Diesel HCCI Combustion Control Parameters Study using n-Heptane Reduced Chemical Kinetic Mechanism 2008-28-0036

Homogeneous Charge Compression Ignition (HCCI) combustion concept has potential of reducing NOx and PM emissions simultaneously and it is being considered as a future technology for diesel engines to meet tightened emission norms imposing by national governments. However, HCCI is limited to a narrow band of operating region due to difficulties in controlling combustion phasing close to Top Dead Center (TDC).

From literature study, Exhaust Gas Recirculation (EGR), Intake Temperature, Boost Pressure, Equivalence Ratio and Compressions ratios are considered as most critical parameters for HCCI control. The chemical kinetics study was conducted to understand the HCC combustion using N-heptane mechanism with 162 species and 732 reactions. At lower equivalence ratio (lean burn combustion) higher CO and HC emissions were observed. The combustion efficiency was poor at lower temperatures, which resulted in high HC and CO emissions with less than 10ppm NOx. Poor combustion was observed at low intake temperatures less than 25°C for which auto ignition temperature is less than 950K, which is threshold temperature for diesel auto ignition. Higher cylinder pressures were observed with increased boost pressure at constant equivalence ratio. Experiments show that at high boost pressures and higher EGR rates the gas exchange efficiency was poor due to higher pumping losses, hence it requires efficient EGR controller. At higher EGR rates greater than 65%, both NOx and soot emissions were close to zero, but very high CO and HC emissions with poor combustion efficiency. It was also observed that high EGR rates and lower compression ratios are most effective control parameters to control the dP (pressure rise per crank angle) below the designed value. The air fuel mixture auto-ignition is extremely sensitive to variations in intake air and EGR mixture temperatures. Low NOx emissions were observed at combustion temperature less than 2000K. Experimental results observed very low NOx (< 10ppm) low soot (less than 2%) under controlled low temperature combustion. Trends of simulation results matched with experimental results with EGR and boost pressure. This study concentrated the reasons for formation of high CO and HC emissions in HCCI engines.