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The world’s first variable cylinder management system, VCM system, for large motorcycles has been developed. The system uses an in-line four-cylinder, 1 137 cm3 engine as the base, and allows changing of the number of working cylinders from 2-cylinder to 3-cylinder to 4-cylinder. To eliminate an abrupt change of power delivery when shifting number of working cylinders, the electronically-controlled throttle valve (Throttle-By-Wire: TBW) is employed. Coupled with the harmonized control of the intake-and-exhaust valve deactivating system, the smooth switching of number of working cylinders suitable for motorcycles has been realized. As a result, while maintaining the same power output characteristics of the conventional engine, EURO 3 exhaust emission regulations are satisfied while attaining approximately 46% better fuel economy in the New European Driving Cycle mode (NEDC mode).

It has been considered that a backfire occurs due to such a phenomenon that the ignition at the overlap top dead center or the reversing of the hot burnt mixture during the valve-overlapping period triggers ignition. We have recently determined that there is a different type of ignition source that cannot be explained by the conventional theories. That is, there is an extremely slow burning cycle that continuously burns even at the exhaust stroke. That extremely slow burning lasts even after the exhaust stroke and ignites the fresh mixture during the following intake stroke, causing a backfire. In this study, we clarified procedures of the extremely slow burning that serves as a source of ignition to trigger a backfire by the pressure measurements at the intake and exhaust ports and visualizing the phenomena with the high-speed CCD camera. In this article we report the outlines along with the considerations on the factors that affect the extremely slow burning cycle.