Effects of intake oxygen mole fraction on the near-stoichiometric combustion and emission characteristics of a CI (compression ignition) engine

• Near stoichiometric diesel combustion characteristics were investigated.
• Single-cylinder engine experiments were performed.
• Combustion and emission characteristics were plotted on Φ-SOI (start of ignition) map.

In the present study, the combustion and emission characteristics under various equivalence ratio up to stoichiometric condition were experimentally investigated in a CI (compression ignition) engine. In order to control the equivalence ratio, the O2 mole fraction of intake gas was reduced from 21 to 11.16 % by adding only N2 to intake gas while the injected fuel quantity was held constant. The test was used a single-cylinder compression ignition (CI) engine with a displacement volume of 373.3 cc, a compression ratio of 17.8, a re-entrant type piston-bowl. The emission measurement instruments consisted of emission bench (MEXA-9100D, HORIBA) and smoke meter (AVL-415S, AVL).In the result, the maximum IMEP (indicated mean effective pressure) and fuel conversion efficiency gradually decreased when the equivalence ratio was richer. In particular, the stoichiometric condition (i.e. 1.0 equivalence ratio) had around 15% and 11% losses, respectively. The stoichiometric diesel combustion had a positive ignition dwell over the whole operating condition, leading to a large amount of THC (total hydrocarbons) and CO emission. Especially, the level of IS (indicated specific)-soot for stoichiometric combustion is significantly higher than the maximum level of IS-soot under the same operating condition without reduced O2 mole fraction.