A low swirl, low compression ratio engine with narrow fuel spray angle injector was used to investigate the cylinder wall wetting process of early direct injection strategies. A methodology was developed in order to detect liquid fuel impingement on the cylinder wall oil film.
First, single injection tests were performed in order to investigate the effect of injection pressure and start of injection on the amount of fuel that can be injected in the combustion chamber without liquid fuel cylinder wall impingement. Then double injection strategies were performed to verify the existence of interactions between successive injections for early injection thermodynamic conditions. Finally an optimization of the injection strategy maximizing the fuel quantity without cylinder wall wetting is proposed.
Results obtained for single early direct injection show that all conditions of injection timing during the intake stroke lead to cylinder wall wetting above a given injected mass. Moreover during this stroke the injection pressure has no effect on the maximum injected mass without cylinder wall wetting. During the compression stroke, the earlier the injection timing or the higher the injection pressure, the less fuel mass can be injected without cylinder wall wetting.
Results of double early direct injection show that in the operating range investigated (low temperature, low density), no interaction between successive injections was detected.
The injection strategy proposed showed that it is not possible to reach a realistic charge using only very early direct injections for the conditions of this study: later direct injections were added in order to reach an objective of ~ 6bar IMEP.
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