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Experimental study on ignitability of lean CNG/air mixture in the three-stage cascade engine combustion system

Lean air-gas mixtures combustion systems seem to be very promising solutions for IC engines in terms of lower emission indexes and higher thermal efficiency. The main problem however is the necessity to provide the bigger activation energy for ignition of lean and very lean mixtures. In the preceding investigations reported in several publications and in research performed by the authors of this paper it has been confirmed, that the implementation of turbulent jet ignition (TJI) results in significantly faster inflammation of main CNG/air mixture due to the improvement in on-ignition mechanism. The TJI-system consists of two chambers connected with nozzles, which provide charge exchange between them. The orifices generate complex charge movement during the mass transfer to the pre-combustion chamber and decrease indeed the charge homogeneity in the pre-combustion chamber, interrupting therefore first ignition process. For improving this unfavorable property authors conducted investigations on pre-combustion course modification using additional auxiliary combustion chamber integrated with spark plug in order to change inflammation mechanism in pre-chamber. The investigations have been performed using pneumatically driven RCM (Ɛ=15) with both indicating measurements and broadband chemiluminescence optical method. The images have been recorded using high-speed camera and post-processed with Davis software for the parameterization of inflammation intensity and flame front velocity. Investigations have been performed at λ=1.5. On this basis the on-ignition capability of three-stage combustion system has been proven. The occurrence of luminescent jets in the main combustion chamber indicates delay in comparison to the prechamber system with conventional spark plug. However, in the further time instances, three-stage combustion resulted in maximal bigger flame luminescence confirmed by bigger heat release rate obtained in indicating measurements, finally combined with earlier completing of combustion. Introduction of third stage resulted in shorter combustion of defined fuel quantity. The delay in ignition of main charge indicates further potential to be compensated with ignition advance adjustment. Key words: CNG, lean-burn, three-stage combustion, turbulent jet ignition, pre-chamber, flame development, heat release rate
Topic: Combustion processes in SI and CI engines
Author: Wojciech BUESCHKE, DEng.
Co-authors: Krzysztof WISŁOCKI, Prof. DSc. DEng. Maciej SKOWRON, MEng. Filip SZWAJCA, MEng.