stratocharge motor and muffler mods

[JUDGE1162]

I posted this under another thread but it seemed to have gotten lost in the mix, so I started a new thread for it.

Why would they need a CAT if the 455 is a stratocharged design?? Something isn't right... and yes, if it is a true stratocharge, then it won't respond well to the muffer mods of old...

Please explain to a 2 stroke newbie, why if it is a true stratocharge, then it won't respond well to the muffler mods of old?

From my understanding and please correct me if I am wrong!!!!!!!!

All stratocharged mean is there is an extra "cycle" where air with no gas/oil mix is pushed into the cylinder to push out the exhaust gas to cut down on "blow by" wasted extra fuel/oil being pushed out with the exhaust which happens in a standard 2 cycle motor.

I know it causes lower max rpms as the extra cycle of air with no fuel takes time to happen. But I am not sure why a standard muffler mod would not work.

 

[Lakeside53]

There is no extra cycle... at least not in Stihl design. I'll try to uploads some docs explaining this.

 

[bwalker]

There is no extra cycle, but rather a burp or slug of fresh air that procedes the fuel are charge. This way the engine is scavenged by fresh air instead of fuel/air mix which leads to a huge reduction in unburnt hydrocarbon emmissions.

 

[bwalker]

There is no extra cycle, but rather a burp or slug of fresh air that procedes the fuel air charge. This way the engine is scavenged by fresh air instead of fuel/air mix which leads to a huge reduction in unburnt hydrocarbon emmissions.

 

[eyolf]

Lakeside will hopefully explain it fully, but I can't wait.

What I do understand about these various stratified charge schemes is that there are usually modifications to the inlet tract where a part of the air entering the engine bypasses the venturi and picks up no fuel. It is still throttled, however, so that the relative amount of air/fuel mix and plain air (relative to each other) remains the same. Within the engine, there will be zones where the charge will be mostly air or mostly fuel-air mix; porting in the cylinder and geometry of internal components help maintain this separation such that the first gasses that follow the spent charge out of the exhaust port will contain significantly less fuel.


that part is easily understood. It seems to me the engine is actually using some energy to pump more air through itself, and that designers may be actually allowing a bit more air (but very much less fuel) to escape out the port than before...especially at what was formerly peak power range, so the peak volumetric efficiency may be lower. We aren't seeing much hard figures relating to this idea, not yet.

In short, I think the strat charge engines will prove much cleaner, but also less powerful...it may take a 75cc engine to equal a 70cc older style engine as far as peak specific output. To partly overcome, of course, the strat charge engines seem to exhibit flatter torque curves...the sweet spot where optimum fuel/air mix retention is realized in the RPM range is spread out a lot. Trying to tune an exhaust system to these types of engines would be pointless

 

[JUDGE1162]

Thanks for the input, I knew there was not really another "stroke or cycle" but did not know how to explain it, like I said 2 cycle newbie.

I still don't see how opening up the muffler will not increase power, but again like I said I am a 2 cycle newbie

I look forward to Lakeside53 explaination of stratocharge, maybe if I understand the process better I will understand why the mod will not work:bang: :bang: :bang:

 

[manual]

Yes a muffler mod will work but all that I understand is you won't be opening up the muffler as much.
One guy on the site has already modded his 441, Although he cannot tells us of any type of gains only because he did the mod as soon as he bought it.
the point is his saw is running well and he is happy with it.

 

[manual]

It seems to me the engine is actually using some energy to pump more air through itself, and that designers may be actually allowing a bit more air (but very much less fuel) to escape out the port than before...especially at what was formerly peak power range, so the peak volumetric efficiency may be lower. We aren't seeing much hard figures relating to this idea, not yet.

In short, I think the strat charge engines will prove much cleaner, but also less powerful...it may take a 75cc engine to equal a 70cc older style engine as far as peak specific output. To partly overcome, of course, the strat charge engines seem to exhibit flatter torque curves...the sweet spot where optimum fuel/air mix retention is realized in the RPM range is spread out a lot. Trying to tune an exhaust system to these types of engines would be pointless

Hmmmmm.
The "energy" used is in the stroke. the emmision of air is through the carb.
When you look at the carb it apears to look like a two barrel.
the air / fuel mixture is not effected. one part of the carb introduces a shot of air to clean out the cylinder while the other part introduces the air /fuel mixture.

 

[eyolf]

Moving the piston and recipro assembly requires energy input, which we get in the form of the fuel. Since we're separating the air into two streams, some of that air isn't drawing in fuel, and therefore cant help in converting that fuel into heat energy. But we still need to pump it in and out.

So far, we probably haven't sustained much net loss...but what happens if we sacrifice a greater proportion of that air back out through the exhaust? The engine can only pump so much air in and out, and if we use a relatively greater proportion to make sure we're running clean, that leaves a little less for power.

Of course, any two old-style stroke engine is going to run effciently as regards to specific fuel consumption over a fairly narrow RPM range. The peaks in energy conversion/BSFC will naturally occur at or near the torque peak for any particular engine. I believe the strato charged engines will exhibit a little less peak power in comparison to conventional two-strokes for any given displacement, but they may make it up by being less peaky..the torque curve will be flatter. A conventional two-stroke runs efficiently and produces peak power over a fairly narrow range because outside of these ranges it discards a portion of the fuel charge or suffers from mixture dilution with spent gasses.

I'm mostly interested in seeing some comparisons.

 

[Lakeside53]

In short, I think the strat charge engines will prove much cleaner, but also less powerful...it may take a 75cc engine to equal a 70cc older style engine as far as peak specific output. To partly overcome, of course, the strat charge engines seem to exhibit flatter torque curves...the sweet spot where optimum fuel/air mix retention is realized in the RPM range is spread out a lot. Trying to tune an exhaust system to these types of engines would be pointless

This hasn't been the case in the Stihl design. The 441 has slightly more power than the same size 440 (70.7cc), and a wider power band. I'll make a wild guess and say that more complete combustion generates the power at greater fuel efficiency (20% better economy). If anything they detuned it to stay with the same product positioning.

 

[manual]

This hasn't been the case in the Stihl design. The 441 has slightly more power than the same size 440 (70.7cc), and a wider power band. I'll make a wild guess and say that more complete combustion generates the power at greater fuel efficiency (25-30% better economy). If anything they detuned it to stay with the same product positioning.

Yes a fire needs oxygen to burn. getting rid of the old gasses creates more space for fresh oxygen. Now what i want to Know is.
By cleaning out the cylinder and introducing fresh gasses. is this allowing unburnt hydrocarbon's to escape into the air ?

 

[timberwolf]

I wonder if part of the application here is similar to the air compensated carbs, where an air jet is added to the carb bore to even out the fuel curve. If the control on the extra fresh air was made in such a way to send in extra fuel at times when the carb ran nuturaly rich it would lower unburned HCs as well as flatening out the power curve.

I think you would need direct injection to make this work, but fuel can be burnt in a lean state efficiently and with low HC polution if a pocket of richer fuel surounded the sparkplug. HCs go down but I think oxides of nitrogen and sulfer are formed.

 

[Lakeside53]

Here's how the 441 version works...

 

[manual]

Seems HC would be higher.

 

[Lakeside53]

Seems HC would be higher.

Sure... that's why the muffler is just a big open can! The EPA is completly fooled by the slick advertising.

Read the very last sentence of the third picture above...

 

[JUDGE1162]

OK, I now better understand how stratocharging work, air without fuel is used to clear the cyclinder of the exhaust gas then the fuel/air mix enters the cyclinder thats all good and I will let others more knowedgable then me argue the pros and cons of this type of 2 cycle motor

Now that we have a better idea how it works, how does this effect a muffler mod?? Will it work and if not why? or what would you need to do diffrently to get it to work.

 

[Lakeside53]

My understanding is that there is no significant back pressure to relieve. The muffler didn't need to be choked off in the first place. There are some small gains, but modest in comparison to the prior designs.

This is new ground for modders, and will require some experimentation before anyone understands where the new limitations are imposed. Complicating this is that there are several different way to go about strato-charging... and they may respond differently. Even Stihl has two approaches - reeds on the TS700/800, and piston "ported" on the 441.

My information on modifying the 441 muffler came from a guy I respect (no names) and he had little luck in improving the output by conventional means.. He's looking at it... It might require changing both the input and exhaust to maintain system mixture balance.

 

[JUDGE1162]

OK now it is starting to make sense

Correct me if I am wrong but this is what I think is happening

In non-stratocharged motors the muffler cause a lot of back pressure to keep the fresh fuel/air mixture in the cylinder and out of the exhaust gas and by opening the muffler you increase HP/RPMs by reducing the back pressure but also increase the amout of fuel/air mix in the exhaust gas

In stratocharged motors due to the fresh air with no fuel being used to clear the cylinder instead of the fuel/air mix there is less wasted fuel in the exhaust gas to begin with so they did not need to make the mufflers as restrictive and with so much back pressure that by opening up the muffler on a stratocharged motors there is no extra backpressure to release so there is not any real HP/RPMs to gain.

 

[Lakeside53]

I have some really detailed thermal images of what happens inside a regular 2-stroke, in about 5-10 degree increments, and the same for a stratocharge... but I need to get permision for the publisher first... working on it...

 

[bwalker]

By cleaning out the cylinder and introducing fresh gasses. is this allowing unburnt hydrocarbon's to escape into the air ?
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This isnt the case at all. Unburnt HC levels are greatly reduced because the cylinder is being cleared of exhaust by fresh air instead of fuel/air mix like a standard two cycle.