Why does the stihl 151 have two chokes?

[Eyebeefa]

Does anyone know what the second choke on the stihl 151 is for? 

thank you!!

[openspaceman]

23 minutes ago, Eyebeefa said:

Does anyone know what the second choke on the stihl 151 is for? 

thank you!!

Never seen one but it is probably because the engine is a modern stratified charge one. As the piston rises to compress the charge a depression is formed in the crankcase, this sucks fuel and air into the crankcase. As the piston reaches the top the main choke is cut off by the piston but windows in the piston allow air only through the other choke to pass into the tops of the transfer ports.

 

This means after the charge fires above the piston and the piston descends and opens the exhaust port the  first gases coming out of the transfer ports are just air to scavenge the cylinder. This reduces the carry over of unburned fuel to the exhaust.

[pleasant]

If you are referring to the two butterfly valves in the carburettor, then this is because it isn't a traditional 2-stroke engine, but the Stihl 2-mix 2-stroke engine and operates slightly differently, predominantly to reduce emissions.

 

It is generally referred to as a 'stratocharging' port, and is designed to put a layer of fresh air on top of the fuel/air mix in the cylinder to minimize emissions of unburnt fuel.

 

Blue is straight air- no fuel.

 

Hope this helps.

 

 

[Joe Newton]

Are you getting the fast idle setting on the master control confused for a choke?

[Stubby]

I think he could mean 2x venturi ?

[spudulike]

God knows.....he knows everything.

Us mere mortals can only guess!!

[GarethM]

Wouldn't the extra air be pretty much unmetered, unimpeded or however you will call it ?.

 

As the choke butterfly and throttle butterfly valves are what controls the fuel and the rpm.

 

Saying that YouTube shows one with three valves.

 

https://youtu.be/FYeE7QNyn9Q

 

Edited January 6, 2023 by GarethM

[spudulike]

2 hours ago, GarethM said:

Wouldn't the extra air be pretty much unmetered, unimpeded or however you will call it ?.

 

As the choke butterfly and throttle butterfly valves are what controls the fuel and the rpm.

 

Saying that YouTube shows one with three valves.

 

https://youtu.be/FYeE7QNyn9Q

 

Two strokes, of traditional design, are pretty dirty as they use fuel vapour to eject the exhaust gasses from the combustion chamber through a scavenging process and some of this fuel vapour is pushed out of the muffler during this process.

The strato type saws do as mentioned earlier, the piston and inlet design allows the engine to load the very top of the transfer port with a charge of fresh air which is what pushes the exhaust out so vastly reducing the amount of unburnt fuel vapour in the exhaust.

That is how they work - the air is metered by carb, inlet manifold design and the timing of the piston and cylinder.

[spudulike]

Just seen the vid - two air ports and one fuel air port (middle one). The air ones are for the top of the transfer air charge. I am sure that is what the fella is saying can't be bothered to watch it TBH!

[GarethM]

I get how they work and why, just seems like a lot of design complexity and things to go wrong compared to say using fuel injection across more of the ranges.

 

Whilst it wouldn't solve the unburnt fuel issue entirely, it can control the mix more accurately.