Ms461 saw design

[MasterMech]

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No one wants to tell me this....

This layer of exhaust gas that is being used as a boundary layer. How is it any different than what is already in the combustion chamber and muffler. If a person takes time to think, rather than just quote what Stihl has said about this engine, they would see that there is no difference and that this "design" is just a marketing ploy.

It's not any different. If the goal is to prevent raw fuel from being discharged into the exhaust port, you have to either close the exhaust port before the incoming charge fully displaces the spent combustion gases or slow the departure of those gases to intentionally force incomplete cylinder scavenging.

 

[Chris-PA]

Randy and Brad provided the timing numbers and the information that the muffler isn't unusual or different from the the 660. KG441c provided the document which shows what Stihl claims - both for how their delayed scavenging supposedly works and for what that bracket thingy under the piston is for. With all of that we know a lot more about what Stihl is up to. Isn't that what a technical discussion is supposed to accomplish?

I'm pretty skeptical that the system does what they claim, but not having taken one apart personally does not prevent me from understanding the claims, and if I had all the pieces in front of me I still wouldn't know any more about how well it works (or not). In the same way I could understand the function of the strato system on a 576XP by reading the description contained in the Master's Thesis I was directed to on line, without ever having seen one. I reject the idea that only those who have personally dissembled a saw can meaningfully contribute to understanding such systems - I think it's more a matter of respecting what each brings to the discussion, and the limits of our own knowledge.

While I am skeptical of the claims of delayed scavenging, there are two things that make me cautious. First, they seem to be allowed to sell it. Second, there is apparently one physical difference - the longer transfers. Whatever positive pressure is in the combustion chamber when the transfers open (relative to the case) will push exhaust down the transfer runners. The fresh fuel/air mix will not be able to enter the combustion chamber until that exhaust gas is pushed back up, which is a delay. But if the timing and muffler are the same, then there isn't any more backpressure or volume than before. If the transfers were short and the exhaust made it all the way to the case, then it would just mix some exhaust with the fresh charge and not delay it the same way.

 

[Mastermind]

Chris......I appreciate your input here. You make posts that reflect that you think deeply about the matter being discussed.

I took offense to a couple of guys, and took my frustrations out on the whole. For that I offer an apology.

The fact they Stihl is able to sell these saws means one of three things to me.

1. They greased the skids. Campaign donations in the right pocket go a long way in this country.

2. It does work as described and I am totally wrong.

3. The carb, ignition, case, and transfer design differences make it more efficient.

#3. is the most likely in my mind.

I'll go back to this......

This layer of exhaust gas that is being used as a boundary layer. How is it any different than what is already in the combustion chamber and muffler.

 

[Chris-PA]

I'll go back to this......

This layer of exhaust gas that is being used as a boundary layer. How is it any different than what is already in the combustion chamber and muffler.

I'm not sure quite what you mean - if the timing and muffler isn't any different between the 460 and 461, then you'd have the same amount of exhaust gas and pressure left in the cylinder right before the transfers open. But after they open different things could happen - the long transfer runners might allow that gas to push down the runners towards the case. Then the fresh charge would have to wait until the pressure differential reversed and all that gas was pushed back up into the cylinder before it could enter into the combustion chamber. Basically you'd have the runners filled with separated exhaust gas on top of the fresh charge.

The whole purpose of strato is to delay in time the arrival of fuel into the combustion chamber so that there is less opportunity for it to go out the open port.

But if the timing and muffler are the same, then that exhaust gas must have always been blowing down into the transfers on 460s - how did that work? If the transfer runners were shorter and of larger volume, then maybe it just did not cause as much of a delay. Or did it blow all the way into the case? That would seem to be bad news.

Even if it does work this way, I really don't like the idea as much as using fresh air to fill the transfers and delay a much smaller fresh charge.

 

[Mastermind]

What I mean is this.......if the exhaust has too much hydrocarbons in it to be acceptable to the EPA, then using that same exhaust gas as a buffer between the exhaust and the incoming fuel/air charge seems silly.

 

[Chris-PA]

What I mean is this.......if the exhaust has too much hydrocarbons in it to be acceptable to the EPA, then using that same exhaust gas as a buffer between the exhaust and the incoming fuel/air charge seems silly.

Apparently the biggest issue with OPE 2-strokes is raw, unburned fuel in the exhaust, not the stuff in the burned fuel like they are worried about in 4-strokes. They're not yet bothering with how well it burns the mix that stays in the combustion chamber, since so much is just blowing right out. If you delay the arrival of fuel to the combustion chamber until very late in the fill portion of the cycle, then you lose less out the exhaust port. You, or the feedback carb, adjust the mixture so the burn is correct, but you have to put less fuel in because less is blowing out the port.

So if delayed scavenging works properly then what comes out the exhaust would be different - exactly the same combustion products, but less unburned fuel. What is inside the chamber after ignition should be the same.

 

[Mastermind]

I'm still not buying it.

 

[Chris-PA]

I'm still not buying it.

I'm not really either!

 

[Mastermind]

It does make for an interesting conversation though........as long as we discuss what is really on the table.

 

[Marshy]

This layer of exhaust gas that is being used as a boundary layer. How is it any different than what is already in the combustion chamber and muffler.

I will tell you hands down that the best source for seeking an answer to all your (their) questions is with the US Patient office.

Heres an interesting read, not sure if this Patent is owned by Stihl or someone else but non the less it gives specific detail on a stratified scavenging 2-cycle engine. http://www.google.com/patents/US6216650

Here is a Patent owned by Walbro for a Layered Scavenging Carb... http://www.google.com/patents/US8261775

Reading through some of the comments in this thread I feel there is some misconception in the operation of a stratified engine and what is taking place. The intent of a stratified 2 stroke is to put a boundary layer of fuel free air in between the fuel charge within the crank case and the exhaust gasses exiting the cylinder. Im uncertain how Stihl actually accomplishes this with their design as I have not read their patent nor have I disassembled one of their engines but if the patent is found then we would understand their claims a lot better.

Maybe this is the one..? http://www.google.com/patents/US6267088

 

[Mastermind]

I understand exactly how a strato engine works.......that's why I say that the 461 isn't a stratified engine.

 

[Marshy]

Maybe the stratified layer is being acomplished by the carb and not some fancy secondary prot runner?

 

[bryanr2]

I have nothing to contribute as this is WAYYYYYYY over my head, and has nothing to do with keeping my family warm and pissing off my neighbors.... that said, I brought some opcorn: to share. Ported saws Rule!

 

[MasterMech]

The issue with two strokes and the EPA has always been raw, unburned fuel escaping the cylinder during scavenging, at least as far as OPE is concerned. It's how/why engines like the 4-mix came about. From a pure performance standpoint, we don't really care if a little fuel/air charge escapes as this means the cylinder is completely purged of exhaust.

Forgive me for going into territory of which I am no expert, but is it possible that the differences in transfer port design between the 460 and the 461 are the key to the reduced fuel usage? If the cylinder scavenges more efficiently with less loss out the exhaust port, fuel consumption for the given power output would also decrease. Regardless of any "delayed stratification" or other marketing phenomena.

 

[tree monkey]

The whole purpose of strato is to delay in time the arrival of fuel into the combustion chamber so that there is less opportunity for it to go out the open port.

But if the timing and muffler are the same, then that exhaust gas must have always been blowing down into the transfers on 460s - how did that work? If the transfer runners were shorter and of larger volume, then maybe it just did not cause as much of a delay. Or did it blow all the way into the case? That would seem to be bad news. .

this is correct

 

[tree monkey]

why do you get big gains from a muffler mod?

 

[MasterMech]

why do you get big gains from a muffler mod?

While I know a muffler mod is worth a heap of power on a 460, some say it does little on the 461. So a question for guys that have had more than a couple of these apart, is there truth to that? That would make sense if the 461s induction system is indeed much more efficient than the 460.

When there are big gains to be had from a muffler mod, it's because cylinder scavenging has improved due to decreased back pressure in the exhaust. Incomplete scavenging, while it might keep the EPA happy, leaves inert exhaust gases in the cylinder, taking up volume that could be filled with fresh air/fuel mix. Strato engines stuff the transfers with fresh air (sans fuel) ahead of the rich fuel mix. So ideally we want to close the exhaust port sometime between that fresh air exiting the cylinder and the fuel mix reaching the exhaust port.

 

[Chris-PA]

why do you get big gains from a muffler mod?

If it works as they describe, then the only thing driving it is backpressure from the muffler (as the timing is not unusual). So removing the muffler restriction would disable the whole process and advance the fresh mix flow into the cylinder, as well as probably cooling the upper end a lot. Also increase fuel lost out the exhaust.

IF it really works as described.

 

[KG441c]

If it works as they describe, then the only thing driving it is backpressure from the muffler (as the timing is not unusual). So removing the muffler restriction would disable the whole process and advance the fresh mix flow into the cylinder, as well as probably cooling the upper end a lot. Also increase fuel lost out the exhaust.

IF it really works as described.

My thoughts exactly Chris. As awesome as the 461 is ported, (stock after gutted), the system seems to peform pretty dang good without burning up as testimony from many loggers and users with no reported problems?

 

[Mastermind]

Now we are cooking.

An excellent discussion.