Does the mixture knob in your cockpit confuse you? Most of the time in training, it seems like the other knobs like throttle and carb heat get all the attention while the red knob on the right gets just the standard "mixture full rich" check prior to takeoff. Setting the mixture properly is just as important as throttle and other settings and goes a long way toward long term care of your engine. Here's why:
Whether your engine has a carburetor or is fuel-injected, you control the ratio of fuel to air going into your cylinders with the mixture knob. This is important because the engine needs just the right ratio of fuel to air to produce the most power. If there's too little or not enough fuel for the amount of air, the engine will run rougher and at a lower power. In that way, it's a lot like keeping a campfire going, too much wood (fuel) and you'll suffocate the fire, and too much air and the fire will blow out.
On takeoff and at other critical phases of flight, we'll typically have the mixture setting all the way forward for "full rich." This provides enough fuel for the engine to produce power, and then some surplus fuel just for good measure. This gives us a safety buffer so we know the engine won't start coughing on us at a critical moment. Here's what the engine looks like at full rich:
The extra fuel in the cylinder isn't just there for safety though, it also has a cooling effect on the engine. At full power, the engine gets hot. Typically, the air and oil flowing around the engine is enough to keep the cylinders cool, but running hot like on takeoff, that extra fuel goes a long way to keeping the engine at normal operating temperatures.
So what's wrong with having all that extra fuel? For starters, as you climb and the air gets thinner, you could risk running the engine rough as the mixture gets excessively rich (think too much wood on the campfire).
There's also the problem of what happens to all that unused fuel - it typically gunks up on the leads from the spark plugs which could affect how well the ignition is working.
Lastly, it hurts the range and endurance of the aircraft - you're literally throwing fuel away! Once we're up at altitude and we don't need an excessively rich mixture, we'll want to lean it out for best performance and fuel economy. Here's what's going on inside the engine as you lean it:
1) We've come out of the climb into cruise flight at full rich. The abundance of fuel has a cooling effect on the engine, which we can see if we have a gauge for the Exhaust Gas Temperature (EGT). This measure the very hot gases that are the after effect of combustion and are expelled from the cylinder through the exhaust valve. Here, we have a relatively low EGT.
2) We'll start to lean the mixture by pulling or turning it back. This reduces the amount of fuel in the cylinder. With less fuel cooling the cylinders, the EGT starts to rise. Power isn't affected, though, because we still have enough fuel for the amount of air to produce cruise power.
3) As we lean even further, we start to notice a roughness in the engine and a reduction in power. We've starved the engine of the amount of fuel it needs to produce peak power, so RPM starts to drop. Since we're not producing as much power, the EGT drops as well, even though we've taken more of the cooling fuel away.
4) If we then enrichen the mixture by moving or twisting it forward a bit, until the power surges again and the roughness goes away, we'll be operating at a temperature slightly lower than the peak we saw between step 2 and 3. We have enough fuel for power production, plus a small surplus that acts as a coolant and a small safety buffer. By finding this mixture we're not wasting fuel and we're keeping the temperature under control.
Assuming you have a reliable EGT, why not just lean until you get peak temperature? Won't this be the most fuel efficient while still producing the needed power? Engine manufacturers like Lycoming don't recommend running at peak EGT. Doing so can have negative effects on the engine, such as pre-igntion, where a hot cylinder wall will start to glow, and cause the fuel and air to ignite in advance of the piston reaching the top of the cylinder.
For this reason, we'll find peak temperature by leaning until we get a slight drop in RPM, and then enrichen slightly until the power returns. This approximates an EGT on the rich side of peak. Your engine manufacturer may recommend running at around 50-100°F rich of peak. In a carbureted engine, this isn't an exact science because of uneven mixing of fuel and air in the carburetor. Even with sophisticated probes in your cylinders, it's a good idea to listen for roughness as you lean. Remember, this is more like a campfire than a nuclear reactor!
In fuel-injected engines, you may be able to more precisely meter the mixture and even run at lean of peak to get better fuel economy and engine life. Always follow the recommendations of the manufacturer. But in any case, don't be afraid to lean the mixture on your next flight and save your flight school some gas money!
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