Luiz,

On a Fox Stunt 35, I like cylinder port Exhaust duration of 230°, Bypass duration of 220°. This seems to work well in the 9,000 to 10,500 RPM range, on this engine. "Blowdown" is the total 'time' difference between how long (in degrees) the exhaust and bypass ports are open- just an accepted definition. It is twice the 'lag' time between port openings or closings. 

These numbers allow a longer period with the cylinder closed - extracting power from the combustion - than many modern engines use. Also, the Bypass(es) open sooner after the Exhaust, so they stay open longer than with typical 'sport engine' timing - so a longer time to move the new charge upstairs to the combustion chamber.

We can't do much about shaft inlet timing unles we are into racing or combat. Even there, I think the current engines are best left as built; they're built for those jobs. Messing with that expertise would require more expertise than the guys who make them...

Determining what the stock engine's timing is is not difficult, but certainly involves a few steps. MAKING a change to set in "my" numbers, or any others that are different from the stock numbers, requires care. Too easy to ruin a part unless you are certain of what you are doing...

Mounting a degree wheel to the shaft is a crude way to check sleeve timing - there's always some slack in the rod bearing. I use length-type measurements using a vernier caliper good to 0.001" or better. I have a spredsheet I wouldn't inflict on anyone else to analyze the measurements and find how much to 'adjust' them for what I want. Details get a bit,well, detailed, so - another time if ever, ok?

Lou and Brian, very gratefully for the answers.
 
Brian, when you mention for Super Tigre 51 stunt, for example, the numbers 70/60, are you meaning that the exhaust timing from BDC normally is between 70° and 60°, or that the exhaust timing is normally near 70° and transfer time normally near 60° ?

By the numbers you gave, we see that the antiques stunt engines had significant shorter exhaust timings than the moderns. Does this explain the more significant 4-2-4 performance of the antique engines? Could we generalize this?

Thanks, Luiz.

Luiz
Sorry if I caused a bit of confusion there but the 70/60 (or whatever) figures I used was just my shorthand way of giving both the exhaust and transfer timings so the 70 was exhaust opening time (or 140 degrees total) and 60 was the opening time for the transfer port (or 120 total). The difference between them indicates the blowdown period (in this case, 10 degrees). I use that method for timing because virtually all the exhaust happens between when the port first opens and BDC and similarly for the transfer. Personally I'm not convinced that timing is the be-all and end-all of a good 4-2-4 break because flying a (dark side) model with a Rossi 45 had a beautiful 4-2-4 break doing things like square loops and eights . What I found interesting with my two Enya 35's was the considerable difference in timings between the two of them. Their crankshaft timings were exactly the same so it showed a running change was made at some point in their production and given Enyas quality I doubt it was a tolerance stack up of that magnitude .

Lou
I understand your method but that involves a total strip down first so the rod length (centre to centre) can be measured. I find with the degree wheel I get an accuracy of less than 1 degree and will always repeat a reading 3 or 4 times to ensure consistency. I realised you'd made a mistake earlier but I figured you'd pick up on it fairly soon .

Brian, except for specially reworked engines, where an initial teardown inspection would undo the magician's touches, I generally do open and strip an engine. For cleaning as necessary, and for measuring as I need for analysis of stock relationships/consideration of possible timing and other mods (as in partial rebalance of crankshaft), and occasionally reshaping portions of ports and flow paths.

Perhaps more recent engines have less bearing slack than those available when I gave up on degree wheel measuring. ...muchos years ago...

Luiz, first, you notice that Brian's 70°/60° figures from BDC would be the same as 110°/120° from TDC? Sample numbers, possibly, but interesting.

Most "sport" or "sport RC"  type engines I've looked at have had longer exhaust-open periods and shorter transfer-open periods than I prefer for a modest RPM stunt engine. Whether the port has to be raised or lowered to meet the intended timing depends on the dimensions of that sample, although most of the same (e.g., 40FP, 46LA, etc.) engines are closely similar these days.

The longer ex-open period works well in Sport RC flying, usually including some time at RPM well above what we want for dependable stunt flight. Longer 'blowdown' IMHO is mostly a consequence of the 'higher' exhaust port opening edge, and certainly does no harm at those upper RPMs.

There's no practical way to add metal to the sleeve to lower the exhaust port opening edge, so shaving the lower side of the sleeve flange, or of the crankcase deck, is a practical way. Moving the sleeve lower in the crankcase also, of course, moves the bypass port edge(s) the same distance. In so many engines, the as-stock blowdown was more than we wanted, and lowering the sleeve makes it moreso. That's why you often hear that the bypass opening edge needs to be raised to go with the  modified exhaust opening timing.

A bit off the center of this topic - baffled-piston engines famous for their 4/2 runs were often 4-cycling near their max RPM on prop, fuel, plug, load and needle setting. Fox Stunt 35 is a classic example. It did not gain a lot - if any - RPM when maneuver loads caused the mode shift. There wasn't an immediate punch of acceleration, rather, the torque increase helped (a bit) to reduce slowing.
 
...Sounded like a lot more RPM because we could hear about twice as many firing sounds when it went from misfiring about every other time to firing just about every time.

Modern engines "hold power" much more noticeably, even when a low-2 cycle run makes it harder to hear an RPM change.

Lou, thanks once more for the good information about the issue.

Thanks Chris for the Stalkers specs.

Luiz

Hi Luiz,

You may want to take a look here

http://www.mh-aerotools.de/airfoils/hdi_enginetiming.htm

Let me know if you need any help with the Enya 61CX.

Martin

Martin and Brian, thanks very much.

Brian, I am sure that these data will be very useful for all of us.

Martin, I have many and good information about Enya 61 CX, most from you in this Forum, Stuka Stunt Main Forum and in VCC on Line(Brazil).  Thank you for the offer. I will not hesitate in asking you when needed.

Luiz

I think that looking at timing numbers is a fairly limited view, its more about flow.

Agreed Chris, timing is important to some extent, mainly in having lots of torque at lowish revs. I think the reason that any engine can 4 stroke (all the way from our stunt engines to an FAI speed engine) has been pretty well agreed on as being caused by charge contamination but the reason why they can switch into a 4-2-4 is more uncertain. It may be from a very slight change in fuel delivery caused by a manoeuver leaning it slightly or a little extra change in internal heat from added load allowing the contaminated charge to ignite (slightly more advanced ignition?) or maybe both. One thing I do know (from experience) is that an undercooled engine can suddenly go from a 4 stroke to a 2 stroke that won't revert to 4 stroking until given a chance to cool down by flying level for a while. So I think heat plays a major part.

As far as compression goes, what I've found is that it has no real affect on 4-2-4 other than changing the power output in both modes. Low compression gave low 4 stroking power but also low 2 stroking power. High compression gave both modes higher power. For instance, with my Enya 61 RE and the standard head (8.8 compression) it went into a 4-2 switch between 7.8-7.9K revs and peaked at 8800 revs (Bolly 13.5x5 (2 blade) CF prop and zero nitro) but with a HC head button (12.4) the 4-2 switch was at 7.5-8.2K revs with a peak rev of 9500 which equates to an increase in max HP of 26%!

Hello All,
I have been lurking around this thread and soaking up most of the comments. One thing that interests me is the comment that one can go from a 4 cycle mode to a two cycle mode in certain engines and this is caused by an increase in heat within the engine. I have no reason to disagree with that analysis. I presume that this is caused by a difference in timing, to be more precise a difference in the point where the fuel / air charge starts to ignite.
Now I have an interest in old spark ignition engines and here you can prevent this variation in ignition timing that ones gets with a glow engine. You fix the firing point by the amount of advance that you set the ignition advance / retard lever.
One of our resident spark ignition men (I think it was Floyd Carter), said you can get an Orwick 64 to run in a constant 2 stroke or a 4-2-4 style of run. To be honest I was very sceptical when I heard that. But he was absolutely correct, as I found out to my amazement (one should listen to those that have the experience!) So maybe the break from a 4 to a 2 cycle and vice versa, is a little more complicated than I thought.
Apologies if I am talking rubbish or off topic,

Regards,

Andrew. 

"One thing that interests me is the comment that one can go from a 4 cycle mode to a two cycle mode in certain engines and this is caused by an increase in heat within the engine."

That is not happening in the engines I run, in order for that to be true they would have to heat up and cool down in less than 1 second, Heat can/will make an engine go into a 2 cycle, but it will not come back once overheated immediately.  Load and Gravity forces maybe more of a factor than heat,

Randy

At the point of ignition I see that a rapid temp change could easily happen but in the mass that is the entire engine, obviously not.

The most blatant example of ignition point temperature change would have to be a flame out.

Right Chris
BUT, the increase in temperature would be because the mixture leaned when it went **into** a 2 cycle.  A 2 cycle is hotter than a 4 cycle, the exhaust gas temp will rise and the cylinder temp will also.
Point is it doesn't happen in reverse. The engine did not go into a 2 cycle because of heat, it heated because it went into a 2 cycle..leaner mode of running.

A while back I took a 12 inch wood ruler, drilled a hole in the 6 inch center and glue wood shims front and back, put it on a 40 on the test stand, ran it in a fast 4 cycle, when I turn the motor stand up , the engine 2 cycled. there is no extra load or extra heat at that point. When return to level it goes into an instant 4 cycle.
My stand will rotate up, and down.

Regards

Randy

"One thing that interests me is the comment that one can go from a 4 cycle mode to a two cycle mode in certain engines and this is caused by an increase in heat within the engine."

That is not happening in the engines I run, in order for that to be true they would have to heat up and cool down in less than 1 second, Heat can/will make an engine go into a 2 cycle, but it will not come back once overheated immediately.  Load and Gravity forces maybe more of a factor than heat,

Randy

Hi Randy,

Please clear up a misty area for me.  What characteristics concerning timing allows an engine to 4-2 break at maybe 1000 or more rpm than another engine?  Kinda like what happens
when comparing a Fox .35 to an Aero Tiger .36?

Thanks,
Bill

Bill
If you are asking what will make a motor break harder and at a higher RPM, that typically is the difference in blow down period,. or overlap. Also comp ratio can be involved
when you reduce the blowdown period you normally reduce the amount of 2 cycle power above the 4 cycle power, or the RPM difference.

Randy

So an engine with more blowdown will break at say 9800 and an engine that has reduced blowdown will break at say 8800.  Is that correct?

Thanks!
Bill

Randy,

Since with a shorter blowdown you reduce the amount of 2 cycle power above the 4 cycle power, is it easier for an engine that has a shorter blowdown, to break, than an engine that has a longer blowdown, considering that they have the same other characteristics, props, etc?

Luiz

While it will not tell you what "the best" timing is, it will tell you what the timing is. All done with a depth mic.

 Look at  www.doov.com



Double Deuce

A bit late, but ...

The matter of heat causing the mode break is pretty much right, properly viewed.

Our schnuerle engines flow the fresh charge over almost 3/4 of the piston and sleeve circumference. That has to cool better than the not quite 180° of the old baffle piston engines.

The abundant fuel/air mixture not only helps cool and lube those zones, but also delivers more to the combustion chamber for the next burn. So the combustion chamber that exists perhaps 10-15° each side of TDC may heat rapidly, but may also be cooled quickly by the next recharge mixture.

As the piston moves downward, the volume above it quickly leaves the compact, highly compressed conditions in the chamber where the 'burn' occurs efficiently... At our typical RPMs, that efficient combustion chamber may exist for 0.00004 seconds. A lot happens very quickly. If, say, drag loads cause the prop to slow 10% for a split second, that combustion chamber condition will be 10% longer. Not much time, but perhaps significant in relationship to the unloaded duration of ideal combustion conditions. 10% more burn? At least some more heat released and trapped in the combustion chamber for that fractional extra time. Sure, heat; also sure, on loads dropping, quick cooling of that small zone.

So, combuston heat usually doesn't have time to overheat the crankcase, particularly the zones surrounding the bypasses. If THOSE zones do get too hot, you're more likely to trigger a 2-cycle runaway. (Bill Netzeband's studies from when he was helping develop the Testors/McCoy Series 21 engines. I never read these, but he did mention them in conversations, and possibly occasional mag articles.)

Blowdown, remember, is defined as the difference between total port open durations. It isn't the lag between exhaust port opens and bypass port opens - it is double that time in degrees.

Hi Lou

There is much more to it than "heat" The attitude -gravity-G-force fuel head etc has a dramatic effect.
You can do so much to affect/change the  break in the exact same engine by changing C/R , Venturie size, timing, needle settings, etc
The heat cause the 4/2 break is not what is going on. Take a flat wood prop with no pitch "zero" , and install on a motor, turn the engine straight up and then straight down, there can and was a dramatic difference in the "break" and there is no difference in load to heat or cool the motor, What seemed to be happening was gravity acting on the fuel tank. The test I did also showed another big difference if the tank was moved farther away from the engine.

Randy

Is there a definitive text about 2 stroke timing?

It's a subject I have an interest in, but would like to get to some published info so that I can demystify & get less opinion on a 4- 2 break (something I don't need at present)

Regards

Greg

Is there a definitive text about 2 stroke timing?

It's a subject I have an interest in, but would like to get to some published info so that I can demystify & get less opinion on a 4- 2 break (something I don't need at present)

Regards

Greg

There is no definitive text about the correct way to make a model airplane stunt engine

Randy

Chris,

Thanks mate that would be great.


Regards

Greg

Glad to help, I will bring them along with me next time I ( pester you lot) come to the field.

Chris,

Thanks mate that would be great.

Randy

I don't need to know about a stunt run just now , thanks

Regards

Greg

Greg 
then what you want to get is Gordan Jennings book on 2 strokes

Regards
Randy

Finally back to the keyboard after mangling my left ring finger nail... (Don't ask...)

Several answers come to mind on recent comments...

Definitive 2 Stroke Manual: There is Gordon Jennings (I think) "Two-Stroke Tuners' Handbook" on-line somewhere. It is dated - published about 1974 - and related to gasoline-burning 2-cycle motorcycles - and in mostly metric measures, but goes into more detail than most of us really want. Excellent section in it on designing tuned pipes, it just needs adjustment for the different heat of gasoline and methanol exhaust gases.

Chris, yes, I'm sure that blowdown is the difference between port total open timings. That was the definition in a few engine theory courses I had in HS in the 1950's. Wrote something in a Stunt News wa-a-a-y back, and GMA corrected me on it. I had referred to opening/closing lag as blowdown, while I DID know better, it just slipped my mind ..

Randy, of course there's more than just heat involved in the 4/2 and 2/4 break. As you mentioned, loads have a lot of impact. I mentioned the extremely brief time duration of our ideal combustion chamber conditions - which is even briefer with a severe squish&bowl chamber. After that very few hundred-thousandths of a second, contained volume increases rapidly and burning mix contacts the cooler cylinder wall.

... consider maneuvering loads: Induced drag results from lift, and its drag coefficient changes with the square of the Lift Coefficient. Straight, low, level flight (1g of lift) needs only a very small Lift Coefficient, so the Induced Drag Coefficient is also quite small.

... Round loops take about 10g lift for the shape, which varies around that by the height, angle and bank of the wing relative to gravity's 1.0g (always straight down.) That's 10 times the lift, ± about 1g. Induced Drag Coefficient increases to about 100 times its value for 1g level flight. Even as small as it may be in level flight, that's quite a change in drag. I figure the engine slows some, at least at the very start of the figure.

... It probably regains much of any RPM loss in the course of the figure, but at the very start, there is a significant drag load. If the prop RPM is slowed 2% to 5% for those few hundred-thousandths of a second when the combustion chamber is at its best conditions, burning fuel there has a significant % of longer burn time.  With a baffle-piston engine, that can help trigger the mode-break. For modern schneurle engines in low-2, it isn't as easy to hear the effect.

.... Squares, per Bill Netzeband, who studied them for decades, may need up to about 20g lift for the fraction of a second they take. If pure, clean aerodynamics were in play, that would mean an induced drag jump of 400 times  the unloaded level flight value. I doubt we fly square corners in clean, aerodynamic conditions. (Stop motion studies by Igor Burger; photo of Brett's stunter vertical about one fuselage length above 5' level flight altitude). Even so, there's a lot of drag in a square corner - ever feel the slam of air near such a turn?

Also, flight is a dynamic (-moving-) condition. Things like static fuel draw don't really fit. Sure, lower the tank several inches and the engine leans out because it has to draw fuel up to the spraybar. To simulate the prop slowing effect on the run, carefully, with a glove or rag squeeze the spinner lightly while the engine is running 4-cycle. Load like a maneuver drag load, get it? The engine WILL switch to 2-cycle, while being slowed, right? And will drop back to 4-cycling when you let go.

These aren't the only things involved, of course, and you mentioned several of the others. We are fortunate that we can fiddle with prop, fuel, plug, setting, load, to find useful combinations,if we make the effort...

Chris, thanks for the compassion... (It actually hasn't hurt much yet, perhaps the pain nerves went with the 80% of the fingernail.)    Doctor says there's a good chance of regrowing the entire nail!

The bandage does hit the turn signal lever. Sometimes with a sharp reminder to NOT do that.

On the definition of blowdown, one of the texts in my HS courses was from the US Government, and dealt with much more than the esoterics of sleeve, or poppet, valve "timing." Opportunity providing, I'll reread the references you cited. (I do agree that for sleeve-timed 2-cycle engines, lag is a much more significant factor. It does, however, affect both ends of the 'port open' condition. That leaves some validity to comparing TOTAL times.)

George Aldrich had, apparently, also studied from the same definitive sources - and it was my oversight that called "lag" "blowdown."

A thing I've observed, and mentioned here, on SSW (if I may refer to it), and other fora (proper plural of forum.) --

There can be a break to 2-cycle on the turns down, in a square figure. Most noticeable, if you are ready to hear it, for classic 4/2-2/4 break engines. No other explanation occurs to me except the rapid drag increase at the sharp turn. Again, the "steady" sound of "constant-2" engines masks this...

...Bandages MAY all be off early next week! I have engines to bench... Hallelujah!