HI all
OK, so this may help as it relates to how the spark reacts under pressure changes. There are a lot of other factors.
https://www.highvoltageconnection.com/articles/paschen-curve.html
The ignition system, especially powered from a simple magneto, is limited inn what it can develop. Old hot rod tricks were to lower the heat range of the plug and then narrow the gap. Today on high performance race engines we just jam more power to bridge that gap under higher pressures.
The exhaust plays a big part, especially at altitude. The Rotax has not only a max back-pressure value but also a minimum. At altitude that becomes difficult as Jeff has noted. The ambient air is so slight that when the valve opens the cylinder pressure can just dump out.
Another issue is the turbo itself. If we look at the smallest turbos Rotax uses on our engines, the type 914 is basically a Garrett T25 variant, it hits its max at 5000 meters or about 16000 feet. After that point it can no longer maintain the pressure due to the turbo compressor wheel size. As you fly higher the turbo shaft speed also then becomes a factor and the shaft goes into overspeed reducing the effectiveness. The turbo manufacturers have literally hundreds of possible combinations to work with and it is dependent on the application the turbo is being used for (what is the design altitude) You may read high performance builders talk about the turbo compressor map.
If you want to climb into a rabbit hole this is a fun one. Link in next post
The good part is you will see that we always discuss air in weight, that is the essence of fuel and air ratio, it is by weight to burn effectively.
Now from before we talked about "blowing out the candle" in that at high altitude it reacts like high pressure in the cylinder. It is really that the spark in a low KV power system cannot bridge the gap in our high pressure filled cylinder. The carb version engines only had in the range of 20 KV power for the plugs. In the old race car years they would reduce the spark gap and run a lower heat range plug to try to allow the spark to fire across the gap. The type 915 and 916 engines did a lot of changes to solve that and they have a remarkably higher operational altitude. (forged pistons with lower compression, higher output coils, for example) In the case of the standard 915 and 916 engine the mapping in the ECU is done to cover the small aircraft manned range we have, no problem up to approx 24 to 26,000 feet. (at least for the engine) There is a great deal of issue when you go higher, another long topic. This regards oil pressure but best left for another thread. (lack of ambient pressure) With mapping and addressing some of the other concerns flights higher that 26,000 are possible. This gets into those problem areas and why we see stacked turbos or multistage turbos to stuff the cylinder pressure. In the Perseus B they also have the ignition from an Indianapolis race engine supplier for spark and use LoX injected into the heads. That is the engine picture you see at the start of the thread. The Altus 2 was slightly later and they solved a lot of the issues and that aircraft was just shy of the 60,000 ft in flight but was more simple and had a 2 stage single turbo with a 300 lb payload for the aircraft, an impressive bit of engineering. ( it took literally hours to bring back down from that altitude due to the extreme wings and prop used.)
You can have a lot of fun doing some research into the topics. In theory it was speculated a piston engine could get to 90,000 feet. This however has never happened and that range is best left for mega expensive craft like the Global Hawk.
lets try this ...never had the page warp before
