Effect of Mixture Condition on Combustion of Fuel〜Air Inhomogeneous Mixture in a Constant-Volume Vessel

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Recently direct injection stratified charge spark-ignition engines have appeared in the commercial market. The thermal efficiency of these engines increased enormously around light duty of the driving. The combustion in these engines is probably due to the flame propagation under concentration gradient of the fuel from rich or stoichiometric to lean mixture, however, precise mechanism has not been revealed yet. Research of such propagation has been hardly done. Therefore, the purpose of this study is to investigate the combustion without turbulence fundamentally. The vessel of 25mm in diameter and 200mm in length was used. Propane was supplied from near the lower side of the vessel during a certain period. After that, the fuel of propane diffuses gradually to the upward direction and forms gradient of fuel concentration. At first, the change of the fuel concentration field along the vessel was confirmed with laser induced fluorescence (LIF) without combustion. Second, the mixture was ignited by a spark at the bottom of the vessel and the flame development was observed by high-speed Schlieren photographs. The initial and main stages of the combustion and the maximum of the pressure were analyzed from the pressure history in the vessel. The effects of the fuel concentration near the spark plug and fuel concentration field in the vessel were discussed. In homogeneous mixtures, the flames have the tendency of changing to the tulip flame in the middle of the vessel. However, in stratified concentration field, the tendency becomes weaker and the flame quenches on the way of the development due to the existence of the lean mixture before the end of the vessel. And radical species of CH, OH and C2 from the flame were detected with a monochrometer by changing the wavelength. In the near future, the observation of the quenching of the flame will be needed in detail and the relation between the flame and the Lewis number or diffusion coefficient will be discussed.