Model-based combustion pressure computation through crankshaft angular acceleration analysis

Many faults caused by faulty combustion and mechanics in multicylinder engines can be detected through measurement and analysis of the crankshaft's angular acceleration. Using a low-cost magnetic sensor near the flywheel teeth, this method is easily applicable for production, service and on-board diagnosis. Errors in angular acceleration signal due to noise and limitations of the measurement procedure can be minimized by appropriate digital signal processing. The measured waveforms are interpreted based on a model describing the relationship between input variable (pressure in the combustion chamber) and output (angular acceleration). Research efforts for a single-cylinder engine (four-stroke spark-ignition) with an additional reference pressure transducer in the combustion chamber permit numerical comparison between measured and computed results without superposition of effects of forces from several cylinders. In addition to gas pressure and inertia forces, the angle-dependent variat ions of torque due to friction and valve gear are taken into account. Comparisons between measurements and model results indicate a high accuracy (error equal smaller than 3%). By inversion of the developed model it is possible to estimate the combustion pressure from measured waveforms of the crankshaft's angular velocity with an overall accuracy of plus minus 200 mbar (conditions: no load/1500 rpm).