My airplane has a choke, just like your grandfather's old Packard. Or as we call it, the mixture knob. One consequence of GA's antique engine technology is the mixture has to be manually adjusted during flight. Getting it wrong has potentially disastrous consequences, yet most pilots have no idea how to set the mixture. There's even a religious debate about it.

The fuel/air mixture in the engine needs to be in the right proportion, roughly 15:1. If there's too much fuel ("too rich") the engine produces less than optimal power, fuel is wasted and you get carbon deposits in the cylinder. If it's way too rich you can flood and kill the engine, like I embarassed myself in Santa Fe once, but fortunately flooding is not really a risk while flying. Conversely if there's too little fuel ("too lean") the engine also produces less power. Worst case, you can starve the engine and have it quit. This is a flight risk, specifically if the pilot forgets to richen up on descent. You won't notice being too lean at idle power but if you decide to go full power to go around you can kill the engine and, soon after, yourself.

The mixture also affects engine cooling. Common pilot wisdom is to fly a little rich of peak because the extra fuel evaporating helps cool the cylinders. If you let the engine get too hot you risk causing detonation or pre-ignition, in which case the engine can run away and get so hot it blows a cylinder. This part's all a little mysterious, particularly in planes without much instrumentation, and mostly we avoid it by trying not to fly over 75% power. The way I was taught to set the mixture was to get to cruise, then lean it out "until the engine runs rough", then nervously push it back in a little bit. High tech!

In the past few years engine monitors have become affordable, little temperature probes stuck in the engine that tell you that the exhaust gas is 1475°F and the cylinder head temperature is 332°F. Now pilots can know exactly how hot the engine is running and set their mixture accordingly. You can run at peak EGT, like the manual says, which is efficient but pretty much as hot as possible. Or you can run 50° rich of peak EGT, the common wisdom, only it turns out that places you most at risk of detonation. Or you can run about 30° lean of peak which seems freaky to old timers but definitely saves gas and may keep the engine cooler and cleaner. There's no consensus on what's best, just a religious war, but the guys with the most advanced understanding seem to prefer lean of peak.

One more complication: each cylinder stands alone, there's four or six little engines all running in parallel. Each with its own temperature and fuel/air mixture. And there's only one mixture control and it's not properly calibrated between the cylinders. No one noticed this before engine monitors became available so the manufacturers got away with it. Our plane is quite out of balance: one of our cylinders peaks about 1gph sooner than the others so there's no way to set the mixture right for everything. I just ordered $700 worth of fancy GAMI balanced fuel injectors to fix this problem.

There's a lot more to say on the topic but I'm out of my depth. The best stuff I've read on engine operation are John Deakin's Pelican's Perch articles. Specifically "Where should I run my engine?" (1, 2, 3, 4). Also Go ahead, abuse your engine!, Mixture magic, and Detonation myths. These articles only present one side, they're the foundational documents of the lean of peak religion, but they're clear and make more sense than anything else I've found.

  2010-11-16 19:20 Z