| Combustion process of internal combustion engines is the most critical factor that influences the dynamic, economic and emission performances of the engines. Combustion process monitor can detect combustion faults in the early stage to make sure the engines operate around design conditions. There are several combustion monitor methods, such as methods based on in-cylinder pressure, instantaneous speed and vibration signals. In-cylinder pressure is one of the most efficient signals to estimate the combustion states. Unfortunately, the piezoelectric pressure transducers are not convenient for install and not feasible for all situations, especially for non-intrusion application. Instantaneous speed signal is more available for on-line applications. However, when the engine operates at high speed and small load situations, the rotating unevenness and the signal-to-noise ratio becomes lower, also the influence of the torsional vibration becomes larger. This limits the application of the instantaneous speed. Compared to the in-cylinder pressure and the instantaneous speed, vibration signal is one of the most potential methods to realize non-intrusion combustion process estimation. Combustion process estimation using vibration signals has great significant for the future non-intrusion application.Earlier research shows that the vibration responses that excited by combustion can be divided into several segmentations. In order to study the relationship between the combustion and the vibration responses, simulation and experimental methods are applied in this paper. Also the methods of how using different vibration signals to monitor the combustion process is talked about, the influence of non-combustion excitations on combustion vibration responses is studied. The main works of this paper are described in the following sections.1. Relationship between combustion process and vibration signalsIn order to analyze the relationship between vibration signals and combustion process, a single-mass force model and a finite element model are constructed according to the 195 diesel engine structure. The amplitude-frequency and phase-frequency characteristics are obtained using these models. Combined with the spectral analysis of the in-cylinder pressure, the characteristics of the vibration displacements are analyzed. Results show that the vibration responses can be divided into two segments. Before the location of the peak pressure, the vibration displacement is approximately linearly related to the in-cylinder pressure; after the location of the peak pressure, the system becomes to vibrate, the vibration displacement is determined by the system amplitude-frequency characteristics, phase-frequency characteristics and the spectrum of the cylinder pressure. For vibration velocity and acceleration are separately the derivative and the second derivative of the vibration displacement, the vibration velocity and acceleration can efficiently describe the trends of the rate of pressure rise and the second derivative of the cylinder pressure before the location of the peak pressure. According to this segment feature, the research work is focused on the vibration analysis before the location of the peak pressure.2 Relationship between combustion characteristic parameters and vibration parametersBefore the location of the peak pressure, there are similar tendencies between the cylinder pressure and the vibration displacement, the rate of pressure rise and the vibration velocity, the second derivative of the pressure and the vibration acceleration. Based on this relationship, the combustion characteristic parameters can be identified by the vibration responses. Results indicate that when supposing the engine supporting system to be a rigid body, the location of the vibration displacement, the displacement peak value and the mean displacement are separately related to the location of the peak pressure, the value of the peak pressure and the indicate mean efficient pressure. The turning point of the vibration velocity in compression stroke, the first zero-crossing point after the location of the peak velocity, the location of the peak velocity and the value of the peak velocity are highly corresponding to the combustion timing, the location of peak pressure, the location of the maximum rate of pressure rise and the value of maximum rate of pressure rise. The first zero-crossing point before the location of the peak vibration acceleration can identify the combustion timing and first zero-crossing point after the location of the peak vibration acceleration can identify the combustion timing and the location of the maximum rate of pressure rise.3 Influence of non-combustion excitations on vibration responses excited by combustionExcluding combustion, there are several non-combustion excitations include piston slap, piston side pressure, crack shaft main journal load and valves opening and crash of adjacent cylinders. The influence of these non-combustion excitations on vibration responses excited by combustion is necessary to study for combustion vibration extracting.The vibration responses of the piston slap, piston side pressure and crack shaft main journal load are simulated on the 195 FEM model. The time-frequency characteristics of the vibrations excited by these non-combustion forces are compared to the vibrations excited by combustion. Results show the vibration responses'amplitudes excited by the side pressure and the crack shaft main journal load are two orders smaller than the vibration amplitude excited by combustion. The side pressure and crack shaft main journal load will not affect the combustion vibration response. The vibration displacement and velocity excited by the piston slap is separately two orders and one order smaller than the vibration displacement and velocity induced by combustion. However the vibration acceleration excited by piston slap can't be ignored. In frequency domain, the vibration acceleration excited by piston slap is mainly distributed on the frequency band upper 2000Hz and the vibration acceleration excited by combustion is mainly distributed on the frequency band below 2000Hz. The influence of the piston slap can be removed by digital filters.For multi-cylinder engines, the influence of valves opening and crash of adjacent cylinders on combustion vibration responses is also need to analysis. Study is based on the 495 FEM model. The vibration responses excited only by combustion and the vibration responses excited by combustion and valves opening and crash of adjacent cylinders are conducted on the model. The influence of the valves opening and crash of adjacent cylinders on combustion vibration responses are analyzed based on these two simulation results. Results show that in time domain, when combustion is intensive, the signal-to-noise ratio is high and the influence of the valves opening and crash on combustion vibrations can be ignored. However when combustion is poor, the signal-to-noise ratio is low. The valves opening and crash will affect the combustion parameters extraction. But the influence of valves opening and crash can be removed by digital filters.4 Influence of supporting system constraints on vibration responses excited by combustionThe restraints of the supporting system may also influence the vibration responses excited by combustion. To analyze the influence of the restraints of the supporting system, simulation is conducted on the 195 FEM model under different supporting system stiffness. Results show that the vibration of cylinder head is a combination of the vibration excited by combustion and the free vibration induced by the restraint forces. The restraints of the supporting system have great effects on combustion vibration displacement. In some operating conditions, the combustion vibration displacement may be submerged by the displacement induced by the restrain forces. That mean the measured vibration displacement can't be effectively used to monitor combustion states. Compared to the vibration displacement, the influence of the restrain forces on vibration velocity is much smaller, and the vibration velocity can reflect the combustion information effectively. The free vibration can be removed by proper time domain or frequency domain methods. The influence of the constraints on the combustion vibration acceleration can be ignored and the vibration acceleration can reflect the combustion information efficiently.5 Combustion parameters extracting based on vibration velocityExperiments are conducted on the 195,495, C6190ZLC and CP12V190ZL diesel engines. Results indicate that the vibration velocity is a combination of the free vibration velocities induced by constraints and the vibration induced by combustion. The free vibration velocities are mainly concentred on the frequency band below 250Hz and its influence can be removed by proper time domain and frequency domain methods. The vibration velocity after treatment is used to identify the combustion parameters. Results show that the combustion timing, the location of the rate of pressure rise and the location of the peak pressure can be fixed effectively by the velocity. But there is a phase shift between the parameters fixed by velocity and by the rate of pressure rise due to the phase-frequency characteristics of the system. The phase shift can be seen as system deviation for a special engine. Also the tendency of the maximum value of the rate of pressure rise can be expressed by the maximum value of vibration velocity.6 Combustion parameters extracting based on vibration accelerationTime-frequency characteristics and the influence factors of vibration accelerations measured on different diesel engines are analyzed under different conditions. Results show that the influence of the supporting system constrains can be ignored. The vibration accelerations excited by the piston slap and the valves opening and crash are mainly concentrated on the frequency band upper 2000Hz, while the acceleration excited by combustion is mainly concentrated below 2000Hz. And the energy of the vibration under 2000Hz is increased obviously with the rise of the combustion load. The vibration acceleration excited by combustion can be extracted by a low-pass filter with a 2000Hz limiting frequency.7 Application of vibration signals in HCCI engineCombustion timing is one of the most important feedback parameters for HCCI combustion control. Base on the results of the prior chapters, the vibration signals are used to determining the combustion timings of the HCCI engine. Experimental results show that the vibration velocity excited by the combustion of the cylinder head is also influenced by the constraints of the supporting system. However the influence of the constraints can be removed by least square fitting method or by high pass filter. In addition the vibration signals also disturbed by environmental noises. Analysis indicates that the noises signals are mainly distributed on the band exceeded 2000Hz, and the noise signal can be removed by a low-pass filter. The vibration velocity and acceleration can be effectively used for combustion timings'identification in the HCCI engine.
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