Side Port(vintage) Design

[GONZO]

A question to those who are knowledgeable on design aspects of side port IC model engines, like the venerable Mills 1.3 etc. To the best of my knowledge and experience(I have numerous examples of each type of engine: Irvine Mills 1.3 & 0.75, Indian Mills 1.3 & 0.75, MP Jet 040 & 1.8, CS Deezil, ED Comp special, etc) the induction is controlled by the piston skirt uncovering the induction inlet hole/s in the liner side wall. All my engines bar one, recently acquired (no names no pack drill), do not uncover the liner holes with the piston crown when at BDC. To me this is a serious design/manufacturing flaw as any residual combustion pressure could/would vent into the inlet against the flow of any fresh incoming charge thus adversely affecting the breathing of the engine.

There are two other failings with this engine, one fixable the other like the above not really fixable. 1/ the front piston skirt obstructs the gas flow into the transfer ports from the crankcase as the piston nears BDC; fixed by cutting away part of the skirt. 2/ the piston crown goes way lower than the bottom edge of the exhaust ports leaving a significant volume that will not be scavenged of exhaust gas; only fix I can see would be is by lowering the cylinder in the crankcase casting, this would also address the inlet port issue.

It's almost as if the wrong liner for the stroke of the engine has been used.

Any comments from those who are knowledgeable on engine design welcomed.

[J D 8]

 Why not name the item ?  Could be as you think, it is a bitsa, done to be able to say " good compression " but not the right parts

[GONZO]

It's new not SH or a bitsa. Currently on sale. Modern version based on a 70 year old design. Don't want to cast bad rep when there may be a reason like recent design innovation, although I doubt it. 

[Jonathan W]

From your description, it does sound like the ports in the liner do not match the stroke of the piston. If you bought it new, have you inquired with the supplier or manufacturer?

[GONZO]

It's supposedly test run before sale. No doubt it will run but power will be low( information I have indicates no more than 2/3 the power of the 70 year old original design which had plain bearing crank and new has twin ball race) 

Edited April 8, 2022 by GONZO
additional info

[Don Fry]

Bin it, run it, or return it. It’s a motor, designed to lift airframes to heaven. 

[Jonathan W]

Guessing it's a Redfin engine, why not just give them a call and discuss your findings. They might be able to offer a solution, or maybe not, but you won't know unless you ask them.

[GONZO]

You may think that but I couldn't possibly comment. No names no pack drill as I said earlier. I purchased the engine aware of the piston skirt/transfer port issue as I'm capable of rectifying that. But, the other ones are a no go.

[GONZO]

You may think that but I couldn't possibly comment. No names no pack drill as I said earlier. I purchased the engine aware of the piston skirt/transfer port issue as I'm capable of rectifying that. But, the other ones are a no go.

[Jonathan W]

There's little value in you being so obtuse via your keyboard when the manufacturer is only a phone call away.

[Simon Chaddock]

Gonzo

Without seeing the engine are you sure the fact that the piston crown drops below the exhaust ports and also uncovers the inlet port holes is that serious?

Any residual exhaust gases in the cylinder are displaced by the induction transfer which is never a 100% process in any two stroke. In addition the mixture flow in the inlet venturi will have been cut off by the descending piston so will have built up a slight positive 'inertia' pressure by the time the piston crown descends to expose the inlet hole so avoiding exhaust ingestion.

Two stroke porting does have a bit of 'suck it and see' as to what works, particularly on long stroke relatively low revving diesels.

 

IMHO you would have to run the engine first 'as is' under test conditions to give you a base line to determine if the modifications you propose make a significant difference.       

[Jonathan W]

Here is cross-section of the SAM125 diesel, as produced by Redfin. The porting appears to be made intentionally as Gonzo describes. However, consulting Redfin themselves would give a definitive answer.

 

 

[GONZO]

Jonathan W

If we are going to use the SAM 125 as an example then yes the porting is made intentionally this way. Because it is intentional doesn't mean it is correct(many model engines have been produced correctly but are poorly designed). If the stroke is reduced from 14mm to about 12mm it becomes a 1cc engine and porting falls into place. Perhaps an existing con rod was used where a longer version would be more appropriate to keep the porting correct. Although, this may then cause issues with the contra piston and the proportions of the whole top part of the engine. Power curves produced by Maris Dislers indicate a max of just under 0.08 bhp with a very flat bhp curve from 9000rpm to 11500rpm indicating, to me, breathing restrictions. The original, 70 years back, Mills 1.3 peaked at 0.12 bhp (50% higher) with a plain bearing crank. On some props the SAM 125 is 1000rpm down on one of my good Indian Mills 1.3. It is obvious from pictures of the internals, the piston, that the skirt does not have the cutaway in the front face to improve mixture flow into the transfer ports. This is an easily applied mod if all else was well. In fact Maris noted this and was going to do and then report results, but not so far. Perhaps he encountered the other issues.

If I owned this engine(?)? I would be loath to carry out a mod then find it only effected a partial cure or no cure at all. One would then be stuck with a nice looking paper weight.

To restate, I have no intention or desire to slate off any manufacturer. I was thinking that maybe some new scavenging/porting arrangement had been found to work, that I was unaware of, and was seeking conformation/information if this was so.

[Jonathan W]

I don't think either of us is slating off the manufacturer. Looking at the SAM125 example, if the "ideal" porting arrangement was to be achieved with the given stroke/capacity, then probably the whole engine will have to be made taller in order to space out the ports "perfectly". Perhaps the less than ideal porting is a compromise in order to keep the overall height within a desired limit.

 

For example, if you move the inlet port down so that it does not overlap with the exhaust, then you have made the inlet timing longer so you then need an even longer piston skirt to pull the timing back, if there is room to do that without a taller crankcase.

 

Personally, I don't see the in/ex overlap as a big problem. This occurs at BDC when the inlet is not active in any case. The inlet is only functional either side of TDC.

 

How is the skirt length/profile w.r.t. to sub piston induction via the exhaust port at TDC on your engine?

 

If you simply install a longer con rod to this particular engine, then your inlet timing will become longer, your exhaust timing shorter, unless you then make the piston skirt longer, adjust the port heights etc which leads on to other issues. And you do have to make the head taller. It's all a compromise, depending on what parameters and constraints the manufacturer had to work to, perhaps indeed utilising existing parts for economy. In the end, the manufacturer is best placed to explain these compromises. I very much doubt that there any new developments for piston-ported model 2-strokes. Realistically, they are being produced as a nostalgia piece. The ultimate performance is inherently limited, due to the constraint of the inlet timing being symmetrical about TDC. If one is worried about 2-stroke power output, that is why disc valves (or crankshaft porting) and reed valves came about.

 

I don't own any Redfin engines, but I have chatted to Alex Phinn on one occasion. He was most approachable and pleasant. I'm sure he would be the same with you, assuming you do have one of his engines and wanted to politely quiz him a little.

 

I can understand your disappointment over the power being even lower than an equivalent old Mills engine. If you own such a Mills, you might discover something by taking comparative measurements between the two engines and then educate us all. ? For example, most simply without any disassembly, is the Mills 1.3 significantly taller than the SAM125?

[Jonathan W]

PS, Looking at pictures in the current Aeromodeller article, the SAM125 does look to be a considerably lower overall height than the Mills 1.3. The more compact size of the SAM has perhaps lead to the porting compromises. Look at the height of the crankcase below the inlet duct of the Mills compared to SAM.

 

[Engine Doctor]

Get your engine off to Red Finn and fixed before parts run out. I believe the factory has been destroyed during the conflict.

It does sound as if an incorrect con rod has been fitted but impossible to confirm without having engine or some decent pictures.

Good luck and hope you get it sorted.

Edited April 9, 2022 by Engine Doctor

[brokenenglish]

TBH, I find all this theorising without even trying to run the engine is intellectual overkill, not to say a bit of a waste of time.

At least try to run the thing and see what happens.

If it runs OK, no problem, and if it doesn't, then is the time return it, along with your opinions.

I must have run several hundred old diesels and I've never even thought of this kind of nonsense. At least give the thing a chance...

[GONZO]

It's not an old engine, it's brand new. If I run it returns would be complicated, thus the theorising. Easier to return an unrun engine under the distance selling regulations. The same model of engine has been test run(prop rpm figures) by Maris Dislers and has shown a low level of performance as detailed on a sheet supplied by the vendor. Maris commented on the test sheet that he was going to modify the front lower edge of the piston skirt to give improved gas flow to the transfer ports and then report back, nothing so far. I can carry out this if there was nothing else and I was confident it would fixed the low performance. But inspection has shown there are other issues and modifying the piston skirt may only be a partial cure at best. It would also negate my option to return the engine. I've compared the Maris Dislers prop/rpm figures with one of my Indian Mills 1.3 and the Indian shows up better across the range, some props 1000rpm faster. To me there is something not right with the engine design/production. I'm not too old(74) or have operated too many engines(100's) to believe that there is not the possibility that I don't know everything, that's why this little chat to explore possibilities. Although, I'm definitely falling on the side of regrettably returning it.

[Jonathan W]

Yes, the theorising has its attractions! I have found engineering plans for the Mills 1.3 on Outerzone

 

The Mills is a taller engine than the SAM125, but it also has a longer stroke of 15.875mm vs 14mm and smaller bore for much the same capacity.

 

The top of the Mills cylinder head (excluding the compression screw) looks to be about 10mm taller than the SAM125, so they are quite different designs of the same general engine type. Hopefully the manufacturer of the new engine can shed some light on performance figures.

[brokenenglish]

There's some confusion here. You say "If I run it, returns would be complicated".

That means that you want to return it even if it runs, or did I miss something?

I don't think anyone would question the return of an engine that you were unable to start.

Also, returning an engine without even trying to run it would hardly stand in the event of a dispute.

If the engine is a Redfin, call Alex, ask for his opinion and do whatever he recommends.

I still think all this discussion without even trying to run the engine is (insert your own mod-compatible term).

[Don Fry]

I love conspiracy theories. If it helps I have a good recipe for Yellowfin.

[PatMc]

It's an engine designed & intended for F/F, or RC models that emulate F/F only using the radio to keep within easy retrieval distance. Does the low power really matter ? I would have thought that the more robust build than it's predecessors was more important than power so long as it's just as easy to start. 

Although the Indian Mills are more powerful they are notorious for their easily bent crankshafts. 

 

[Martin Dance 1]

I've just reread Maris Disler's review of the SAM 125 diesel, in which he seems to conclude that it is in the mix with the similarly sized original Mills engines and the subsequent replicas. I would suggest a bench run of the motor in question is the simplest way of determining whether or not it compares with the figures Maris obtained with the test engine. If there is a significant difference then contacting the vendor or Alex Phin would be a logical next step. If however the motor compares favourably with the Mills/ and clones then it is performing as intended just enjoy it. If you consider the design to be at fault the obvious approach would be to design build and test your own motor, have it manufactured and become famous!?

[GONZO]

brokenenglish

The distance selling regs in the UK allow you to return any item purchased 'at distance', online by phone etc for a full refund as long as it is within a certain time limit, is as received and unused. Plus you need no other reason than you have just changed your mind and no longer want it.

As to whether it runs being the deciding factor this is false. Engines have been made and sold in the past that have been less than acceptable, the DC Bantam was one - looked OK easy to start and ran OK but had trouble pulling the skin off a rice pudding.

As I stated the data sheet(not the aero modeller) sent to me detailing Maris Dislers findings were "less than expected" his words. Compare with the 70 year old test of a Mills 1.3 MkII      Mills MkII test.

As I have stated one of my Indian Mills exceeds the prop/rpm figures across the board, on some props by almost as much as 1000rpm. This is a new engine with ball race crankshaft that does not perform as well as a 'curry' Mills an engine generally accepted as agricultural. I see no point in running a series of prop/rpm tests on one of my four Irvine Mills 1.3 for comparison if it cant match or outperform a 'curry' Mills.

[GONZO]

I've had a response on the Barton forum that explains the inlet ports opening above the piston crown (in 67 years of modelling I've only ever seen the inlet ports in the liner side wall being opened and closed by the rear lower edge of the piston skirt). Although the comment urged caution as this could be taken too far and be detrimental. It appears that in high performance side port engines this has been used to a very limited degree for improved performance. As I've said, I'm not too old to think I can't learn something new.

I think the Aero Modeller comments re comparison with others may have been generous. Especially if you compare the 70 year old test performance curve with the published Aero Modeller data( which I believe is the same as I received on the separate data sheet).

http://sceptreflight.com/Model Engine Tests/Mills 1.3 Mk II Throttled.html

Edited April 10, 2022 by GONZO