Study On Knock Detection And Control Strategy For 468Q Gasoline Engine

The increasingly decrease of petroleum resource and the greatly increase of fuel price makes more and more automotive manufacturers to adopt higher compression ratio for higher fuel economy. In the mean time, emission regulation gets stricter and stricter , and unleaded petrol is required in these years. All of these will cause the probability of engine's knocking. While people are optimizing the engine for its power, fuel economy and low emission, they are restrained from the engine's knock inevitably. So it's very important to make a research on the knock detection and control for optimization.Hafei DA468Q gasoline engine is used as the test engine, research on the engine's knock detection and control strategy to restrain the occurrence of knock is made in this thesis. The main work in this thesis includes:1. High-speed data acquisition and reliable data storage are achieved by means of high-performance PCL-818HG data acquisition card and its PCLD-8115 writing terminal board made by Advantech Co. Ltd. Signal storage and analysis platform for electric gasoline engine, which is based on MATLAB and LabVIEW, is used to achieve high-speed data acquisition and reliable data storage and online signal analysis.2. A non-resonance knock sensor is chosen. To clearly detect the knock signal, the knock sensor is fixed on the protuberance between the cylinder 2 and cylinder 3 of the block, according to the fixation principles of knock sensor.3. Module design is adopted in this thesis so as to apply this knock control module in other trials about the engine's knock characteristics. The module includes: knock signal processing panel, control panel and other circuits. The research on knock signal characteristics and knock control strategy has been made using this knock control module.4. The knock signal sampling rate is 35kHz based on the scope of knock vibration signal and the sampling law so that the signal can be sampled without distortion. The knock signal has been analyzed in time domain and in frequency domain respectively while it's sampled.In time domain, the engine is working in the following operating conditions: speed is 1500r/min, 2000r/min, 2500r/min respectively with 50% TPS(throttle position sensor), and the knock signal of one combustion cycle with knocking is compared with non-knocking cycle. And these phenomena are discovered: speed increase causes fiercer vibration of block and knock signal amplitude increases accordingly. And the engine's knocking takes place between 10℃A～75℃A ATDC in expansion stroke. After that, the knock detection window is set to 3℃A～81℃A ATDC in expansion stroke.In frequency domain, Fourier Transform and Power Spectral Density is applied to analysis of the knock signal. MATLAB is utilized to apply FFT function and PSD function for knock signal analysis. The knock's center frequency is found, which is 8kHz, through analysis in frequency domain. So the detecting pass-band frequency of HIP9011 is 8.02kHz.5. The knock real time control system is designed to restrain the engine's knock. This system includes open-loop and closed-loop control. In open-loop control, the micro controller reads ignition map and directly control ignition advance angle. While in closed-loop control, the knock signal is considered as a feedback parameter, the micro controller modifies the ignition advance angle by the occurrence of knock. Referring to the statistics of HIP9011 integral voltages with knock and without knock operating condition, the voltage keeps between 0.8V~1.2V and the average voltage tends to 1V in non-knock operating condition, while the voltage changes greatly in knock operation. According to the analysis, the voltage 1.2V is the knock threshold for HIP9011 integral voltage.The concept of knock rate is adopted as a measurement of knock intensity. In these engine's working condition: speed 1500r/min,2000r/min,2500r/min with 50%TPS, and 30%TPS,50%TPS,80%TPS at 1500r/min, a research on knock rate is made through adjusting ignition timing. Through these trials and analysis, it is found that knock occurs easily in low speed and in high load, and it is also found that knock can be restrained by decreasing ignition advance angle. The target of knock control is to keep the engine work with light knock and the knock rate is about 1%~5%. In this operation, the torque is maximum, and the fuel consumption is minimum. To achieve this target, a control strategy is designed. According to the calculation of knock rate, the ignition advance angle is modified. In case that knock rate equals to zero, it is clear that the engine is far from knock limit. And the modified ignition advance angle is A=+1℃A every 10 cycle so that the engine works near knock limit.In case that the engine works with light knock, the ignition advance angle is kept, and it is A=0℃A, so that it keeps the engine work in knock limit. In case that the engine works with medium knock, the ignition advance angle isslightly decreased, so that the engine can leave off the knock field. In case that the engine works with severe knock, the ignition advance angle is A=-6℃A. Through the trials, the ignition advance angle decreased by 6℃A is much enough to restrain the knock.