Subscriber question:
"I don't have an engine monitor. I've always been taught to lean until rough and then just enough to run smoothly for cruise. Am I harming my engine this way? Should I richen a bit more for safety?" — Matt A.
Ryan:
“That’s a perfectly acceptable way to lean for cruise. You’re not harming your engine as long as you’re at reasonable cruise power settings—but you should not richen a bit more for safety.
The roughness you feel occurs because each cylinder receives roughly the same amount of air but not the same amount of fuel. That means each cylinder produces a slightly different amount of power. At rich mixtures, those differences are negligible. But at very lean mixtures, power drops off rapidly with decreasing fuel and you can feel the differences between the richer cylinders and the leaner ones from one power stroke to the next. It’s not harmful, but you still want to fix it. So you’ll richen just enough for smooth operation.
This technique puts you on the lean-of-peak side of the mixture curve, where leaner mixtures are cooler and richer mixtures are hotter. That means that ‘richening just a bit more for safety’ is misguided. Adding a little extra fuel doesn’t provide protection—it actually leads to hotter cylinder temperatures with higher internal pressures.
So when is this technique appropriate? With most engines, it’s reasonable at around 65% power and below. Engines that have well-tuned fuel injection systems can use this technique at even higher power settings, since they can get even leaner (and therefore cooler) before the difference in power output across cylinders is noticeable. Note again: it’s not running “too lean” that’s the potential problem—it’s running “not lean enough.”
In all cases, monitor cylinder head temperatures if you can. For Lycoming engines, consider 420°F a personal redline and 400°F the start of a personal yellow arc. Reduce those numbers by 20°F for Continentals. If you can’t keep CHTs in your personal green range at cruise, you’re either not lean enough, or you should to switch to a rich-of-peak technique instead.
And if you don’t have an engine monitor, consider getting one. Or convincing the folks renting you the airplane to get one. It’s the best way to promote the longevity of these rather expensive engines.”
Check out Ryan’s latest webinar for more on simple engine operations.
Do you use an engine monitor for mixture control?