When you put a Davis Diesel Converter Head on a standard glow engine (in place of the glow head that comes with the engine), you gain several advantages; i.e., the engine sound is quieter in flight. The engine will develop more usable power. The power can be used to turn a larger (and much more efficient) prop just as real aircraft engines, which are often geared to turn a bigger prop for gained efficiency. The engine sucks in less air and the venturi air sound is lessened. The engine needs no glow plug and there are other gains, such as doing “something different” which many of us enjoy as R/C modelers.
What’s different?
The fuel is different. The average life of the glow plug engine is estimated by experts to be almost 15 air-hours. Glow fuel’s methanol has no natural lubricity; model diesel fuel’s kerosene is from the petroleum family and adds natural lubricity to your engine. You use non-silicone fuel line like neoprene or gas line as sold at lawn mower shops. Your fuel tank (if it contains a silicone plug) may require substituting a Du-Bro #400 gas conversion stopper. Engine life is normally longer when it’s dieselized.
How does it work?
Ignition in your glow engine starts when the fuel comes in contact with your heated glow plug. With a diesel engine diesel fuel, the heat of compression (the result of the piston rapidly compressing air/fuel in the cylinder) causes the fuel to self-detonate and the engine starts! The fuel is blended with either and other additives that contribute to easy starting. I use my 12-volt electric starter to turn the engine over, and the engine’s needle valve works just the same as with glow fuel.
How complicated is it?
The Davis Diesel Converter Head add one more control to your engine. The added control is a hex screw where the glow plug would normally be. The hex screw drives down what’s called a contra-piston that fits on the bottom of the Converter Head. When screwed down, the contra-piston makes the combustion chamber smaller (the compression ration actually increases a bit) and the engine’s timing is advanced much like a distributor advances timing on a car engine. Adding nitromethane to your glow fuel serves to advance the timing too. Advanced timing normally goes with higher rpm. Backing up the screw (and using nitro fuel) retards the timing so the combustion cycle is easily matched to the size of the prop and the rpm at which it turns. Backing up the screw retards the timing and prevents pre-ignition or detonation, which we all know ruins all engines in our models and in our cars too. I find that with the same diesel fuel and the same prop. I don’t normally alter the compression control once it is set. Simply said, a diesel conversion is not complicated.
Where does the added power come from?
Simple, methanol in glow fuel has about 10,000 units (called BTU’s) of energy per pound. Kerosene has about 18,000 units per pound and the ether that’s in the diesel mix has about the same amount of energy as methanol. The added power comes from using a much more powerful fuel: kerosene.
How does it start?
I choke for a few flips, flip the prop with a chicken stick to be sure there’s not too much fuel in the engine, then use my electric starter, it’s that easy!
How do you handle the fuel?
Model diesel fuel comes only in metal cans for safety. Quarts and gallons are the common size at your hobby shop (the hobby shop can get it from Great Planes Model Distributors). I use a 3-ounce ear syringe sold by Walgreen drug stores, with a length of smooth NyRod on the top to reach the bottom of the can. Fuel is drawn from the can and the can’s top is quickly replaced/tightened so the ether won’t quickly evaporate and result in poor starting.
Being a petroleum product, model diesel fuel is not compatible with silicone fuel line, so I use neoprene from the hobby shop. Du-Bro’s #400 Gas Conversion Stopper (the black one) converts any Du-Bro tank to diesel-compatible. The Hayes tank is okay as is. Also, Sullivan’s #S171 6-ounce fuel bulb is a good choice too, in place of the ear syringe. When through flying, I squirted the syringe contents back into the can, then rinse the syringe contents back into the can, then rinse the syringe with alcohol to clean it for the next flying session.
How about the smell?
Yes, some R/C modelers may not like the odor of the fuel, I happen to think it’s a “hospital-clean” aroma.
How about the black exhaust?
Eighteen wheel diesel trucks often put out black sooty exhaust. Your dieselized model engine will too. You can handle the soot that’s trapped in the exhausted oil with a piece of hardware store tubing over your muffler’s exhaust outlet wire-tied to a landing gear strut below the model. It’ll only cost 200-300 rpm and the model stays cleaner as the sooty exhaust oil exits down the tube.
Just how much is the power increase?
It’s a major power increase for your model with a Davis Diesel Converter added to your glow engine. My experience says different engines have different variations in the increase. My Fox.15 was broken in as a glow engine and the 8x4 Master Airscrew peaked at near 12,400 on glow. With the Davis Converter Head it went up to 13,400 which is a major horsepower increase burning Davis Diesel fuel. About .152 horsepower versus .210 as diesel.
Additional performance figures!
The most popular R/C engine size is a .40. We’ve had a fine performing thunder Tiger GP-40 glow engine that’s been comfortably turning a Master Airscrew 10x6 at 14,400 for high rpm and 1900 for low. This is the low cost sport engine that’s about like the O.S. .40FP and the new Tower .40 engine, with no ball bearings and the small diameter throat through the carb. With an 11x6 on glow, the engine was “uncomfortable”: with a 12x6, it showed severe overheating signs right away. The bigger props caused preignition detonation and overheating as expected. With the 10x6 on glow, it put out about .55 horsepower at 11,400. We installed the Davis Diesel O.S. .40FP Converter Head (they’re about the same price from either your local hobby shop or Tower Hobbies) and got the results on diesel (shown in chart below.)
Final thoughts
Heat, speed and lack of lubrication ruin engines. A glow engine with a Davis Diesel Converter Head running on Davis Diesel fuel is more efficient running a bigger prop at the same rpm as it turned best as a glow engine. The 11x7-1/2 as a diesel would be a good choice to hold the rpm down to the glow-like rpm in the figures in the chart. Dieselized R/C engines are different as far as power, use, appeal and fun. Many people love diesels, you might too!
Conclusion
Bob Davis has received AMA’s Technical Award, a well-deserved honor.
