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 by Frank Granelli |
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The main operational point was that ABC engines are more tolerant of lean fuel/air mixtures than ringed engines are. This is because of the thermal expansion differences inherent in this design. But ringed engines usually outlast the ABC type if the fuel/air mixture is always correct and the engines are always properly maintained. And it is true that ringed engines have a bit more torque than corresponding ABC engines. Regarding torque, I mentioned that four-stroke engines have more than corresponding two-strokes. I meant more usable torque but wanted to avoid using extra space to explain what that meant. In fact, two- and four-strokes have roughly the same amount. But the four-stroke produces its maximum torque at sufficiently low rpm so that most sport fliers can "prop" their engines to reach this speed. Many two-strokes (not all, since older, so-called long-stroke engines did not) have their "torque band" or "curve" (the rpm range at which maximum torque is produced and remains nearly constant) at relatively high rpm. Many times the torque band is higher than 13,000 rpm and may require impractical propeller selection to reach—especially for sport pilots using low-nitromethane-content fuel. In "Engines 101" I wrote that in theory, a two-stroke engine should have twice the power of an equivalent four-stroke. This assumed that the two-stroke was 100% efficient. As I have pointed out, it is far from that efficiency level. I also wrote that modern four-stroke engines have roughly 70%-80% of the "power" of an equivalent two-stroke. That is true, but I should have added that I was considering only sport 40-60 engines since the article was addressing only noncompetition pilots. Some supercharged, fuel-injected competition four-strokes can reach power parity with the two-stroke, but at a much higher cost. I didn't mention these engines because they are not usually relevant to new RC pilots, but they are fine engines that are worth more than their cost in the long run. I know because I use them in RC Aerobatics (Pattern) competition. "Engines 101" contained two major bloopers in addition to the two errors I already mentioned, one of which was that ball bearings provide "more" crankshaft support than bronze bushings. I meant that they provide "better" support since ball bearings reduce crankshaft friction loads. The bushing actually provides "more" support since more area is in contact with the crankshaft. The point I made was valid, but the image described was incorrect. If you are interested, you can see in that article's photo of the K&B .65's insides that it does indeed have bushed crankshaft bearings. An embarrassing mistake was that I identified the oil-retention groove as a score mark. When looking at the photo, I noticed the thin line. Since that engine had not been moved in nearly 15 years, I thought the camera had captured oil buildup from the cylinder onto the long-stationary piston. My eyes are no longer good enough to see that thin line without the camera's magnification, so I never noticed it before. I learned something new here. The oil groove serves to retain some lubrication in the piston/cylinder contact area. I am happy to correct this mistake and thank all those who wrote in to point it out. The last error involves the K&B .65 shown in "Engines 101." I assumed that the bushed engine followed the older, non-Schnuerle-ported K&B design, but it does not. The non-Schnuerle-ported engine should have been the K&B .61 shown here in the comparison photo that I originally thought was too dirty to print. Not all Schnuerle engines show the boost transfer port on the outside. Some enlarge the entire upper cylinder case to fit the boost port, as shown in the photo comparing the two K&B "60s." These engines are more difficult to spot. Revisiting "Engines 101" has taken time and space that could have been used for the third article in this segment of the "From the Ground Up" series, which will be published next month. I thank all those who sent in their suggestions and comments. Although the important operation steps I presented in "Engines 101" are valid, those comments helped identify those areas that needed more explanation. MA
Frank Granelli
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