This will be my final report on the 60 degree Vee Twin design.
Having made 2 engines, one a sideport, the other a conventional shaft induction port and now having had the chance to appraise them with various "experiments" - I can say that the inefficiencies inherent in the plain 2 stroke design cause the engine to only have the power of the single cylinder from which it was derived.
These are my observations.
Sideport
Based on an ML Midge which allows no control over the timing the induction phase of the engine, it actually produces less power than the single cylinder upon which it is based. It will run and run quite nicely if in my case inverted, but weight for power it is only about 40% efficient compared to that single cylinder version - or - power wise 80% efficient as a single but with double the capacity.
Shaft induction
Based on a single again, this time David Owen's wonderful Owen Mate. This engine showed a lot of promise when it was initially run as a single with the No2 hole blanked off. In fact using a 10x6 wood prop as the benchmark, the Owen Mate gave 5,500 rpm and the 2 cylinder/piston combos gave 6100 and 6150, quite a bit more power in what was a bigger crankcase volume and with much more conservative inlet timing. I think the ball bearing at the conrod end of the crankshaft has helped here, but this is very interesting and maybe worthy of some discussion later. However as a 2 cylinder engine the best I could get was 6100, so in fact again the second cylinder was contributing only enough to overcome its friction in my opinion. Further experimentation with blanking off the transfer ports to try to force some of the mixture into No2 cylinder only seen it diminish the power and with 50% blockage it struggled to get 5200rpm. In all cases the No2 cylinder was not firing, or if it was not enough to heat the muff enough to make you let go of it with your fingers, whilst No1 was the opposite. I believe where No2 contributes is with increasing the crankcase pressure meaning that No1 is like being supercharged and the effect is about equal in more power to the loss through friction of the other components.
I did say at the outset that I beieved there was a 50% loss with a 90 degree twin and a 25% loss with a 60 degree twin - now I realise that this was without friction losses - and that the friction losses look like robbing most of the extra 25% so that the end result is 50% loss for the 60degree Vee Twin - or in other words 50% loss from a double 2cc engine gives the same power as a single 2cc engine - which is what we have proven here.
I certainly don't regret going down this path, I now have 2 nice looking engines that actually run, but not with quite the power I was hoping for.
Overall the vibration was not too bad, with double the reciprocating weight you would expect vibration to be worse than a single, but in both cases I tried to remove as much weight around the crankpin as possible and I am sure this helped.
The V-HOT I would class as a success, however the 60 degree Vee twin two stroke diesel design is a definite failure, fun to build, a bit of a curiosity as I said before - but it does look good and as No8 engine now made - it is my favourite so far.
The photo here is of all 8 engines made, left to right back row the first ones then left to right front row the more recent ones.
It is a very rewarding hobby and my next engine will be either a inline or horizontally opposed twin, but meantime I have an idea ...
I believe there are many guys out there that would like to build an engine but just need a bit of help to get started ... thinking along the lines of a BollAero18 and developing a class/clinic online through HMEM where the "pupils" can share ideas, ask questions, and encourage each other to "give it a go"
Engines
L-R back row BollAero 18 - Owen Mate - Mills 1.3 - Holly Modded Owen Mate
L-R front row Holly Sideport - ML Midge - Butterfly - V-HOT
Ed
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