How an Engine Turns Fire Into Motion · Day 1 of 21

Fuel Is Not the Main Ingredient

A 50-litre tank of gasoline weighs about 37 kilograms. To burn all of it, the engine has to breathe in roughly 540 kilograms of air. At sea level, air weighs about 1.2 kilograms per cubic metre, so that is about 450 cubic metres — the air inside a room ten metres by nine with a five-metre ceiling. One tankful of fuel, one room-sized volume of air, and the air outweighs the fuel about fifteen to one.

That ratio is the single most useful fact about an engine, and it is worth understanding why it is true rather than just remembering it.

Start with the burning. There is a habit of speaking about fuel as if it were a container with energy inside, like a battery, and combustion as the act of opening the container. That picture is wrong in a way that matters. Gasoline is a mixture, mostly molecules with eight or so carbon atoms strung together with hydrogen atoms hanging off them. A standard stand-in for it is octane, C8H18: eight carbons, eighteen hydrogens. Those atoms are held together by chemical bonds, and breaking a bond costs energy. If all you did was take a gasoline molecule apart, you would have to put energy in, not get it out.

The energy comes from what happens next. The loose carbon and hydrogen atoms grab oxygen, and the bonds they form with oxygen are stronger — meaning lower in energy — than the bonds they had before. Each carbon ends up in carbon dioxide, double-bonded to two oxygen atoms. Each pair of hydrogens ends up in water. The books are simple: you spend energy pulling the fuel apart, you get back more when the pieces bond with oxygen, and the surplus comes out as heat. Written as a balanced equation it looks like this: two octane molecules plus twenty-five oxygen molecules give sixteen carbon dioxide molecules and eighteen water molecules. For gasoline the surplus is around 44 megajoules for every kilogram burned, which is a lot — it is why we put up with engines at all.

Now count the mass on the left-hand side of that equation. Two octane molecules weigh 228 units. Twenty-five oxygen molecules weigh 800. So each kilogram of fuel needs about 3.5 kilograms of pure oxygen to burn completely. But there is no pure oxygen lying around. Air is only about 23 percent oxygen by weight; nearly all the rest is nitrogen. Divide 3.5 by 0.23 and you get roughly 15. The exact figure used for pump gasoline is 14.7 kilograms of air per kilogram of fuel, and you will meet that number again and again, because a huge amount of engine design is about hitting it.

So the fuel is the minor ingredient by weight. And notice what the nitrogen is doing: nothing useful. About three-quarters of the mass the engine drags in through its air filter takes no part in the burning. It gets heated, it gets pushed out, and at high enough temperatures some of it stops being inert and combines with oxygen to make nitrogen oxides, which is a problem for a later day. Mostly the nitrogen is freight.

This is why it is more honest to describe an engine as an air pump. A four-stroke engine takes in one cylinder-full of air for every two turns of the crankshaft. Take a two-litre engine turning at 3,000 rpm with the throttle wide open. It fills two litres every two revolutions, so 1,500 fills a minute, so about 3,000 litres — three cubic metres — of air a minute. That is about 3.6 kilograms. To match it at 14.7 to 1 you need about a quarter of a kilogram of fuel a minute, which is roughly a third of a litre. The air is what the engine physically moves. The fuel is a squirt.

You can see the imbalance in the hardware. The fuel filter on a car is a cylinder you could close your hand around. The air filter is a pleated paper box the size of a briefcase, and behind it sits an intake tract as thick as your wrist, and behind that a set of valves and ports shaped with real care. Nobody agonises over the shape of a fuel line. People spend careers on the shape of an intake port.

It also explains what the accelerator pedal actually does. On a gasoline engine, pressing the pedal does not open a fuel tap. It swings a flat plate — the throttle — inside the intake pipe, and that plate controls how much air can get in. The fuel system’s job is then to measure out however much gasoline matches the air that arrived. The driver commands air. The fuel follows. Everything about making more power comes back to the same question: how do I get more air into the cylinder? Bigger cylinders, more cylinders, faster revolutions, better breathing, or a pump that forces the air in under pressure. Those are essentially the only answers there are, and the rest of this course is mostly a tour of them.

One last consequence, and it catches people out. If most of the mass entering the engine is air, then most of the mass leaving it must be air too, plus something extra. Burn one litre of gasoline — about 750 grams — and you get roughly 2.3 kilograms of carbon dioxide out of the tailpipe. The exhaust weighs three times what the fuel did. Nothing has been created. The carbon came from the tank, and the two oxygen atoms bolted onto each carbon atom came out of the sky.

The tailpipe is mostly returning the atmosphere, slightly rearranged.