An Experimental Investigation on the Effect of Diluent Addition on Flame Characteristics in a Single Cylinder Optical Diesel Engine 2015-24-2438

The present work investigates the effect of low levels CO₂ addition on the combustion characteristics inside a single cylinder optical engine operated under low load conditions. The effects of dilution levels (up to 7.5% mass flow rate CO₂ addition), the number of pilot injections (single or double pilot injections) and injection pressure (25 or 40 MPa), are evaluated towards the direction of achieving a partially premixed combustion (PPC) operation mode. The findings are discussed based on optical measurements and via pressure trace and apparent rate of heat release analyses in a Ricardo Hydra optical light duty diesel engine. The engine was operated under low IMEP levels of the order of 1.6 bar at 1200 rpm and with a CO₂ diluent-enhanced atmosphere resembling an environment of simulated low exhaust gas recirculation (EGR) rates. Flame propagation is captured by means of high speed imaging and OH, CH and C₂ line-of-sight chemiluminescence respectively. Each of the above species is a proxy of a fundamental combustion property; OH* is related to the oxidation zone, CH* to the heat release zone and the flame front, while C₂* is indicative of fuel-rich areas. The combined analyses of the obtained results were made under the perspective of identifying the induced alterations in flame structures and, possibly, combustion modes, and their manifestation at the global in-cylinder conditions. The increase of CO₂ addition results in lower peak pressures and in an overall delay of the combustion process, while also influences the spatial characteristics of reaction and oxidation zones, as well as differentiates the extent of fuel-rich pockets. Multiple injections advance the main combustion event and an increase in injection pressure enhances fuel evaporation and mixing while spatially confining the observed flame structures. Overall, operation under relatively slightly diluted conditions with more pilot injection events at higher rail pressure appear to enhance mixing, proving thus an indication of lower emission levels.