Effects of Mixing on Early Injection Diesel Combustion

Ignition dwell is defined as the interval between end of fuel injection and start of combustion in early injection diesel combustion that exhibits HCCI-like characteristics. In this project, the impact of in-cylinder temperature and fuel-air mixing on the ignition dwell was investigated. The engine cycle was simulated using the 3-D CFD code KIVA-3V. Work done by Klingbeil (2002) has shown that ignition dwell allows more time for fuel and air to mix and drastically reduces emissions of NO_X and particulate matter. Temperature is known to have a direct impact on the duration of ignition dwell. However, initial fuel-air distribution and mixing (i.e. at the end of fuel injection) may also impact the duration of ignition dwell. To investigate this, variations in EGR, fuel injection timing, engine valve actuation and swirl were simulated. The aim was to use these techniques to generate varying levels of fuel-air mixing and to check if ignition dwell was affected. In order to verify if fuel-air mixing had been affected by these techniques, the equivalence ratio and temperature distribution, intermediate species formation, mixing timescales and fuel vaporization were analyzed. The results showed that in-cylinder temperature distribution was primarily responsible for controlling duration of dwell. However, the initial fuel-air distribution also affected ignition dwell although to a smaller extent. It was also seen that methods that affected fuel-air distribution (in addition to just temperature), resulted in a more homogeneous mixture and potentially would allow for a faster response time. Overall, this study helped to evaluate the relative influence and importance of various mixing control strategies to achieve ignition control though increased dwell.