As a former Chemistry student, I find that the best way to understand the workings of an internal combustion engine is to start with the chemical reaction that is taking place, and show how the engine serves to facilitate the reaction, and then to make use of the reaction to create power.

The basic reaction that occurs with a standard, gasoline (or petrol, for the Europeans) driven engine is actually not very complicated at all. Octane, like most other fuels we use, is called a hydrocarbon, because it's made up of hydrogen and carbon. Carbon makes up the core of the molecule, and hydrogen atoms bond along the perimeter to complete the molecule. The chemical formula of Octane is C8H18. It gets the name octane from the Latin root "Oct" which means "8", in reference to the 8 carbon atoms. Many hydrocarbons use this nomenclature system, giving them names like "pentane" (5 Carbon), heptane (7 Carbon), and so on.

Anyway, the primary chemical ingredient into the reaction is Octane, which reacts with Oxygen (O2) gas from the air, and heat, to produce Water (H2O) and Carbon Dioxide (CO2). The balanced reaction is 2 C8H18 + 25 O2 -> 18 H2O + 16 CO2.

Before I go further, I should point out that this theoretical reaction is not the ONLY reaction that goes on in a real combustion engine. This is for a number of reasons. First of all, the gasoline we use is not perfectly pure Octane. When you purchase your fuels, you can buy differing grades of gasoline. The grades are based on purity of octane, by percentage of weight. So gasoline with an 87 rating is (we hope) 87% pure octane. The rest if other hydrocarbons, and various impurities that the refining process was unable to remove. The engine is set up for Octane, so mixing in other ingredients will foul the reaction slightly, causing other by-products in small amounts.

Another reason that the reaction is not perfect is that certain environmental variances can have an effect on the reaction. Such uncontrollable factors as air temperature and air pressure will affect the density of the O2 being taken into the system, which will affect the combustion reaction as well. Also, engines will wear over time, causing further variation.

Most importantly, however, there is no perfectly efficient combustion engine. Our engineering has improved over the years, but we still have yet to perfect the process and make a mechanically perfect engine.

Let's discuss how the engine functions to facilitate this

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