Visualization results show that pilot ignition occurs in the vicinity of the glow plug, and strongly influences main combustion initiation. Main combustion starts from the pilot flame, and propagates to the rest of the combustion chamber showing a strong visible reaction zone. After end of main injection, the rapid leaning of the mixture suppresses the strong radiation, and OH radiation is observed to progress to the rest of the combustion chamber. The combustion process shows a strong scattering, which has been quantified

ElsevierDesantes Fernández, JM.; García Oliver, JM.; Pastor Enguídanos, JM.; Ramírez Hernández, JG. (2011). Influence of nozzle geometry on ignition and combustion for highspeed direct injection diesel engines under cold start conditions. Fuel. 90 (11) tion rates increase when using nozzles which induce a higher momentum, improving mixture conditions. For these reasons, the solution under these conditions necessarily involves a trade-off between ignition and combustion progression.

Cold start at low temperatures in current direct injection (DI) Diesel engines is a problem which has not yet been properly solved and it becomes particularly critical with the current trend to reduce the engine compression ratio. Although it is clear that there are some key factors whose control leads to a proper cold start process, their individual relevance and relationships are not clearly understood. Thus, efforts on optimization of the cold start process are mainly based on a trial-and-error procedure in climatic chambers at low ambient temperature, with serious limitations in terms of measurement reliability during such a transient process, low repeatability and experimental cost. This paper presents a novel approach for an experimental facility capable of simulating real engine cold start, at room temperature and under well-controlled low speed and low temperature conditions. It is based on an optical single cylinder engine adapted to reproduce in-cylinder conditions representative of those of a real engine during start at cold ambient temperatures (of the order of −20 °C). Such conditions must be realistic, controlled and repeatable in order to perform systematic studies in the borderline between ignition success and misfiring. An analysis methodology, combining optical techniques and heat release analysis of individual cycles, has been applied.

Glow plugs are currently the most employed solution to promote ignition in light duty diesel engines during low temperature cold start. Improved knowledge about the mechanisms that control ignition and flame development under such conditions is necessary for design purposes, especially with current trends to reduce engine compression ratio. This paper aims to analyze the influence of the glow plug configuration (disposition and temperature) on cold start combustion. Experimental tests carried out in an optical engine with high speed visualization have confirmed that the spray-glow plug configuration influences the whole combustion process through pilot ignition.Ignition of pilot injection is controlled by glow plug temperature and by the fuel and air motion after the end of injection. Nevertheless, the glow plug temperature effect starts to be negligible over certain value since chemical ignition delay cannot be further reduced. For this reason, the path to follow is to increase the amount of fuel that reaches the glow plug, which is directly * Corresponding author Email address: jpastor@mot.upv.es (J.V. Pastor ) related with the flow motion in the cylinder after injection.