To exploit a fuel’s energy, the bonds between the atoms need to be shattered. And the only practical way to do that is via burning, a.k.a. combustion. Energy is released because the combustion products contain less stored energy than their parents.
Technically, burning is the same thing as adding oxygen, or ‘oxidising’. (This, in part, explains why carbohydrates and partly oxidised hydrocarbons like ethanol don’t produce as much energy as petrol.)
Oxygen from air, plus petrol vapour, compressed in a combustion chamber, is like a hand grenade. The spark is like pulling the pin. The octane molecules disintegrate; some oxygen combines with the carbon to make carbon-dioxide, and the rest hooks up with the hydrogen to make water (as steam, a colourless gas). These are the two basic combustion products, so you can see that carbon dioxide, hydrocarbons and energy are inextricably linked. Each of the eight carbon atoms in octane goes off and collaborates with two oxygens to form eight molecules of CO2. Every molecule of octane yields eight molecules of CO2. Every kilogram of petrol (1.43 litres) produces a little more than three kilograms of CO2. (The ‘extra’ two-ish kilos is donated from consumed atmospheric oxygen.)
No amount of clever engineering can subvert the fuel-CO2 relationship; it is intrinsic to the energy-extraction process, and perhaps the biggest problem currently facing humanity. And it’s not just petrol. Similar intrinsic CO2 relationships exist for all other carbon-based fuels. If efficiently burnt, every atom of carbon in every fuel becomes part of a new CO2 molecule, which explains why everything from a campfire to a coal-fired power station inevitably yields CO2 to the greenhouse effect.
Meanwhile, back in the combustion chamber, something marvellous is occurring. Because water and CO2 contain far less stored chemical energy than petrol, a great deal of energy has just been liberated, in the form of heat. In practice, this forces the violent mechanical expansion of all the gasses in the chamber (read: controlled explosion). The result is what physicists call ‘work’ – the piston is shoved downwards, hard. This process takes place 50 times each second – per cylinder – at 6000rpm, and from the driver’s seat, things start getting really interesting…