Engine Clinic
By Clarence Lee, MAN 2006

This past month I received a package from Bob Davis who heads Davis Diesel Development/Davis Model Products. Dave sent me one of his diesel conversion heads for the O.S. 91FX engine, along with a diesel-compatible Sullivan fuel bulb, fuel line, fuel tank conversion items, and a gallon of David Diesel fuel. Although at one time I had an O.S. .91FX, which I reviewed back in 1998, I no longer had the engine having traded it to a fellow engine collector for an old-time spark ignition engine that I had been looking for. However, although I could not run and test with the O.S. .91FX, I have over the years run quite a few diesels as well as several glow engines converted to run on diesel with Bob’s conversion heads including the K&B 3.5, and more recently, an O.S. .46XF, so, I knew pretty much to expect.

Both of these engines converted to diesel operation extremely well swinging larger propeller sizes and, surprisingly, idling lower than with glow ignition. Although the larger props would be contributing factors to the lower idle due to more flywheel action, the major factor here would be the higher combustion pressure that generates the heat that, in turn, ignites the air/fuel mixture. A glow engine on the other hand, although semi-diesel in operation, depends on the heat of its glow plug for ignition. If the glow plug’s element cools off during idle, the engine dies.

The first diesel engine I ever ran was a Drone Diesel manufactured by Leon Shulman back in 1947. This was a .29 displacement size, fixed compression, I.E., no compression adjustment, engine intended for control-line aerobatic models. The first engines were of sleeve bearing design that wore out pretty quickly. Later engines had a single ball bearing at the rear of the crankshaft and even later engines had variable compression heads. The last engines were sold with glow heads before production ceased in 1949. Leon Shulman, incidentally, is the grandfather of Jason Shulman, now one of the country’s top aerobatic and pattern competition fliers.

Those that read the manufacturer’s advertisements will have noted that Bob claims a 50% increase in power with his diesel conversion, in turn, making that little .25 into a .40, and the .40 engine into a .60, etc. A month seldom goes by in which I do not receive one or two letters from readers who have seen Bob’s ad inquiring if this is really true. In many cases this is true, but we must qualify that by saying a lot also depends on the particular engine, its timing, porting, compression ration, etc. Generally, when you run a diesel engine, or convert a glow engine to diesel operation with one of Bob’s conversion heads, you can increase the propeller to one size larger, i.e., if your engine normally turns a 14x8, as is the case with the O.S. .91FX, you can now use a 15x8, which the engine will turn at approximately the same rpm placing the power range in the 1.20 engine displacement range, which is pretty impressive!

In some cases, diesels are similar to 4-stroke engines in that diesels are strictly low-rpm engines, as many believe. A modern day glow engine that has been converted can turn up in rpm just as high as their glow ignition cousins, as such, ducted fan engines utilizing Bob’s conversion heads turn up just as high in rpm as non-converted ducted fan engines, and I am told that in some cases even higher. Bob offers diesel conversion heads for all of the most popular ducted fan engines such those in the K&B, O.S. and Rossi lines, as well as special diesel fuel intended for ducted fan use.

So, you may ask, how does a diesel develop more power than an equivalent glow or gasoline engine? It is due mainly to considerable higher compression ratios and higher combustion pressures. Whereas glow engines run compression ratios in the 7.5 to 8.5:1 range, and gasoline engines up to about 10:1. diesel engines run as high as 22:1! Diesels also operate more efficiently in converting fuel into heat energy with the majority of the air/fuel mixture burning, whereas, a glow engine expels considerable unburned fuel out the exhaust. Hold your hand close to the exhaust of a 2-stroke glow engine while it is running and you can actually feel raw fuel on your hand. Especially if running slightly rich. Not so with diesels. This accounts for diesels getting about twice the fuel economy of a 2-stroke glow engine.

Due to more complete burning taking place inside the combustion chamber prior to the opening of the exhaust port, diesels also run quieter. A glow engine, on the other hand, will still have the combustion process occurring as the exhaust opens, and the sooner the exhaust opens, the higher the noise level. Ever run a glow or gasoline engine at night and notice the flame coming from the exhaust? That’s burning fuel. Many smaller diesels will be as quiet without a muffler as a similarly sized glow engine running with a muffler.

Diesel Fuel
For those not familiar with model engine diesel fuel, it is not the same type of fuel sold at your local gas station for automotive and/or tuck use. Model engine diesel fuel is composed of three major ingredients: kerosene, ether, and lubricating oil, with kerosene being the main ingredient. A small percentage of amyl nitrate is added which smoothes the combustion process, makes adjusting the engine easier, and more or less acts like tetra-ethyl lead used to work in automotive gasoline before it was banned. A typical diesel formula would consist of 40% kerosene, 30% ether, 20% oil, and 2% amyl nitrate. The lubricating oil can be either castor, synthetic, or a combination of both. In years past, diesels required high oil content fuel, but the is not necessary with modern engines. The different displacement size engines do have different fuel requirements; little 1/2A engines require more ether than the larger SuperTigres, etc., and bob has fuel mixes to meet these needs.

With all the good aspects of diesel engines pretty well covered, what are the bad? First you have two adjustments to make: the compression ratio and the fuel mixture. The compression ratio is razed and lowered by turning an adjustment screw in the head of the engine. Bob’s heads use a socket head screw that is prevented from turning by a smaller setscrew and a length of friction material, i.e., no lock nut is required as some diesel engines use. When starting a diesel, the compression adjustment screw is turned in about one turn. Then, after the engine is running, it is backed out the same amount. The fuel mixture is adjusted much the same way as a conventional 2-stroke engine. There is some interaction required before achieving the correct engine adjustments, which is covered, very extensively in Bob’s operating instructions. Although this may seem a little intimidating at first, after running a diesel a few times, it actually becomes quiet easy.

Second, burning kerosene, and ether has a distinct smell that not everyone finds a beautiful perfume. Even with various fragrances added, it is hard to mask the diesel fuel odor.

Bob markets over 100 models of his conversion heads for just about every make of engine ranging in size from the Cox 1/2A engines to the big SuperTigre 4500. Prices range from $19.95 to $99.95. Bob also markets an extensive line of German-made Soundmaster mufflers, CO2 engines, and other model related products. Check out his website, Davisdieseldevelopment.com for the full product line as well as a considerable amount of other interesting information.