Piston Engines - Fuel — Page 77, Lesson 103

Let’s start with the fuel itself, because everything downstream depends on what the fuel does when it meets air. I want to walk you through the properties of aviation gasoline that we care about as pilots and engineers, and then we’ll go through the combustion process itself. First, volatility. A volatile liquid is one which is capable of changing readily from the liquid to the vapour state, and it does that either by the application of heat, or by contact with a gas into which it can evaporate. So think of it as the fuel’s eagerness to become a vapour. Now, fuel is added to the air at the carburettor, and the efficiency with which the fuel mixes with the air is largely determined by the volatility of the fuel. That’s the key link — the better the fuel vaporises, the better it mixes with the air. However, the time involved is so small that some of the fuel remains in the form of minute droplets, and the evaporation of those droplets occurs in the induction system. So the mixing isn’t instant; it carries on downstream. Now, high volatility. A liquid boils when its vapour pressure is greater than the atmospheric pressure acting on the surface of the liquid. That means that, as the atmospheric pressure reduces with altitude, the fuel vaporises at a lower temperature. This is generally referred to as ‘low pressure boiling’. So at altitude, with less pressure pushing down on the fuel, it boils more easily — that’s the phenomenon we call low pressure boiling. Next, stability. A number of the hydro-carbon compounds which are present in gasoline have a considerable attraction for the oxygen in the air. When they come into contact with air, they oxidize and undergo chemical changes to form heavy resinous gummy compounds and corrosive bodies. So these unstable hydrocarbons, if left alone, would turn into gum and corrosive stuff inside your system. It is essential that these potentially unstable hydro-carbons are not allowed to oxidize, and this is prevented by the addition of oxidation inhibitors. So we add chemicals to stop that oxidation from happening. Then sulphur content. Sulphur and sulphur compounds, when burnt in air, form sulphur-dioxide. That sulphur-dioxide combines with the moisture content of the exhaust products to form a sulphurous acid, which is extremely corrosive to the exhaust system. So it is important that the sulphur content is kept as small as possible, and in aviation gasoline the maximum amount of sulphur permitted is 0.001%. That’s a hard limit — 0.001% maximum. Now the combustion process itself. Combustion is a controlled rate of burning — it is not an ‘explosion’. That’s a critical distinction. The mixture induced into the cylinders consists of gasoline vapour, which is 84.2% carbon and 15.8% hydrogen by weight, and air, which is 78% nitrogen, 21% oxygen, and 1% other inert gases. When combustion has been completed, the hydrogen in the fuel will have combined with the oxygen in the air to form H2O, which is water vapour, and the carbon in the fuel will combine with the oxygen in the air to form CO2 — carbon dioxide. So the two products of complete combustion are water vapour and carbon dioxide. Now, the nitrogen and other gases play no active part in the combustion process, but they do form the bulk of the gas that is heated and expanded to create pressure energy. So they’re not chemically involved, but they’re the working substance that expands. The nitrogen also slows down the rate of combustion — without nitrogen, combustion would be an explosion with far too rapid a temperature and pressure rise to be harnessed to do useful work. So the nitrogen is what keeps the burn controlled, turning what would be a detonation into a steady, usable burn. Let me tie that together. The fuel’s volatility determines how well it mixes with air. Its stability determines whether it turns into gum. Its sulphur content determines how corrosive the exhaust will be. And the combustion process itself is a controlled burn, not an explosion, with the nitrogen in the air acting as the brake on the burn rate, and the products being water vapour and carbon dioxide. That’s the foundation of the fuel system.