Couldn’t have said it better!
I think I have heard it referred to as delayed scavenging. The design uses longer transfer tunnels and a relatively short blowdown. The short blowdown allows for some spent gases to enter the transfers before the case pressure gets high enough to push the charge into the the cylinder. Similar to a strato engine, there ideally is a layer of spent gases pushing the remaining exhaust out of the cylinder before the fresh air/fuel charge can escape the exhaust port. The longer transfer tunnels prevent spent combustion gases from getting into the crankcase. I recently rebuilt a Stihl Ms271 that used this approach. Interestingly enough, early MS271’s were a regular strato saw like a 261...not sure when Stihl ditched the strato on these.
This is the general idea on the 461, 661, and 500i as well
I believe it has something to do with the V shaped uppers that we turn into —— or —_ shape. I think the tip of the primary V opens up and some exhaust enters there first. Which pressurizes the secondaries
When the piston is moving up I am still seeing a negative pressure before the transfers close and the strato ports are sucking in air first, then once the piston moves further up past this point the standard intake port sucks in the fuel/air charge.
You see the cutouts in the piston through the strato ports open first, but nothing happens until the top of the piston cutout meets the bottom of the upper transfers. This is the part you can’t see. And it happens at the same time the intake port starts to open, unless the intake timing has been modified.
Always wondered if it was a way of directing charge more than pressurizing, or whether they figured out which portion of the incoming charge is most apt to be siphoned out the exhaust port. Certainly the design isn’t a casting flaw.
Wow that gives me some good insight and I had no idea how the exhaust played a role in the layering. Then again still trying to grasp the layering principle, thanks for that info!!!Ol' sausage fingers about nailed it, twice
The reason I posted the WiKi link was to help people understand the process of the layer stacking and voodoo gas charge pressure coming off the last power cycle. This a three layer transfer port stack comprised of hot spent exhaust, warm fresh air for a buffer to lean out the third and following cool overly rich fuel mix near 10-1. Last two are a couple and mix well together. The first two are devoted enemies like having a chit ex.
Most people are never going to understand this triple action stacking process happening unless they can see the transfers all opened up like a five series Husky or something with removable covers. Even then the slotted piston that is the key to it all working just gets overlooked I'd guess. A basic geometry lesson, some sketches on paper and arrows sketched for flow direction helps most people pick it up.
To me the most critical part in these engines is the the exhaust event and case dwell time. As stated when doing machine work the strato timing will be chasing the intake duration down at about the same rate based on the piston position. Too early coming on to the gas transfer and you might be ok. Too late and you have no pressure left to charge the transfers and add to the case pressure. When it does reverse direction and drag the fuel charge behind it the weak shot from low case times and high exhaust is not going to clear the spent gasses properly.... like a gun with not enough port pressure, things get weak. A weak dirty mix ain't what they were after. Reused exhaust is the second thing to leave behind the regular exhaust. So then the fresh air rolls over with the mix and pushes out all the hot bs as it passes the combustion chamber area. Maybe you forget about the thermal separation between leading fronts... I didn't.
The temps matter and this where the layer stacking occurs so neatly and leans out the incoming fresh charge while helping to kick out the old hot chit.
Best I'll ever do in print.
and... gfy
The temperature difference between the spent gases and incoming charge also contributes to layering.
Nothing to do with the engine. Just talking about temperature changes on a large scale like blow torch heat vs room temperature. They tend not to mix and create boundary layers like water does when on the move. Both carrying velocity and liquid suspensions with great force.
