, along with 10 to 15 other properties.
4. Cetane Indices: This method uses the distillation points and density of the fuels in a formula to determine cetane rates.
Why Cetane Rating is Important
In many ways, your diesel engine is only as good as the fuel you put in it. If you ran a high-performance European diesel engine on fuel made for North America, it might smoke, sputter, and lose power. If we want the best-performing, smallest, and cleanest engines, we’ll need better fuel. This could come in the form of a diesel fuel cocktail. One promising additive is dimethyl ether (DME).
Dimethyl ether (DME) is only about half as energy dense as diesel fuel, but at certain mixtures it helps lower NOx emisisons, makes the engine run quieter, and helps produce more power. This fuel can be made from fossil fuels, waste, or renewable sources. The diesel fuel of the future will probably be a dynamic thing, and we’ll be able to buy different blends of it based on the elevation, temperature, and population density of where we buy it. The one thing we can be sure of, though, is that all our future options will come with a higher cetane rating than is currently offered in North America.
Cetane Rates of Different Fuels
It’s generally accepted that the higher a diesel fuel’s cetane rating, the better the fuel will burn in any diesel engine. You’ll note from our chart below that the United States has some of the lowest cetane rated fuel in the world. Also take note that synthetic diesel fuel promises to offer improved cetane ratings and as an added benefit, less contaminants, which is needed for today’s sensitive injection systems.
United States Petroleum-Based Ultra-Low-Sulfur Diesel- 40 (minimum) (1)
European Diesel- 51 (minimum) (2)
Biodiesel (B100)- 47 (minimum) (3)
Biodiesel (B20)- 40 (minimum) (4)
Synthetic Diesel (made from natural gas, coal, sewage and industrial waste, biomass, or from a reaction of CO2, H20, and electricity)- 60 to 80 (5)
Dimethyl Ether (DME)- 55 to 60 (6)
1. ASTM D 975, ASTM International. www.astm.org
2. EN 590, European Standards (EN) www.cen.eu
3. & 4. ASTM D 613
5. Currently in ASTM Subcommittee DO2. www.astm.org/SNEWS/APRIL_2007/cherrillo_apr07.html
6. International DME Association