Numerical investigation on the effect of oxygen in combustion characteristics and to extend low load operating range of a natural-gas HCCI engine

Abstract

The effect of increasing oxygen on extending the low load operating range and combustion and emission characteristics of a natural-gas homogeneous charge compression ignition engine with pre-chamber is studied numerically. There are five oxygen percentage selected to investigate the influence of oxygen in homogeneous charge compression ignition combustion. The five selected Oxygen percentages are 21% (pure air), 40%, 60%, 80% and 100% (pure oxygen), which have 0.3 (pure air), 0.158, 0.105, 0.079 and 0.063 (pure oxygen) equivalence ratios, respectively. 60% oxygen is the optimum amount due to the earliest start of combustion and CA50 in all cases that directly affect the indicated mean effective pressure and efficiency. It has the maximum pressure, temperature, and heat release rate compared to the other oxygen percentages. Increasing oxygen up to 60%led to producing more OH radicals, while more increasing oxygen, causes a lack of H atoms from the decomposition of the CH bond in natural-gas caused to reducing OH concentration. Increasing oxygen makes the temperature distribution more uniform and shorts the combustion duration. It also leads to drop the temperature for the start of combustion. CO and UHC drop with excess oxygen, the maximums decreasing for CO and UHC are almost 100% and the minimums are 50.11% for CO and 80.4% for UHC occurring in 40% oxygen. Oxygen can extend the load range to the dilute regions, 40% oxygen would expand about 47.33% while by 60%, 80%, and 100% oxygen it would expand about 65%, 73.67%, and 79% respectively.