Piston Engines - Fuel — Page 85, Lesson 105

Let’s pick up right where the combustion process leaves off. We’ve got the spark, the flame, and now we need to talk about how fast that flame moves and why the timing of the spark has to change as the engine speed changes. First, the flame rate. When normal combustion takes place, the compressed charge is ignited by the spark and burns rapidly and steadily with a flame speed of 60 to 80 feet per second. That gives a steady and smooth temperature and pressure rise in the combustion chamber. So the key number here is 60 to 80 feet per second — that’s the normal flame speed. Now, maximum pressure will be generated when combustion has been completed. And ideally, this should occur when the crank is at 8° to 10° after top dead centre — that’s ATDC. Why there? Because at that point, because of the ineffective crank angle, the volume of the combustion chamber is still at a minimum. In other words, the piston has just passed TDC and hasn’t moved down much yet, so the chamber is still tiny, and that’s where you want peak pressure. Here’s the critical warning: should maximum pressure conditions obtain in advance of this — that is, at or before TDC — the engine would tend to run backwards. That’s a real danger, and it’s why timing matters so much. Now, variable ignition timing. Since combustion takes a short period of time, in order for combustion to be completed when the piston is at 8° to 10° ATDC, the spark must occur before the piston reaches TDC. The flame rate remains reasonably constant, but the engine speed varies considerably. So at low engine speeds, it’s necessary for the ignition to be retarded — that means the spark happens later — to prevent the maximum pressure building up before the piston reaches TDC. As the engine speed increases, both the flame rate and the time required for complete combustion remain constant, but because of the increased piston speed, it’s necessary to advance the ignition so that the maximum pressure still occurs at the right time, again 8° to 10° ATDC. So the logic is: flame takes a fixed time, but the piston moves faster at high RPM, so you must fire the spark earlier to still finish burning at the same crank position. Now, variations in flame rate. The flame rate does vary slightly. For instance, the mixture will burn faster if it is made richer, or if the pressure in the cylinders increases. It’s necessary to increase the mixture strength of all aircraft engines when they are producing high power to ensure stable combustion. So the increased flame rate that results from selecting a rich mixture shortens the time required for combustion. Therefore, to obtain full power, it’s necessary to retard the ignition slightly — or alternatively, not to make any further advance of the ignition. Next, anti-detonation properties. The higher that the pressure of the fuel/air mixture can be raised before combustion, the higher will be the pressure of the burning gases. Consequently, the greater will be the power output and thermal efficiency of the engine. The compression pressure is governed by the compression ratio of the engine, and it is limited by the tendency of the fuel to detonate, or knock. Finally, detonation — also called knocking. Detonation occurs after ignition and is unstable combustion. During normal combustion, the flame travels smoothly and steadily through the mixture as the advancing flame front heats the gases immediately ahead of it, so that they in turn burn. Progressively, there is more and more heat concentrated in the flame front, which is brought to bear on the remaining unburnt portion of the mixture — that’s termed the end gas — and its temperature is raised. So we’ve got the flame speed of 60 to 80 feet per second, the ideal peak pressure at 8° to 10° ATDC, the danger of running backwards if it peaks too early, the need to retard at low RPM and advance at high RPM, the richer-mixture effect that speeds the flame, and the start of detonation with the end gas heating up. That’s the foundation for the next part, where we’ll see what happens when that end gas gets too hot.