ORIGINAL: Ed_Moorman
ABC engines get looser when hot. Here's the theory and is works very well in practice. The piston is made from a mix of aluminum and a little silicon. This lowers the thermal expansion of the aluminum so it is slightly less than the brass sleeve. The sleeve is bored in a taper, tighter at the top when cold. When the engine heats up to normal running temperature the clearance opens up to what is specified for best running.
The ABC idea was originally conceived by Super Tigre for their control line speed engines. An additional feature was when you got a lean, hot run, the sleeve would open up even more, losing compression, but saving the engine from seizing up.
Thanks, I understand now, Im not convinced by ABC engines!! Just doesnt seam to make good engineering sence for maximum performance, at opperating temperature the "blow by" past the piston skirt must rob the engine of power and efficency. Im not clever enough to know if they are more efficent than a ringed engine in this respect, possibly they gain in other areas like reduced friction from the piston to the sleeve? I guess at the end of the day most of the high performance glow engines are ABC construction, but they also are far higher reving than the 10,000 rpm that is used for sport flying on a noise controled site and presumably any blow by effect will deminish with higher RPM???
From an opperational point of view, its a pain to not be able to re hand start an engine at the flight line when it cuts just before take off!!
Thanks for all the advise!
Regards Angus