Piston Engines - Mixture — Page 100, Lesson 121

Let’s start with the big picture. In a piston engine, the mixture is the ratio of air to fuel that goes into the cylinders. That ratio is not fixed — it changes with the operating condition. And the reason it changes is that the engine’s needs are different at starting, at slow running, and at take-off. So I want to walk you through why a rich mixture — that is, one with extra fuel relative to air — is required in each of those phases. First, starting and slow running. There are two distinct reasons, and I want you to hold both of them. Reason one is about vaporization. Fuel will only burn when it has vaporized and is mixed with air. When you start a cold engine, there is very little heat to assist the vaporizing process. So only the lightest fractions of the fuel — the most volatile parts — will actually vaporize. That incomplete vaporization can show as white smoke at the exhaust. Now, white smoke can also appear because water is a product of combustion, leaving condensation inside the exhaust, and because the engine is breathing in cold, moist air. Either way, that white smoke will gradually disappear as the engine reaches normal running temperature. The key point is this: to make sure there is sufficient fuel vapour in the cylinders to support combustion, you need a rich mixture. You’re compensating for the fact that most of the fuel hasn’t vaporized yet. Reason two is about exhaust valve lag and gas inertia. The exhaust valve is given a certain amount of lag — it stays open a little longer than you might expect. That lag lets the engine take full advantage of the considerable inertia of the gases at normal engine speeds. That inertia does two useful things: it gives efficient scavenging of the burnt gases — that is, clearing them out — and it gives impetus, or push, to the incoming charge. Now here’s the problem at low speed. As engine speed reduces, the gas velocity falls, and more of the burnt gases remain in the cylinder. At still lower speeds, there’s a tendency for exhaust gases to be sucked back into the cylinder by the descending piston before the exhaust valve closes. That means the induction gases — the fresh charge coming in — get diluted with burnt gas. The dilution is so significant that, to maintain smooth running, you need a rich mixture to compensate. So for starting and slow running: rich mixture, for vaporization reasons and for dilution reasons. Now take-off power. When you select full power for take-off, the mixture must be further enriched — to about 10 to 1. That’s ten parts air to one part fuel, by weight. Now, why? Apart from the cooling effect — and I’ll come back to that — the excess fuel is wasted, because there is insufficient oxygen available to burn it all. So you’re deliberately putting in fuel that cannot be burned. The reason you do that is the cooling effect: the extra fuel absorbs heat as it vaporizes, which cools the engine and the combustion process. That’s the trade-off — wasted fuel in exchange for cooling. Let me show you what this looks like on a typical air/fuel mixture curve. That figure is Figure 7.1, a typical air/fuel mixture curve. It plots the mixture ratio against engine power or speed, and you can see how the required mixture changes across the operating range — rich at the low end for starting and slow running, leaning out in the cruise range, and then enriching again toward take-off power. So to summarise the three points: starting and slow running need a rich mixture because of poor vaporization when cold, and because of burnt-gas dilution at low speeds. Take-off needs an even richer mixture — about 10 to 1 — primarily for its cooling effect, even though the excess fuel is wasted because there isn’t enough oxygen to burn it.