Ignition and combution in a twostroke

You can read much more in my notes on Facebook.
/ notes
Below a short explenation of a twostroke:
A twostroke works on the pulses of the exhaustpipe. When tuning is made high or very high the exhaustpulses becomes far more important. A 2t racing engine without exhaustpipe is more like a turtle. The powerband of a twostroke lies between the moment the backwave starts to comeback not to soon, so with closed transfers and the moment the rpm is so high that the backwave starts to come to late. To exeed the last one the ignitioncurve is going down closer to the TDC. to heatup the burned gasses for the moment they flow into the exhaustpipe. So pipe become hotter and backwave become faster. If the last one is going to far the piston will melt.. The backwave is needed for push back the fresh gasses who went to the exhaustpipe. So when the piston close exhaustduct the pressure wil go up, so cilinderfilling is much more with a good exhaustpipe.
If you understand it all what I described, you will also understand why tuners comes to different conclusions when using the same changings. And also why it tooks so many years to reach the level of the Aprilia RSA in 2007, which is realy about the maximum of the current twostroke design. Even after so many years.
The only way to come futher with the currect 2 stroke it is to get lower the whole temperature of the engine and may be new materials and more dedicated electronic devices.

[@andykaw1]

As you described I confirm your conclusion, every time I need to reduce spark energy (by changing the size of the capacitor) or changing the ignition point at higher RPM I will get more power only if the exhaust system was tuned for lower RPM... the same if I need to mantain higher temperature with insulating band around the belly, or final cone.

[@50ccRacing]

Yes Andrea, isulate the pipe from the outside air, will heatup the inside gas and gave, apart from a few other benefits, the same result as a pipe which is calculated for a higher rpm.

[@guidorollard2944]

hallo Luc, at 0:45 reading the text; are you talking about scavenging cylinder, in other words, creating a vacuum from the outgoing exhaust pulse, just before closing the port? I do not know so much about 2stroke, but more of 4 stroke engines. Seems to be a tricky point to match this up, with all parts in relation, and designing the exhaust.

[@50ccRacing]

Hello Guido, A twostroke works on the pulses of the exhaustpipe. When tuning is made high or very high the exhaustpulses becomes far more important. A 2t racing engine without exhaustpipe is more like a turtle. The powerband of a twostroke lies between the moment the backwave starts to comeback not to soon, so with closed transfers and the moment the rpm is so high that the backwave starts to come to late. To exeed the last one the ignitioncurve is going down closer to the TDC. to heatup the burned gasses for the moment they flow into the exhaustpipe. So pipe become hotter and backwave become faster. If the last one is going to far the piston will melt.. The backwave is needed for push back the fresh gasses who went to the exhaustpipe. So when the piston close exhaustduct the pressure wil go up to about 140% so cilinderfilling is much more with a good exhaustpipe. .

[@guidorollard2944]

many thanks Luc, sounds difficult establishing what exhaust design and sparktiming/duration, to with backpulse is forcing spilled fresh mixture back before it gets to close. Is there still room for developement? Or did you get to the limits of it now?

[@50ccRacing]

For the current twostroke tuning is almost at the limits only materials and electronics can may be help to get higher. A change in principe is realy possible and opens new ways, so there is absolutely hope for 2 stroke in the comming years..