ja mislim da nisu, nisi link otvorio, to bi bilo blowdown time-area
evo kopirano objašnjenje toga
Many people talk about a port area as a target value and indirect refers to a header area, with an area factor for the port. That approach is a completely waist since port area don't give any information about port shape. Time area is still the standard unit for 2-stroke ports since at least 1971. An area change far up on the port will give a completely different change than on the bottom of the port. It would also affect the pipe differently. The time area distribution is simply different over the port height because the port is open different periods of time during the stroke.
In the Bimotion Advanced Port & Pipe program we can see this distribution in a chart. (Se pictures below.) We should be more careful with the machining precision at the upper part of the port since that area is opened for a longer time.
The distribution curve will also show the effect of auxiliary exhaust ports which adds blowdown time-area. We need a certain blowdown time-area to equalize the cylinder pressure to the crank case when the transfer ports opens. If this not works, we get blow back to the crank case and that will drop the power rapidly.
Auxiliary exhaust ports adds time area which can be seen in the distribution chart.
The same port but without auxiliary ports.
Now, the blowdown timearea is not just a figure to match, it is dependent on how strong pulses the pipe deliver. If the pipes tuning degree is high (strong pulses), then we get away with less blowdown time area !
And the transfer ports don't need so much time for scavenging since the pipe suction wave is strong, pulling out the gas from the crank case. We could then have wide and low transfer ports. With low ports we can go for higher rpm without getting blow back into the transfers.
Why do we need to take all this in consideration for the exhaust port design? The answer is that it is not only a time area target value, the whole system needs to be investigated since there is an interaction, a balance between pressures as the pipe is affecting even the reed valve and the carburettor at BDC!!!
The normal procedure for race engine design would be to keep the exhaust port as low and wide as possible and to match the blowdown target (depending on pipe). The exhaust blowdown target is a statistical value for the tuning degree decided by the bmep target (braked mean efficient pressure). The port duration should not exceed the recommended value.
