Author: Joe Wagner
Edition: Model Aviation - 2011/04
Page Numbers: 76,77,78
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The Engine Shop

Glows vs. diesels in detail

Throughout the years that I’ve been writing this column, I’ve often mentioned the many virtues of diesel power plants for model airplanes. But until now I had never directly compared a diesel with a same-size glow, spinning the same-size propellers, on the same day. And I hadn’t yet discussed in much detail the minor adverse quirks of diesels and how those can be overcome.

I’ll do that now.

Test engines and setup

For these comparative tests I chose four of my own well-broken-in and currently available engines: a pair of .19s and a pair of .40s.

  • Diesels: British-made Progress Aero Works (PAW) .19 and .40.
  • Glows: Thunder Tiger “Green Power” .18 (.19-class) and NV (formerly Norvel) Revlite .40.

These tests were not laboratory-grade, high-precision procedures. Instead, I carried them out under roughly the same conditions as the average modeler would at his or her local flying field—with one major exception: I used the same propeller on both .19s and another same prop on both .40s. That forced all of the engines to run at less-than-optimal efficiency: the diesels ran slightly smaller props than they’d prefer, and the glows ran slightly larger ones. Still, as experienced model fliers know, the airplane has more influence on optimum prop size and pitch than the engine’s size and type. All I hope to show here is a basic relationship—not a set of cast-in-concrete aeronautical engineering formulas.

I hand-started all of the engines without difficulty.

Fuels and propellers

  • Glow fuel: Morgan’s new specially blended for CL — 15% nitro, 22% oil (half castor, half synthetic).
  • Diesel fuel: commercially prepared sport-type — 30% ether, 22% castor oil, 43% kerosene (aviation-grade jet fuel).
  • Props: Graupner 9 x 4 for the .19s and Graupner 12 x 5 for the .40s.

Performance results

Performance differences showed up most markedly with the .40-size power plants. The NV .40 is a short-stroke design, intended for maximizing rpm; the PAW .40 is a long-stroke design, built to generate torque.

  • NV .40 (Revlite): reliable rpm ranged from 11,000 to 3,100, sound output 93–71 dBA. Weight: 13.6 oz (mostly aluminum).
  • PAW .40: 12 x 5 prop spun at 9,200 rpm top and 2,200 rpm reliable idle, sound 98–78 dBA. Weight: 14.1 oz.

The .19s were more closely comparable in design and performance:

  • Thunder Tiger .18 (.19-class): 9 x 4 prop at 11,800 rpm top and idled at 3,100 rpm, sound 92–78 dBA. Weight: 7.4 oz.
  • PAW .19: 9,300 rpm top, 3,100 rpm low idle, sound 90–82 dBA. Weight: 6.9 oz.

Diesel advantages and common concerns

Diesels offer several advantages:

  • No glow plugs.
  • Runs are about 70% longer per ounce of fuel.
  • Unaffected by humidity.
  • The same fuel (and easily mixed homemade fuel) can be used for every size engine.
  • Can spin any-size propeller you can fit on its shaft.
  • Small, unobtrusive muffler.

A commonly complained-about problem is dark, greasy exhaust. Dark exhaust usually indicates excess compression, which causes extra stress, heat, and lost power. However, exhaust needn’t spray rearward over the airplane: diesels tolerate exhaust back pressure, and PAWs in particular will accept extensions that carry the exhaust well behind and below the aircraft without significantly reducing power output.

You might have heard that model diesels always require hand-starting. That is suggested to eliminate the chance of hydraulic lock during electric starting, which can damage moving parts. That problem can be averted in various ways:

  • Mount the engine with its cylinder pointed sideways (most convenient).
  • Even with an upright or inverted cylinder, you can electric-start if you begin with a dry (nonflooded) engine and start cranking with the compression backed off. Slowly ease the compression up until the engine fires, then proceed with normal running adjustments.

Fuel mixing and handling tips

If you want to mix your own diesel fuel, many John Deere dealerships sell cans of Starter Fluid. That is effectively ether in model fuel form, though it doesn’t evaporate as quickly.

Ways to extract the fluid from these cans without using the spray head:

  1. Place the John Deere can in a freezer overnight so the propane propellant dissipates.
  2. Puncture a small hole in the upper part of the can and let the propellant escape.
  3. Enlarge the hole and add a vent hole; then pour the fluid into a glass container.

I like a straight-sided jam jar for measuring and mixing diesel. Mark a length of tape on its outside to represent proportions (using generic ingredient names) for each jarful: 22% castor, 43% kerosene, 30% ether.

The biggest operational tricks with model diesels are avoiding flooding and making incremental adjustments between compression and needle settings. That produces the smoothest, most efficient operation.

As a diesel warms up, its compression usually needs to be lowered slightly to prevent preignition (the mixture firing while the piston is still rising). After reducing compression, you lean the mixture a bit; that raises temperature, which typically calls for further reducing compression. Generally, about three back-and-forth adjustment sequences will be needed to fine-tune needle and compression settings.

Troubleshooting: glow plug grounding issue

While running these comparisons I encountered a puzzling problem with glow and spark engines that I’ve seen occasionally over the years. A reader, Abe Gallas (Kansas City, Missouri), described it:

“In preparing to run one of my glow engines, I made a battery pack of two D-size cells in parallel. I put an alligator clip on each terminal of this battery pack, plus an alligator clip on ‘ground’ and the top of the glow plug.

“The plug did not light. I then used a commercial glow clip, and the plug lit right up. I was using the same battery setup, so electrically the circuit was the same. I asked several modelers for an opinion as to why the first arrangement did not work, and none of them had a clue. Do you?”

The cause: oxidized aluminum is electrically nonconductive. Many model-engine cases are aluminum; when anodized (an oxidative process), their surfaces are insulated. Hooking up a glow-plug lead to the outer surface of an anodized case—i.e., using that surface for ground—won’t light the plug because the anodizing encapsulates the case in an insulating skin. Electricity does travel via non-oxidized aluminum (for example, where a glow-plug hole has been drilled and tapped through otherwise anodized metal), which explains why a commercial glow clip that makes good metal-to-metal contact works.

Sources

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