2-4 cycle

[Al Williams]

I have read almost all threads and searched and can not find an answer.
Please explain what the 2-4 cycle is and how to adjust an engine for it.
Thanks
Al Williams

[Dan McEntee]

     
      This is in reference to what speed a 2-cycle engine is running in, and can be confusing to new comers. It is regarding how the needle is set also to get the correct engine speed you want.  If an engine is leaned out to a peak setting it is thought to be running in a "2-stroke mode" and generally stay in that mode. If you open the needle a bit to the point where the engine richens up and slows down, it may sound like that it is only firing on every other stroke, and thus the "4-stroke mode"  You get the "4-2 beak" or some call the "2-4 break" if you have the engine speed set at the higher end of the 4 stroke mode, and then when you g into a maneuver and the nose raises up, the engine will lean out and "break" into a 2-stroke mode. This is caused by gravity holding back the fuel flow a bit, and the engine sensing a different air density. The engine will stay leaner the nose of the engine points down, and when the model gets back to level flight, it should go back to the original needle setting if all is well with the fuel system. Different features of a particular engine will determine how "hard" an engine will break , like compression and the specific timing of that engine. Needle valve setting, glow plug, nitro and oil content in the fuel, prop size all have a small part to play in it. The more modern engines with higher timing tend to not run well in this fashion and need to be more in the higher RPM ranges to run smoothly and make their best power. That is where some guys have the "runaway" issue if they try to run a modern engine in the same fashion as an older vintage engine. I hope this helps and makes sense. To a lot of guys, the sound an engine makes when running like this is very nostalgic to them and that is what they want to hear. Engines like the Fox.35 and McCoy Red Heads are classic examples of engines that like to run in this fashion.
   Type at you later,
   Dan McEntee

[John Park]

Right: time for a bit of controversy here.  I do not think that when a two-stroke engine switches from four-stroking to two-stroking, the 'switch' has much, if anything, to do with air density or the effect of G on the fuel.  I think, from my experience with motorcycles as well as control-line model aircraft, that the chief factor is LOAD. The simplest way of demonstrating this with a model aero engine is to bench-mount it - I used an old Merco .29 - and run it on a largish prop. (I used a Tornado 10x6) with the needle set to a two-stroke but not leaned right out.  Stop it without touching the needle, and change the prop. to one that imposes rather less load (a Master 9x4 in my case).  Re-start, and you will find that the engine four-strokes: the needle will have to be closed somewhat to make it two-stroke - at, of course, a much higher speed than with the larger prop..
Alternatively, ride a two-stroke motorcycle in a low gear on a level road, on a light throttle so that it four-strokes.  Without moving the throttle, head up a slope: you will hear the engine 'switch' to a two-stroke.
My belief is therefore that the 'switch' is triggered by the extra load imposed on the engine by, typically, pulling the model from low-level flight into a climb.  I've noticed, with some model/engine combinations, that in a dive from overhead, the engine will slip back into a four-stroke BEFORE you pull out into low-level flight.
One final thing: as a musician (I'm a blues and ragtime guitar picker), I have a fairly good sense of pitch, and I am not convinced that the increase in RPM when the switch occurs is all that great.  On the mororcycle, in fact, it doesn't alter at all.
There: that's my 'two penn'orth', as we say in England.  It's based solely on my own observation and the above-mentioned experiments, and I shall be very interested to hear what those more expert than me hve to say about it.

John

[L0U CRANE]

These are nice answers, so far, but I think it will also help to consider prop load and ignition "duration."

If the whole thing was due to raising the venturii above the fuel tank to lean out the setting, why does it not flood off when we dive the model so the fuel is above venturii?

My answer is prop load. When we maneuver, the wing's lift  makes more drag. The prop is - in rough effect - like a car's tires on the road. It is the means of pulling the model. More drag means more load on the prop. Now, ignition: it takes an actual length of time to burn fuel as the piston reaches and passes through top dead center. Very short time, only several hundred thousandths of a second. Load slows the prop, that allows a bit more time to go through the burn zone. More of the 'fuel-air charge" has time to burn, and does. When the maneuvering drag load drops away, RPM increases to more or less what it had been before you added the load. The fuel mixture doesn't have time to burn as completely - that's the misfire every other revolution that changes the sound to resemble that of a 4-stroke engine.

So, even if the RPM sounds like it has doubled, it hasn't . You hear twice as many "bangs" because they are there. But that doesn't mean the RPM doubles, just that a clean burn is happening every rev, not every other.

You could check this on a test bench. Put a spinner on the engine and set a "4-cycle" RPM. With a lot of care and, say, a leather glove, squeeze the spinner just hard enough to cause an RPM drop. It WILL go into a "2-cycle" burning mode! And, it will drop back to the "4-cycle" mode when you remove the load.

If you still don't get it tach the 4-cycle RPM, then the 2-cycle RPM you caused with the added  drag...

Older engines that act like this do not have the broad range in 2-cycle that modern engines have. Recent (i.e., recent decades)
engines will not shift firing mode as easily at lower RPM because they 'fire' cleanly at such lower RPM. Trying to get modern - usually Schneurle ported - engines to run in "2-4" merely forfeits power across a range with better, more consistent power that's available - IF you use it. 

[Dan McEntee]

   Well, when I typed my answer, it was early, and I forgot to add in engine load as a factor !!  I usually use the chain saw analogy. If you watch some one tune a chain saw that knows what they are doing, it will usually sound like crap as they rev it, until they put the chain into some wood, then it cleans right out and sings. The same for a dirt bike engine but not as pronounced. Sitting still it isn't too clean but when you come to a hill it should clean up and pull nicely if it's jetted and adjusted correctly. My defense for the effects of gravity are the old stand by test, where if the engine is running with the model held level in a four stroke mode, then the nose of the model is raised, it will lean out and break, and then if the fuselage is held level again, it will go back to it's previous tone. There is no real change in load during this test, just a change in gravity affecting the fuel flow.
   Type at you later,
   Dan McEntee

[Brett Buck]

My defense for the effects of gravity are the old stand by test, where if the engine is running with the model held level in a four stroke mode, then the nose of the model is raised, it will lean out and break, and then if the fuselage is held level again, it will go back to it's previous tone. There is no real change in load during this test, just a change in gravity affecting the fuel flow.

   That's true, but that is only because that is the only thing going on. This effect gets swamped by load effects. For example, a typical 4-2 break engine set normally, say, a Fox 35, you set just into a two-stroke on the ground, then when you release it, it goes into a 4-stroke. With a conventional engine and a uniflow tank, the fuel pressure at the needle is lower in level flight that it is on the ground, but it still goes into a 4-stroke. With a suction tank, the pressure goes *up* in the same situation, it still goes from 2-stroking to 4-stroking. Another example is flying around at 60 degrees - the fuel pressure is higher than level flight, but goes into a 2-stroke.

    Go into a corner, the pressure goes up dramatically as the airplane slows down, but the engine breaks into a 2.

    Brett

[L0U CRANE]

...and then, there's my old wig-wag ground check of the "tank height..."

It's crude simulation of the direction and location of the forces on the maneuverable CL model in flight.

Set things up so that you can start the engine and hold the model with fuselage centerline level and wings vertical, outboard tip down. Start the engine. set a 4-stroke (for older engines) or a medium high RPM for more recent engines. (For modern schneurles, it's likely better to note that RPM.)

Roll the model roughly 45° either left or right. Listen for an RPM change (for the oldies) or tach any change from the initial  setting. Then roll it to about the same angle to the other side. Note the sound or RPM change, if any.

The engine will go "richer" on the side  where the tank is "higher." Confirm this by checking for a "leaner" response at the opposite  rolled attitude.
set the change in your mind.

Stop the engine -rolling it outboard wingtip up should uncover the fuel pickup ,and the engine should stop promptly. Make an adjustment to tank height towards reducing the difference in the rolled attitudes.

Repeat the test, more than once if necessary, until there's minimal RPM difference.

This will not be a perfect solution, but it will be close enough that you can make initial flights without risk of starving or  flooding the engine off. Final height dialing is,  as ever, based on getting the run equal in inverted and outsides   and upright and insides,

I've done this for years, and it is a decent start.

[Paul Van Dort]

I am also convinced that more load makes the engine want to run leaner, less load will make it richer. This has to do with mister Bernouillie and the venturi effect. Bernouilli states that the pressure changes with the square of the air velocity in the venturi. When the load drops, the engine speeds up some, pumping more air trough the venturi, increasing the rate fuel/ air. Causing a richer condition. When the load increases, the opposite occurs. the rate fuel/air will decrease and the engine will run leaner. This whole concept gives stability to our engines. This principle allows to set a steady RPM with the NVA.

This principle is also causing an important issue in stronger wind. We all know that in level flight, going into the wind, the engine slows down  (richer condition)and with the wind in the back, the engine speeds up (leaner condition) . This is also noticable with muffler pressure, so ram air effect on the fuel is not causing this. We need to understand that a model turning into the wind experiences this condition as LESS load on the engine. (This contra intuitive) The relative speed of the model to the air is increasing, so the engine  speeds up and runs richer. This one of the hurdles to take in the quest for constant speed relative to the ground .
I can imagine that props with non linear pitch over the diameter create less load variations back to the engine, but that is something I need to figure out a bit better :-) 

Any hints towards reaching the constant speed over ground are more than welcome :-)