husqvarna 575/576 strato musings

[heyduke]

we were on page umpteen of a thread about some vaporware known as the husqvarna 590. i mentioned that i think i understand husqvarna/remax strato toechnology fairly well but whenever i sit down and meditate on a 575 piston, i just need to flip the cap on a cold ipa. chris-pa was kind enuff to send me a document from husqvarna that goes into it in depth:

If you want to wade through the terminology and the language translation issues, the info you're looking for is in the attached Master's Thesis doc. It's really about the use of computer modeling to show how the fuel/air flows through an engine, but it happens to be based on the 570/575/576 series of engine. Towards the end (Section 3) there is a series of graphics showing air and fuel location in the ports by crank angle. The "passive scalar" refers to the air with fuel in it, or maybe the boundary between fuel and plain air.

and included this attachment: MikaelMasterThesis.pdf

it's full of some fascinating graphs and CAD illustrations and a lot of squiggly calculus stuff and a few nuggets of very interesting text.


here are some fotos of a 575 piston and cylinder:


the main feature here is the external transfer duct running from below the exhaust port to the upper cylinder. i think it's bent to make it longer. it's only about 1/2 inch in diameter. theses saws don't need much to drink.


this is the carb flange. the square hole in the middle is for fuel intake, sucked in when the piston rises. the two round ports are for fresh air, supplied by a second throttle valve linked to the carb. at a certain part of each cycle, when there is a vacuum in the case, fresh air is sucked backwards into the tops transfers.



the cavity at the bottom allows a conduit for the fresh air to flow backwards into the top of the transfers when the piston is in the correct position. The slot at the top is what has tormented me for years. i couldn't figure out what it did.



here's another view. you can see a hole is bored all the way thru the thing. that's a 5mm allen. wtf? i was amazed to find out that "...the function of the piston pockets in Figure 2.7 are to reduce the weight of the piston.

see page 18 of:

MikaelMasterThesis.pdf

 

[Chris-PA]

"...the function of the piston pockets in Figure 2.7 are to reduce the weight
of the piston.

You know, I saw that too - but I don't really believe it:

First of all, I don't see how it reduces the piston weight at all. The piston is not solid, it's basically a thin wall cylinder. Deflecting a portion of the wall inward and adding a top and bottom panel cannot be reducing the required material.

Second, every strato I've seen has that feature, although I've not seen one where they are connected before. Sometimes they are above the air passage slots, sometimes below, but they usually line up with the strato air passages.

I think they are for cooling and/or lubrication. I suspect that since they are a dead-end passage they accumulate oil over time, which helps to lubricate the cylinder walls near the dry air strato inlet ports.

********

What I was really surprised about in that document was just how late in the cycle the fuel actually arrived at the cylinder - pretty much right before the transfers closed off. By that time the piston has been rising for quite a few degrees, and you wouldn't think things would still be moving up the transfers. Apparently there is enough momentum to keep it moving.

 

[heyduke]

You know, I saw that too - but I don't really believe it:

First of all, I don't see how it reduces the piston weight at all. The piston is not solid, it's basically a thin wall cylinder. Deflecting a portion of the wall inward and adding a top and bottom panel cannot be reducing the required material.

Second, every strato I've seen has that feature, although I've not seen one where they are connected before. Sometimes they are above the air passage slots, sometimes below, but they usually line up with the strato air passages.

I think they are for cooling and/or lubrication. I suspect that since they are a dead-end passage they accumulate oil over time, which helps to lubricate the cylinder walls near the dry air strato inlet ports.

********

What I was really surprised about in that document was just how late in the cycle the fuel actually arrived at the cylinder - pretty much right before the transfers closed off. By that time the piston has been rising for quite a few degrees, and you wouldn't think things would still be moving up the transfers. Apparently there is enough momentum to keep it moving.

i guess the fuel has to get in line behind the fresh air, sort of a just in time supply system. it's probably part of the reason these saws burn so little fuel. also, i mentioned in another thread that i suspect there is some use of shock waves. maybe they help stuff the fuel in.

another issue that was brought up was effective stroke vs actual stroke, though they didn't use that terminology. i first read about this back in the 1970's when we were building two stroke crotch rockets that went over 200mph. i've been trying to discuss this problem as it relates to strato saws but no one seems to get it, especially not the "porters" on this site. the cad modelling in that document was really useful.

on another note, i just finished resurrecting a 575 that had been run over by the truck. after i took it apart i was surprised to see that the intake side of the piston had failed and both main bearings were in bad shape. my theory is that someone straight gassed it and then put it under the wheels so the boss wouldn't find out. anyway, looking at all that stuff made me decide to switch to 40:1. i think the 575/576's use so little mix that the bearings are on the bleeding edge.

 

[cus_deluxe]

just started reading that pdf, looks like a great resource to me.

 

[Chris-PA]

just started reading that pdf, looks like a great resource to me.

I believe that was referred to me by @one.man.band

 

[cus_deluxe]

Nice, ill probly have to read it a couple times to really absorb it well, but a ton of great info and theory in there so far.

 

[Marshy]

Cool document. If you read the conclusion it tells you that there is a sharp lip at the opening of the stratifying channels that create a lot of turbulence. They had to change the geometry in the model to get it to compute correctly. That suggests to me that is a great starting point for builders to gain efficiency on this engine. Since they were modeling just the cylinder and not the engine case it would be interesting if you could somehow tell how closely the ports match the case and blend them to eliminate any miss-match.

"Regarding Figure 3.30 in section 3.2.2 it is obvious that the sharp edge in the beginning of the stratifying channels is a source of turbulence. This behavior, turbulence dueto sharp edge, is previuosly mentioned in section 2.1.1. In this case the sharp edge in the beginning of the scavenging channel resulted in such a turbulent behavior that the solver did not manage to get the solution to converge. The geometry was thus changed. These are two areas of the engine geometry which gives rise to high turbulence."

 

[Chris-PA]

Cool document. If you read the conclusion it tells you that there is a sharp lip at the opening of the stratifying channels that create a lot of turbulence. They had to change the geometry in the model to get it to compute correctly. That suggests to me that is a great starting point for builders to gain efficiency on this engine. Since they were modeling just the cylinder and not the engine case it would be interesting if you could somehow tell how closely the ports match the case and blend them to eliminate any miss-match.

Also interesting to note that (I think) the transfer entrance is one of the main things they changed for the 576.

 

[Marshy]

Also interesting to note that (I think) the transfer entrance is one of the main things they changed for the 576.

Depends on what your are comparing it to I guess. It is vastly different than the 570/575.

 

[heyduke]

Also interesting to note that (I think) the transfer entrance is one of the main things they changed for the 576.

my 575 transfers look just like the ones shown for 576's, smooth and angled. i'll have to check. they may use the same part number.

correction: the cylinder pictured is an after market item (note the rough castings). it does look like the 576, with the transfers smoothly angled into the case. the oem cylinder may be different. the 575 and 576 use different part numbers for cylinder and piston. i think they would be interchangeable though.

 

[heyduke]

Cool document. If you read the conclusion it tells you that there is a sharp lip at the opening of the stratifying channels that create a lot of turbulence. They had to change the geometry in the model to get it to compute correctly. That suggests to me that is a great starting point for builders to gain efficiency on this engine. Since they were modeling just the cylinder and not the engine case it would be interesting if you could somehow tell how closely the ports match the case and blend them to eliminate any miss-match.

"Regarding Figure 3.30 in section 3.2.2 it is obvious that the sharp edge in the beginning of the stratifying channels is a source of turbulence. This behavior, turbulence dueto sharp edge, is previuosly mentioned in section 2.1.1. In this case the sharp edge in the beginning of the scavenging channel resulted in such a turbulent behavior that the solver did not manage to get the solution to converge. The geometry was thus changed. These are two areas of the engine geometry which gives rise to high turbulence."

it wasn't the stratifying chanel that was improved. it was the scavenging (transfer) channel, where it joins the case. perhaps some prussian blue would show if the scavenging channels meet smoothly with the case. you could do the same thing with the strato channels too.

 

[Marshy]

my 575 transfers look just like the ones shown for 576's, smooth and angled. i'll have to check. they may use the same part number.

correction: the cylinder pictured is an after market item (note the rough castings). it does look like the 576, with the transfers smoothly angled into the case. the oem cylinder may be different. the 575 and 576 use different part numbers for cylinder and piston. i think they would be interchangeable though.

You cannot interchange cylinders between the 575 and 576. Well, technically you can and it will fit but the location of the scavenging inlets (transfers) do not align to the cases very well at all.

570/575 Cylinder:



576 Cylinder:


570/575 Case:


576 Case:

 

[Chris-PA]

You cannot interchange cylinders between the 575 and 576. Well, technically you can and it will fit but the location of the scavenging inlets (transfers) do not align to the cases very well at all.

570/575 Cylinder:
View attachment 510463


576 Cylinder:
View attachment 510464

570/575 Case:
View attachment 510465

576 Case:
View attachment 510466

That junction between the case and transfer is what I was referring to as the difference between the 575 and 576 - I believe this is the high turbulence area that was mentioned in the document.

 

[heyduke]

You cannot interchange cylinders between the 575 and 576. Well, technically you can and it will fit but the location of the scavenging inlets (transfers) do not align to the cases very well at all.

570/575 Cylinder:
View attachment 510463


576 Cylinder:
View attachment 510464

570/575 Case:
View attachment 510465

576 Case:
View attachment 510466

Thanks for the information and the great fotos. I haven't worked on a 576 yet, but I'm nearly finished with my third 575, which came in with a busted case. (had to design my own splitter) the cylinder in my fotos was an aftermarket item which is still nearly new though i bought it 5 years ago. obviously it isn't being used in a saw. i require oem for these saws. The 576 cases really are different. the 575 cases just have two little 1/2" holes for the transfer/scavenger ducts. the CAD images in the paper weren't completely accurate for the 575.

I'm also confused about the 570. do they all use a 49mm piston or did some come with 51mm pistons?

anyway, I now feel more comfortable about recommending the 576 to anyone using it professionally, though i wouldn't recommend the auto tuning option, especially for high elevations like where I live. it looks like the 576 is actually an improvement, not always the case for a newer model.

 

[Marshy]

Thanks for the information and the great fotos. I haven't worked on a 576 yet, but I'm nearly finished with my third 575, which came in with a busted case. (had to design my own splitter) the cylinder in my fotos was an aftermarket item which is still nearly new though i bought it 5 years ago. obviously it isn't being used in a saw. i require oem for these saws. The 576 cases really are different. the 575 cases just have two little 1/2" holes for the transfer/scavenger ducts. the CAD images in the paper weren't completely accurate for the 575.

I'm also confused about the 570. do they all use a 49mm piston or did some come with 51mm pistons?

anyway, I now feel more comfortable about recommending the 576 to anyone using it professionally, though i wouldn't recommend the auto tuning option, especially for high elevations like where I live. it looks like the 576 is actually an improvement, not always the case for a newer model.

The 570 has the 49mm bore vs the 575 and 576 has the 51mm bore. IDK why they could make such a small change in bore size, trying to reach different price markets? All 3 have the same 36 mm stroke.

 

[heyduke]

The 570 has the 49mm bore vs the 575 and 576 has the 51mm bore. IDK why they could make such a small change in bore size, trying to reach different price markets? All 3 have the same 36 mm stroke.

yeah, it had to have been a marketing ploy, sorta like dolmar, who build the same saw in 64cc, 73cc and 79cc. couldn't be more that an few cents difference in production costs but sales prices are incremented in the hundreds of dollars. seems dumb to me but there may be some eurozone regulations that require it.

i've always wondered how they settled on the 49cc and 51cc bore sizes. as far as i know they're the only saws with that bore. must have something to do with all that calculus...

 

[7sleeper]

subscribed! GREAT thread!

Thanks guys!

7

 

[heyduke]

The 570 has the 49mm bore vs the 575 and 576 has the 51mm bore. IDK why they could make such a small change in bore size, trying to reach different price markets? All 3 have the same 36 mm stroke.

so i think i figured out why they went with the 51mm piston. the 371/372 uses 50mm piston . both 372 and 575 use a 36mm stroke crankshaft which probably share some components. they didn't want to try to market two saws with the same dispacement so the 575 got bumped to 51mm which was probably the max that their modeling would safely allow.

 

[heyduke]

You know, I saw that too - but I don't really believe it:

First of all, I don't see how it reduces the piston weight at all. The piston is not solid, it's basically a thin wall cylinder. Deflecting a portion of the wall inward and adding a top and bottom panel cannot be reducing the required material.

Second, every strato I've seen has that feature, although I've not seen one where they are connected before. Sometimes they are above the air passage slots, sometimes below, but they usually line up with the strato air passages.

I think they are for cooling and/or lubrication. I suspect that since they are a dead-end passage they accumulate oil over time, which helps to lubricate the cylinder walls near the dry air strato inlet ports.

well, i went down to the shop and meditated on the piston again today, and i think you're right. a major challenge with this design is getting enough lubrication to the bearings and the cylinder walls. the problem is that the case volume is microscopic and not much fuel flows thru the system. that's great for mileage but not so good for lubrication. the two holes that penetrate the piston allow some of the cool fuel vapor from the case to wash over the cylinder walls and keep the rings slick.

 

[Chris-PA]

i think you're right. a major challenge with this design is getting enough lubrication to the bearings and the cylinder walls. the problem is that the case volume is microscopic and not much fuel flows thru the system. that's great for mileage but not so good for lubrication. the two holes that penetrate the piston allow some of the cool fuel vapor from the case to wash over the cylinder walls and keep the rings slick.

Well, I don't quite agree with that. The reduced case volume does not effect the amount of fuel/oil mix that is drawing into the case - if anything it improves pumping efficiency and increases it. The amount of mix is only reduced by the reduction in scavenging losses, so maybe 20%. I don't buy that this effects the bottom end, as once the surfaces are coated then additional mix just passes through, and the oil coating the bearings is not going to wear out - you can't expect mix to keep accumulating on the case surfaces.

The main difference is the piston & cylinder walls because of the dry air pulled down through the transfers. Still, the fuel mix passes through there every cycle anyway, and the piston gets cool air pulled across those cavities every cycle, which does not happen in a non strato engine. I think they added those dead end cavities just to accumulate some oil in there that gets smeared over the cylinder walls.