Piston Engines - Mixture — Page 90, Lesson 119

I want to walk you through the heart of mixture control in a piston engine. We're starting with the chemically correct ratio, and I want you to hold onto this number because it's the anchor for everything else. Air and fuel vapour will actually burn across a fairly wide range — anywhere from 8 parts air to 1 part fuel by weight, which we call rich, all the way out to 20 parts air to 1 part fuel, which we call weak. But here's the key: complete combustion only happens at one specific ratio — 15 parts air to 1 part fuel by weight. That's the chemically correct ratio. At exactly 15:1, every molecule of oxygen in the air combines with every bit of hydrogen and carbon in the fuel. Nothing is left over, nothing is wasted. Now, you'd think the chemically correct mixture would give you the best performance, but it doesn't. The reason is temperature. At 15:1, the combustion temperature gets so high that you can actually lose power through detonation — that's the uncontrolled, explosive burning of the fuel-air charge rather than a smooth flame front. So the theoretically perfect mixture is, in practice, too hot to be ideal. That brings us to the practical mixture ratio. The chemically correct strength would theoretically produce the highest temperature and therefore the highest power, but in the real engine, mixing and distribution are less than perfect. What that means is some regions of the charge end up richer than optimum and others weaker than optimum. And this variation can exist from one cylinder to another — cylinder number three might be running weak while number one is running rich, even though the carburettor is delivering the same mixture to all of them. So let's look at what happens when you deviate from 15:1. A slightly rich mixture — say, more fuel than chemically correct — doesn't hurt power much. Why? Because all the oxygen is still being consumed. The excess fuel just slightly reduces the effective volumetric efficiency — that's the engine's ability to fill the cylinders with fresh charge — and in fact, that extra fuel has a cooling effect, which can actually be beneficial. It helps keep combustion temperatures down. Weak mixtures are a completely different story. A weak mixture rapidly reduces power, because now some of the inspired oxygen is not being utilised — there isn't enough fuel to burn all the air, so you're wasting oxygen that could have produced power. And here's the important contrast: that power reduction from weakness is much greater than the reduction from slight richness. Richness costs you a little; weakness costs you a lot. That's why, when maximum power is the objective rather than best fuel economy, it's common practice to run engines at somewhat richer than chemically correct — typically around 12.5 parts air to 1 part fuel. The logic is simple: by running slightly rich overall, you ensure that no individual cylinder is left running severely weak. You're deliberately trading a small power loss from richness to avoid a large power loss from any cylinder going weak. Now, the excerpt cuts off as it's introducing the problems caused by weak mixtures. What we've established is that a weak mixture — anything weaker than the chemically correct ratio — burns at lower temperatures. And that's the opening point of the next part, which we'll pick up from there.