Aoyama, Y., Hasegawa, R., Yamada, T., Itoh, T. et al., "Development of Closed-Loop Robust Control System for Diesel Engines - Combustion Monitoring by Crank Angular Velocity Analysis and its Applications -," SAE Technical Paper 2012-01-1157, 2012, https://doi.org/10.4271/2012-01-1157.
Closed-loop robust control system that can monitor combustion state and control it into optimal state using crank angular velocity analysis was established. The system can be constructed without any change of the current hardware. It can avoid engine stall, deterioration of drivability and white smoke emission by misfire after filling low cetane fuels. This study was attempted to grasp the frequency characteristics of crank angular velocity both normal combustion and misfire with FFT (Fast Fourier Transform) and Wavelet Transform. FFT used for frequency analysis is generic method to acquire the frequency characteristics of steady oscillation, however is unsuitable for acquiring the frequency characteristics of transient oscillation. Therefore authors adopted Wavelet Transform and succeeded in grasping the phenomenon in misfiring in time sequential. With this knowledge, this study was attempted to determine the combustion instability by extracting frequency element of the 0.5 order of engine speed that is the characteristic frequency element in misfiring from pulse signal of the crank angle sensor with digital filters. With this method, misfire and combustion instability with white smoke caused by excessive EGR (Exhaust Gas Recirculation) rate or filling low cetane fuels can be detected. In addition, deterioration of combustion after filling low cetane fuel can be recovered by reducing EGR rate to the value of misfire index calculated from digital filtered crank angular velocity that is equal to that of using normal fuel. Moreover, HC (Hydrocarbon) and torque fluctuations can be recovered to the almost same levels.