See the Fuel Quality Control section on the opposite page — Page 85, Lesson 109

Let’s start with the recognition of detonation, because that’s the danger we’re trying to control. Detonation is spontaneous combustion. In normal running, the flame front sweeps smoothly across the charge. In detonation, the remaining end gas — the unburned mixture ahead of the flame — ignites all at once, violently, instead of being consumed progressively. That sudden, explosive burning sets up violent vibrating pressure waves that strike the walls of the combustion chamber. You recognize it by a metallic knocking sound, which we call pinking. That knocking is literally the pressure waves hammering the chamber walls. Now, the serious problem in an aircraft: under high power, detonation can do a lot of damage. But because the propeller is so noisy, you may not hear the knocking until it’s too late. So you cannot rely on your ears in flight — that’s why prevention matters so much. Let me give you the four ways detonation can be controlled. The first is a compact combustion chamber. A compact chamber reduces the distance the flame front has to travel, so the charge burns faster. And you can cut the burn time even further by initiating flame fronts from two sparking plugs — two fronts meeting in the middle means the whole charge is consumed sooner, leaving less time for the end gas to detonate. Second, if possible, start the flame from the vicinity of a hot spot, such as the exhaust valve. The idea is to push the end gas away from the hotter parts of the chamber and compress it into a cooler part. So the last unburned gas ends up in the coolest region, where it’s less likely to self-ignite. Third, running conditions can delay the onset of detonation. Here’s the key example: you can get the same power at a higher engine speed by using a finer propeller pitch. That lets you use a smaller throttle opening. This helps in two ways. The smaller throttle opening reduces cylinder pressure. And the higher running speed cuts down the time available for the charge to detonate. Both pressure and time are reduced. Fourth, and this is the summary principle: anything which can reduce temperature, pressure, or time will be instrumental in reducing detonation, or at the very best, preventing its creation. Keep those three — temperature, pressure, time — as your mental checklist. Now, fuel quality control. One of the easiest ways to control detonation is to improve the fuel itself. There are two chemically pure fuels used as reference fuels when determining the anti-detonation qualities of a fuel under laboratory conditions. They are Iso-octane and Normal Heptane. Iso-octane has very good combustion characteristics — it shows little tendency to detonate when mixed with air and ignited at high temperatures. It is given a rating of 100. Normal Heptane, by contrast, detonates very readily, and it has a rating of 0. So you have your scale: 100 for the best, 0 for the worst. The combustion characteristics of any blend of fuel can be compared with those of the two reference fuels by using each in turn under standardized conditions in a special single-cylinder engine. That’s the laboratory test — you run the blend, then you run Iso-octane and Normal Heptane under the same standardized conditions, and you see which reference fuel the blend matches. That comparison gives you the fuel’s anti-detonation quality on that 0-to-100 scale. That figure shows the effects of detonation — the damage pattern you’re trying to avoid. And the earlier figure on the opposite page shows how moving the ignition point to the optimum position changes things for idling rpm versus high-speed running. So to tie it together: detonation is spontaneous combustion of the end gas, recognized by metallic knocking or pinking, dangerous because propeller noise can hide it. You control it by reducing temperature, pressure, or time — through a compact chamber, dual sparking plugs, starting the flame near a hot spot, or adjusting running conditions like propeller pitch and throttle. And fuel quality gives you a laboratory scale, from Normal Heptane at 0 to Iso-octane at 100, to measure how resistant a fuel is to detonation.