Influence of Injection Timing on Exhaust Particulate Matter Emissions of Gasoline in HCCI and PPC 2016-01-2300
In order to reduce nitrogen oxides (NOx) and soot emissions while maintaining high thermal efficiency, more advanced combustion concepts have been developed over the years, such as Homogeneous Charge Compression Ignition (HCCI) and Partially Premixed Combustion (PPC), as possible combustion processes in commercial engines. Compared to HCCI, PPC has advantages of lower unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions; however, due to increased fuel stratifications, soot emissions can be a challenge when adding Exhaust-Gas Recirculation (EGR) gas. The current work presents particle size distribution measurements performed from HCCI-like combustion with very early (120 CAD BTDC) to PPC combustion with late injection timing (11 CAD BTDC) at two intake oxygen rates, 21% and 15% respectively. Particle size distributions were measured using a differential mobility spectrometer DMS500. Additionally, to get knowledge of the effect of injection timing on particle size distributions in PPC mode, measurements were performed in injection timing sweeps at engine speeds 800 rpm and 1600 rpm. Results show that, without EGR, throughout the injection timing from very early to late injection, a unimodal particle size distribution dominated by nucleation mode particles can be observed. Adding EGR, similar unimodal particle size distributions are obtained for early injection timings whereas bimodal size distributions appear for late injection timings when injecting goes into the bowl. The corresponding accumulation mode particle number rapidly increases and results in a high engine-out soot output. In PPC mode with high fuel stratification, a similar trend is found at engine speeds 800 rpm and 1600 rpm during injection timing sweeps. Retarding injection is generally found to reduce particle numbers in nucleation mode and increase numbers in accumulation mode, which leads to a higher soot mass output despite of a reduction in total particle numbers. It can be concluded that charge stratification and fuel impingement influence particle size distributions during injection timing variations.