First, the instrument itself — Page 100, Lesson 123

Let me walk you through the exhaust gas temperature method of mixture adjustment, because this is the accurate way we set mixture in cruise — and it's a technique you'll actually use. First, the instrument itself. The Exhaust Gas Temperature Gauge — we call it the EGT — consists of a thermocouple fitted into the exhaust pipe of the hottest cylinder. Now, a thermocouple is a device that produces a voltage directly proportional to its temperature. So as the exhaust gas gets hotter, the thermocouple produces more voltage. That voltage is then indicated by a gauge that's calibrated to show temperature. So the gauge reads out in temperature units, but what it's really sensing is the voltage from that thermocouple. Now, the operating technique. The mixture control should always be moved slowly. This is important — you never yank it. If you move the mixture toward lean, the exhaust gas temperature will rise and reach a peak at a specific air/fuel ratio: 15:1. That's the chemically correct ratio, where all the fuel is burned with all the available air. But here's the critical point: that 15:1 ratio IS NOT USED. I want to emphasize that — the peak EGT setting is not your cruise setting, because detonation can occur at that ratio. Detonation is the uncontrolled, explosive burning of the fuel-air mixture, and it can destroy an engine. So we never cruise at peak EGT. Instead, on reaching the peak EGT, you move the mixture control back toward rich, and the temperature will drop. The flight manual will specify a temperature drop — a certain number of degrees below peak — and that gives you the rich cruise setting. So the procedure is: lean slowly until EGT peaks, note that peak, then enrich until EGT drops by the specified amount. That's your rich cruise mixture. Now, what about leaning beyond that? Weakening the mixture beyond the chemically correct value — that is, going leaner than 15:1 — will lower EGT and raise CHT. CHT is cylinder head temperature. So past the peak, exhaust gas temperature falls, but cylinder head temperature rises. And if you keep weakening excessively, both EGT and CHT will lower. So there's a point of excessive weakening where both temperatures drop together. Again, the flight manual will specify the temperature drop required to set the economy cruise ratios — that's the leaner setting you'd use when you want to save fuel. One more operational rule: mixture is normally only adjusted at cruise power settings. And whenever you change the power — for example, when you're climbing or descending — you should return the mixture to Fully Rich. That's the safe default. Now let me shift to diesel engines, because they're a different animal entirely. Diesel engines generally run lean. Why? Because the air supply is not throttled — there's no throttle plate restricting airflow. Air is fed unrestricted into the cylinder, and the amount of fuel delivered controls the power. So the mixture is set as a function of fuel delivery, and diesels naturally run lean. Problems such as detonation do not feature as they do with conventional piston engines. That's a key contrast — no detonation concern in diesels. However, running temperatures are generally higher, which requires a reliable and effective cooling system. So you need good cooling to manage those higher temperatures. And here's the operational difference: there is no mixture lever on an aero-diesel. Aero-diesels operate with a 'single-lever' concept, similar to some turboprops. One lever controls everything — you don't manually adjust mixture because the engine manages it automatically. So to summarize the contrast: in a petrol engine, you use the EGT gauge to set mixture at cruise, leaning slowly to peak then enriching to a specified drop, and you return to Fully Rich on any power change. In a diesel, there's no mixture lever at all — it runs lean by design, with a single-lever control, and you just need to manage the higher temperatures with effective cooling.