Vega V twin Aero engine

[creast]

Its been a while since my last post and I have to admit to being very close to chucking in the towel, I abandoned the project for a month as I was close to throwing it in the bin.
However, the rest did me good and I have learnt a few things about engine construction over the last few weeks.
I had always suspected the compression wasn't good enough and after chatting to a colleague who races two stroke motorcycles I decided to follow his advice and remake the pistons and rings. I went a stage further and made the pistons with two separate ring grooves as opposed to the plans two rings in one groove.
This meant making the pistons 1 mm longer to accommodate the second groove and hence a spacer was required to prevent the piston hitting the head.
The piston was made a closer fit, half thou under the bore diameter. The rings were made size to the bore and lapped in an identical bore to the cylinder.
I had discovered that as perfect as the ring may be, when spragged to give the gap it does not form a true circular contact when fitted. I think this was the crucial thing affecting compression and was born out by the results of the new method as I now had decent compression and a good bounce on the propeller.
Individually both cylinders now run fine on their own and starting is much much easier. However, put together I cannot get any life out of it and suspect I am not going to be able to hand flick start this beast. There may also be insufficient inertia in the prop so I will try a larger item.
Interestingly, the engine is very sensitive to the glow plug type. OSF 4 stroke plugs are a bit borderline and the engine really starts well on an Idle bar plug 'Taylor Made' brand so I will probably stick with those.
Hopefully I will get this twin running soon!!

 

[petertha]

I admire your tenacity & trust it will ultimately pay off. I'm making my 'prototype' cylinder tower now. The plan is to chicken out & use a commercial ring to confine potential problems to the other 101 things that will likely go sideways.

One thing I wanted to ask. In Terry's 18C radial build, he shows his flashlight method, shining up through the bottom of the sleeve. The idea is any eccentric gap between ring & bore allows light to peek through. I bought a new OS-56 piston/liner/ring combo to reference/replicate the dimensions on my build & just about to perform this test to see what a new 'factory' engine looks like. Hopefully I can take a pic & post. Did you ever try this trick during your ring iterations?

Also, maybe you mentioned already, what were your target dimensions:
- piston groove width (ex 0.001" wider than axial ring thickness)
- piston groove depth (ex 0.005" deeper than compressed ring ID)
- piston OD reduction relative to bore

 

[creast]

Hi Peter,

No I didn't try the torch trick but I did hold it up to a light but couldn't see any gaps. The main indicator was the uneven polishing of the ring when it was dismantled after hours of flicking which lead me to believe something better had to be done.
Careful lapping in an identical bore did the trick.
I have tried several different ways to make the rings including temperature contolled kiln with fixtures and simple blowtorch method.
To be honest, my successful rings have simply been spragged onto a 2.5mm thick bar and just a quick heat to dull red with a blow torch. No clamping has been used and the four rings came out great. The kiln ones @ 10 minutes actually came out more brittle.
The main problem with spragging for heat treat is that the highest stress is opposite the gap and this inevitably means its here the greatest stress relieving occurs so curvature changes more at this point than the sections near the gap so there is no way you will get uniform pressure when its compressed in the bore.
For the record, my piston is 0.0006" under bore size (0.0003" radial).I know that sounds really tight but 50cc racing two strokes run close as this and so far it hasn't seized. If it does I will post on here
Ring thickness is around 0.002" under groove width and the groove depth is 0.003" more than the ring radial thickness.
The piston was made size to bore and did need a little dressing of the break ends with wet and dry abrasive to get it to enter the bore before final lapping/polishing.
I can also recommend the Nemmett method of breaking the ring by pushing it down carefully on a morse taper until it cracks at the weakest point.
I have also learned my lesson and now fit and remove the rings using shim stock strips, its more reliable than not doing so.

 

[petertha]

For the record, my piston is 0.0006" under bore size (0.0003" radial).I know that sounds really tight but 50cc racing two strokes run close as this and so far it hasn't seized.

Thanks for all the parameters. Sheesh, everything looks to the book. Now I'm really worried when I have to cross that bridge.

Re the piston, this topic came up in another post. I can't recall your bore size, but can you check my sample math. One would thing getting pretty tight, but maybe some parameters are off?

1.23 E-5 in/in/deg F= thermal coeff aluminum
0.5" = piston dia
250 deg F = running temp (assumes nil liner expansion)
= 0.0015" = thermal expansion
0.0006" = clearance at ambient
=<0.0009"> = interference!

 

[creast]

Yes I was worried too... LOL
Fact is, I have just ran the tighter of the two cylinders at 6000 rpm for 10 minutes continuous and no problems. I will check the figures and for the cylinder liner too. I have to admit I did worry about the expansion but thought hey.. what the heck... if it runs I am happy even if short lived.. but that's not what has happened so far!

Thanks for all the parameters. Sheesh, everything looks to the book. Now I'm really worried when I have to cross that bridge.

Re the piston, this topic came up in another post. I can't recall your bore size, but can you check my sample math. One would thing getting pretty tight, but maybe some parameters are off?

1.23 E-5 in/in/deg F= thermal coeff aluminum
0.5" = piston dia
250 deg F = running temp (assumes nil liner expansion)
= 0.0015" = thermal expansion
0.0006" = clearance at ambient
=<0.0009"> = interference!

 

[creast]

Peter,
I am pretty much in agreement with the figures you posted even though you didn't state the value of coefficient of expansion used. I got around 0.001" for 120 deg C differential (250F).
My bore is actually 19mm and calculating for 100 deg C differential and including liner expansion I would get 0.0003" interference.
Now there are two things that could explain why the piston isn't seizing.
Firstly, perhaps the piston has worn slightly? I can check this when I disassemble the engine agin.
Secondly, I have no clue what the actual temperature is. The propeller is certainly force cooling the engine.
I could put a thermocouple somewhere but that won't tell me what the piston is seeing. Maybe I could borrow a thermal imaging camera from work to give a better picture of whats happening.
Or maybe bond a thermal indicator strip inside the piston?

 

[petertha]

even though you didn't state the value of coefficient of expansion used.

Its this ---> 1.23 E-5 in/in/deg F= thermal coeff aluminum

Secondly, I have no clue what the actual temperature is. The propeller is certainly force cooling the engine...

That's the part I'm not sure of either. I've seen reference in the 200F range, but not sure where that came from. The piston crown is right adjacent to combustion front which is very much hotter. But that's only brief time duration, then extended cycles of exhaust & intake, so what's the average? I've always wanted to try & mark the inside of piston ceiling with some sort of temp crayon, but not sure if that would work.

 

[digiex-chris]

hmm. I'm tempted to put a thermocouple port into my engine's head to measure combustion chamber temp directly. Wouldn't be able to measure piston temp though.

200F is a common RC two stroke methanol fueled engine head temp, measured externally to the engine, often with an IR thermometer off of the fins.

 

[creast]

Since this is a four stroke engine I guess it also has more time to dissipate the heat.
I am a bit reluctant to mess with my heads after making them twice

Back on the piston clearance theme, my friends Yamaha 50cc racer 2 stroke has a 40mm piston and clearance is only 0.0015" total. I know commercial pistons are high silicon and the expansion is approx 18% lower than the alloy I am using (6082) but in proportion that's really a close fit.
And its air cooled too.. no propeller cooling here!

Maybe I should just accept it works and stop worrying about the expansion?
That's what my mate suggested.. otherwise it could become an obsession.

hmm. I'm tempted to put a thermocouple port into my engine's head to measure combustion chamber temp directly. Wouldn't be able to measure piston temp though.

200F is a common RC two stroke methanol fueled engine head temp, measured externally to the engine, often with an IR thermometer off of the fins.

 

[xpylonracer]

You may find HE15 better for your piston material, available from CES in UK.
Emgee

 

[petertha]

Maybe I should just accept it works and stop worrying about the expansion?

Works for me!
BTW, I measured the OS-56 piston/liner FWIW.
0.9451" = liner bore dia (4mm nominal)
0.9389" = piston crown dia (reduced to ease ring installation)
0.9421" = piston dia immediately under ring
0.9423" = piston dia at skirt base

So notwithstanding the crown reduction, the main piston blank is 0.003" smaller than bore, or 99.68% of bore, or 0.0015" annular gap. What aluminum alloy piston is made of I can't say. The liner is chromed & slightly hatched, typical RC stuff I suspect.

 

[digiex-chris]

I think the crown reduction might be primarily to allow gasses behind the ring a little easier to push the ring out against the cylinder wall. I've heard that theory in a few model engine design discussions and it certainly happens on some full scale engines that have multiple rings, and therefore no advantage on ring installation.

Here I go on off topic theorizing again. Sorry.

 

[creast]

Thanks Guys,
This is all useful stuff Thm:

 

[petertha]

I think the crown reduction might be primarily to allow gasses behind the ring a little easier to push the ring out against the cylinder wall..

I've heard that too, but I don't quite get it. In order for combustion gas pressure to act on the ring radially outward, seems to me it has 3 distinct connected paths or chamber components:
a- down (gap between piston OD wall & liner ID)
b- over (gap between top of ring & top of piston ring groove)
c- down (gap between ring ID & depth of piston groove)

I would have thought a couple of thou open aperture in these connected pathways to a low viscosity fluid (expansion gas) is equivalent to the patio door wide open. ie. it would see this pressure response right away, so 'opening it up' wouldn't do a lot of good especially over such short distances. OTOH, its probably more complicated than that. Maybe they are liquid (lub oil) filled & that changes the gas only assumption.

I do know playing around with that Trimble calculation that it doesn't take much over-expanding of the ring beyond the (heat set) pin diameter before yield is reached & permanent deformation or fracture. That's what made me think maybe its just a safer way of keeping rings intact if the crown reduction doesn't adversely affect performance.

Good discussion!

 

[digiex-chris]

I've heard that too, but I don't quite get it. In order for combustion gas pressure to act on the ring radially outward, seems to me it has 3 distinct connected paths or chamber components:
a- down (gap between piston OD wall & liner ID)
b- over (gap between top of ring & top of piston ring groove)
c- down (gap between ring ID & depth of piston groove)

I would have thought a couple of thou open aperture in these connected pathways to a low viscosity fluid (expansion gas) is equivalent to the patio door wide open. ie. it would see this pressure response right away, so 'opening it up' wouldn't do a lot of good especially over such short distances. OTOH, its probably more complicated than that. Maybe they are liquid (lub oil) filled & that changes the gas only assumption.

I do know playing around with that Trimble calculation that it doesn't take much over-expanding of the ring beyond the (heat set) pin diameter before yield is reached & permanent deformation or fracture. That's what made me think maybe its just a safer way of keeping rings intact if the crown reduction doesn't adversely affect performance.

Good discussion!

Given that these model engines are often worked on by people with less interest in model engineering than model engineers, I'm betting it's a little of both.

I'm thinking that spring tension helps bias the relatively equal gas pressure such that it'll tend to close the gap with the cylinder wall and open the gap with the groove minor diameter. Plus the down force closes the gap between the bottom of the ring and the bottom of the groove. I think o-rings tend to seal the same way, being shoved into the crack by pressure. I'll have to look that up again at a later time, I can't remember where I found that.

Tough to test effectiveness at the scale we're using, and so it probably doesn't matter at all.

 

[creast]

After all this time I am now happy to say the Vega runs on both cylinders. There is still more to do to finish this beast but I am relieved the gal runs.
Crucial thing is it does need an electric starter, I had hoped to flick start but there is no way as a twin you would stand a chance. As individual cylinders no problem.
I was also fooled by my power supply not delivering enough current for two glow plugs so that did not help. Can you believe I have got through 1/2 gal of fuel over the last 3 months of trying to get it to run?
Anyway, I am glad I didn't quit even though I was close to many a time.
Its been a good learning curve.
If anyone decides to build this engine I am happy to help with advice despite the lack of support I had from all the other Vega builders I contacted.
I am off on vacation now but hope to post a video completion soon.

 

[petertha]

Awesome to hear & look forward to the vid! When you get settled back in, I'd like to learn more details of your PS current levels; what you were drawing when it wasn't firing so good vs. 'the fix'.

 

[creast]

Finally!!! I have now finished the Vega and consigned it to the finished projects section of this site. There are some parts which I should have paid more attention to but hey I am not exhibiting this project at a model exhibition and and it was something I wanted to do and yes it almost beat me.
Overall, I am quite pleased. I have learned a lot from this torment. :wall:

 

[digiex-chris]

Very nice!