Worm, J., Naber, J., Duncan, J., Barros, S. et al., "Water Injection as an Enabler for Increased Efficiency at High-Load in a Direct Injected, Boosted, SI Engine," SAE Int. J. Engines 10(3):2017, doi:10.4271/2017-01-0663.
In a Spark-Ignited engine, there will come a point, as load is increased, where the unburned air-fuel mixture undergoes auto-ignition (knock). The onset of knock represents the upper limit of engine output, and limits the extent of engine downsizing / boosting that can be implemented for a given application. Although effective at mitigating knock, requiring high octane fuel is not an option for most markets. Retarding spark timing can extend the high load limit incrementally, but is still bounded by limits for exhaust gas temperature, and spark retard results in a notable loss of efficiency. Likewise, enriching the air-fuel mixture also decreases efficiency, and has profound negative impacts on engine out emissions. In this current work, a Direct-Injected, Boosted, Spark-Ignited engine with Variable Valve Timing was tested under steady state high load operation. Comparisons were made among three fuels; an 87 AKI, a 91 AKI, and a 110 AKI off-road only race fuel. The engine was outfitted with a sequential port water injection apparatus. Under full-load, water injection enabled operation with 87 AKI fuel at near optimal combustion phasing, and with a stoichiometric air-fuel ratio. This resulted in up to a 5% improvement in Net Mean Effective Pressure and up to a 34% improvement in thermal efficiency compared to the production ECU Calibration. A detailed comparison on combustion phasing, burn rate, Mean Effective Pressure, water requirement, and efficiency is presented in this paper.