Greetings all!
There have been questions flying around lately about diesel engines, and I have been asked about: How do I get one running? How do I tune it? How do I decide what airplane to build? What about prop selection? Fuel?
Ok, lets learn to run a diesel engine.
First off, flying with a diesel engine is a rewarding experience. They are a fantastic for any sport flying, and the equipment you need to bring out to the field is very minimal.
Think you could handle a small flight tote containing lines and handle, fuel, fuel bulb, rubber bands, prop wrench and spare/ different props, "chicken stick", and most important of all, paper towels and cleaner?
No batteries to be dead, and no expensive glow plugs to have on hand.
Lets talk about your flight box gear.
Diesel fuel contains ether, and evaporates very quickly. You MUST keep your fuel in a metal can with a tight cap. If you don't, your either will evaporate and you engine will be hard to start, or won't run at all.
I pour only what I think I might need into a clean empty quart can and leave the gallon can in a cool place. I can always go get more if its needed.
Then there is your fuel bulb. You need a black fuel bulb that is rated for gasoline and diesel fuel. These are still commercially available, so you will be able to obtain one easily. A clear plastic "applicator" bottle from hair coloring or permanent can also work well. If you try to use one of the colored fuel bulbs for diesel fuel the bulb will rapidly deteriorate, and not only destroy the bulb but also contaminate your fuel and fuel system. This is also a problem for rubber bands, hence the spares. You will need to replace them periodically if you use them to mount your fuel tank.
A note on tanks, if you want to use a plastic "klunk" tank, you must change the rubber stopper to the gas and diesel rated one, and use clear tubing inside for the pickup.
Fuel line is also an issue. The best is the clear vinyl type (yes the stuff that hardens up). Not all black tubing is impervious to diesel fuel, so to be safe just use the clear stuff.
A "chicken stick" of some kind is highly recommended as diesel engines must be hand started. Do not use an electric starter on a diesel engine, as engine damage could happen very easily! Do not use your bare finger to start a diesel, as finger damage could happen very easily! If you decide to start your diesel with your fingers, get a heavily padded leather glove or heavy finger protector of some kind. Make your life easy, just use a chicken stick. Snoopy sez so!
Propellers, ahhh,yesss. Have several on hand to try. Diesel engines run at full power. There is no "4-2 break" with a diesel engine, so if want to adjust your airspeed, a prop change is in order. A diesel's power is produced at lower RPM with a lot more torque. Most .15's will easily turn anything from a 7x7 to a 10x4. It just depends on your airplane, and the speed you want it to go. You may need to get a prop reamer and open up the shaft bore to accommodate the shaft on your engine. Many diesels use a type of "sleeve nut," thus requiring the large hole.
Allrighty then, lets talk about that engine.
For now, I am going to assume you have a brand-new engine that needs to break in. Break-in time is critical for any model engine, and even more so for your diesel. The method for breaking in is similar to a glow engine, large diameter small pitch prop, the difference is instead of slow running, run it undercompressed. I recommend at least 30 minutes of bench run time before going flying. It is much easier to mount your engine in a test stand and do this than have to do it mounted in your airplane. The test stand run is much more controlled, allowing you to make adjustments safely. Never, EVER mount any engine in a vise!!!!
OHHHKAAAY
You have your engine securely mounted in a safe test stand, you have a propeller mounted, you have a full fuel bulb, a shop towel or paper towels, and most important, a chicken stick.
First, turn the prop over a couple of times to make sure everything is free, and that the piston will not bind. If it is very hard to turn over, loosen the compression screw on the top of the head. Now give the engine a couple of drops of fuel in the exhaust port, and 1 drop in the intake. The 1 drop is just to make sure the crank has a bit of lube. You don't want a bunch of fuel in the case just yet.
Give the prop a smart flip. If the engine fires, you are ready to proceed. If it doesn't fire, give it another prime in the exhaust port and try again. If after a few flips it still doesn't fire, increase the compression by turning in the compression screw on top of the head 1/4 turn. Give the prop another smart flip with no prime. If it still doesn't fire after 6 or so flips, give it another prime and try again. Keep turning the compression screw in 1/4 turn at a time until you get the engine to fire. Be careful that you don't flood the engine, as you're just trying to establish a starting point, and a flooded diesel will not help anything, except to raise your blood pressure.
Once the engine fires and runs on the prime you are ready to proceed. Fill the tank with fuel, open the needle valve about 4 turns and carefully "choke" the the engine until you see fuel in the line come up to the venturi. Take care that you don't flood it. Prime the engine in the exhaust port and give the prop a smart flip.
Your engine should start and continue to run. If it just runs off the prime, open the needle 1 turn and try again. Hooray! Ya got it running!
Ok, begin by turning the compression screw out a bit. You want the engine to break in easy and not get too hot. Turn the compression screw out slowly until the engine "surges" or "sputters." This "sputtering" sounds like a burst of power, followed by dying out and quickly repeating. Turn the compression screw back in until it runs smooth. The "surge" indicates an undercompressed condition. All right, now the needle valve. Slowly turn it in, and the engine will make a "hard" sound, sort of a spitting sound, and may die suddenly if you go too far. If it dies, open the needle a half turn, give it 1choke and flip the prop. It should restart as long as its still warm. It may also start to "surge" again. If this happens, turn the compression screw in a bit until the engine runs smooth again. Once you get the needle and compression close it's easy to fine-tune. Turn in the needle until you hear that "hard" sound, then back until that sound stops. Then back out the compression screw until it "surges" then back in until it smooths out again. This is almost the setting that you want to fly your airplane. (A flight setting will require turning the compression up about 1/8th of a turn or so)
At this point, shut your engine down, and allow it to cool. Pinch the fuel line to do this. Never throw anything into the prop to stop the engine!
Ok, your engine has cooled, and you're ready for the next run. Fill the tank, choke the engine until fuel comes up the line to the venturi, turn the compression screw in about 1/3 of a turn, give the engine a prime in the exhaust port, and flip the prop smartly. The engine should start and run within a few flips. If it doesn't, the compression screw may need to go in a bit more to get it to start. Once it starts and begins to warm up, start backing out the compression screw again, until the engine "surges," then back in until it smooths out again. Allow the engine to run 5 or 6 mins, then shut down and cool.
Repeat this procedure 5 or 6 times. You will notice that the compression and needle settings will change a bit during this time. This is good, means the engine is breaking in nicely. You will also notice the oily mess left over. You will suddenly realize the need of your favorite cleaner and paper towels. Throw away the soiled paper towels in the outside trash, as your significant other will most likely disagree with the smell if you toss them in the bin under the kitchen sink. Use a degreasing cleaner such as Windex to clean up. I use a mixture of windshield washer fluid water and ammonia to clean up. I will discuss more clean up tips later.
The exhaust should be a light gray / brown color. If the color is black, the engine is overcompressed, or too rich at the needle, and maybe both. Do not run your diesel like this, as it creates excess heat, and will shorten the life of the engine.
Now then, if your engine is used, or "vintage" you may or may not have an easier time getting it running for the first time. If it is clean, then getting it running should not be a big issue. The compression screw should be somewhere in the ballpark, and one bench run should do. Expect an amount of odd colored "crud" to come out of the exhaust if the engine has been sitting for a while. This is normal. If the engine is very stiff and full of crud, you should flush the engine with a solvent cleaner of some type. I use brake cleaner, it breaks up the crud and doesn't leave residue behind. You may also need to remove the backplate if you feel the crud is very excessive, and this makes it very easy to flush clean. Replace the backplate and add a few drops of light oil or ATF down the intake to lube everything. As the brake cleaner will remove all oils.
Then you should be able to bench run the engine. Expect a large amount of odd colored crud to come out of the exhaust, even after a thorough flushing.
Stay tuned...
We will discuss building, and flying tips next lap around!
~Snoopy
This page was upated Feb. 14, 2007