Piston Engines - Mixture — Page 100, Lesson 122

Let’s pick this up right where the mixture curve leaves off. We’ve just covered why a rich mixture is needed for starting and slow running. Now I want to walk you through what happens as we move up through the power range — climbing, cruising, and how we actually measure the mixture setting in the cockpit. First, let’s be clear on a fundamental point about power. The engine power output is the product of engine speed and the mean effective pressure in the cylinders during the working cycle. Mean effective pressure — that’s the average pressure pushing down on the piston during the power stroke. So higher power outputs involve increases in both of these factors — you spin faster, and you push harder. Now, as the speed and the pressure increase, there is also an increase in the temperature of the gases, and therefore their tendency to detonate. Detonation — that’s the uncontrolled, explosive burning of the fuel-air charge, and it’s destructive. So temperature is the enemy here. Now, climbing power. When higher power is required for climbing, the mixture is enriched to about 11:1. That’s roughly 11 parts air to 1 part fuel by weight. Why enrich it? The extra fuel, in vaporizing, cools the mixture and reduces the tendency to detonate. Think about that — the fuel absorbs heat as it changes from liquid to vapour, so it acts as a coolant inside the cylinder. That’s the whole reason we go rich for climb: not for power, but for cooling and detonation control. Now let’s contrast that with cruise power. During cruising conditions, only moderate power is required from the engine. So the mixture can be leaned to around 18:1 — much leaner, much less fuel. That allows the minimum expenditure of fuel to achieve economy. So you trade the cooling margin for fuel savings when you don’t need the power. Now, how do we actually know what the mixture is doing? That’s where the exhaust gas temperature gauge comes in. As the mixture control is moved from fully rich to a weaker setting, the air-fuel ratio approaches the chemically correct value of approximately 15:1. At this ratio, all the air and fuel are consumed — that’s the stoichiometric point — and the heat released by combustion is at its maximum. More heat means more power. So as you lean toward 15:1, you get more heat, more power. Now here’s the practical part. With a fixed pitch propeller, as you lean the mixture, rpm will rise. And airspeed will increase as more power is produced. Both these indications — rpm and airspeed — can be used to adjust the mixture. But a more accurate method is to indicate the change in exhaust gas temperature as the mixture is varied. That’s the whole point of the exhaust gas temperature gauge — it gives you a precise, direct read on combustion temperature, which peaks right at that chemically correct 15:1 ratio. So let me tie this together. Rich for start and slow running — for combustion stability. Rich for climb — about 11:1 — for cooling and detonation control. Lean for cruise — about 18:1 — for economy. And the exhaust gas temperature gauge is your instrument for finding that peak temperature, which corresponds to maximum heat release at roughly 15:1. That’s the full picture of mixture management across the power range.