lml@lancaironline.net Mailing List Archive

<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> Rather than get into a long discussion about the physics of the internal combustion engine I would strongly suggest that, if you are interested, you read up on the subject. The most cited authors on the subject are Ricardo, Heywood and Taylor. Check the SAE web site. One very interesting example of engine comparison is given in "The Internal Combustion Engine in Theory and Practice" by Charles Feyette Taylor where he compares a large marine diesel engine (26,500 cuin/cylinder) to a Cox 049 (.10 cuin) model airplane engine. Both engines are loop scavenged two stroke. Of particular interest is that smaller engines make more horsepower per cubic inch displacement (per cylinder) than larger engines. The ratio is approximately proportional the cube root of displacement. What I found remarkable is that several factors are almost identical between the behemoth and the flea. They are: Brake Mean Pressure (average pressure on the piston), Mean Piston Speed, Specific Output (horsepower per square inch of piston area) and the ratio to their Weight to their Displacement. If you want a lot of horsepower out of the lightest package you want a lot of little pistons. Little pistons make for short strokes and short strokes make for high crank RPM. Motorcycle engines make this point well. The bad news in airplane engines is that high RPM and propellers don't get along so now you need a gearbox and the gearbox must be counted into the engine weight. Did I mention that the thermal efficiency of small cylinders is considerably poorer that large cylinders? After a little investigation you will find that engine comparison is a much more complex matter than it would seem at first blush. Comparing horsepower and displacement or weight only tells a fraction of the story. Regards Brent Regan LML website: http://www.olsusa.com/Users/Mkaye/maillist.html Builders' Bookstore: http://www.buildersbooks.com/lancair Please send your photos and drawings to marvkaye@olsusa.com.