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2- vs 4-cycle motors

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auerbach
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2009-09-04          165451

Those 2-cycle engines on trimmers and chainsaws are giving way to 4-cycle ones. Given the same displacement, do they have the same power?

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earthwrks
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2009-09-04          165452

My buddy has a 4-cyl homeowner trimmer and it's a beast--stops at nothing.

By power I think we'd need to distinguish between torque and HP. I have to think the 4-cyl has more torque. And I be curious about usable RPM--trimmers and chainsaws seemingly would benefit from a 2-cyl.

But considering technology advancements that diesels have seen in recent years, diesels are on-par RPM wise with gas engines, so, conceivably, 4-strokes may someday seethe same lower RPM that 2-strokes do. Just my opinion. ....

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greg_g
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2009-09-04          165456

Light duty 2-strokes are simply not as efficient as 4-strokes, which pretty much levels the playing field from a power perspective. But I haven't seen a carbureted 4-stroke yet that will run upside down.

Unfortunately, I think it's the EPA that is forcing the shift away from 2-stroke applications. They think the silly "tamper-proof" carbs on these new fixed-rpm 4-strokes stop tinkering owners from increasing pollution levels. In my estimation, the power just isn't there anymore (compared to previous carb/throttle configurations). But back to oil burners. On heavier duty 2-strokes, the EPA's already mandated oil injection. That takes improper oil/gas ratios out of the equation. But owners that can still blend their own oil/gas mixture for lighter duty engines - remain beyond their control. EPA solution? Get rid of light duty 2-strokes.

//greg// ....

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kwschumm
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2009-09-04          165457

Stihl makes a 4-stroke, called the 4-mix, that runs on 2-cycle mix for lubrication. Those run upside down and the dealer told me they had excellent power and meet EPA regs.
....

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hardwood
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2009-09-05          165465

Saw a show on Discovery or History about railroad locomotives. Some of them are two cycle diesel, don't remember why now if it was more power from the same displacement or more efrfecient fuel use. ....

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Murf
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2009-09-05          165473

Quote:
Originally Posted by greg_g | view 165456
But I haven't seen a 4-stroke yet that will run upside down.


I fly a plane with a 4 stroke engine certified for inverted flight, and I roll it over on a regular basis too! LOL

I agree though, the new small 4 strokes are very good, and only getting better.

Best of luck. ....

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greg_g
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2009-09-05          165474

yeah, but we're talking earth-bound carbureted engines Murf. Not fuel injected airbornes.

//greg// ....

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auerbach
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2009-09-05          165476

If I may re-word the question, it's time to replace my 31-cc brush-bladed rotary trimmer, and I need to know this: In a 4-stroke, would the same cutting ability go with a smaller, the same, or a bigger displacement? The answer from dealers depends on what they're selling. ....

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hardwood
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2009-09-05          165481

Auer;
Rather than me giving a shot in the dark answer about like dispacement engines being capable of like output being 2 or 4 stroke, somewhere some must have a rule of thumb chart about that.
My GUESS is that a four would be roughly half as capable, but not sure. ....

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kwschumm
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2009-09-05          165482

That's an interesting question and I have no idea what the rule of thumb might be. Two cycles don't extract as much power per power stroke due to more valve overlap but they have twice as many power strokes. Maybe the only real world specs that matter for hand held equipment would be power, weight and fuel consumption so displacement may be rather unimportant but I'd be curious to know the answer as well.

....

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earthwrks
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2009-09-05          165488

Being able to run upside down really isn't a feat--for years hand-held equipment has been made to do that. All it is is a brass (for weight) fitting and a fuel strainer connected to the flexible fuel line inside the tank which keeps the inlet submerged.

My road grader has a 2-stroke Detroit diesel, but it's supercharged too. 2-stroke diesels have been around a long time--if memory serves even before 4-strokes (I'm going research this now to be sure) ....

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Murf
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2009-09-06          165503

Greg, my bird is carbureted, not fuel-infected. LOL

Regardless the technology is the same. A way to, as EW mentioned, deliver oil & gas in any orientation. Not hard to do at all.

Best of luck. ....

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earthwrks
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2009-09-06          165505

Murf, assuming the bird has a carb with a float, just how does it regulate fuel? ....

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auerbach
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2009-09-07          165515

Earth's question brings back something I once came across (forgot where). It seems the Spitfires could not fly inverted but the enemy ME109 could, and a European came up with something (a carb?) that would allow the Spit to also do so, and had difficulty getting the Brits to listen to him.

To this day, every aircraft either is or is not certified for inverted light. Another factor is fuel-flow design (gravity feed wouldn't work). And wing design, pilot restraint system, etc.

Been researching 2- vs 4-strokes. The latter mainly have a cleaner (and quieter) exhaust, but are heavier, costlier, and harder to start. They use less fuel but require oil changes. Longevity is similar: the 4s have more parts but work at lower speeds. And they ARE weaker. So I'll get a replacement 2-stroke while still available. ....

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kwschumm
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2009-09-07          165516

Quote:
Originally Posted by auerbach | view 165515
Been researching 2- vs 4-strokes. The latter mainly have a cleaner exhaust. Longevity is similar: the 4s have moreparts but work at lower speeds. And they ARE weaker. So I'll get a replacement 2-stroke while still available.


How much weaker are the 4 strokes? ....

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auerbach
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2009-09-07          165518

Hard to quantify. On the face of it, a "2" would have twice the power of a "4" (a power stroke for EVERY crankhaft revolution, vs one power stroke for every TWO crank revs).

But it's not that simple. The big variable is getting the fuel in and the exhaust out, so it depends on the engine design. And designers of small engines have much more experience perfecting the 2-strokes. But the fuel/exhaust system on a "2" is inherently less efficient, so there's less power per stroke.

....

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earthwrks
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2009-09-07          165519

Auer: "On the face of it, a "2" would have twice the power of a "4" (a power stroke for EVERY crankhaft revolution, vs one power stroke for every TWO crank revs)."

No, the power stroke on a two stroke is ONE power stroke for every TWO strokes, or, more properly every other stroke; and ONE power stroke for every THREE strokes (which is why 4-strokes have larger, heavier flywheels to keep them spinning while they work for only one power stroke). ....

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Murf
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2009-09-07          165522

Quote:
Originally Posted by earthwrks | view 165505
Murf, assuming the bird has a carb with a float, just how does it regulate fuel?


Jeff, I must admit to be just a little sneaky, you caught me. Inverted is different from negative 'g' flight. I can fly upside down, and the plane doesn't know it's upside down. This is possible because as long as you maintain a +1g situation you can be oriented any way you like. For instance, if I want to invert all I need to do is make a wide slow rolling loop keeping a perfectly balanced attitude. At that point things don't fly around, I don't even spill my coffee! Best of all SWMBO doesn't 'critique' my flying! LOL

Best of luck. ....

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earthwrks
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2009-09-07          165525

So is it fair to say that at anything less than +1G fuel isn't regulated properly, again assuming you do have a carb with float? Basically, or really you're talking about centrifugal force on a big scale, and not necessarily G-force, depending on what part of flight you're in--diving versus wide, sweeping arcs--whether inverted or not and not losing your coffee, right?

And hey whatever happened to (me?) "transporting toys" to the bridge? ....

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Murf
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2009-09-08          165532

Quote:
Originally Posted by earthwrks | view 165525
So is it fair to say that at anything less than +1G fuel isn't regulated properly, again assuming you do have a carb with float?


Not quite, but really close to that.

The carbs on such aircraft as mine are equipped with an extra restrictor plate in the carb that limits fuel flow to just slightly more than that which the engine can burn in a WOT situation.

This was an invention developed by a young British (female) engineer during WWII. The German aircraft were fuel-injected and the Allied aircraft were carbureted so it didn't take long for the Germans to learn that all they had to do was make a sudden dive to get away from a pursueing Allied aircraft. If the American or British pilot followed his plane would quickly bog down due to flooding. The quick & dirty solution was a restrictor in the carb to stop (or nearly so) the flooding. To this day it's known as a "Miss Shilling" plate.

In aircraft terms centrifugal force is almost always called "g force" and measured in units of 'g', regardless of the direction or orientation of the aircraft. It is usually described as an imaginary piece of string from the "C of G" (Center of Gravity) and leading off to the direction of the force in a singular linear direction which is the result of all the varying forces being blended into a single resultant force. This is much easier to calculate and deal with in that form.

So then if properly done, a turn (or any other maneuver) is called a "balanced" maneuver if it results in a +1g force straight down, the same as if the plane was parked on the ground.

The 'toy' thing is moving along but very slowly, LOTS of people are doing the same thing, and there aren't (at the moment any 'bargains' in your (MI, IN, W.OH.) area. If you see something that looks good drop me a note though, it doesn't all make it to the web where we can see it from here.

I just got back from a week in Vancouver and score some deals out there, enough to fill a transport truck coming back next week.


Best of luck. ....

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treeman
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2009-11-01          166646

A 1000cc 4 stroke snowmobile has about the same power as a 600cc 2 stroke one. ....

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