Periodic engine speed fluctuations of an Electronic Fuel Injection (EFI) engine system at idle conditions are analyzed by examining the two engine dynamics, filling dynamics and rotational dynamics of the engine. Utilizing simple models of engine dynamics linearized around the nominal operating point, it was predicted that the periodic engine speed fluctuations were caused by the instability of the derived vibration system.
These predictions are verified by the engine test, utilizing the frequency response method. The low frequency vibration at idle is confirmed to correspond to the idle hunting phenomenon, and its characteristics such as the resonant frequency and the stability depend on the control system as well as the engine mechanical structure.
Among the practical EFI system, the L-J (Mass-Flow) system is shown to be more stable than the D-J (Speed-Density) system.