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Author Topic: Piston deflector height  (Read 1365 times)

Online Lauri Malila

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Piston deflector height
« on: May 24, 2020, 12:14:29 PM »
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« Last Edit: September 11, 2022, 02:07:09 AM by Lauri Malila »
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Offline Tim Wescott

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Re: Piston deflector height
« Reply #1 on: May 24, 2020, 01:48:50 PM »
Why not Schnuerle?

Given that you're doing a bunch of experimentation to make the World's Greatest Stunt Motor, I'd suggest that you make a bunch of different pistons with different baffle heights.

Or do like some of the manufacturers did in the 1930's and make a bolt-on baffle, not so much for manufacturing convenience, but so that you can have one piston/liner with good fit, and you can experiment with different baffle geometries, just by popping the head off and replacing the baffle.
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Offline Dennis Toth

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Re: Piston deflector height
« Reply #2 on: May 24, 2020, 02:01:30 PM »
If you google Gordon P. Blair, QUB cross scavenging two stroke engine you should be able to see a unique stepped piston/baffler design that cuts a step in the piston to form the baffle with a flat top (very much like the Drone Diesel stepped piston/flat top). In his book "The Basic Design of Two Stroke Engines"  May 1989, Gordon indicated that the QUB design was a very good match for the loop scavenging directional porting even at full throttle. In general they indicate the baffler is up to 50% higher then the port opening.

Best,  DennisT
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Offline Tim Wescott

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Re: Piston deflector height
« Reply #3 on: May 24, 2020, 05:12:30 PM »
Quote from: Lauri Malila on May 24, 2020, 02:14:12 PM
...With this technology, you just don't make several pistons to one cylinder at least not if you want repeatable and logical results...

That's why I suggested bolting on the baffle -- you'd only need to make new baffles, and perhaps matching heads, while keeping the piston fit the same.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline RandySmith

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Re: Piston deflector height
« Reply #4 on: May 24, 2020, 09:26:10 PM »
Quote from: Lauri Malila on May 24, 2020, 12:14:29 PM
Hi Randy,

As discussed earlier, I'm looking for your recommendation for the height of my piston baffle.
I finally had time to finish the drawing. As I don't know what I'm doing, I stole the main proportions from my well-running Retro.60 (port area, opening duration, baffle location..) transfer port area is scaled up from Retro (0.77:0.60=1.28x).
Because of ring, the transfer port is divided to 3 smaller ports. 1.5mm (0.059") wide bridges between ports seems about right.
Here are the main dimensions:

-Stroke 28mm (1.102")
-Bore 24mm (0.944")
-Transfer port height 4.67mm (0.184"), 16.7% of stroke.
-Distance between baffle front edge and cylinder wall at centerline 4.1mm (0.161").
-The corner of baffle front edge and piston top surface will be rounded with 1.5mm (0.059") radius.
-In Retro.60, the baffle height is same as port height (4mm/0.157"). There is no corner radius and the shape is also not curved (1st picture)

So, could you recommend a baffle height to start with, please?

Cheers, Lauri

Hi Lauri

That is about the exact same height as the  OLD ST 60,  Use a baffle height of .200 inch, it will use less fuel, make more power, and run better. I used this on 65 and 74 engines.  If you look at the  Super Tiger built  COMO engines, you would see  they  increased  baffle height.  Nice  looking  machine work !

Regards
Randy
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Offline RandySmith

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Re: Piston deflector height
« Reply #5 on: May 25, 2020, 11:43:15 AM »
Quote from: Lauri Malila on May 25, 2020, 02:53:11 AM
Thanks, Randy!

.200 makes sense. That would be 0.41 above port edge at bdc.
How should it ideally be at tdc, how close to combustion chamber ceiling? L

Hi Lauri
I assume your asking about the slot cut into the head, or button to clear the baffle, I use .020 thou in mine, I do not want it super close in case  carbon or  debris comes into play
Sleeve looks  awesome,  post a pic of the piston when it is finished

Randy
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Offline Trostle

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Re: Piston deflector height
« Reply #6 on: May 25, 2020, 03:43:24 PM »
Quote from: RandySmith on May 24, 2020, 09:26:10 PM

Nice  looking  machine work !

Regards
Randy

Randy,

I have had the pleasure to hold several engines that Lauri has made.  They are indeed works of art.

Keith
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Offline RandySmith

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Re: Piston deflector height
« Reply #7 on: May 25, 2020, 07:54:16 PM »
Quote from: Trostle on May 25, 2020, 03:43:24 PM
Randy,

I have had the pleasure to hold several engines that Lauri has made.  They are indeed works of art.

Keith

Hi Keith
 You are  100% correct, the  work I have seen of his  is  first class  stuff,  It will be  interesting  to see how this  next motor comes  out

Regards
Randy
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Offline Olli-Matti

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Re: Piston deflector height
« Reply #8 on: May 26, 2020, 12:46:26 PM »
Damn that it looks good Lauri! Hats off  H^^
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Offline Steve Helmick

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Re: Piston deflector height
« Reply #9 on: September 13, 2020, 11:51:37 PM »
Start high and mill off .010" at a time. If you go too short, it won't 4 cycle at all. I suspect 1mm too high wouldn't cause any trouble, except that as you cut the baffle height down, eventually compression would reduce enough to lose a little power and maybe require making a new head button.

What's "Tufnol"...Delrin, perhaps? It's been ages since I've seen an AeroModeller and even then didn't know what it was equivalent to on this side of the Atlantic.  ??? Steve
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