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Author Topic: Four Stroke Happens - Why?  (Read 946 times)

Online Peter in Fairfax, VA

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Four Stroke Happens - Why?
« on: September 23, 2021, 09:27:56 AM »
What is the theory on why a two stroke model engine will change its tone to four stroke when rich?  No practical reason for asking, just curious.

Online Dan McEntee

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Re: Four Stroke Happens - Why?
« Reply #1 on: September 23, 2021, 09:48:17 AM »
  Well, my simple understanding of it is that when the mixture is so rich that it can't completely burn the fuel/air charge, and some times that excess can ignite and pop on the way out of the exhaust if you have an open stack. The sound and the lower RPM makes the engine sound similar to a four stroke engine, where there is a power stroke every other cycle of the piston. When the mixture is leaned out and it cleans up, and the RPM is much faster, you hear the explosions closer together. In some instances, at the right mixture, compression ratio and such, it misses the power stroke and accents this. You don't see this with a glow plug engine but can with something that is spark ignition. The old fashioned way of describing a two stroke run is the usual "Ring, ding, ding, ding" sound, and I have heard four strokes described as "Suck, squeeze, bang, blow!!"  There are some old motorcycle engines out there that sound just like that!! 
   That is my simplistic may of understanding it.
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Offline Mike Alimov

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Re: Four Stroke Happens - Why?
« Reply #2 on: September 23, 2021, 11:06:10 AM »
Brett summarized this in the large prop thread:
"This is the cause of the 4-2 break in vintage engine systems. It's running along in level flight with a mixture that is excessively rich, to the point it misfires on alternate cycles (4-cycles). Put a load on it, and that slows the rpm enough that it has enough extra heat to fire even the rich mixture every time - breaking into a 2-cycle. "

Online Brett Buck

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Re: Four Stroke Happens - Why?
« Reply #3 on: September 23, 2021, 12:32:26 PM »
Brett summarized this in the large prop thread:
"This is the cause of the 4-2 break in vintage engine systems. It's running along in level flight with a mixture that is excessively rich, to the point it misfires on alternate cycles (4-cycles). Put a load on it, and that slows the rpm enough that it has enough extra heat to fire even the rich mixture every time - breaking into a 2-cycle. "

  All gleaned from the classic Scott Bair experiments and paper on the topic (and some minor additional testing and analysis by me - a minor expansion of Scotts seminal work).

    This should be required reading for anyone interested in how things work in detail - why it 4-strokes, why it breaks, and why it doesn't suddenly have twice the power when it suddenly starts firing twice as often (each individual firing in a 4-stroke is *much stronger* than almost the same RPM and mixture in a 2-stroke). All very basic notions that you should know about, even if you aren't running a 4-2 break engine.

      Brett

   

Online Peter in Fairfax, VA

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Re: Four Stroke Happens - Why?
« Reply #4 on: September 23, 2021, 08:21:55 PM »
That paper does sound interesting.  Is it available?

The part I am not getting is how the engine recovers from a misfire.  If it misfires once, what changes that allows it to fire again?

Online Dan McEntee

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Re: Four Stroke Happens - Why?
« Reply #5 on: September 24, 2021, 07:28:42 AM »
That paper does sound interesting.  Is it available?

The part I am not getting is how the engine recovers from a misfire.  If it misfires once, what changes that allows it to fire again?

       On a C/L model, once the nose goes up, gravity has it's effect on fuel flow and it leans out a bit. The load increases on the engine also and causes it to heat up a bit more clean out and fire accordingly. After the nose comes back to level, the previous fuel flow returns and if the engine cooling is sufficient, so does the 4 stroke mode. Depending on conditions, you can sometimes hear the engine go back to a rich condition on the down hill side of a loop when gravity helps out again with fuel flow.

     Watch some one that knows what they are doing tune a chain saw carb. While running static on the work bench, it doesn't sound clean and will be four stroking. But get it revved up and cutting a log, it will clean up and sing a nice tune. I tune my two stroke dirt bikes the same way, so they clean up and run strong when loaded up climbing a hill or pulling some high RPM on a long road stretch in high gear. It's no fun having an engine seize when you are 100 miles from your truck!

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Online Brett Buck

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Re: Four Stroke Happens - Why?
« Reply #6 on: September 24, 2021, 09:36:38 AM »
That paper does sound interesting.  Is it available?

The part I am not getting is how the engine recovers from a misfire.  If it misfires once, what changes that allows it to fire again?

  If it misfires, no exhaust is left behind, and that entire stroke is used just to scavenge out the exhaust remaining from the previous firing. Next time, you are starting over again from nearly scratch. This also explains why the two-stroke firings at the same mixture are much less powerful than the 4-stroke firings - when you are just lean enough or loaded enough to firing every time, lots of exhaust it left from the previous firing, making the current firing partially pure charge and partially old exhaust. The difference in the pressure is very large, 50% difference or so.

   That's also why 4-strokes (that is, valved engines that can only run in a 4) are so much more efficient, run hotter, and have extremely powerful power strokes. In a proper 4-stroke, you have an entire stroke used just to clean out the old exhaust, which does a very good job. On a 2-stroke, a fair bit of the incoming charge just shoots out the exhaust, and, you are just hoping most of the exhaust products are driven out when the exhaust port opens. At least some of it is still hanging around when the port closes, meaning it gets trapped and you are not firing a clean charge every time.

    When you finally get the load or mixture low enough, it doesn't fire every time, meaning you get nearly nothing on that stroke* but you do force almost all the old exhaust products out, meaning the next one fires very strongly even though it is too rich to be ideal. Rich enough or low enough load,  and it misfires on the next firing, and all that stroke does is add some scavenging.

   Note that both of the run modes with 2-stroke engines end up doing things very inefficiently in terms of fuel consumption, which is why they tend to use  A LOT more fuel, and, why changing the various scavenging features can have a huge effect on the power, efficiency, and running temperature. And why you nearly never see 2-strokes in motor racing aside from motorcycles, despite the fact that they are generally *much* more powerful for their displacement than 2-strokes. (typically 50-100% more, that is, *twice the power for a given displacement*). 

      Brett


*but not entirely nothing. After looking at Scott Bair's cylinder pressure data, it seems like the "dead" stroke on a 4-stroking 2-stroke engine actually adds a bit of energy/RPM - that is the compression stroke slows the prop down, the expansion (with no firing, mind you) speeds it up more than it was, slightly. My interpretation is that this engine is stealing some of residual heat to the unfired charge, like a Stirling engine. It's not free, because it also cools the rest of the engine and causes the next actual firing to be slightly less energetic.
« Last Edit: September 24, 2021, 10:33:16 AM by Brett Buck »

Online Brett Buck

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Re: Four Stroke Happens - Why?
« Reply #7 on: September 24, 2021, 09:44:57 AM »
       On a C/L model, once the nose goes up, gravity has it's effect on fuel flow and it leans out a bit. The load increases on the engine also and causes it to heat up a bit more clean out and fire accordingly. After the nose comes back to level, the previous fuel flow returns and if the engine cooling is sufficient, so does the 4 stroke mode. Depending on conditions, you can sometimes hear the engine go back to a rich condition on the down hill side of a loop when gravity helps out again with fuel flow.

   I would note, without belaboring it again, that the load effect seems to be *much more powerful* than the mixture changes in many real-life situations, with the engine going from 2 to 4 or vice-versa following changes in loading/airspeed,  despite fairly big changes in the fuel pressure that would suggest the opposite. There are plenty of places in the pattern where the engine goes into a 2 when the fuel pressure is going up.

     Also, there are plenty of examples where adding load makes it go from a weak 2-stroke to a strong 2-stroke based entirely on the load, and opposite direction the fuel pressure variation. If it doesn't, the engine *is not operating properly* for stunt. *Something* has to make those firings stronger, and it's not the mixture.

      Brett

Online Dan McEntee

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Re: Four Stroke Happens - Why?
« Reply #8 on: September 24, 2021, 10:14:59 AM »
   I would note, without belaboring it again, that the load effect seems to be *much more powerful* than the mixture changes in many real-life situations, with the engine going from 2 to 4 or vice-versa following changes in loading/airspeed,  despite fairly big changes in the fuel pressure that would suggest the opposite. There are plenty of places in the pattern where the engine goes into a 2 when the fuel pressure is going up.

     Also, there are plenty of examples where adding load makes it go from a weak 2-stroke to a strong 2-stroke based entirely on the load, and opposite direction the fuel pressure variation. If it doesn't, the engine *is not operating properly* for stunt. *Something* has to make those firings stronger, and it's not the mixture.

      Brett

      That is why I included the chain saw example. Most modern two stroke lawn and garden equipment use some sort of crank case pressure to the tank to help with fuel feed in abnormal positions. Also, how many times have you seen some one just raise the nose of a model after setting the needle to see how it breaks? It's a part of the whole scenario, and I think, depending on the actual needle setting and how rich the engine really is, can initiate the process. The whole set up is really a pretty fine balance of conditions.
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Online Brett Buck

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Re: Four Stroke Happens - Why?
« Reply #9 on: September 24, 2021, 05:06:33 PM »
Quote from: Brett
*Something* has to make those firings stronger, and it's not the mixture.


It has to be turbulence, or more precisely the variations in the magnitude of turbulence.

   It's heat, probably radiant heat, from the hot engine parts (like the glow plug) into the charge, slow it down, more heat, so it fires sooner than it would have otherwise, effective advancing the timing.

      Brett

Online Peter in Fairfax, VA

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Re: Four Stroke Happens - Why?
« Reply #10 on: September 24, 2021, 05:59:54 PM »
Makes sense that after a misfire, the exhaust of the previous firing can be well scavenged, allowing for another firing with less previous exhaust.

Online Brett Buck

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Re: Four Stroke Happens - Why?
« Reply #11 on: September 24, 2021, 08:16:10 PM »
That paper does sound interesting.  Is it available?

The part I am not getting is how the engine recovers from a misfire.  If it misfires once, what changes that allows it to fire again?

    Randy probably has a scan. I have a OCR  and cleaned up electronic version that I need to assemble and convert to .PDF for compatibility. I submitted as a article for the PAMPA web site, but I cannot find it now.

        Brett

Online Brett Buck

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Re: Four Stroke Happens - Why?
« Reply #12 on: September 27, 2021, 11:30:42 AM »
If you look at succesful stunt engines, they tend to have a built-in inefficiency in the scavenging. That is, ”dead” zones caused by either turbulence or porting geometry.
That way you have an unburned stock of mixture in cylinder that either does or doesn’t participate in combustion, depending on scavenging pressure, -speed or heat radiation, as you suggest. L

   That doesn't explain why virtually any 2-stroke engine can be induced to run either apparently "cleaner" or actually transition from 4 to 2 or back, just by varying the load, with no other factors changing - fuel pressure, etc, - even to a degree on spark plug engines where the spark timing is exactly the same.

   BTW, schnuerle engines do perfectly good 4-2 breaks. It's just that most of them do if *far beyond any usable rpm* for a stunt airplane. Unless you use the pipe to alter the operating range with a tuned pipe.  The PA or OR-Jett series, with just a muffler, do perfectly good 4-2 breaks with port angles and dimensions that could not be more different - *gigantic and very deep with steep angles* on the PA, and ports that look like mail slots that have very shallow angles on the RO-Jett. That appears to have dramatic effects on the run symmetry and the internal ballistics, but it certainly will chug around  at 8500 and transition back and forth reliably.

    No one competitive does that, because running along at 8500 with a 13-6 hasn't been competitive for 30 years - not because it won't do it.

    Brett


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