Literature Survey of Water Injection Benefits on Boosted Spark Ignited Engines 2017-01-0658
The automotive industry has been witnessing a major shift towards downsized boosted direct injection engines due to diminishing petroleum reserves and increasingly stringent emission targets. Boosted engines operate at a high mean effective pressure (MEP), resulting in higher in-cylinder pressures and temperatures, effectively leading to increased possibility of abnormal combustion events like knock and pre-ignition. Therefore, the compression ratio and boost pressure in modern engines are restricted, which in-turn limits the engine efficiency and power. To mitigate conditions where the engine is prone to knocking, the engine control system uses spark retard and/or mixture enrichment, which decrease indicated work and increase specific fuel consumption.
Several researchers have advocated water injection as an approach to replace or supplement existing knock mitigation techniques. Water, having high latent heat of vaporization, acts as a heat sink and reduces temperatures in the end gas zone, thereby reducing the tendency for auto-ignition. The added water also changes the ratio of specific heats of the charge mixture, and slightly dilutes the oxygen concentration. These changes greatly reduce the tendency to knock or detonate, in addition to reducing NOx emissions.
The optimum strategy for injection to maximize benefits is still debatable, due to the fact that the latent heat of vaporization decreases as pressure increases. The ability of water to improve anti-knocking properties can potentially allow engine designs with higher compression ratio and boost pressure, and this will enable operation closer to maximum brake torque (MBT) spark timing under all operating conditions. It is worth to note that most of the research work done on water injection focuses on extending permissible engine power output. However the current trend toward boosted and downsized engines demands extending the knock limit without increasing exhaust temperatures or specific fuel consumption (which are the major limitations of existing knock mitigating techniques). This paper examines the prior research in using water injection to extend knock limit in boosted spark ignition engines, and its potential effects on performance and emissions.