Stratto saw life

bwalker

bwalker

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Its a single throat carb that feeds both the stratos and the charge ports, not your usual set up but does create an exception.
If your feeding the "stratos" with fuel/air mix it would seem to me its not a strato. I haven't seen an 881 so I can't say for certain. Maybe someone who has can chime in.
 
bwalker

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If your feeding the "stratos" with fuel/air mix it would seem to me its not a strato. I haven't seen an 881 so I can't say for certain. Maybe someone who has can chime in.
I just looked at some pics of the 881 intake tract and carb. It appears to me that what Stihl did was use the fact that fuel leaving the carb nozzle will tend to hang on the bottom side of the carb. The intake boot is split so the fuel rich side at the bottom goes into the crank case and the fuel lean top side goes to the strato ports.
 
bwalker

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They use 2 stroke mix for their strato function, not fresh air.
Here is what Stihl says about the 2-Mix engine.

"The two-stroke engine with 2-MIX technology sets new standards. During scavenging a layer of air without any fuel is created between the burnt charge in the combustion chamber and the fresh charge in the crankcase. The exhaust is separated from the fresh fuel to minimize the scavenging losses, eliminating the components of the mix that pose a health and environmental hazard"
So, no air fuel mix is not being fed into the strato ports. They just figured out how to do it with a split intake and a single barrel carb.
If it did feed air and fuel into the strato ports it would totally defeat their purpose in the first place.
 
Vintage Engine Repairs

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Very interesting, I’m sure there will be atomised fuel within the stratified area of the engine, but maybe not much? Not sure. However I agree, it surprised me too.
 
frank_

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the stihl model c strimmer uses a single choke carb with a plastic divider down the middle,so that the jet side is directed to the crankcase
and i fitted a cheaper old type single choke carb to one last week, and had to screw the h jet down fully shut to get it to run properly
 
Slotracer577

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That's not how a strato charged engine works. The fresh air side is only used to push exhaust out.
The carb settings are larger because the signal is to the carb is diluted.
On my makita 6000, that is how it works. Strato charge is pulled in through the top of the transfers and stacks on top of the fuel/air charge. So that way it goes into the cylinder first when the ports open. The rest of the charge has to carry enough fuel for air only charge.
 
bwalker

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On my makita 6000, that is how it works. Strato charge is pulled in through the top of the transfers and stacks on top of the fuel/air charge. So that way it goes into the cylinder first when the ports open. The rest of the charge has to carry enough fuel for air only charge.
Thank of it from a differential pressure standpoint and then what stratified charge actually means. You will get what I'm saying.
 
Slotracer577

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Thank of it from a differential pressure standpoint and then what stratified charge actually means. You will get what I'm saying.
That doesn’t change how it works, on the strato saw I have the strato port runs from the air cleaner to the intake port. It has a throttle blade and a reed valve. The throttle blade is so it doesn’t work at all throttle openings. during the intake stroke fresh air is pulled into the transfers while air/fuel is pulled into the bottom end. The regular intake port closes earlier and the strato port being reed valved continues to flow loading the transfers with fresh air. Then as case compression starts the charge is compressed as normal. Then when transfers open the fresh air which is in the transfers goes into the cylinder first, then it’s followed by the air/fuel. Since the first charge forcing the exhaust out is fresh air, less unburned fuel will end up in the muffler. Once the exhaust closes it’s all mixed up and burned as normal.
I expect you could make the saw run quite well by taking advantage of the strato port and its reed valve. Extra fuel can be added as needed to match the extra fresh air. No need to get crazy on the piston port intake and suffer the issues with long intake timing.
now I have not researched how other saws achieve strato ports, but the makita design seems to work quite well.
 
bwalker

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That doesn’t change how it works, on the strato saw I have the strato port runs from the air cleaner to the intake port. It has a throttle blade and a reed valve. The throttle blade is so it doesn’t work at all throttle openings. during the intake stroke fresh air is pulled into the transfers while air/fuel is pulled into the bottom end. The regular intake port closes earlier and the strato port being reed valved continues to flow loading the transfers with fresh air. Then as case compression starts the charge is compressed as normal. Then when transfers open the fresh air which is in the transfers goes into the cylinder first, then it’s followed by the air/fuel. Since the first charge forcing the exhaust out is fresh air, less unburned fuel will end up in the muffler. Once the exhaust closes it’s all mixed up and burned as normal.
I expect you could make the saw run quite well by taking advantage of the strato port and its reed valve. Extra fuel can be added as needed to match the extra fresh air. No need to get crazy on the piston port intake and suffer the issues with long intake timing.
now I have not researched how other saws achieve strato ports, but the makita design seems to work quite well.
Fresh air via stratos purges the cylinder of exhaust, transfers open and fuel air mix purges the cylinder of fresh air. Spark plug fires, repeat.
 
Slotracer577

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Fresh air via stratos purges the cylinder of exhaust, transfers open and fuel air mix purges the cylinder of fresh air. Spark plug fires, repeat.
How do the strato purge the cylinder when the transfers are closed? the strato port only goes to the transfers and there are no ports in the piston. When the transfers open, the cylinder pressure has dropped due to the exhaust being open, then the compressed air and air/fuel go from case to the cylinder. Flows from high pressure to low. Other designs might be different, but that’s how my 6100 works. Simple design imho.
 
bwalker

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How do the strato purge the cylinder when the transfers are closed? the strato port only goes to the transfers and there are no ports in the piston. When the transfers open, the cylinder pressure has dropped due to the exhaust being open, then the compressed air and air/fuel go from case to the cylinder. Flows from high pressure to low. Other designs might be different, but that’s how my 6100 works. Simple design imho.
They don't start purging until the transfer open if the motor you are speaking of doesn't have dedicated strato ducts into the cylinder. That doesn't change what's going on. That is a cushion of air separates the fresh charge from the exhaust port until most of the fresh air cushion is expelled into the exhaust port and the port closes. This layering of exhaust, fresh air and fuel/air charge is why the design is called stratified charge.
 
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