Author: Joe Wagner
Edition: Model Aviation - 2009/02
Page Numbers: 90,91,92
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The Engine Shop

Joe Wagner | [email protected]

Model diesel engines' advantages

A brief mention of model diesel engines in one of my recent columns prompted Ivo Lucchita (Flagstaff, Arizona) to e-mail me and ask why this type of model engine—his favorite—never became popular in the U.S. He also noted that even in Europe, long a stronghold of model diesel usage, its popularity has declined considerably.

That seems strange to me too. I've done a lot of flying with diesel power. It has several distinct advantages over glow. These advantages aren't obvious at first, so I'll describe them in detail.

Model diesels are adaptable. A diesel can happily spin any size propeller that will fit onto its shaft. I demonstrated that once to a skeptic with a 10 x 4 propeller installed on a .03 diesel. I've also had a .09 diesel turning a 3-inch-diameter impeller in a Kress ducted-fan unit at roughly 27,000 rpm. Both engines started readily and responded normally to throttling. They used the same fuel, too.

Speaking of Kress jets, Bob Kress briefly manufactured a geared-down version of the Cox Pee Wee .020 to allow it to spin larger propellers. It worked, although it was heavy and cumbersome. Model diesels need no such complications. That amazing adaptability comes from a unique feature of model diesels: variable compression. This type of model engine has a movable contrapiston in its head that is adjustable via a screw. Diesels need no spark or glow plug to ignite their fuel-air mixture—the heat generated by compression does that job. Adjusting the compression allows the ignition point to be precisely optimized for the best running performance. That's what allows a tiny .03 diesel to run on a 10-inch propeller and a .09 to spin a 3-inch ducted-fan impeller.

The model diesel's adjustable compression makes it especially useful to model fliers such as Ivo Lucchita, who live at high altitudes. A diesel starts and runs nicely on the same fuel at 7,500 feet (where Ivo flies) as it does at sea level. I demonstrated that several years ago on one of my cross-country car trips. I had a small, easily portable test mount with me, with one of my favorite glow engines on one side and a favorite diesel on the other. As I traveled from one environment to another, from time to time I'd take my dual test mount out and run both engines. As expected, my glow engine's performance varied noticeably. Using the same fuel throughout the trip, I had starting difficulty in high humidity (Shreveport, Louisiana) and at high altitude (La Junta, Colorado). The glow's performance varied too, but the diesel started easily and ran almost the same everywhere. Adjustable compression made all the difference.

True, at high altitudes the air is less dense and the diesel's actual horsepower output is lessened. However, the decreased air density also reduces the aerodynamic drag on the spinning propeller blades. That's why my diesel could turn the same propeller at the same rpm as at sea level. Yes, its thrust output went down. But so did that of the glow engine—except the glow engine's rpm declined too.

Diesel fuel is different; it's roughly 40% kerosene, 30% ether, and 30% castor oil. Its distinct aroma bothers some people, but it reminds diesel-power fliers of enjoyable days at the flying field. The proportion of the ingredients in diesel fuel is noncritical; they can vary quite a bit.

Eric Clutton, "Doctor Diesel," once told me that if you can smell ether in your diesel fuel, it has enough. And Andy Woitowicz (Timmins, Ontario, Canada) has succeeded in starting and running model diesels with no ether content in the fuel. However, doing so required preheating the engine.

Obtaining commercial fuel for model diesels used to be more difficult than for glow engines. Every neighborhood hobby shop carried glow fuel. But the recent decline in America's hobby shops—there isn't one within 80 miles of my home now, where there were a dozen 10 years ago—has made glow fuel a mail-order commodity in much of the country.

However, it's easy to "home-brew" diesel fuel; the ingredients are readily available. John Deere-brand Starting Fluid provides the ether. Kerosene comes in many forms: auto diesel fuel, home-heating fuel, aviation fuel for jets, and even Wal-Mart's Odorless Lamp Oil. As for lubricating oil, castor has long been the standard in model diesel fuel. I've had a few reports of successful use of SAE 70 auto oil in diesel fuel, but for several reasons I prefer castor. Drugstore castor oil works fine—in glow fuel too—and with today's high shipping costs, it's no more expensive than mail-ordered castor oil. Also, model engines using all-castor-oil lubrication don't rust.

One advantage of model diesel fuel that I particularly like is that it doesn't harm any kind of model finish. It won't even affect decals. Another advantage is that diesels run roughly 70% longer per ounce of fuel than similar-size glow engines.

Now for sound. Most of us longtime piston-power proponents like our models to be audible. Yet the general public dislikes model-engine noise, although its decibel output seldom exceeds that of power lawn mowers. That's because of the quality of the sound.

It's like the difference between a clarinet solo by Benny Goodman or Artie Shaw and the squawking of a rank clarinet beginner. I once measured the decibel output of fingernails scraping down a slate blackboard. That never exceeded 60 dBA but was almost unbearable to listeners.

Model diesels put out as much power as glows, with close to the same decibels. But their sound quality is far less annoying. The screechy, high-frequency overtones that glow engines too often produce are absent.

For that reason, model diesels don't need the massive mufflers that we have become used to seeing on our glow engines. PAW diesels in particular have exceedingly small mufflers that barely extend outside the engine's mounting lugs.

Yes, electric power is far quieter than a muffled model diesel. At recent S.M.A.L.L. (Small Model Airplane Lovers' League) meets in Little Rock, Arkansas, it seemed almost spooky to see the sky crowded with powered RC models—but with hardly a sound to be heard. That silence is doubtless one reason for e-power's popularity.

Yet piston power has its advantages too. It's nice to be able to hear your engine changing speed in response to throttle commands, for one thing. And since you need to start a piston engine by spinning its propeller, there's no danger of an accidental turn-on.

Piston engines cost less than outrunner electric systems of equivalent power output. Also, refueling a piston engine for another flight takes less than a minute. There's no need to remove its "fuel tank" for recharging, either.

My recent mention of starting problems with a couple of Magnum four-strokers with unusually high compression brought in considerable e-mail from readers. This seems to be a common problem, and various solutions for it were offered.

One suggestion was to add extra head gaskets, but doing that alters the valve timing. The engines run just fine after startup, so their high-compression design can't be faulted. Several readers came up with special starting techniques that solved the problem:

  • Hand-flip the four-stroke's propeller backward several times, and then apply the electric starter. This reduces the amount of fuel-air mixture in the combustion chamber for at least one revolution, allowing the electric starter to gain enough rotational momentum to turn over the engine until it fires.
  • A two-person technique: the person at the transmitter pulls the throttle stick all the way back and then moves the throttle trim to the cutoff position. The other person at the airplane hand-flips the propeller several times, connects the glow plug, and applies the starter. As soon as the engine fires, the person at the transmitter instantly advances the throttle. This takes quick responses, but it works because with the carburetor's barrel fully closed, the intake stroke draws in significantly less fuel-air mix, reducing effective compression and letting the starter spin up the engine.

I enjoyed reading a unique model-engine book by David Janson. Unlike anything I've seen in this field, his Model Engine Designer and Manufacturer Profiles is devoted to interesting details and backgrounds of mostly unusual power plants—112 of them—each the subject of an individual short article.

Among the engines covered in Dave's book—which is more about the engines than about their designers and manufacturers—are:

  • Technopower five-cylinder radial
  • 1940 Syncro B-30 (first of the "cheapie" all-cast model engines)
  • Herb Wahl replicas
  • Moki M6
  • Krasnoruftskij .60 inline single
  • G-Mark .30 five-cylinder radial
  • Laser .45 four-stroke
  • Fitzpatrick .60
  • Davies-Charlton .29 Twin
  • Hirtenburger (HP) .61

This book is plastic-comb bound and is printed on only one side of each page. It is available for $28 postpaid, direct from the author. MA

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.