Let's Talk Transfers

[huskihl]

Still enjoying the heck out of this thread! Can someone help me out with "blow down"? My simple, inexperienced mind defines it as the distance that the piston travels in degrees between the exhaust port opening and the upper transfer opening. Is this correct? And not enough blow down is basically the upper transfer opening before the pressure in the crank case is greater than the pressure in the cylinder causing exhaust to back feed into the transfers? If I've got it twisted somehow please advise. Thanks!

Yep. Pretty much. The basis of this thread is the whys, wheres, and whens of all that

 

[Terry Syd]

The newer engines with long transfer tunnels looping towards the front work a bit differently (say on a Husky strato). It appears the engineers have actually designed the engines to utilise less blowdown so that the transfer tunnels get a big hit of exhaust gas down the tunnels. When you pull the jug, you can see the carbon from the exhaust going down almost to the crankcase.

It appears what they have done by using the long tunnels is to capture the exhaust gas to use in the loop-scavenging. With a conventional two-stroke, when the transfers first start to flow there is considerable turbulence and mixing of the mixture coming out of the transfer port with the residual exhaust gas in the cylinder. It takes a while for the 'freight train' of mixture to get established in a flow. The result is that there can be some 'short-circuiting' and dilution of the mixture.

By using the captured exhaust gas in the tunnels, they don't have to worry about turbulence and mixing - it is just exhaust gas mixing with exhaust gas. When the flow gets established, then the air from the strato can enter the flow and finally the fuel mixture from the intake port.

 

[Mastermind]

I need to study that design a bit more brother Terry. The transfers fill with fresh air via the cutouts on the sides of the piston, then you'll get a some exhaust gasses on top of that. When the flow reverses......well you see what I'm saying.

 

[drf256]

The newer engines with long transfer tunnels looping towards the front work a bit differently (say on a Husky strato). It appears the engineers have actually designed the engines to utilise less blowdown so that the transfer tunnels get a big hit of exhaust gas down the tunnels. When you pull the jug, you can see the carbon from the exhaust going down almost to the crankcase.

It appears what they have done by using the long tunnels is to capture the exhaust gas to use in the loop-scavenging. With a conventional two-stroke, when the transfers first start to flow there is considerable turbulence and mixing of the mixture coming out of the transfer port with the residual exhaust gas in the cylinder. It takes a while for the 'freight train' of mixture to get established in a flow. The result is that there can be some 'short-circuiting' and dilution of the mixture.

By using the captured exhaust gas in the tunnels, they don't have to worry about turbulence and mixing - it is just exhaust gas mixing with exhaust gas. When the flow gets established, then the air from the strato can enter the flow and finally the fuel mixture from the intake port.

I need to study that design a bit more brother Terry. The transfers fill with fresh air via the cutouts on the sides of the piston, then you'll get a some exhaust gasses on top of that. When the flow reverses......well you see what I'm saying.

I'm trying to figure out how there's any time for all of this to happen at 250 times a second.

 

[XP_Slinger]

I'm trying to figure out how there's any time for all of this to happen at 250 times a second.

I'm tearing down my 372 XT this weekend for different reasons and if my degree wheel gets here in time I'll get some numbers and share them on this thread.

 

[drf256]

Sounds like a transfer almost makes itself what it wants to be.

Make them too high, they back stuff, compress, and release. So they'll act like a lower transfer in a way.

 

[Terry Syd]

The transfers fill with fresh air via the cutouts on the sides of the piston, then you'll get a some exhaust gasses on top of that. When the flow reverses......well you see what I'm saying.

When I first looked into a Husky strato I was surprised how far down the tunnels the exhaust gases had travelled. Initially, I thought it needed a heap more blowdown, but it only took a couple of degrees to raise the cutting speed a fair bit higher. I had to re-examine my ideas about blowback into transfer tunnels, the extra long tunnels then made a lot more sense.

Another thing to consider, when the exhaust does blowback down the tunnels, it is also raising the crankcase pressure. When the flow finally reverses, it probably reverses back with a vengeance.

The strato design is very clever, I wish the design had been developed earlier so that the dirt bike scene wouldn't have been so adversely affected.

I'd still like to play with a Dolmar 6100 and see what I could get out of that engine. I can visualise a rear boost port behind the reeds to give the additional time/area - and with a better loop-scavenging. Maybe I can blow up one of my saws to justify the 'need' for a replacement.

 

[drf256]

Terry,

Do you think carbon in transfers is absolute evidence of back flow?

Is it possible that what your seeing is staining from back flow during deceleration or only at Idle?

 

[Terry Syd]

If it was deceleration or idle then there wouldn't be much back pressure in the cylinder to force the exhaust down into the transfers. - It is likely that it is the heat of the gases between cycles that bake the deposits onto the walls. Theoretically, the walls of the tunnels are constantly being washed by fresh mixture and should be nice and clean.

Take a look at a few cylinders and see how far the back flow extends down into the transfers. Even a cylinder with lots of blowdown (and a free flowing muffler) will show a little carbon near the port.

 

[XP_Slinger]

If it was deceleration or idle then there wouldn't be much back pressure in the cylinder to force the exhaust down into the transfers. - It is likely that it is the heat of the gases between cycles that bake the deposits onto the walls. Theoretically, the walls of the tunnels are constantly being washed by fresh mixture and should be nice and clean.

Take a look at a few cylinders and see how far the back flow extends down into the transfers. Even a cylinder with lots of blowdown (and a free flowing muffler) will show a little carbon near the port.

So with there being somewhat excessive blowback into the transfers on strato saws, I wonder if widening and slightly raising the exhaust on a strato would yield more results when compared to a traditional cylinder. Just a thought.

 

[drf256]

Do stratos have excessively high transfers so that their initial charge makes it out the exhaust?

 

[XP_Slinger]

Do stratos have excessively high transfers so that their initial charge makes it out the exhaust?

Not sure at this point. Hopefully I can get some numbers this weekend to evaluate these theories regarding strato saw blow down. It would be interesting to compare numbers to an OE 372 but I don't have one. Maybe someone else could post the stock numbers up?

 

[srcarr52]

The Husqvarna strato port design fills the crankcase with fresh fuel/air like normal but the transfers are open to a fresh air intake via groves in the piston. So they get filled with just air. So when the transfers open it pushes the exhaust out with fresh air from the transfers, then comes the fuel/air charge. This avoids mixing the fuel charge with exhaust gasses and also keeps a lot of wasted fuel/air from escaping out the exhaust, these features keep lower emissions. To get all this done they have to have a short blowdown period and high crankcase compression.

 

[XP_Slinger]

The Husqvarna strato port design fills the crankcase with fresh fuel/air like normal but the transfers are open to a fresh air intake via groves in the piston. So they get filled with just air. So when the transfers open it pushes the exhaust out with fresh air from the transfers, then comes the fuel/air charge. This avoids mixing the fuel charge with exhaust gasses and also keeps a lot of wasted fuel/air from escaping out the exhaust, these features keep lower emissions. To get all this done they have to have a short blowdown period and high crankcase compression.

Makes perfect sense to me now. I bet getting some fuel into the strato charge would do it some good. Not to concerned about the EPA. Ok I'm done derailing the transfer port thread with XT talk. Thanks for the responses. Fire it up!

 

[shorthunter]

In light of the recent 262 thread, I thought I would share a little info from Gordon Blairs, Two Stroke Design.

"The pressure at the cylinder end of the transfer duct exhibits a backflow during the first 10° of transfer port opening, as the cylinder pressure exceeds the crankcase pressure at that juncture during blowdown. The consequences of the backflow on the local purity and temperature can be found in Figs. 5.17 and 5.18. In Fig. 5.17, the purity at the cylinder end of the transfer duct drops temporarily from 0.95 to below 0.90 and, in Fig. 5.18, the temperature rises from 180°C to 280°C. When scavenging actually commences, it is with gas which is both hotter and less pure than it might otherwise be."

"The conventional spark-ignition two-stroke engine burns a homogeneous charge. The air-fuel mixture is supplied to the cylinder via the transfer ports with much of the fuel already vaporized during its residence in the "hot" crankcase. The remainder of the liquid fuel vaporizes during the compression process so that by the time ignition takes place, the combustion chamber is filled with a vapor-air-exhaust gas residual mixture which is evenly distributed throughout the combustion space. This is known as a homogeneous combustion process."

 

[Keith Gandy]

The newer engines with long transfer tunnels looping towards the front work a bit differently (say on a Husky strato). It appears the engineers have actually designed the engines to utilise less blowdown so that the transfer tunnels get a big hit of exhaust gas down the tunnels. When you pull the jug, you can see the carbon from the exhaust going down almost to the crankcase.

It appears what they have done by using the long tunnels is to capture the exhaust gas to use in the loop-scavenging. With a conventional two-stroke, when the transfers first start to flow there is considerable turbulence and mixing of the mixture coming out of the transfer port with the residual exhaust gas in the cylinder. It takes a while for the 'freight train' of mixture to get established in a flow. The result is that there can be some 'short-circuiting' and dilution of the mixture.

By using the captured exhaust gas in the tunnels, they don't have to worry about turbulence and mixing - it is just exhaust gas mixing with exhaust gas. When the flow gets established, then the air from the strato can enter the flow and finally the fuel mixture from the intake port.

Stihl calls that delayed stratification scavenging technology!! Lol

 

[Al Smith]

My theory is because some of these new designs use basically ambient air for the purge portion of the transfer it might slow the burn rate down until that air it heated by the combustion process .If that holds true it would explain why for instance certain Stihls like the 201 and 441 are slow on the uptake acceleration .It seems to me with either one of those saws they run better after a portion of usage because of that .
It also makes sense that because of the use of pure air they would use less fuel than conventional designs which blows the unburned fuel right out the exhaust------best I can come up with on that subject .

 

[Keith Gandy]

My theory is because some of these new designs use basically ambient air for the purge portion of the transfer it might slow the burn rate down until that air it heated by the combustion process .If that holds true it would explain why for instance certain Stihls like the 201 and 441 are slow on the uptake acceleration .It seems to me with either one of those saws they run better after a portion of usage because of that .
It also makes sense that because of the use of pure air they would use less fuel than conventional designs which blows the unburned fuel right out the exhaust------best I can come up with on that subject .

Would gutting the STRATO help or hurt from a performance standpoint?

 

[XP_Slinger]

Would gutting the STRATO help or hurt from a performance standpoint?

I'll let you know...I'm gutting mine tomorrow. I've read about good results in other places. Really helps tame the idle

 

[Keith Gandy]

I'll let you know...I'm gutting mine tomorrow. I've read about good results in other places. Really helps tame the idle

What r u gutting? I gutted a 241 from the plastic divider on the back of the carb to the port window. It ran good but on a long idle it would stumble when u dumped the trigger. I added a stock intake manifold with baffle intact back in and it stopped. The #s were 105/118/74. I sold the saw the the other day and the guy was shocked how well it ran and ask if the high rpm would damage it? Lol!!