lablover

Just got a new OS 55AX and been reading about breaking in...WOW, info overload. I'm pretty new the the hobby and am reading something about 2 strokes four stroking. Not sure what that is or how it happens.

I tend to read the manual and consider it gospel....Now I see couterdictions from OS on the FAQ of their website..Anyway, different subject

Can someone explain to be what 4 stroking a 2 stroke is..What it may sound like etc.

I have several four stokes that I broke in the way the manual says..But this is my first 2 stroke MAy have 2 of them in my B-25.........

Thanks

Joe

EDIT
I may have put this in the wrong forum..Mods please move if I did

blw

My Feedback: (3)

Joe,

A two stroke engine fires every time the piston comes up to the top of the cylinder. Firing at the top is one stroke, and reaching bottom is the second. When you get the engine set rich enough, it won't fire every time so they call that 4 stroking. That is the story in a nutshell.

You will be down a lot on power when it is set rich enough to 4 stroke. Lots of smoke. You can hear the change.

Stick to the manual and you'll be okay. You pick tidbits and techniques along the way. Sounds like you are doing fine.

jeffie8696

Like he said.
It is just jargon descibing an very rich needle. Iusually set my needles on the very edge of four stroking for breakin flights and afterwards tune with a tach to 200-300 rpms rich from peak as you are probably already doing.

mighty9

My Feedback: (10)

[youtube]http://www.youtube.com/watch?v=0wzph3hL6EE[/youtube]

Konrad

Often one will read about the 4 cycle to 2 cycle break. This can be very difficult for a beginner who has little or no experienced help to understand. I have also seen many instruction use the terms but offer no explanation as to what it is. What follows is my attempt to explain the phenomena.

The 4 cycle to 2 cycle break is a magical thing.

When a 2 cycle engine is running so rich the exhaust gas is traveling so slow that it can't all leave the cylinder. This leaves exhaust in the cylinder contaminating the inlet charge for the next cycle resulting in what is known as oxygen deprivation. This contamination is so bad that the cylinder will not fire on the upcoming power stroke. So on the next inlet cycle this "contaminated" unburned charge is pushed out by the next inlet charge. This inlet charge scavenges the diluted/contaminated charge enough that it will burn on the power stroke. This looks sort of like a four cycle (one power stroke for every other rotation of the crankshaft).

Now as one leans out the mixture the velocity of the exhaust gets high enough that it scavenges the cylinder well enough that the inlet charge will burn with each power stroke 2 cycling.

As an engine tuner this happens rather abruptly. Just by leaning out the mixture the engine will appear to suddenly develop a lot more power (rpm). It is usually best to break in an engine in the rich 4 cycle mode just prior to the 2 cycle break.




Konrad

coriolan

One of the first engine to achieve that was the FOX 35 for C/L stunt. The 2-stroke /4-stroke set-up allowed the engine to run rich when level and going down and lean when going up in a figure.
Kind of a automatic speed control very usefull in C/L stunt since the speed remain the same during figures and permit precision and more points Engines with baffle lend themselve better to that setting than modern flat piston schneurle. A alternative now is to use piped engines, were the pipe act as a regulator. Its not really needed in r/c and would interfere with trottling the engine.

w8ye

My Feedback: (16)

modern day Schnurle ported two strokes will not hardly four stroke

The old baffle piston two stroke engines from 40-50 yrs ago had a two stroke - four stroke break point up near to their peak rpm.

Konrad

Yes, the schnuerle ported engine is much more efficient at scavenging the exhaust byproducts from the cylinder (no dead spot behind the baffle).

"The old baffle piston two stroke engines from 40-50 yrs ago had a two stroke - four stroke break point up near to their peak rpm."

Can't say I agree with that statement. Many engine designers thought this was a desired trait (such as that found in the Fox 35 stunt). But as one started to strive for more power the blow down period was increased and that statement became much less relevant. Many of the higher powered Super Tigers would have a long 2 cycle power band.



Konrad

downunder

Pretty much what Konrad said. Even when fully peaked out (leaned out) there's still some small residual exhaust left in the cylinder which contaminates the next lot of fresh mixture so when the mixture is made richer the amount of contamination slowly gets more and more as the needle is opened. Eventually the contamination gets so bad that the plug can't ignite it so the engine misses one firing stroke. When the exhaust port then opens, most of this charge escapes allowing a fresh charge in which of course is well suited to being ignited giving a power stroke. The cycle then repeats and the engine is 4 stroking. This isn't what's described in that video unless by "gas" he means residual exhaust gas, but I doubt it seeing that Americans refer to petrol as "gas" .

The curious thing though is that the fresh charge that can be ignited is less contaminated with residual exhaust gas than the normal 2 cycling charge so it actually gives a larger rise in pressure which means the 4 cycling power stroke is greater than the usual 2 cycling stroke. You'd think that if an engine only fires on every second stroke then it would only have half the power it would normally have but in fact it has about 70% the power which is why there's not a great difference in revs when an engine slips between 4 cycling and 2 cycling.

If the engine is running quite rich and 4 stroking the exhaust sound is lower, something like a Harley, but as the mixture gets leaned out there'll come a point where you hear the normal 2 stroke higher pitched scream which is what you can hear in that video. It's fairly easy to set the mixture to a point where the engine will be undecided if it wants to 4 or 2 stroke so it'll switch between the two at a fairly regular interval and that's what's called the 4-2 break point. That's where I tune an ABC type engine for its initial runs when running in.

If you go [link=http://www.holdfastmac.com.au/instanttorque.html]here[/link] you'll find a graph of the actual torque produced by an ST 46 running in a 4 stroke and you'll see just how regular the 4 stroking is (note that this torque is before any internal losses due to friction so it's not crankshaft output torque). If you click on the link at the top to Instant RPM the next graph shows how the propellor changes speed according to torque produced. I had to get permission from the originator of these graphs (Brett Buck) to put them on my web page which is why I can't actually reproduce them here.

w8ye

My Feedback: (16)

I was a control line guy back then and the Fox 35's, McCoy red head stunt 35's, OS Max control line S35's, Johnson Stunt Supreme 35's, all had a nice 2-4 break as described.

I didn't like the Super Tigers for stunt except the Super Tiger 46. It didn't have a 2-4 break worth anything but had enough power to fly nicely in control line.

The good stunt engines would four stroke in level flight. If you put the nose up, they would break into two stroke. Nose down and they would four stroke. Worked very well to keep the speed steady

The Super Tigers would run away on you and go lean. Didn't seem to hurt the engine but they were terrible for control line stunt except for the ST G21/46. The G21/40 would run away....

w8ye

My Feedback: (16)

Downunder, cannot get your page to load tonight but very interested in the G21/46 chart

downunder

Seeing there's some talk about baffled piston v Schneurle, a baffled piston engine hasn't got as good scavenging so there's more chance of charge contamination which is what's needed for 4 stroking. That said, I've never yet found a Schneurle ported engine that won't 4 stroke, in fact I set up my Rossi 45 to 4 stroke in level flight but break into a fast 2 stroke as soon as it went into a loop or steepish climb. It had a lovely 4-2-4 break . In general, modern engines will 4 stroke up to around 2-3000 revs below peak. Run rich enough they'll go down to about 6000 before getting unstable enough to cut out (without power to the plug).

(Edit)
Jim, I had the page up while I was typing that post and saw your post after I put it up so I checked. Well ****** me, seems the site has gone down for a while .

Konrad

And as most here are now R/C fliers the ability of an engine to change power states by aircraft attitude is NOT a desired trait. So if you set up an engine for C/L you might choose a cylinder timing that has a very short blow down period. This means that the ability to scavenge the cylinder is rather narrow. In an R/C engine you want the carb to control the power output NOT the exhaust velocity (heat). So most R/C versions of the same C/L engine will have a longer blow down period (higher exhaust timing).


Konrad

downunder

The only engine available that comes in both RC and dedicated CL forms is the ST G51. In CL form its exhaust is 70BBDC while RC form is 74 or so. AFAIK the transfer is 60 in both versions so blow down is 10 for CL and 14 for RC. My Rossi 45 has a blow down of only 9 degrees (75/66) but an early round head ringed ST S45 has 20 degrees (80/60)!

However I agree that for RC flying the throttle should be used but I'm so used to CL flying (and so bad at RC ) that I tend to set the engine where it leaves me with less throttle fiddling while I concentrate on other controls .

w8ye

My Feedback: (16)

Modern control line stunt no longer is concerned with the 2-4 break.

Even the lowly discontinued OS LA 40 C/L has much more power than the old famous 35 stunt engines of years past. They are not used in the 2-4 break mode.

Four stroke Saito stunt engines are always going to four stroke.

Now it is done with the larger diameter flat pitch props like the APC 12 1/4 X 3 1/4 prop that was actually brought out for control line but often used by the 3D R/C guys

coriolan

downunder looked at your site, very interesting stuff!
http://www.holdfastmac.com.au/tech.html

Lnewqban

Joe,

Running a 2-stroke model engine in its "4-cycle" mode harms nothing.
It only increases fuel consumption, and reduces power output somewhat, but it reduces operation temperature and lengthens engine life.

Many pilots adjust the needle valves of two-stroke engines for maximum, running them fast and lean.
However, they wouldn't run their automobile engines that way.
Model engines are no different.

MJD

My Feedback: (1)

As a blanket statement this is just not so. There is no better way to bugger up the piston/liner fit than to run a tapered bore engine too cool.

It is true that no engine benefits from being run over lean. But there is a difference between setting engines to max rpm on the ground so they cross the "lean/overlean" threshold in the air where power diminishes, temps rise rapidly, and lubricating oils break down - versus setting the needle properly so the engine runs in a clean fast two stroke in the air without going over lean.

Tapered bore engines wear more quickly when the operating temperature is too low, due to interference between the top of the piston and the cylinder wall - this is a fundamental issue with modern ABC/AAC engines. They require proper operating temperature so that the sleeve expansion results in proper operating clearance. This is why you don't run them in a rich four-stroke during break-in - you are causing more harm than good. Loosely fit general purpose sport engines may not suffer as badly, and many are operated this way in the field, but that doesn't mean they have not played havoc with the P/L fit.

Ringed or lapped engines don't care, true, they run cooler and gets lots of lube, and this is generally how they are broken in. But which type is the vast majority these days?

The 4-stroking phenomenom is fully documented, I think it was Dave Geirke who bench ran an engine with a pressure transducer to plot cylinder pressure to prove that the engine was truly firing on alternate strokes, for a MAN article. Maybe others have too. At the break between 2 and 4 stroking, the exhaust note appears to change by an octave - because by definition an octave is the interval we hear when the frequency doubles or halves. i.e. an octave above A=440Hz is A=880Hz, an octave lower is A=220Hz.

Lnewqban

Agree about the lack of need for running a two-stroke on "four-stroke" mode as a regular practice, at least in RC.

Disagree about the dangers associated to eventual "four-stroke" mode; there is nothing to get alarmed about when it happens.

Actually, I have broken-in all my engines overrich and cool, and they have served me well for years.

This article describes a test performed regarding the mythical harm of "four-stroke" mode on two-stroke model engines (read the long version if you have time):

http://www.holdfastmac.com.au/Rich%20ABC6.html

Konrad

A funny story. In the racing classes that required stock engine there was one guy who always had engines that were a little stronger than mine. One day I just needed to know "How could this be possible"? I asked him and he said he followed the direction. So I went home and read the directions to my ABC Rossis and there it was, run it rich during break in. As these use Hydrodynamic principles they will be fine as long as the glow plug insulators don't blow. It is a bad idea to turn over the engine without the glow plug for this very reason. (Do NOT turn over an ringless ABC engine without the glow plug to clear a flooded crankcase!)



Konrad

gkamysz

My Feedback: (19)

This is Brian's (downunder) article if I'm not mistaken.

downunder

You're not mistaken Greg.
Actually I was prompted to carry out that experiment after a new guy made a post here in RCU saying he'd just run half a tank through his LA in a 4 stroke and wondered what to do next. Someone who shall be nameless told him he'd just ruined his engine and might as well throw it away and buy another one. We never heard from that guy again and he's probably collecting stamps now. It never sounded right to me but because I had nothing to back up my thoughts I decided to find out once and for all as soon as I had a chance. That came when I bought a new ABC very cheaply off Ebay and the rest is history but it certainly caused a stir when I wrote about it in RCU .

MJD

My Feedback: (1)

How tight is the P/L fit on a new Blue Bird .51? Is it a higher quality engine with a close piston/liner fit designed for high performance, or a run of the mill sport engine set up so the average guy can run it out of the box pretty much how they want?

I said in so many words that it is probable that a large number of sport engines are not set up so tightly that this is really such an issue - and I have seen comments from Clarence Lee et al along the same lines. But, again, as a blanket statement I still do not agree. On another RCU forum a modeller complained that his new Jett was not performing up to expectations, then explained his slow, rich break in procedure - in stark contrast to the manufacturer's instructions. He mentioned that it had loosened up nicely, but was spinning a 10-6 APC, as I recall, around 2000rpm shy of the norm, and these are engines that are proof-run to rpm specs at the mfg before shipping. In this case he had essentially ruined the p/l fit due to cool running.

It is probably safe to say that, for a large percentage of the current sport engine crop, i.e. modest performance engines, it's really not such a big deal. But I'd have to see some quantified testing on more than one engine to be convinced that it is safe to do this on any tapered bore engine.

In the end run 'em how you want, but I've treated all my better engines - Jett, West, OPS - per traditional ABC practice and they are still tight and run like demons after many flights.

Sport_Pilot

Most modern sport engines don't have enough taper for this to be a problem. OS manuals (includingABN) recommend a brief four stroke during break in, from the amount of pinch in OS engines I would tend to believe them.

MJD

My Feedback: (1)

For sure, there is a tremendous amount of difference in TDC pinch between a new OS sport engine and a high performance sport engine like a Jett or West .50 which are definitely "squeaky". Tremendous performance difference too. It makes perfect sense that a typical sport engine is set up loosely enough that they won't care much, otherwise every third guy would be having fits. But again.. as a blanket statement I don't agree with it being okay, you just need to understand what you are dealing with - but I sure agree it likely makes little difference with many sport engines.