Ready To Go|
Here's the completed engine -- after its first successful test run. I meant to take some pictures before the run but I was too impatient and just had to get it making some noise, heat and flames first.
I could have lied and said these pictures were taken before testing but the more observant amongst you would have noticed that the original silver color of the stainless exhaust tube has been changed to a nice bronze color after a long run during which it glowed red hot.
Note that my test stand is not exactly a masterpiece of high-tech sophistication.
It's a converted school-desk with a weight off my dumbells thrown on "just in
The Intake End
Initial attempts at starting the engine quickly showed that the fuel jet was in the wrong place. That brass fitting on the side closest the camera is the original fuel jet but it enters the intake at the narrowest point of the venturi -- my reasoning being that this would be the point of lowest pressure when the engine was running (highest air velocity = lowest air pressure = better fuel draw.
Unfortunately I forgot that you've got to blow air in the front of the engine to start it and with the venturi in this position the fuel was getting blown back up the pipe so that when the engine fired there was no fuel at the jet and it rapidly flamed out again.
I also found that these engines need a lot more fuel than I'd expected. This engine requires that you get copious quantities of fuel into it before it will fire up -- and with the fuel jet at the narrowest point of the venturi it meant that fuel was running out of the front of the engine rather than flowing back into the combustion chamber when the engine wasn't running.
So, I turned up a new fuel jet (the one on top) and angled it back towards the reed valves so that any excess fuel when starting tends to run into the combustion chamber where it's burnt off pretty quickly once the engine fires.
That arrangement of washers and bolt on the plastic fuel line is the very crude mixture control. You'll note that the original jet had an adjustable screw for setting the mixture but I couldn't be bothered the second time.
The fuel tank is a plastic bottle which is held in position by some brazing
rod bolted to the test stand. It's pretty crude but it is very easy to
adjust the fuel head by sliding the bottle up and down.
The Hot End
After I got the engine running I thought it would be interesting to see what effect changing the length (therefore resonant frequency) of the exhaust tube would have. I cut a piece off the exhaust tube and the engine seemed to run better so I decided I'd try a longer tube and built this simple sleeve out of an old Jolt Cola can, a couple of bits of aluminum strip and some cap-head machine screws. The idea here is that I'll be able to now slide the old piece I cut off so as to make the effective length of the tube even longer than it was when I started. I'll see what difference it makes to the engine's power and handling.
that I haven't tried this yet... I have a strange feeling that the thin aluminum
from the Jolt can will melt :-(
The problem I then faced was that on the Sunday when I decided to fit the spark plug, I found that my 100 piece tap set had every size and thread type you can think of -- except the one used on this spark plug! Ugh!
So... I ended up throwing the spark plug in the lathe, turning it down to remove the existing thread, rethreading to match an available tap and then making a small plate steel piece that the plug screwed into and which was then welded onto the combustion chamber. What a procedure! -- still, what's the point in having all these tools if you don't use them now and then?
The spark is provided by an unbalasted automobile coil which is driven from a relay that is wired to act like a buzzer. There's actually two relays because my box of junk didn't have a three-pole relay that could handle enough current so I had to wire it to buzz and then run a slave relay from it that could switch up to 20 amps of current.
Again, it's a pretty low-tech solution but it works fine.
My Compressed Air Source
Because it runs on 12V I can power it from the same battery as is used to drive the ignition coil.
It also now gets put into service sucking up all the swarf and crud that builds up on my bench and around the lathe, grinder and drill press.
I paid the princely sum of US$9.99 for this little unit so I'm not too
concerned if it expires after a month -- and I don't feel guilty filling
it with metal filings when cleaning the shop.
After half an hour of this I was feeling a little despondant and sick from the smell of the smoke and gas fumes. Fortunately I remembered that I had about three gallons of methanol which is a much sweeter-smelling and cleaner-burning fuel so I tried that.
What a difference! As soon as I switched to methanol I started getting some encouraging pops and bangs from the engine and, once I'd sorted out the fuel system by changing to the new design of fuel jet, I had some good sustained runs.
Another nice thing about methanol is that the inside of the engine is left clean as a whistle. When I dismantled it after trying with gasoline it was black with carbon -- now it's almost spotless.
One thing to watch with methanol though is that it has a lot of water as its primary combustion byproduct and that means that the steel reed valve will rust if you don't strip the engine and give it a light oiling.