Aviation Training Tip: Pre-Ignition vs Detonation

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1:03flight syllabi, lesson plans, homework assignments, binders and tracking, quizzes, logbook audits, endorsements, and more. And you can try every single one of our courses for free for as long as you would like at Wi-Fi-CFI.com. All right, there are a couple errors that we can get within our combustion system. And these could occur if the engine is not being properly cooled, if the engine gets too hot. There are other things that can cause these issues as well, but that is one of them. So that's kind of our segue from our last point to this point. During normal combustion, we know the fuel air mixture burns in a controlled and predictable manner. It's nice, it's controlled, and it's smooth. However, with that nation, we get an uncontrolled explosive ignition of the fuel air mixture within the chamber, and it will lead to excessive engine temperatures and pressure. It can lead to excessive engine damage. Essentially, looking at it

2:03like this, here is our piston. During normal combustion, this piston would be moving up and down nice and smooth. All right, and then you can see where you can see on the left hand side here, he's got his hand, he's pushing it down nice and smooth. So when we get that explosion, boom, we push it down nice and smooth. However, with that nation, we would have the situation depicted on the right. So once we get the compression event, and then we go to the power stroke, it's like slamming it with your fist. Instead of it being a nice, smooth push, it's like a slam with your fist. And when you're in the cockpit, if you have this, you would probably be able to hear it in small general aviation aircraft. You hear like a bang, bang, bang, you would hear your engine banging. That is not a good sign. So what the heck causes this? Because we do not want to have this happen. You could have engine damage, you could have engine failure, you could have some real bad problems, a real bad day if this starts to occur. So common operational causes are using a lower fuel grade than specified by the aircraft

3:04manufacturer. Right after this, we're going to get into the fuel system, and we'll talk about the different fuel grades. Operation of the engine with high manifold pressures and conjunction with low RPM. If you've got an airplane that's got a controllable pitch propeller, so you've got a throttle and a manifold pressure gauge and then a prop and an RPM gauge. If you've got really high manifold pressures and really low manifold pressures, or sorry, really high manifold pressures and really low RPM, you could get this issue. Operation of the engine at high power settings with an excessively lean mixture, we're getting it too hot. That that mixtures too far back. Remember, the fuel from the from the mixture does help to keep our engine a little bit cooler. So if we have an excessively lean mixture, this could be getting too hot in here and we could get detonation. Maintaining extended ground operations or steep climbs in which cylinder cooling is reduced. And again, another time in which we're making the engine too hot. If we have to do extended ground operations, have your throttle back. Make sure your make sure is not too lean. Try to park your

4:08airplane so it's face into the direction of the wind. So you get some of that wind coming through giving you better cooling, better airflow. With climbing, sometimes there's nothing you can do. If you've got to make a steep climb to get over train, you've got to make a steep climb to get over train. But once you're clear of the train, go ahead and lower your nose a little bit. Do a cruise climb instead of climbing at an angle like this where you're not getting as much cooling. When lower your nose, you won't get to your new altitude as fast. But you'll have better forward visibility from the cockpit and you'll have better engine cooling. With detonation, that's what we got to do. We got to increase our mixture. We've got to cool the airplane engine down. We got to get it cooler because it is exploding in here instead of pushing nice and smooth. Preignition is the next one. So this occurs when the fuel air mixture ignites prior to the engine's normal ignition prior to the power event. You're getting an explosion in there too soon before you should be. Good look like this because it is usually caused by residual hot spot in the combustion chamber.

5:11Usually you've just got your spark plug or you know an airplane engine you might have a spark plug on each side or two spark plugs, whatever. But you've got your spark plug delivering the spark. With preignition, you've got these hot spots in here that are also exploding and look. If you look at this picture, the spark plug is not sending out a spark because these are happening before the spark plug even gets a chance. You can even see that the piston is a little bit lower right here. So the piston hasn't made its travel all the way up yet. It hasn't fully compressed this fuel air mixture and we're already getting to the power stroke. That is not good. Again, you're going to hear some banging. That is not a good day. That would be preignition. And they can lead to one another. Detonation can lead to preignition, preignition can lead to detonation. When you're up flying in the airplane, like I said before, if you're having one of these events happen, you're going to hear like bang, bang, bang. You're going to hear some banging. You might see some stuff on your engine gauges in the cockpit, whatever. But are you really going to know if it's detonation

6:16or preignition? I don't know. Probably not. The odds are you're probably not going to be able to tell the difference between whether you're getting detonation or preignition and it doesn't really matter anyways. What we've got to do is we got to bring some power back. We got to probably increase a little bit of mixture to try to cool the engine down, start a descent. And honestly, if you're having this happen, find an airport to land at. Hopefully there's one nearby. But you are looking for a safe spot to get down because you could be, if you continue on in this situation, you could cause a lot of engine damage and it could lead to engine failure as well. So continued flight is probably not recommended. Even if you get this under control, even if you cool the engine down, whatever, again, you do that descent, you increase your mixture, you get some more cooling and you're not getting the banging or the knocking anymore. I still didn't chance it personally. I mean, you look at your crappy OH and you talk to your flight instructors

7:16and flight school, whatever. I wouldn't chance it. If I had any of that, I'm fine in the closest airport. Here we go. We're going to land. We'll get a mechanic once we're on the ground to look at it. But those are the two, the two different things that you could have in the combustion system that could be very bad for your airplane and they could be caused by not cooling the engine enough.