Author Topic: Sub piston induction question.. (Read 5428 times)

Lauri Malila · « **on:** December 19, 2015, 03:03:41 PM »

Randy,

Maybe you have some experience on this;
I still haven't tracked the origin of the problem, but as I just assembled my new engine for first time, I noticed a small amount of sub piston induction when piston is in tdc. I more or less know the effects it causes in engines without silencer but with a muffler, it's a bit unknown.
I think it's not a catastrophe, I can get rid of it with shimming the cylinder 0,2..0,25mm or in worst case by making a new piston. Should I worry?

Lauri

FLOYD CARTER · « **Reply #1 on:** December 19, 2015, 04:19:33 PM »

What engine? Most of the Cox 1/2-A engines have it. Also, Orwicks, Super Cyclones, Anderson Spitfire. Is beneficial for high speed engines when intake port opening time gets too short for adequate air/fuel intake.

Floyd

Lauri Malila · « **Reply #2 on:** December 19, 2015, 04:47:29 PM »

A stunt engine, .77.
It is quite simple without muffler, but with one, I guess it can get more unpredictable. L

Air Ministry . · « **Reply #3 on:** December 19, 2015, 06:26:32 PM »

Youd have to shim the cylinder DOWN !? .

With the 21/40 wondered about a ' cold air feed ' to the exhaust to feed in clean air for the s.p.i. .
Dratted automobiles used air pumps to clean exhaust by afterburning. Like the PINTO .

A Venturie tube Like a p-40 air intake to a feed in the adaptor , as youve a short stack , could get the best of both worlds .

the T120 used a perforated baffle megaphone to augment ex. gas. ignition and promote scavenging - tough it wasnt realy a silencer

A cold air feed could be a whole new line of tooned zorst development . The Old ' through ' type mufflers might feed to spi if set up right ?

Air Ministry . · « **Reply #4 on:** December 19, 2015, 06:41:01 PM »

Quote

The front end of the Sea Fury fuselage quickly became so complicated in design that I had to build a forward fuselage mock-up to properly locate the firewall, engine mounts, 6.5 oz. adjustable fuel tank, cooling bypasses and the muffler installation. It soon became apparent that no commercial muffler would work without making the fuselage impossibly difficult to build; so I decided to design my own. It would have to be short - only two in. long - to stay entirely in the engine compartment. Only the tailpipe would run down the left cooling air bypass. Hoping to avoid power loss from my 2/3 length muffler, I increased its diameter to 1 1/2 in. The extra 50% diameter would increase the internal volume of my "shorty" to 140% of average commercial .60 mufflers. To further reduce power loss, I made my muffler a "flow through" or "extractor" type with a minimum tube ID of 1/2 in. The muffler was machined for me by Bob Wilder. It weighed 2 1/4 oz. installed (later reduced to 1 7/8 oz.). I was surprised at the sound suppression effected by my rather breezy contraption on the test stand and considering that it is completely housed inside the cowling of the Sea Fury, it rates as far more effective that just a "legalizer." In practice, it performed exactly as I had hoped, by effectively muffling the engine without causing any obvious power loss or increase in operating temperature.

http://www.airplanesandrockets.com/airplanes/sea-fury-march-1973-american-aircraft-modeler.htm

http://www.stunthangar.com/evolution_of_a_thoroughbread.htm

Phil Krankowski · « **Reply #5 on:** December 19, 2015, 06:48:15 PM »

Good ole EGR. Exhaust Gas Re-circulation. It *should* settle in an run nicely, though with an open exhaust it likely will make some more RPM.

Those air inducing exhausts are pretty neat looking. Interesting ideas.

Phil

Chris Wilson · « **Reply #6 on:** December 19, 2015, 11:40:07 PM »

Here is a quote that I had with one of the boys at Jett -

"SPI is not really a black art. Many engines use it. Simply helps with fuel transfer and utilizes the energy of the moving system a bit better. There is also a benifit of having the piston a bit lighter too, and less mass makes it all run smoother.

Bob"

Basically if SPI has much effect on running then your normal induction isn't doing its job.

And I am of the opinion that the much talked about Cox SPI issues were more about the close proximity of a ring type muffler collector and reflective waves than sucking in exhaust gases.

Lauri Malila · « **Reply #7 on:** December 20, 2015, 01:55:31 AM »

Quote from: Matt Spencer on December 19, 2015, 06:26:32 PM

Youd have to shim the cylinder DOWN !? .

No, up.

Still no real answers. What kind of phenomens can I expect compared to no spi?
Here is rhe muffler. It is quite neutral to the engine, actually so neutral that in all earlier versions I've had to increase backpressure later.

Lauri Malila · « **Reply #8 on:** December 20, 2015, 01:59:42 AM »

Here's the insides.

Wolfgang Nieuwkamp · « **Reply #9 on:** December 20, 2015, 05:04:31 AM »

Lauri,

it seems that there is no experience available.
Before making a new piston, I would make two bench runs, one without and one with shims.
Regards,

Wolfgang

RandySmith · « **Reply #10 on:** December 20, 2015, 01:01:14 PM »

Hi Lauri
I have found that its a big problem, "if" you have a lot of SPI, if you just have a small amount , at the corners, it seems to be no problem, The more back pressure you have on a restrictive muffler, will cause more problems, if, your having a SPI problem.
I really do not want the spent, hot, exhaust charge going back into my motors, but I have had engines with smalll amounts of SPI show zero probllems, no real effects from the SPI
My recommendation would be to run it before I made a new piston,or jacked the sleeve , you may have no worries here
By the way, some ST 60s have SPI and had no ill effects, of course , it was only a very small amount

Randy

Lauri Malila · « **Reply #11 on:** December 20, 2015, 01:27:22 PM »

Thanks, Randy.

It is really little, I think I can get away with it with a 0,2..0,25mm shim.

Lauri

Phil Krankowski · « **Reply #12 on:** December 20, 2015, 01:41:09 PM »

Quote from: Lauri Malila on December 20, 2015, 01:27:22 PM

Thanks, Randy.

It is really little, I think I can get away with it with a 0,2..0,25mm shim.

Lauri

Run it first. That is thinner than paper. It sounds like you just see it with a strong light source. The castor film will virtually seal that gap.

Phil

RandySmith · « **Reply #13 on:** December 20, 2015, 02:04:11 PM »

Quote from: Lauri Malila on December 20, 2015, 01:27:22 PM

Thanks, Randy.

It is really little, I think I can get away with it with a 0,2..0,25mm shim.

Lauri

Sound like you will have no problems, just run it as is, if you see any ill effect then shim the sleeve. I don;t think you have any worries

Randy

Ken Burdick · « **Reply #14 on:** December 20, 2015, 02:31:34 PM »

Ditto to the no worries. I use it for some applications and it is a good way to get more air into the engine, but the amount you described is minimal imho

FLOYD CARTER · « **Reply #15 on:** December 20, 2015, 02:35:21 PM »

That is an elegant-looking engine! It's doing 20K RPM just sitting there!

I assume a shim would go under the top flange of the liner, to move the bottom of the exhaust port upward a bit.

Floyd

Brian Hampton · « **Reply #16 on:** December 20, 2015, 05:09:05 PM »

Lauri
You could get rid of that SPI by either raising the liner or making a slightly shorter conrod but either of them will change your port timings and compression. Knowing how fastidious you've been with your designing and machining then a new piston or liner is probably the only way you'll feel satisfied. Personally though, I doubt that little SPI would make any difference to how it runs. As for those venturi (flow through) mufflers, they're way too loud!

Air Ministry . · « **Reply #17 on:** December 20, 2015, 05:45:11 PM »

My Blue. out in the sun to long , saturday , it is shim it up .

Test on the C35 pdp ST on prolonged idle it sucked the castor out of the muffler ,
when it was opened up it glooped the plug .

Presume youll check how much rooms under the piston at bdc to ascertain Max. posable skirt length ,
then do it to the ' appropriate ' length , with it a way below port at tdc .

Beautiful workmanship , we'll have to get you out to do the machining on my four valve twin cam twin ,
probly a bit heavy as its 960 c.c. .

Lauri Malila · « **Reply #18 on:** December 20, 2015, 11:47:12 PM »

Floyd,

The shim would go between the case and bottom of cylinder block. There is no separate sleeve.
Anyway, I found the error. It's just the bottom edge of exh. port and quite difficult to spot.
Usually, in this design, the piston top edge in bdc goes 1mm below exhaust port, this seems to be 0,8..0,9.
No it won't do 20K. just to be sure that nothing breaks, the construction has been tested up to 14K but the normal operating range is around 7-7,5K.

L

phil c · « **Reply #19 on:** December 21, 2015, 01:23:43 PM »

Quote from: Lauri Malila on December 20, 2015, 01:27:22 PM

Thanks, Randy.

It is really little, I think I can get away with it with a 0,2..0,25mm shim.

Lauri, I had a recent F2D engine that threw a circlip and broke off about 1/3 of the piston skirt but no other damage. So I put it back together and flew it again. It ran fine. the only change was a maybe 10% loss in power. Otherwise it ran exactly the same.

Lauri, don't know if this will help--I had a recent F2D engine that threw a c.irclip and broke off about 1/3 of the piston skirt- massive sub induction. With no other damage I put it back together and ran it a few times. Thj

Lauri