The Davis Diesel head is a precision engineered replacement for the stock glow head.

When using a modern automotive fuel, for example, in an injected internal-combustion engine, a micro-computer determines the correct time to fire the spark plugs based on sensors that monitor engine conditions. We don’t have anything like that with our carbureted model engines. We only have a few tuning elements at our disposal: Our primary tool is the carburetor needle valve that we use to set the fuel mixture. A leaner setting means more heat inside the combustion chamber; more heat means that the fuel is ignited “earlier”; it’s what can be referred to as “advanced” timing.

We also have fuel with different nitro percentages. In practical terms, more nitro means more power and more heat. We can also install different glow plugs, some of which are “hotter” than others. A “hotter” plug translates into more advanced timing, too. In cases where we need to “retard” the timing, we can use less nitro or install a “colder” plug or additional head gaskets to lower the compression ratio slightly.

Now, here’s something interesting: the compression ratio affects the heat generated during the compression cycle and, therefore, the ignition point of the fuel. This is key to understanding how to tune a diesel engine.

TUNING A DIESEL

In a diesel, the only tuning elements are the fuel, the compression ratio and the fuel-mixture setting (needle valve). Since you should be using the fuel recommended by Davis Diesel for your particular application, let’s concentrate on the compression ratio and fuel mixture. A Davis Diesel head allows you to adjust the compression ratio and, therefore, the “timing” of the ignition point simply by turning a hex capscrew on the top of the cylinder head. The instructions included with the new cylinder head describe the process in detail, so I won’t repeat it here.

What’s neat about this is that you can change this important tuning element very easily without taking your engine apart or buying additional parts. You’ll always have peak performance, regardless of atmospheric conditions and prop selection. Yes, the propeller affects your engine’s timing requirement. If you change props, you need to retune the engine for peak performance. There’s no way around that, but with the Davis Diesel conversion, you can tune the engine to run with a much wider range of props than a glow engine ever could. And with a diesel, you don’t need to worry about replacing glow plugs either!

Setting the mixture is not difficult when you have the compression and timing dialed in. If you can tune a glow engine, you shouldn’t have any problems with a diesel as long as you understand the fundamental differences between the two types and try to “unlearn” your glow-engine tuning technique. It took me a while to get used to the different sound of the diesel, but it does give you some clues if you do something that makes it unhappy. I learned that if you set it a little too lean, when you increase the load e.g., by flying an upline, the engine will surge. Unlike a glow engine that will simply lean out and quit, the diesel will give you a warning.

CONVERSION OVERVIEW

So, this all sounds great, but how difficult is it to convert your glow engine to diesel? Davis Diesel has done all of the hard work for you. They have heads for all of today’s popular engines. They also have accessories to make your diesel engine experience hassle-free. In addition to removing your existing cylinder head and replacing it with the new head from Davis Diesel, you’ll need to make sure that your fuel system is compatible with diesel fuel. This is easy because you use the same components as you would use for a gasoline engine.

Most fuel tanks are compatible with glow and diesel fuel, but the fuel lines and soft parts vary according to which type of fuel you use. Davis Diesel offers everything you need, or you can use the Tygon fuel lines offered by Du-Bro and the Sullivan gas conversion kits for your fuel tank (available at most hobby shops). To pump fuel into the tank, I use a Dave Brown Six-Shooter pump that’s mounted on a metal fuel can and made specifically for gasoline. You don’t want to store diesel fuel in a clear plastic container.

Diesel fuel costs a little more than glow fuel, but you’ll use a lot less. Some people report using half as much diesel as glow fuel but I’ve found it to be closer to two thirds as much. It depends on your
airplane and flying style.

GLOW & GAS COMPARISON

An O.S. 46LA spins a 13x4 APC prop at 10,000rpm, easily hovering a 4.5-pound 3D ship!

To see whether this conversion was worth the effort, I wanted to compare the glow-fueled version of an O.S. 46LA with its diesel counterpart. I installed the engine in a Lanier R/C Radical 330 profile 3D airplane. I started with a brand-new out-of-the-box O.S. 46LA. This is a reasonably priced sport engine with bronze bushings and a relatively small, air-bleed carburetor. Nothing fancy here.

The real performance gains when using a diesel conversion show up when you increase the prop size to the outer limits and beyond what a glow engine can spin.

The plane had plenty of top speed; I estimated about 70mph, which is faster than you should fly a profile 3D airplane. Grass-field takeoffs were short and the landing speed was reasonably slow. I easily flew aerobatic maneuvers from level flight, but vertical performance was somewhat limited. Basically, the airplane flew very well, but long uplines with rolls presented a bit of a challenge. Hovering was not easy and required almost full throttle. This is where you need thrust, and an 11x6 clearly was not enough for this 4.5-pound airplane. As for noise, I measured 87dB at 9 feet from the muffler. No surprises here, and then it was time for the Davis Diesel conversion.

While the engine was cooling, I removed the fuel tank and converted the fuel system to diesel with a new stopper and new fuel lines. Don’t forget to replace the fuel line on the remote needle valve! Having done that, I removed the original head and head gasket and installed the Davis Diesel head with a new head gasket. I began with the prop that Davis Diesel recommended for this engine: an APC 13x4. I must admit that I had my doubts at this point, since I’ve never seen a .46 engine that could turn a 13-inch prop without a tuned exhaust!

After reading the instructions (always a good idea), I filled the tank with diesel fuel and cranked the engine. After some cranking with the starter, the engine came to life (you have to flush all of the glow fuel out of the crankcase), and I dialed in the compression setting with my tachometer. The first flight proved that I was a little on the lean side because I heard the engine surge on uplines. I landed and opened the needle slightly. Much better. Now I was able to hover at about 1/2 to 3/4 throttle. The punch-out wasn’t great, but it was much better than with the glow-engine setup. In fact, it was almost comparable to an electric setup.

This little engine turns a 13-inch prop at 10,000rpm, and the idle is extremely low. How does an idle of 1,800rpm sound? Remember, this is a little .46! The diesel is quieter than the glow setup, too (3 to 5dB). This is partly the result of turning a larger prop at a slower speed, but it also has to do with the way the diesel operates. Throttle response is excellent, and you don’t have to worry about flameouts cause by the glow plug’s cooling off because there isn’t a glow plug. I’m impressed.

DIESEL BEATS GLOW IN 3D

In fun-fly and 3D, the diesel was clearly superior to its glow-fueled counterpart. It swings a larger-diameter prop, increases throttle response and has a super-low idle speed, so it makes the plane easy to handle. I must admit that when I started this project, I had my doubts, but the Davis Diesel conversion setup performed much better than I expected once I had the engine running right.

There’s a bit of a learning curve, but once you get past that, you shouldn’t have any problems with setup and tuning.

So why don’t we see more diesels at the flying field? I don’t know. When you look at its advantages—being able to swing a larger prop, better fuel economy, lower idle speed and quieter operation—you have to wonder. There is, of course, a downside: diesel fuel doesn’t exactly smell pretty. But some people find the odor of glow fuel offensive, so I guess it’s just a matter of preference. The slight discharge of burnt oil and fuel that leaves a properly tuned diesel system’s muffler and leaves a dark residue on your plane is not something you want. The solution is a hose extension: add a piece of latex tubing, automotive fuel line, or something similar to the muffler; unlike with a glow engine, this will not noticeably reduce performance.

CONCLUSION

With the current nitro shortage affecting the sales and price of glow fuel, many people are saying that the only choices are gas or electric, but they should think again. There is another option: diesel! I recommend the diesel conversion experience to anyone who is looking for an interesting project that’s sure to attract attention at the flying field. Diesels have a unique sound that you have to hear for yourself. If your club has noise restrictions, you’ll make new friends among the club’s officials and any nearby neighbors. And wait until someone sees your .46 engine with a 13-inch prop. Be prepared to answer many questions. You may even become known as the club’s “diesel guru”!

Please contact Davis Diesel

Links

Dave Brown Products, www.dbproducts.com, (513) 738-1576

Davis Diesel Development, www.davisdieseldevelopment.com, (203) 877-1670

Du-Bro, www.dubro.com, (800) 848-9411

Sullivan Products, www.sullivanproducts.com, (410) 732-3500