Analyses of cycle-to-cycle variation of combustion at low load condition by using high-speed PIV and direct photographing of early flame propagation

Abstract

Cycle-to-cycle variation (CCV) of combustion is an important issue because it affects emissions and drivability. Improvement of CCV of combustion has been carried out using electronic controls (e.g. ignition timing, fuel injection and variable valve timing) in motor vehicle's engines. However, electronic devices are hardly used for motorcycle's engines because of limited space and cost. Therefore, the engine performance itself must be improved to reduce CCV of combustion in motorcycle. Though CCV of combustion is caused by CCV of in-cylinder flow pattern, fuel distribution, temperature and residual gas, and ignition energy, it is difficult to measure and analyze these factors. In this study, the simultaneous measurement of high-speed PIV and direct photographing of flame propagation was carried out. CCV of in-cylinder flow was evaluated as temporally-averaged flow that was obtained by instantaneous flow using low-pass filtering and cut-off frequency. As a result, in-cylinder temporally-averaged flow pattern fluctuated between individual cycles. Especially, the flow pattern on the surface of piston at BDC was different between the highest and the lowest cycle in IMEP. This difference is considered to be due to the location offset of tumble flow. Also the fluctuation of turbulence kinetic energy (TKE) is caused by tumble flow offset. TKE distribution near the spark plug at ignition timing affected the direction and speed of flame propagation.

Keywords

• internal combustion engine
• cycle-to-cycle variation
• high-speed piv
• in-cylinder flow
• time-average
• flame propagation