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Author Topic: Single bypass plugs.  (Read 2256 times)

Offline Chris Wilson

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Single bypass plugs.
« on: January 27, 2015, 04:31:39 AM »
I have a theory on why engines like the Fox 35 benefit from partially plugging their single bypass with a central post.

It really has little to do with increasing the velocity of the charge, as the forward speed of the charge is quite low anyway and it has little to do with reducing the volume of the passage way since the entire lower crank volume can be considered a large "passage way" between the intake and the combustion chamber.

The major effect of blocking with a central column in a single bypass is that of keeping the centrally mounted plug out of the single wet and cool stream that is directed at it and splitting and redirecting it either side instead. In other words it acts to shield the plugs elements from over cooling.

I know that reducing the volume of the passage will give a velocity increase and the same passage can act as a reservoir for coalesced fuel that can be effected by G forces but I am betting that if the the bypass was instead blocked by using two smaller posts either side instead of one up the middle that any perceived benefits would diminish greatly. And conversely if a central plug was shortened from its lower edge it would not change much.

Thoughts?

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Offline RandySmith

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Re: Single bypass plugs.
« Reply #1 on: January 27, 2015, 09:57:04 AM »
Hi Chris
An interesting theory, looks like someone needs to try this and report back what they found. since the baffle is going to deflect a still smaller charge, and the size of the charge will certainly be reduced from a stock setup, I do not see how much , or any, difference would be noticed, but only way to tell is to do it!

Regards
Randy
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Online Brett Buck

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Re: Single bypass plugs.
« Reply #2 on: January 27, 2015, 10:39:15 AM »
Quote from: Chris Wilson on January 27, 2015, 04:31:39 AM
I have a theory on why engines like the Fox 35 benefit from partially plugging their single bypass with a central post.

It really has little to do with increasing the velocity of the charge, as the forward speed of the charge is quite low anyway and it has little to do with reducing the volume of the passage way since the entire lower crank volume can be considered a large "passage way" between the intake and the combustion chamber.

The major effect of blocking with a central column in a single bypass is that of keeping the centrally mounted plug out of the single wet and cool stream that is directed at it and splitting and redirecting it either side instead. In other words it acts to shield the plugs elements from over cooling.

I know that reducing the volume of the passage will give a velocity increase and the same passage can act as a reservoir for coalesced fuel that can be effected by G forces but I am betting that if the the bypass was instead blocked by using two smaller posts either side instead of one up the middle that any perceived benefits would diminish greatly. And conversely if a central plug was shortened from its lower edge it would not change much.

Thoughts?


 
  The flaw with the reasoning is that it works almost the same if you leave a single channel down the middle of similar cross-section.

    Brett
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Online Lauri Malila

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Re: Single bypass plugs.
« Reply #3 on: January 27, 2015, 10:58:58 AM »
Maybe that's the case when port(s) are really aimed towards the cylinder head, like Schnüerle boost port tends to be. But with a single 90 degr. bypass with piston deflector.. I doubt.
I think it's both the higher velocity in bypass and the higher velocity of fuel mixture as it hits the deflector, breaking into smaller drops for better burning. L
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Offline Chris Wilson

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Re: Single bypass plugs.
« Reply #4 on: January 27, 2015, 12:29:35 PM »
Quote from: Brett Buck on January 27, 2015, 10:39:15 AM
 
  The flaw with the reasoning is that it works almost the same if you leave a single channel down the middle of similar cross-section.

    Brett
Hi Brett,
You honour me by saying it is "almost " the same.

So I have an inconclusive flaw? I can live with that and feel tempted to try an inverted V shaped plug in my unused Fox 35 to push this further.
Thanks.
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Offline Chris Wilson

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Re: Single bypass plugs.
« Reply #5 on: January 27, 2015, 12:38:54 PM »
Quote from: RandySmith on January 27, 2015, 09:57:04 AM
Hi Chris
An interesting theory, looks like someone needs to try this and report back what they found. since the baffle is going to deflect a still smaller charge, and the size of the charge will certainly be reduced from a stock setup, I do not see how much , or any, difference would be noticed, but only way to tell is to do it!

Regards
Randy
Hi Randy,
I suppose it does get down to having an identifiable 'problem' to fix in the first place!

Whilst I have you here can I please ask how many boost channels you use in your engines?

Cheers.
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Offline Chris Wilson

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Re: Single bypass plugs.
« Reply #6 on: January 27, 2015, 12:44:57 PM »
Quote from: Lauri Malila on January 27, 2015, 10:58:58 AM
Maybe that's the case when port(s) are really aimed towards the cylinder head, like Schnüerle boost port tends to be. But with a single 90 degr. bypass with piston deflector.. I doubt.
I think it's both the higher velocity in bypass and the higher velocity of fuel mixture as it hits the deflector, breaking into smaller drops for better burning. L
Laurie,
What velocities are we talking about here?

I have it on good authority that a high pumping ratio small ported 2.5cc engine only transfers at about 26kph And the ye olde Fox with a quite poor pump ratio and massive channel would far less.  In other words I don't see that any significant velocities are ever achieved.

Talk soon.
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Online Lauri Malila

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Re: Single bypass plugs.
« Reply #7 on: January 27, 2015, 01:02:47 PM »
Chris,

I'd guess velocity peaks are slightly higher  than piston speed when ports start to open. How much that is in km/h, haven't bothered to calculate. L
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Offline RandySmith

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Re: Single bypass plugs.
« Reply #8 on: January 27, 2015, 03:37:15 PM »
Quote from: Chris Wilson on January 27, 2015, 12:38:54 PM
Hi Randy,
I suppose it does get down to having an identifiable 'problem' to fix in the first place!

Whilst I have you here can I please ask how many boost channels you use in your engines?

Cheers.

Hi Chris, I think you will find, either block the FOX bypass in  the middle or on either side, and you will see little to NO difference . Again, this problem , mostly shows up when your running the FOX side mounted,
In regards to my engines, I have used from zero to 5 boost ports (as many as 8 ports total), of many different configurations , In the later P.A. engines there are 2 boost ports and 2 main intake ports, 1 exhaust port

Regards
Randy
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Offline Brian Hampton

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Re: Single bypass plugs.
« Reply #9 on: January 27, 2015, 06:29:27 PM »
Maybe I should preface this by saying that I've never used any type of Fox engine or, in fact, any side mounted engine but I have to wonder why it only seems to be a Fox 35 that has this burp problem. What about similar style engines like Merco, Enya, OS etc? No one seems to complain about any of them and the Enya, in particular, has quite a massive sized transfer passage. Is there some detail difference between the Fox and these other engines like maybe baffle height?
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Offline phil c

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Re: Single bypass plugs.
« Reply #10 on: January 27, 2015, 08:43:02 PM »
The Fox 35 has a pretty large bypass.  On some variations top of the bypass may end up either above or below the top edge of the port.
Then there is the position and height of the baffle.  I read in one of Duke's ad columns that he tried quite a few different combinations, but mainly in upright mounted engine positions.

Then you can look at some of the higher performance versions.  They all had the baffle more towards the center of the piston.  An then there was the note he put out on, I believe, the first batch of double ball bearing combat engine where you had to file the corners of the baffle down in an angled wedge shape about .02 in.  The baffle shape added hundreds of rpms, stopped it from blowing plugs, and improved fuel economy.

Since all the 35's used the same basic sleeve dimensions it would be interesting to try swapping parts into a Stunt 35 case and see what happened.
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Offline Chris Wilson

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Re: Single bypass plugs.
« Reply #11 on: January 27, 2015, 09:24:32 PM »
Quote from: Brian Hampton on January 27, 2015, 06:29:27 PM
Maybe I should preface this by saying that I've never used any type of Fox engine or, in fact, any side mounted engine but I have to wonder why it only seems to be a Fox 35 that has this burp problem. What about similar style engines like Merco, Enya, OS etc? No one seems to complain about any of them and the Enya, in particular, has quite a massive sized transfer passage. Is there some detail difference between the Fox and these other engines like maybe baffle height?

Hi Brian,
     the Fox 35 has a low pump ratio, low compression head and quite a high oil content recommended for it - so lots of wetness and not much internal pressure exacerbates things.

Add gravitational forces to horizontal trench that doubles as a port and it becomes a bath tub looking for a plug hole to empty out from.
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MAAA AUS 73427

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Online Lauri Malila

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Re: Single bypass plugs.
« Reply #12 on: January 30, 2015, 11:56:48 AM »
By the way,

I don't know if this relates with the original subject of Chris, but to me the origin of burp is not absolutely clear, quite far from it actually. But there is not only one cause, that is sure. Also symmertically built engines suffer from similar behaviour, I mean engines with rear exhaust. And both Schnürle and single port scavenged engines. I'm talking about side mounted engines, it's a little surprising that side mounted tend to have more symmetric running issues.
I have 6 ports to play with, 3 on both sides of cylinder. I have tried all kinds of configurations by blocking the ports.
Quite interestingly, only running direction of engine determines the the direction of asymmetric running, blocking completely ports on either side has no effect on that. But my bypass channels are small, maybe it's differend with Fox. Has anybody tried Fox with reverse crank?
I still have a feeling that some kind of fuel accumulation in bottom end of engine is one candidate for causing the burp. that would explain why it reverses when I reverse engine rotation.

Lauri
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Offline EddyR

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Re: Single bypass plugs.
« Reply #13 on: January 30, 2015, 12:39:47 PM »
 Most of my stunt type motors that I used starting around 1952-53 were Fox.29&.35's. There were a lot of other motors in there but not for stunt flying. I do not remember having the burp,I never heard of it until the first time I saw it mentioned on a CL forum. After that I did have the problem a few times. I wondered what had changed. The only thing that changed was the use of the uniflow tank. A few years back I flew a borrowed profile that had the problem and put a none uniflow tank with both vents open and the problem went away. The vents were straight out the top and bottom. I was amazed how well the motor ran. Also I always mounted the tank as close to the motor as I could get it. The pick up vent cam out the top or bottom of the tank. Three ounce tank in stunt run, not lean.
 When I belonged to the Tampa Bay Line Flyers many years go we had a event that required a profile model with a Fox .35 on it and I do not remember anyone having the burp problem.
 I entered stunt contests in the late 1950's but all those models had upright or inverted motors. Maybe it was there on my profiles and I missed it,but I do not remember it. mw~
Ed
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Offline Russell Shaffer

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Re: Single bypass plugs.
« Reply #14 on: January 30, 2015, 01:49:56 PM »
I have an Enya 29 mounted inverted that burps on outsides.  It shows up in the horizontal eight when you go straight from an inside to outside.  The other maneuvers are fine.  I have tried two different tanks, a plastic clunk with standard venting and a uniflow metal one - no difference.   
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Offline Chris Wilson

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Re: Single bypass plugs.
« Reply #15 on: January 30, 2015, 03:00:18 PM »
Just to add 'fuel' to the fire here I have seen FAR more invert/ upright engines suffer from what I call a 'gear change' and its simply solved by side mounting.

These engines are diesels though and accepted thinking in these types of ignition engines is that it is fuel accumulation in the ports and the effect is exacerbated by a diesels need for a drier fuel mixture (15:1) and intolerance to vary from the ideal.

A methanol based engine in the other hand runs at about half of that stoichiometric ratio and the fuel load is far wetter and denser but the ignition type far more tolerable to variation.

I believe that a 'burp' or 'gear change' (actually an increase in the misfire rate caused by a rapid enriching of the mixture) is present in ALL engines during transitions but its whether or not you notice it or more importantly the model does!
And at what point you bother to class it as an issue.

I have read about Fox 35's having enormously variable back plate depths and posts by a fellow called "BigIron" who fitted many stuffer back plates only to find that some of the 'corrected' engines ended up worse than some of the stock ones!
So in regards to the old Fox we have a variable commodity and its no wonder that run variations exist and cast a cloud over what the problem is and whether we have to fix it.

I still stand by my original thinking, that of any time a transfer stream is directly aimed at the plug it is a bad thing for run consistency during transitions.

Hence some of the single bypass stunt engines recognising this and having plug shields installed in the head and the use of idle bar plugs elsewhere.

Cheers.
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MAAA AUS 73427

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