Cox Diesel Conversion

anm2 Sun Oct 13, 2013 11:40 am

I received my Davis Diesel Fuel (.049) in the mail on Friday so I decided to start my conversion engine. I am using a reed valve with a strong crankshaft, Teflon Reed (tested the seal before I put attached the engine), and Davis Diesel conversion head. I was able to get the engine started by following the directions. I had to play with it a little bit, but she starts fairly easily. The problem is that I can't keep it running for more than about 1 Minute and I am using an 8CC (non-vented) tank. Here is how it goes: I get it started, then I dial in the compression on the head till it is running smoothly. Then I adjust the needle valve to gain a little RPM. The engine sounds great, and then it cuts off at about 45 Seconds. I back the compression off a little, and restart the engine. It generally restarts quickly, but again, it will only run a short time. While the engine is running, it sounds very strong and smooth. Any ideas or suggestion out there in diesel land? Thanks,

Andy

ian1954 Sun Oct 13, 2013 2:04 pm

You have acheived the most difficult hurdle in getting it to start but you may be being a little impatient in making it perform.

I would recommend running Cox engines in on glow fuel before dieselising and get everything bedded in but failing that don't lean it out.

There is a subtle relationship between the fuel mixture and compression on a diesel.

The compression screw and needle valve settings interact with each other. The main adjustment is the compression screw.

When the compression has been set properly, the next step may be to lean the fuel mixture more.

That will make the engine run hotter, which advances the ignition point, and that in turn may require reducing the compression to retard timing slightly.

It is difficult to tell what is going on without hearing it but trying to "peak" the engine at this stage if it is not run in is not a good idea.

If it is running softly, misfiring, skipping, loping .................then the compression is too low. Turn the comp screw in 1/8 turn.

If it running harshly, sounding labored, rattly, cackly..............then turn the comp screw out 1/8 turn.

Repeat adjustments until the engine is running smoothly and then adjust the needle valve.

Is the engine exhaust very oily? Is the engine four-stroking? If so, it is running too rich. Close the needle valve 1/8 to 1/4 turn, allow a few
seconds for the change to take effect, and check again. Repeat this check until the engine is "singing" at a good speed.

When the engine is tuned for nearly full power, it will heat up and this advances the ignition. Has the engine sound become labored, harsh, rattly, or crackly again? If so then reduce the compression 1/8 to 1/4 turn and check the sound again.

Continue to close the needle valve in small steps. If the engine speeds up, you are going in the right direction. Repeat the procedure. If it
begins to misfire, it is probably too lean. Open the needle valve enough to restore smooth operation.

If misfiring occurs, the compression may be set too low. Turn the comp screw in 1/8 to 1/4 turn. If the engine speeds up, you are going in the right direction. If the engine sound becomes labored and harsh, back the comp screw out to the former setting or even beyond.

Do not fly with a diesel engine you have tuned to peak "on the ground".

Usually it is necessary to richen the mixture slightly and increase the compression slightly from the initial settings to develop maximum power in the air.

Richen mixture - increase compression

Weaken mixture - decrease compression

anm2 Mon Oct 14, 2013 5:47 am

Ian, Thanks. The engine responds pretty well to the adjustments I make. I am doing it pretty much the way you describe, however, once I have it running smoothly, it only runs for about 45 - 60 seconds and then it just cuts off. Based on what you describe below, it may be that the engine gets too hot, and the compression point advances. I will fiddle with it today and see what I can do. Thanks again. Andy

Paulgibeault Mon Oct 14, 2013 7:14 pm

Hi Andy,
Well you gotta ask yourself one question...Do you feel lucky??
Err... what I mean is, are you sure where your pick up tube is located? With the engine pick up tube (normally) located at the 3 o'clock position (looking forward), one minute is about all I get on glow fuel. If I want to use up the full tank of fuel, I tilt my test stand on it's side & tilt the nose up a bit. Only then will I use up all the fuel on the ground. Then I get ~ 2:00 - 2:45 min. runs. Unless you "fix" your fuel pick up tube in position, often the neoprene tube can work its way out of position & give you short runs... Just a suggestion. Good Luck!

Cheers, Paul

P.S. I did have a similar problem with a glow recently with a 'good tank'. The "final solution" ended up being a change of the piston/cylinder ass'y & then the run time returned to normal. I think that engine's cylinder might have been out of round or something because otherwise it looked perfect...

anm2 Mon Oct 14, 2013 8:04 pm

Hi Paul,

Thanks for the suggestion. I placed the pick up tube at 3 O'clock for C/L flying. I should have placed it at 6 O'clock for testing this engine, but habits seem hard to break. With an 8cc fuel tank on glow fuel, I seem to do 3 -4 minutes on the ground. I often use the trick of tilting the test stand on my last engine run to get as much fuel out of the tank as I can. I had heard diesel was more efficient so I was expecting a longer run.

I was able to play with it today and got one tank to go 3 minutes and several other tanks to run at about 2 minutes and 30 seconds. That still doesn't seem long enough. With regard to yesterday, I really think my problem was that I used a brand new cylinder and piston. The Davis Diesel instructions suggest breaking in the engine on glow fuel before switching to diesel (as did Ian). The instructions they gave me worked very well, maybe I should follow all of them:) Anyway, I have a lot to learn about diesels, but my first foray was excellent, and I am going to keep at it. The mylar disk is a weakness on this design. I have read it in several posts, but I experienced it first hand these last two days. I think I am going to try the RJL Diesel head and see what that gets me.

Using a reed valve engine with a standard two slit (non-SPI) cox cylinder and piston and the Davis Diesel conversion head, I able to turn a 7X3.5 Prop at around 9800 RPM, and sounded very strong throughout every run. Someone will have to tell me if that is good or not. Thanks again. Andy

Surfer_kris Tue Oct 15, 2013 9:40 am

The performance sounds fine on the 7x3.5 fine prop. The inconsistency in the runs may very well be related to the teflon disk, they never seem to give a reliable seal (in my experience).

For more consistent runs you can put a thin brass gasket instead of the teflon one.
The RJL heads do not have these problems as they use a o-ring seal.

Martin AU Mon Jun 16, 2014 9:45 pm

I have come across this issue as well with my engines.

The problem was the timing. After the engine has warmed up a little it would cut out after about a minute. Easy to restart without a prime but the engine speed would slowly reduce to an idle before stalling.

Solution is to back of the comrpession screw.

Its difficult to describe the sound but you can definitely tell when the timing is too far advanced. When learning the Diesel ropes, I bench ran the engines with earmuffs on and there is a very slight metallic ringing to otherwise stable exhaust note. Its a difficult sound to describe but the best comaptison would be to grab a steel rod and gently drag it over smooth concrete. That kind of sound, but quite low down in the exhaust whine. Retarding the ignition point eliminated that tone, stabilised combustion and the engine no longer stalled.