Answers to common engine questions

Model Airplane News, Jul 2000 by Gierke, Dave

One of the more interesting facets of writing "RPM" is the positive feedback and questions I receive from readers. Because many modelers probably have similar questions, I will devote this month's effort to some recent letters and hope that the answers will be of interest to you.

ENGINE EFFICIENCY

Ray Baker of Wilmington, OH, has a question about engine efficiency: "I have your book, `2-Stroke Glow Engines for R/C Aircraft.' It says a lot about speed but little about economy. Is there a formula or guideline concerning timing, etc., for attaining fuel efficiency?"

Although I don't know of any formulas for reaching this goal, there are some guidelines that should lead you in the right direction.

The best way to extract the maximum mechanical energy from any chemical fuel in a heat engine is to raise its operating temperature. In the case of a piston engine, this is accomplished by raising the compression ratio. There are limits in terms of combustion defects such as "knock" (detonation) and pre-ignition. Some fuels are better suited than others in terms of their ability to tolerate increased compression. Methanol (methyl alcohol), for instance, allows higher compression than straightrun gasoline under ideal conditions. Of course, high compression ratios require sturdy, heavier, engine construction to withstand elevated cylinder pressures.

Although gasoline has compression ratio limits that are significantly lower than certain other fuels, its air/fuel ratio for chemically correct economical burning is about 15 parts of air to 1 part gasoline (15:1). Because lean air/fuel ratios are necessary for economical engine operation, gasoline is hard to beat. Compared to gasoline, methanol is about 9:1, and nitromethane is 4:1. On the negative side, gasoline engines run hotter than methanol-burning engines and are more likely to produce accidental fires.

Lower shaft speeds, smaller transfer and exhaust ports, a longer expansion (power) period and the reed-valve induction system all contribute to more efficient engine design. However, efficiency and power are mutually exclusive; economical engines aren't powerful engines.

If all this sounds like "back to the future," it is! Our earliest production engines from the 1930s were designed very similarly. Unfortunately, engineers couldn't use high compression ratios because metal alloys for pistons, lightweight castings and other critical construction components were inferior; they couldn't withstand the temperatures and pressures.

NOT RE-STARTS, ROD WEAR AND BEARING RUST

Bobby Green of Birdsboro, PA, has several concerns: "What causes the connecting rod on my Fox .40 ABC engine to wear when it's run very rich [4-cycling] for long periods? I thought the extra oil I use in the fuel would help, not cause harm." Bobby, the worst thing you can do to an ABC-type engine-other than running it lean-is to run it 4-cycling rich for long periods. When new, most ABC (aluminumalloy piston with chrome-plated brass cylinder) engines exhibit an interference fit (components that actually touch) as the piston crown passes top dead center (TDC) when turned over by hand. In many cases, you can actually hear the piston squeak; this is normal. Originally designed for fast, 2-cycle operation, the piston and cylinder expand similarly (slightly more for the cylinder), maintaining a good gas seal without enduring the mechanical drag of a piston ring. Unfortunately, when operated at a rich 4-cycle (engine firing once every other revolution), the piston and cylinder never reach design temperature and the piston rubs at TDC, causing great wearing loads (pounding) on the piston wristpin bosses and both connecting-rod holes. The piston fit also is worn away at its crown, further degrading the engine's mechanical condition. In the future, always try to run ABC-type engines (including the AAC and ABN types) in the 2-cycling mode, especially when they're new.

"What causes an engine not to start when it's hot? The Fox .40 starts Fine when it's cold, but it won't even 'pop' when it's hot!"

When hot, most ABC-type engines are relatively difficult to start. The hot clearance between the piston and cylinder reduces compression to a point at which combustion

chamber startup conditions are questionable. This is magnified if the squeaky fit has been worn away because of cold, 4-cycle operation. You didn't

mention whether you are attempting to hand-start or are using an electric starter. By richening the carburetor's highspeed needle valve a bit and using an electric starter, the hot ABC engine can often be coaxed into starting.

"I am having rust problems with the crankshaft ball bearings in all of my engines. Any suggestions?"

You aren't the only one with bearing rust problems, Bobby. A few suggestions:

* At the end of a running session (bench or flying), run the engine completely dry of fuel from a wide-open throttle setting (pinch the fuel line or run the tank dry).

* Load the engine crankcase with after-run oil (I use Marvel Mystery Oil). For a .40-size engine, I squirt at least 1/2 ounce into the venturi while turning the propeller over slowly. Rotate the induction valve to the closed position so the oil won't run out.