The little sideport engine burst into life yesterday about 2.00pm
A lot of lessons learned on the way and I will list them here for any other budding enthusiasts who might be thinking of designing an engine from scratch, but before I go there just a brief mention about the conrod.
I decided I wanted to try machining this metal Titanium, to learn for myself about how it behaves on the lathe and the mill after reading as much as I could about it. Have to say that I found it no problem to work with, admittedly a very sharp tungsten carbide tip on the lathe was used and a HSS slotting mill on the milling machine. However .... this material lasted all of 10 seconds or so it seemed in service. In fact from the moment I turned the engine over by hand there was a decided scratchiness to it, which was surprising as I used a ball bearing on the conrod end of the crankshaft and a plain bearing at the prop end. After about a half hour of trying to start the engine with various pops and a few multiple rotations, it developed an obvious fault. All this would have been the equivalent of maybe a half minutes running. On pulling the engine apart, the bigend of the rod was very oval and there was a real mess inside the engine from powdered Ti. The crankpin looked and felt like it had 1200 wet and dry applied to it as well.
Thankfully the internals cleaned up without any damage to the other parts and all was cleaned a couple of times in the ultrasonic cleaning machine.
A new alloy conrod was made and installed, and the engine started and ran within a few flicks. Obviously the abrasiveness of the Ti being galled was creating so much friction the engine wouldn't run, but now with the alloy conrod, it turned over extremely freely.
The engine has really nice handling characteristics, but not much power. The port timing is about 145 exhaust 110 inlet and 100 for the transfer port. The transfer port timing is about to be increased to around 130 degrees and I am sure this will make the engine run with a lot more power.
The things I have learnt in designing and building the engine are ...
1. There is no substitute for a very close piston to bore fit. I now achieve this by making circular "feeler gauges" out of alloy and the time spent doing this is inconsequential to the knowledge of what the bore size and taper is like. I make them with a hole in the centre and attach them to a 1/8th screw between 2 nuts and with a bit of 30 grade engine oil to stop galling poke them down the bore - they are the perfect way to measure the internal size at various heights within the bore itself. The bore is lapped using a David Owen design of a lap which is simply made by slitting a piece of alloy rod, tapping with a 5mm tapered tap so that it can be expanded and using diamond paste to get the bore shape and size where you want it.
2. Similarly there is no substitute for one of David's piston laps to bring the piston down to the exact size of the bore. Initially when you try to push the piston down the bore with your hand you shouldn't be able to - it should pinch about half way up, but as soon as you put a prop on it the leverage allows it to go over TDC, and you have as near as perfect a piston to bore fit as you could want..
3. The Contra Piston was made again using one of David Owen's ideas by making the solid portion at the bottom a neat fit in the bore, but the top section bigger in diameter with a thin wall. This is achieved by cutting it with about a 1 degree taper. This is the best fitting and most progressive moving contra I have achieved in the 5 engines I have made so far.
4. The design went through a few revisions along the way, some from finding the original design had a problem, some to make it simpler. Having started on the premise that I wanted to base the engine around a 25 x 25 mm piece of square alloy, this really complicated things and having made it now, I can see that it would work better if I had left more overhang at the rear of the engine to screw with 4 capheads the rear cover rather than to use a internal and external screw thread, but then still have to use screws to prevent it rotating as the rear cover is also the engine mount.
5. The engine as stated above does not have a lot of power, and the transfer port timing is to blame I think. I also think it is not helped by the venturi arrangement I have, I think I need less area around the spraybar to create a faster airflow there and maybe pick up a bit more fuel suction.
6. I guess at the end of the day this was a design exercise from an idea to fruition, again wanting to use a piece of square bar. The end result has probably taken about 60 hours of lathe and mill work plus about 15 hours of design and pushing a pencil and ruler etc around the paper. I have downloaded a CAD programme - but after about 2 hours have given up on trying to earn to drive it. If anyone wants the plans for this I will do them freehand again, as the ones I have done I can understand, but they are far from "professional".
7. I think the first aim was to have an engine that was capable of being built in under 40 hours, I still think that is achievable. I also think that it can be made simpler and in time I will try to make another if I can work out how that simpler might be !
8. The engine is a bit heavy for 1.5cc - but having made it now, I know that it can be stretched easily to 2.5cc for very little additional weight. One thing that is a bit excessive is the cylinder muff - but I like the roundness of it, probably 3/4 oz could be pruned from the 6.25 oz the engine weighs. As a 2.5cc engine it would probably only add a half oz to this and with a more modest muff would probably come out at around 6oz.
Have attached a couple of photos of the engine here. If anone wants the plans I will redo them as said, might take a bit of time though.
Finally, I guess out of all this I have found that like with many things if you are prepared to have a go then you most often will get a result near what you want. Also in any engine building, even from plans, don't be afraid to alter the little things that you feel can give a different result - think it through - do a drawing and make it. If it doesn't work out then you can always go back to original, but by changing things a bit along the way, you learn so much more sometimes.
Ed
Yes it does run !!
Photo of the exhaust side
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