Heat Release Rate and NOx Formation Process in Two-Stage Injection Diesel PCCI Combustion in a Constant-Volume Vessel

The objective of the present study is to elucidate the combustion process of partial premixed charge compression ignition (PCCI) combustion using multiple injections in diesel engines. The effects of the ratio of the quantity of fuel used in the first and second injections, and the injection dwell time on heat release rate, soot and nitrogen oxide (NOx) formations are investigated in simulated partial PCCI combustion using a constant-volume vessel. N-heptane is used as fuel. The experiments are carried out under an ambient condition of 2 MPa and 900 K, which simulates a PCCI-like heat release rate with long ignition delays. The oxygen concentration is set to 21 and 15% to simulate conditions without and with exhaust-gas recirculation (EGR), respectively. The fuel quantity in the first injection is varied between 10 to 40% of the total fuel quantity, and the injection dwell is varied between 0.5 to 2.0 ms. Combustion analyses are carried out based on heat release rates and high-speed shadowgraph photographs. Image analyses of luminous flames are conducted to estimate soot formation and decay. NOx concentrations during combustion are measured using a total gas-sampling apparatus. The results show that, for the ordinary ambient oxygen mole fraction, longer injection dwells reduce the peak of the initial heat release rate in the case of a small amount of fuel used in the first injection; however, this effect is not found when a large amount of fuel is used in the first injection. By reducing the oxygen mole fraction, the above effect is obtained regardless of the quantity used for the first injection. An increase in the size of the first-injection quantity shortens the duration of the luminous flame. In a reduced ambient oxygen situation, the duration and area of the luminous flame increase as the injection dwell is longer. Regardless of the ambient oxygen mole fraction, the final NOx mass per released heat is reduced using two-stage injection, especially in the case of a long injection dwell.