Experimental Study on Ignitability of Lean CNG/Air Mixture in the Multi-Stage Cascade Engine Combustion System

Lean air-gas mixture combustion systems seem to be very promising solutions for IC engines in terms of lower emission indexes and higher thermal efficiency, especially in part-load operation. The main problem however is the necessity to provide the bigger activation energy for ignition of lean and very lean mixtures. 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 lean CNG/air mixture due to the improvement in on-ignition mechanism. The TJI-system consists of two chambers connected with nozzles. The orifices generate complex charge movement and decrease significantly the charge homogeneity in the pre-chamber, interrupting therefore first ignition process. The main research question of the investigations reported here was to improve this unfavorable property and to validate better inflammation ability in the pre-chamber by applying additional small auxiliary combustion chamber integrated with spark plug, creating together the 3-stage combustion system. Four types of combustion systems (conventional, PC spark plug, 2-stage and 3-stage) have been investigated for the comparative validation of main combustion efficiency indexes. The investigations have been performed at λ=1.5 using Rapid Compression Machine (Ɛ=15) with both indicating measurements and optical analysis for parameterization of inflammation intensity and flame front development. In the first step the study on the spark plug with additional combustion chamber has been performed, which has been than installed on the top part of the pre-combustion chamber of turbulent jet igniter. It resulted in bigger flame luminescence confirmed by bigger heat release rate, finally combined with earlier completing of combustion. Introduction of third stage resulted in shorter combustion duration of defined fuel quantity. The results achieved in the model engine have been verified in the one-cylinder research engine under real operation and complemented with the measurement of exhaust gas toxic compounds emission. Achieved results according charge maximal temperature, flame front development and combustion duration have been presented comparatively for all four investigated combustion systems.