The Big Bang Theory

So your car runs on this explosive fuel stuff called hydrocarbons. You may know it as gasoline, Diesel, Natural Gas, methane or biofuel etc etc. They’re all some form of the same basic chemical chain called a hydrocarbon chain. The only real difference at the basic level between each kind of fuel is the length of the chain itself. I’ll get into more detail in the chemistry section if you really want to know… but for you haven’t reached that level or nerd-hood yet I’ll cover the more basic stuff too.

In order for an Internal combustion engine to work we need four things: Fuel, Air, Compression and an Ignition Source. Fuel is important because, well, it’s the stuff that explodes to make the car go. Air is important because the oxygen in the air is what reacts with the fuel during combustion. Compression is important because compressing the fuel and air mixture before combusting it is the only way for it to burn fast enough to make any real usable power. Last but not least we need an ignition source to light the whole works off.

Air/Fuel mixture

In a gasoline engine it is very important that for every bit of fuel in the engine there is 14.7 bits of air mixed with it. This ratio is called a “stoichiometric” ratio, which means that it’s the perfect amount of oxygen and fuel so that after it’s all burned up there’s no more fuel and no more Oxygen. Well, there’s oxygen but it’s not in the form of O2 which we breathe… but that’s for the chemistry section. There are, however, times when variation of that ratio is important for combustion stability and for efficiency. For different kinds of fuel the stoichiometric ratio is different, like methane which has a stoichiometric ratio of 17.2:1 and Diesel engines which almost always operate in “lean” conditions where there is extra oxygen left over after all of the fuel is burned.

Compression

Compression is vital for internal combustion engine operation. Modern gasoline engines compress the air/fuel mixture to somewhere between eight and eleven times the ambient pressure outside of the engine. The fuel and oxygen atoms in this compressed mixture are closer together and the temperature is much higher than before it was compressed. With these conditions ignition of the fuel/air mixture can happen faster than in normal pressures which allows more of the energy of the explosion to be used than if the ignition happened at lower pressures and temperatures. Without compression the ignition would happen slow enough that the ignited fuel wouldn’t be able to light the surrounding fuel fast enough to keep the chemical reaction going long enough to burn all of the fuel.

Ignition Source

 There are two ways to ignite the compressed fuel/air mixture inside the engine once it’s ready to burn. One is by introducing an electric spark to ignite the fuel. The other is from the compression we were talking about earlier which is called auto-ignition. Auto-ignition is when there is enough heat made during compression that the fuel/air mixture spontaneously combust. Some fuels like diesel are much more predictable during auto-ignition than gasoline. Diesel engines do not have spark plugs, but rather, the fuel is injected directly into the pre-compressed air so that it lights off immediately after being vaporized during injection. This kind of ignition is more violent and louder than spark ignition, which is why diesel engines are so much noisier than gasoline and have that characteristic “clack clack clack” sound when running. Since diesel is ignited this way the amount of compression in a diesel engine is usually much higher than a gasoline engine which contributes to engine efficiency.

Throttle the Chaos

Gasoline engines need to have very good control of the amount of air coming into the engine since the amount of fuel must always match the amount of air. So in gasoline engines there is a throttle plate in the intake of the engine which is controlled by the throttle pedal which you push to make it go. Diesel engines can have extra air in combustion so there is no throttle plate on a diesel engine. The throttle pedal in a diesel car only controls how much fuel is added and the air is not controlled as precisely. This is all because of the type of fuel and the type of combustion which is happening. Nice to know huh?

Chemistry as Promised

A hydrocarbon chain is basically a chain of carbon atoms surrounded by hydrogen atoms like this picture of a diesel molecule with 16 carbon atoms and 34 hydrogen atoms. The differences from one fuel type to another are mostly determined by the length of this chain. For instance, Methane is the shortest chain of one carbon atom and four hydrogen atoms, gasoline has eight carbon atoms and eighteen hydrogen atoms. This is where the name hydrocarbon comes from, they’re all made of hydrogenated carbon chains.

The Diesel hydrocarbon molecule

So hopefully you learned something from all of this. I have about three more pages of details that I keep having to cut out just to get the main points across without writing a book. I’ve got no shortage of content, that’s for sure! more blogs to come! 🙂

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2 Responses to “The Big Bang Theory”

  1. Bob Says:

    Hello Nate,

    I like your style of writing and the way you present the topics of discussion. I don’t seem to be able to find any of the ‘blogs to come’ and the book load of information you have of hydrocarbon chemistry… I feel ‘left hanging’ and want the rest of the story! – Did you write more, if so, where is it, and if you didn’t, any chance of doing so :-)?

    Regards

    Bob

    • turbonate Says:

      Hey Bob, thank you for the encouragement. If you want more content, I do respond to engine questions on Quora: Same type of content, different format. I am working on some other content generation software that may require links to these types of blogs. If I’m not happy with the content of other linked blogs I may just have to write some more of my own! 😀

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