Gentlemen,
The cetane rating of diesel fuel is a measure of the auto-ignition characteristic of the given fuel. Cetane rating is like octane rating for a spark ignition engine only the opposite end of the auto ignition-spectrum. In a diesel engine the fuel is ignited by hot air, and, in fact, would fit the definition of detonation in a spark ignition engine. In a spark ignition engine you want the fuel to be ignited by a spark at just the right time. Detonation like a diesel engine would not be good. High octane number fuel is a low cetane number fuel- and vice versa.
Typical cetane numbers for #2 diesel are 40-45 and this meets the requirements of Cummins, CAT, DDC and ASTM D-975, DF2. The cetane rating does control the ignition delay period in the power stroke of a diesel engine, but will have no effect on brake specific fuel consumption (fuel efficiency or MPG), because it has no effect on the heating value of the fuel. Diesel engines are designed to have a ignition delay period that results from the use of commercially available #2 diesel fuels. The rate of pressure rise, the rate of injection versus engine speed are taken into account for fuels that are 40-45 cetane. Just because the ignition delay period is longer does not mean that there is unburned fuel in the exhaust. I stated in my first post on this topic that it is well documented that diesel engines burn (oxidize) about 99. 99% of the fuel they use. An energy balance on the engine proves this. So let's say combustion was perfect and the engine burned 100% of the fuel, then the only products of combustion would be carbondioxide, water vapor and nitrogen dioxide from the high combustion temps (+3000 degrees F). But, combustion is never perfect. The question is how much of products of combustion in the exhaust have heating value and are unburned (not oxidized). The answer is very little. In fact, it takes some very sensitive instruments to measure the exhaust products and the values are in the parts per million range.
Cetane enhancer additives are very popular here in Minnesota in the winter to promote better cold starts and limit white/cold smoke. I use them myself, but I have never experienced improved fuel economy as a result.
I hope this answers the cetane question.
Best Regards,