Effects of Mass, Pressure, and Timing of Injection on the Efficiency and Emissions Characteristics of TSCI Combustion with Direct Water Injection

A CFD investigation has been conducted to study the efficiency and emissions characteristics of Thermally Stratified Compression Ignition (TSCI) combustion with direct water injection. The motivation for using this new low temperature combustion mode is its ability to control the heat release process by introducing a forced and controlled thermal stratification beyond what would occur naturally. In this case, TSCI is enabled using direct water injection. The added degree of control over the combustion process allows for a significantly broader operable load range compared to HCCI. The effects of injection parameters including the pressure, start of injection (SOI) timing, and spray pattern have been shown previously to affect the heat release of TSCI and its induced thermal stratification. In the present work, the efficiency and emissions considerations were investigated in detail, and the effects of injected mass are presented. A 3-D CFD model was simulated using CONCERGE CFD software. The results show that while there is never a large difference between the thermal efficiency of pure HCCI without water injection and TSCI with water injection, certain injection conditions can cause the combustion efficiency to decrease due to excessive forced thermal stratification, late combustion phasing, and longer burn durations. However, if the combustion phasing is controlled and the thermal stratification is not increased beyond the necessary level to control the heat release and pressure rise rates by tailoring the injection conditions, the TSCI combustion efficiency can be very similar to HCCI. Additionally, the results show that TSCI with water injection can cause a reduction of NO_X emissions, while the UHC and CO emissions can increase when too much forced thermal stratification is introduced.