11.4.2 Diesel (Compression Ignition) Engines
11.4.2 Diesel (Compression Ignition) EnginesDiesel engines are different from gasoline engines in one very important way: they do not use a spark plug. Instead, diesel engines use compression ignition, also called autoignition.
In a diesel engine, air is compressed inside the cylinder until it becomes very hot. Then diesel fuel is injected into that hot, high-pressure air. Because the air is so hot, the fuel ignites on its own. This combustion pushes the piston downward and produces work.
Diesel engines usually operate at higher compression ratios than gasoline engines, which helps them ignite the fuel without a spark. This is one reason diesel engines are often more efficient than gasoline engines.
Because diesel engines ignite fuel differently, diesel fuel also has different chemical properties than gasoline. Diesel fuel generally contains longer hydrocarbon chains and tends to work better with straighter-chain molecules, while gasoline performs better with more branched molecules.
Diesel fuel is rated using a cetane number. The cetane number measures how easily and quickly diesel fuel ignites under compression. In general, a higher cetane number means the fuel ignites more readily, which can improve engine starting and combustion performance.
Cetane ratings are based on reference fuels, including:
Cetane (C16H34), which has a cetane number of 100
Text description of the Cetane image.
Ball-and-stick model of a cetane molecule (C16H34) showing a straight-chain hydrocarbon structure composed of sixteen carbon atoms and thirty-four hydrogen atoms. The carbon atoms form a long zigzag chain, with smaller hydrogen atoms attached around the outside. Carbon atoms are typically shown as dark gray or black spheres connected by rods, while hydrogen atoms are shown as smaller white spheres.Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.Iso-cetane or 2,2,4,4,6,8,8-heptamethylnonane, which has a cetane number of 15
Text description of the Iso-cetane image.
Ball-and-stick model of a 2,2,4,4,6,8,8-heptamethylnonane molecule showing a highly branched hydrocarbon structure. The molecule consists of a nine-carbon main chain with seven methyl groups attached at multiple positions, creating a compact, irregular shape. Carbon atoms are typically shown as dark gray or black spheres connected by rods, while smaller white spheres represent hydrogen atoms attached around the outside. The extensive branching gives the molecule a wider, more clustered appearance than a straight-chain hydrocarbon of similar size.
Credit: Moleview
Note: You can use your mouse to rotate and zoom in on the Iso-octane molecule.
In simple terms, the cetane number tells us how easily diesel fuel will ignite in a diesel engine. This is different from octane rating in gasoline, which measures how well a fuel resists igniting too early.
Petrol (Gasoline) versus Diesel - Don’t try this at home chemical combustion video.
What's the Difference Between Petrol & Diesel? | Bang Goes The Theory | Earth Science (3:51)
Transcript: What's the Difference Between Petrol & Diesel? (3:51)
Nowadays most cars on the road fall into one of two categories: either petrol or diesel. Now, from the shape of these vehicles on the outside, you wouldn't know which is which, but on the inside their engines just don't work in the same way, and the net result of that is what finally emerges from their exhaust pipes is also significantly different. All but the very latest diesels give off at least twice the toxic nitrogen dioxide and 10 or 20 times the dangerous soot particles that petrol cars do. So why is there such a big difference? It really comes down to the fuel itself.
Petrol and Diesel, they may look the same and their pumps sit side by side on the garage forecourt, but there is a significant difference between the two of them, and to show you I need a bowl of each and a box of matches. By the way, danger alert: do not not try this at home.
Now petrol is a highly volatile fuel. That means it evaporates easily, so Petrol gas effectively comes up and mixes with the air, and so catches fire readily. But diesel, made of longer heavier molecules, gives off fewer fumes and is actually quite difficult to light. Now because of this big difference in flammability between the two fuels, the engines work in fundamentally different ways.
This is my petrol engine. It's very simple. It's just a single cylinder, and into it I've got a nice fitting piston that can slide up and down. The way the fuel gets introduced into the cylinder, because petrol vaporizes so readily, it actually gets injected into a warm chamber just prior to the cylinder. And then, as the Piston Rises, it sucks in a mixture of petrol Vapors and air, and because they're both gases essentially, they can mix very very intimately. So when you introduce a spark, you get quite a nice explosion. And you'll see on that explosion it's a very blue flame, and that blue means that there's very few soot particles in there. It's clean burning.
Now a diesel engine has to work differently because diesel won't readily vaporize into a gas to make it flammable. It's injected into the air as a kind of fine Mist, a sort of High Press aerosol. Now that really does catch fire. So I'll spray a fine mist of diesel droplets into my cylinder. There you go. There's a big difference. The flame is bright yellow, and that yellowness indicates that there are glowing soot particles in there. And you can quite readily see the smoke coming off it, and indeed the soot, and that's the problem because in the operation of an engine there really isn't time for all those droplets of liquid to burn completely.
So what happens is some of it just gets reduced down to particles of carbon soot, and that means that the exhaust system of a diesel engine has a bigger job to do.