2-Stroke Fuel Requirements Vs. Engine RPM?

[Andyshine77]

If you're a dealer and don't properly tune a saw before it leaves the shop, you're not doing your job!! Echo's like most equipment these days are setup on the edge not being able to to run, let alone run properly. They have to be tune out of the box. And yes you can tune them on the rich side do help prevent failure.

 

[Rockjock]

If you're a dealer and don't properly tune a saw before it leaves the shop, you're not doing your job!! Echo's like most equipment these days are setup on the edge not being able to to run, let alone run properly. They have to be tune out of the box. And yes you can tune them on the rich side do help prevent failure.

This I agree with but since Echo is sold in the big box stores, and it just comes in a box ad the end user has to put it together.. I fear they are being set up to fail. To me that is lack of commitment to their customer.

 

[mountainlake]

WHY would they purposely tune them that way so they blow up and offer a 5 year warranty on them

Plain and simple, to get by the EPA regulations. Keep in mind your in Canada or Germany where the EPA regs most likely aren't s strict. Steve

 

[Rockjock]

Plain and simple, to get by the EPA regulations. Keep in mind your in Canada or Germany where the EPA regs most likely aren't s strict. Steve

I believe the machines are the same. The average guy will not know they need to be returned, but each unit we sell gets checked out.

 

[mountainlake]

You really think that Echo is going to send out choked up burn up lean saws to other country's if they don't have to, not a chance. Steve

 

[alderman]

They are trying to meet emissions without the benefit of strato or feedback carbs.

Not trying to hi-jack, just curious.
In a world where emissions didn't matter would you prefer old technology over new as far as reliability and run ability?

 

[Chris-PA]

Not trying to hi-jack, just curious.
In a world where emissions didn't matter would you prefer old technology over new as far as reliability and run ability?

No. Long before I ever worked on a chainsaw I spent a lot of time tuning real carbs, on road vehicles and some on race cars. There is a correct mixture for power, and it does not involve misfiring/4-stroking. I know how real carbs work, and regard any engine that is misfiring as broken and in need of repair. There never was any need for that, it's just that fuel was cheap and nobody understood the environmental issues or cared about the health of the operator, so they took the cheap way out. Then too, unburned fuel blown out the exhaust due to scavenging losses is a waste, and it produces no power.

Reducing emissions need not have any negative reliability or run-ability impacts at all, rather quite the opposite. Both of these problems were/are easily solved, and have been, with designs that are elegant, simple and effective. Strato does not require a single moving part yet drastically reduces scavenging losses, while freeing the designer to chose port timing for more power. The poor carburation could have been solved without electronic feedback systems all along, but AutoTune achieves feedback control with almost no additional sensors.

Still, there is no world where emissions don't matter, as they will always matter to me. I'm old enough to understand that humans are not invincible, and the kind of cumulative effects that breathing the stuff that's in fuel mix can have on a human body.

 

[SteveSr]

These carbs don't respond directly to load at all, they don't know anything about load. When the throttle is held at a fixed position, the fuel delivered is a function of air velocity only (they do not hold a set fuel/air mixture like other carbs). As the air velocity goes up, the mixture gets (much) richer, and eventually the engine misfires (4-strokes).

With a chainsaw running no load WOT the only resistance is the friction of the chain on the bar and the cooling fan, and these do not represent much of a load. When you tune at WOT the main thing that determines rpm is the terminally rich fuel mixture.

So this explains the over-rich tuning at WOT. But what about the engine requirements when running under load at a point below WOT? It appears from your statement that the lower the RPM the leaner the mixture gets. At the same time does the engine fuel needs increase due to leakage due to longer dwell opening of the exhaust port?

 

[Chris-PA]

So this explains the over-rich tuning at WOT. But what about the engine requirements when running under load at a point below WOT? It appears from your statement that the lower the RPM the leaner the mixture gets.

What you'd like a carb to do is hold a constant fuel/air ratio as air velocity (rpm) changes. If you plot that out it would just be a flat line.

But an uncorrected fixed jet and fixed venturi carb gives a volume of fuel that (in theory) is roughly the square of the air velocity, so the fuel/air ratio would look like a parabola if you plot it out vs. rpm. When you tune you shift that curve up or down, and you can make the fuel/air ratio correct at one rpm only. If you move up in rpm it's on the steeper portion of the parabola, and if you move down in rpm from the set point it's to the less steep side - but yes, it is still a slope and it is getting leaner. So anything that increases load and decreases rpm should make it leaner - one of the reasons pushing a saw can cause engine damage. I don't know if it really works out to truly be a squared function, but it power function and a steep slope - the 4-stroking with just a light lift is evidence of that.

Most carbs use an air correction jet to correct this problem, but that was removed in the process of making these all-position.

At the same time does the engine fuel needs increase due to leakage due to longer dwell opening of the exhaust port?

I'm not sure about that. I'd guess that changing rpm say +/-10% about some point does not change scavenging losses +/-10% proportionately. Then again it's been a long day and things aren't making sense any more!