Effective reduction of NOx emissions of a HCCI (Homogeneous charge compression ignition) engine by enhanced rate of heat transfer under varying conditions of operation

Abstract

Periodic revisions in the emission norms demands refinement of engine design as well as combustion process. Of late HCCI (Homogeneous charge compression ignition) has gained interest among the combustion community for its adoption in automotive engines. Control of NOx emissions is rather difficult compared to other emissions as it interferes with in–cylinder phenomena which in turn reflect on the performance of the engine. In the present work, an attempt is made in achieving further reduction in NOx emission in HCCI mode of operation. For this purpose, premixed charge is inducted into the HCCI combustion chamber assisted with a swirl motion to enhance the convective heat transfer inside the combustion chamber. The effect of swirl in enhancing the convective heat transfer and reduction of NOx emissions was discussed in this paper. An extensive numerical experiment are conducted considering a 1.6 L single cylinder engine with a reentrant piston bowl running in HCCI mode employing a validated ECFM-3Z (Extended Coherent Flame Model-3 Zones) (STAR-CD) combustion model. Emphasis was laid on effective reduction of NOx emissions with enhanced heat transfer by simultaneously varying Boost pressure, Compression ratio, EGR (Exhaust gas recirculation) under different swirl ratios. The study revealed that higher swirl ratios play vital role in improving the convective heat transfer rate and reduction of NOx emissions. Also, it was observed that higher boost pressures & higher swirl ratios, lower EGR proportions & higher swirl ratios and higher compression ratios and higher swirl ratios are favorable in increasing the convective heat transfer. Higher compression ratios, higher boost pressures, higher EGR concentrations were observed to be favorable to reduce the NOx emissions. The results showed that apart from adopting higher compression ratios and boost pressures use of high swirl ratios is observed to be contributing to a large extent in enhancing the rates of heat transfer which would lead to significant reduction in in–cylinder temperatures suitable for low NOx emission formation in HCCI mode.