Accelerometer-Based Estimation of Combustion Features for Engine Feedback Control of Compression-Ignition Direct-Injection Engines

An experimental investigation of non-intrusive combustion sensing was performed using a tri-axial accelerometer mounted to the engine block of a small-bore high-speed 4-cylinder compression-ignition direct-injection (CIDI) engine. This study investigates potential techniques to extract combustion features from accelerometer signals to be used for cycle-to-cycle engine control. Selection of accelerometer location and vibration axis were performed by analyzing vibration signals for three different locations along the block for all three of the accelerometer axes. A magnitude squared coherence (MSC) statistical analysis was used to select the best location and axis. Based on previous work from the literature, the vibration signal filtering was optimized, and the filtered vibration signals were analyzed. It was found that the vibration signals correlate well with the second derivative of pressure during the initial stages of combustion. Two combustion parameters were the focus of this investigation, start of combustion (SOC) and crank-angle of fifty-percent heat release (CA50). The results show that, for a wide range of engine conditions, SOC can be obtained solely from the first derivative of the vibration signal with respect to crank angle, with SOC corresponding to the first peak of the derivative. A CA50 determination that assumes cumulative heat release correlates with the integral of the square of the vibration signal derivative was investigated. The approach shows some promise, but its fidelity appears to be limited in part by the system's vibration impulse response. The overall results demonstrate that an accelerometer can potentially provide sufficient combustion information for cycle-to-cycle combustion control.