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The Effect of Direct-Cylinder Water Injection on Nitric Oxide Emission From An S.I. Engine
Published January 01, 1972 by Institution of Mechanical Engineers in United Kingdom
Results of an investigation into the effects of direct-cylinder water injection on spark ignition engine combustion are presented. It was found that while direct water injection does cause appreciable (>80%) nitric oxide reductions, it at the same time causes a severe power reduction. For moderate water injection rates a power recovery can be obtained by adjusting the spark timing to the MBT setting, which in all cases requires that the spark timing be advanced. However, now the attendant NO reductions are 70% or less.
A theoretical model that predicts a linear relationship between the required change in spark advance to maintain MBT and the amount of water injected was developed. Experimental data appear to support this model. From pressure-time diagrams it is concluded that for MBT operation with direct water injection, the phasing of the combustion does not vary so long as the injected mass is not excessive. Analysis of the heat transfer rate to the engine cooling water during MBT operation leads to the same conclusion. Hence water injection must increase the time required to burn the initial portion of the charge, thereby changing the combustion phasing. This causes a power reduction that can be recovered by advancing the spark timing so long as the injected mass is not great enough to cause quenching.