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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.

The aim of this paper is to extend the evaluation of the accuracy of published 1-D pressure loss coefficients which are used in 1-D gas dynamics models, in unsteady compressible flows propagating in the exhaust pulses in manifolds. These pressure loss coefficients were derived from the conservation of linear momentum over finite control volumes based on assumptions including steady flow. The objectives of this work were to evaluate the accuracy of the pressure loss coefficients over the type of flows generated by engine-like pressure pulses propagating in a range of three-pipe junctions. The evaluation was performed by reference to results from unsteady, compressible, 3-D Reynolds-averaged computational fluid dynamic (CFD - open source software OpenFOAM) simulations. Two of the junction branches represented the exhaust pipes from two cylinders and the remaining was the outlet pipe. All pipes had a diameter of 25mm with length ratio 1:2 between inlet and outlet.