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The operation of a cogeneration internal combustion engine with unscavenged prechamber ignition was investigated. The objective was to evaluate the potential to reduce the exhaust gas emissions, particularly the CO emissions, below the Swiss limits (NOx and CO emissions: 250 and 650 mg/m3N, 5% O2 , respectively), without exhaust gas after treatment. The investigation was carried out on a small size gas engine (6 cylinders, 122 mm bore, 142 mm stroke) and required the development of cooled prechambers and the modification of the engine cylinder heads. The approach was essentially experimental, but included a numerical simulation based on the CFD-code KIVA-3V in order to assist and guide the experimentation. The numerical simulation was carried out in order to evaluate the differences in flow characteristics at the location of the spark plug electrodes between direct and prechamber ignition. Further, the influence of the prechamber geometrical configuration was investigated through variations of the nozzle orifice diameter, number and orientation, as well as prechamber volume and internal shape. Based on the results of the numerical simulation, the most promising prechamber configuration parameters were selected for experimentation. Then, variations of the selected prechamber configuration parameters, as well as of the piston geometry, of the turbocharger characteristics and of the engine operating parameters, were carried out in order to determine their influence on the engine performance and emissions. Through the generation of gas jets in the main chamber, the use of a prechamber strongly intensifies and accelerates the combustion process. However, this advantage is conditioned by a significant delay of the spark timing in order to generate substantial gas jets. This results in a large decrease in peak cylinder pressure and in an important reduction of NOx, CO and THC emissions. Minimum emissions are achieved at a spark timing of about 8°CABTDC. The prechamber geometrical parametric study indicates that trends which increase the penetration of the gas jets and/or promote an early arrival of the flame front at the piston top land crevice entrance are beneficial to reduce the CO and THC emissions. In comparison with the direct ignition, the prechamber ignition yields approximately 40% and 55% less CO and THC emissions, respectively. However, this also leads to about 2%-point lower fuel conversion efficiency. The optimisation of the turbocharger results in a recovery of about 1%-point in fuel conversion efficiency, but a consequent change in the exhaust manifold gas dynamics causes an increase in THC emissions. At the rated power output (150 kW), the prechamber ignition operation fulfils the Swiss requirements for exhaust gas emissions and still achieves a fuel conversion efficiency higher than 36.5%.