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Mixture Lab
Binns Flight Services
A simulated normally-aspirated, fuel-injected engine that follows the relationships in John Deakin's
Pelican's Perch #18: Mixture Magic . Drag the knobs: black throttle (manifold pressure), blue prop (RPM), and red mixture.
Use the ± buttons for fine adjustments. The mixture buttons work like a vernier, 0.1 GPH per click.
Things to try
Lean rich of peak: start from "Cruise 8,000 ft, full rich" and lean slowly. EGT and CHT rise, power rises a little, then CHT peaks (about 50°F ROP) and EGT peaks.Lean assist: press LEAN ASSIST before you lean. The monitor marks each cylinder's peak and shows how far past it you are.The big mixture pull: from full rich at cruise, pull the mixture quickly to a lean-of-peak fuel flow. Watch the CHT. Because it lags, you can pass through the hot zone near peak without the heads ever getting there. Lean slowly through the same range and compare.The red box: move the throttle and prop. The red box grows at high power and shrinks to nothing at about 65%. Operating inside it at high power drives CHT up, and in extreme cases into detonation.The trap: load "25" / 2500 right after takeoff" (IO-550). On a TCM fuel-injected engine, reducing the throttle cuts out the full-throttle power enrichment and leans the engine right into the red box. Watch the CHTs climb.
The Lycoming's Bendix/RSA servo meters on airflow and has no such step, so its full rich stays rich at part throttle.Full rich at takeoff is deliberately well outside the red box. Engines are certified with a detonation margin at full rich and rated power.Stock vs balanced injectors: with stock injectors the cylinders peak at different fuel flows, so the engine gets rough lean of peak. Balanced injectors let all cylinders peak together, and LOP is smooth.Low RPM, high MP: try "Deakin's low-altitude LOP." It's cool and efficient at 50°F LOP, but enrich it to rich of peak and the pressure peak moves toward top dead center. That's detonation territory.
What's shown
The engine display shows what a real all-cylinder monitor would show. The Instructor view shows things you can't see in a real cockpit:
mixture relative to peak, actual horsepower, BSFC, peak cylinder pressure and its timing, and detonation margin.
CHT responds over a minute or more (use sim speed to fast-forward). EGT responds in seconds. Detonation makes CHT climb much faster.
About the model
This is a teaching model calibrated to the article's TCM IO-550 data (25"/2,500 RPM: peak EGT ≈1,520°F at 95 PPH, peak CHT ≈425°F about 50°F ROP, ≈250 BHP, best BSFC ≈0.385),
the friction losses quoted there, and its description of peak pressure timing. The "red box" follows the Advanced Pilot Seminars concept.
Numbers are realistic but not specific to any particular airplane. Always follow your POH and engine manufacturer's guidance.
