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A Computational Study of the Impact of Mixing on Homogeneous Charge Compression Ignition
Published September 07, 2004 by CMT in Spain
Event: Thiesel 2004
Ignition dwell is defined as the interval between end of fuel injection and start of combustion in early premixed diesel combustion that exhibits HCCI-like characteristics. In this project, the impact of in-cylinder temperature as well as fuel-air distribution and 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 NOx and particulate matter. Temperature is known to have a direct impact on the duration of ignition dwell. However, initial fuel-air distribution (i.e., at the end of fuel injection) may also impact the duration of ignition dwell. In order 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 distribution and mixing and to check if ignition dwell was affected. In order to determine if fuel-air mixing had been affected by these techniques, the equivalence ratio and temperature distribution, intermediate species formation, turbulent time scales and fuel vaporization were analyzed. The results showed that the in-cylinder temperature distribution was primarily responsible for controlling the duration of ignition dwell. However, the initial fuel- air distribution also affected the ignition dwell in some cases such as certain magnitudes of swirl. It was also seen that methods that affected fuel-air distribution (in addition to just temperature), produced a more homogeneous mixture in the cylinder. Another observation was that, although a long ignition dwell produced a more homogeneous mixture, it did not necessarily produce an extremely low equivalence ratio. Instead, the equivalence ratio approached a value of 1.0 close to start of combustion, which resulted in high peak temperatures and NOx. In addition to this, it seems possible that there may be certain magnitudes of swirl ratio, which affect fuel distribution and vaporization such that the gas mixture is brought to a much lower and homogeneous equivalence ratio throughout the cylinder.