You might think that all engines that burn fuel from crude oil operate similarly, but it isn't so. It turns out that gasoline and diesel engines can have characteristics that are opposite of one another. This article focuses on the important role that fuel cetane plays in diesel engines, easing ignition in the cold and ultimately, helping to save your starter and battery and delay your DPF service time.
High-Octane Toast
Chances are you've heard about octane‚ the characteristic that helps good gasoline prevent your car from knocking - when fuel burns explosively and produces a knocking sound in the engine.
High octane means a fuel that resists breakdown with temperature. Think of a type of bread that you can toast much more thoroughly without it getting burned -- that's like high-octane toast. Low octane fuel, say 87 octane, in a high-compression engine that needs 91 octane will knock when the engine is under a heavy load. When toast burns, the carbon it contains separates from the rest and produces that solid, black powdery substance that tastes bitter, known as carbon. It all has to do with the fact that the compression stroke in the engine heats the air up quite a bit, and that too much heat breaks down the fuel chemically. When fuel breaks down, it burns too fast while also producing soot, which is primarily pure carbon.
Dr. Diesel's Design
In the 1890s, Dr. Rudolf Diesel dreamed of a super-efficient internal combustion engine that could compress the charge of fuel and air much more than the gasoline engines that were popular at the time.
Compressing the gases that burn and force the piston down first, before the burning happens, greatly increases the efficiency of any engine.
Its ability to efficiently convert the heat in the fuel to pressure and power then goes way up.
Engine Efficiency in Higher Pressures
Experiments with engines at the time had already shown that high compression could cause explosive combustion. But, Diesel understood that the most efficient engine would run under extremely high pressures. So he theorized he could build an engine that would compress air to an extremely high pressure, a process which heats it to a very high temperature, without exploding. What would be needed was a special fuel system. He could prevent pre-ignition by designing the system so the fuel would be held out of the cylinder until after the piston reached the top of its stroke. It would then pump the fuel in gradually for, say, the first 20% of the downstroke.
The Process of Injection
We now refer to this process as injection, and say that the fuel was injected over a certain period of time in the engine's cycle. This controls how fast the fuel burns by limiting how much fuel is available for burning at each point in the rotation of the engine.
An incidental significant advantage of the design was that the high compression would make the air so hot that no spark plug or other ignition device would be needed. This is where cetane comes into play.
The Role of Cetane
The science of making good diesel fuel is so complicated that the 1956 Nobel Prize was awarded for explaining why a diesel engine doesn't need spark plugs, what the awardees called "the mechanism of auto-ignition" meaning that the fuel ignites by itself. What is important here is that the experience proved to oil refiners that some of the heavier, less volatile fuels they could make out of crude oil like diesel fuel would ignite much more easily.
Cetane is the particular type of oil that ignites most easily, so that became the standard. Cetane is the opposite of octane in that it's actually desirable for the fuel to break down and develop a tendency to burn very rapidly.
Ignition Delay
That's why it ignites from heat alone. The fact that the fuel undergoes this process for only a very short time, known as ignition delay, minimizes the amount of fuel that burns rapidly. It's all controlled by the injection system.
While modern diesels don't quite perfectly duplicate Diesel's theory, an engine that is in good condition will run smoothly, with little knock, especially after it's warmed up and the air is plenty hot enough to ignite it very quickly. Diesel compression ratios are about twice as high as those in car engines, allowing much higher efficiency.
Cetane Ratings
This brings up the business of the cetane number, referring to the rating of the fuel's ability to ignite quickly. Most engines require a cetane rating of 40, the ASTM D 975 standard for diesel fuel that the oil companies try to meet with every gallon they refine. A special cetane test engine (as shown in the photo above) exists in every refinery. The test engine has a standard compression ratio and can measure how long it takes for the fuel to ignite after the injection system starts to pump the fuel in, giving it a cetane rating. The faster the fuel ignites, the higher the rating.
Notes on Cetane in the Cold
Quick Ignition and Reduced DPF
While cetane helps the engine run smoothly when it's hot, it matters much more when it's cold. Joel Morrow, an officer with Ploger Transportation and an expert who has his own consulting business, says that high cetane really helps a cold diesel start up quickly and then greatly reduces the particulate that can foul the Diesel Particulate Filter (DPF) while the engine is warming up.
Fuel that ignites quickly actually enables the injection system to control the heat in the combustion chamber more effectively, preventing the same kind of explosive burning and breakdown that occurs in a gasoline engine when it knocks.
Manufacturers' Requirements
He also mentioned that cetane requirements in Europe are higher and that some truck manufacturers with European roots may advise higher cetane than 40, so check your owner's manual for recommendations.
Fuel Additives
He reports that his company often tests its fuel and has found it sometimes comes through just below the standard. As a result, they use good fuel additives. Cummins admits that poor quality fuel sometimes shows up on the market and has actually formed a partnership with a manufacturer of fuel additives. A good additive can boost cetane so it's at or above the standard while also providing lubrication for the wearing parts in the injection system, removing deposits that can interfere with fuel system operation, and in winter preventing gelling and freezing due to separated water. The anti-gelling quality is especially important in winter if you have purchased your fuel farther south from where you need to run. For these reasons, it's a good idea for you to explore the use of one of the many fuel additives on the market.
Morrow mentioned FPPF as a good brand of additive, and Cummins works with Power Service, but there are lots of good brands out there. Just avoid alcohol-containing additives, as alcohol may be able to keep water from separating from the fuel, but it also interferes with cetane.
Final Thoughts
Keeping your fuel's cetane rating above the manufacturer's requirement will ease starting in the cold, thus saving your starter and battery and helping to ensure you can get going and haul the cargo you need to haul each day to have a successful business. And, it is also likely to delay the time when your DPF will need service.
Other relevant articles:
Diesel Emissions Controls: Part 1 - History of Standards and Systems
Diesel Emissions Controls: Part 2 - Truck Maintenance Downsides