Butanol-diesel blend spray combustion investigation by UV-visible flame emisison in a prototype single cylinder compression ignition engine

The paper reports the results of an experimental investigation carried out in a prototype optically accessible compression ignition engine fuelled with different blends of commercial diesel and n-butanol. Thermodynamic analysis and exhaust gas measurements were supported by optical investigations performed through a wide optical access to the combustion chamber. UV-visible digital imaging and 2D chemiluminescence were applied to characterize the combustion process in terms of spatial and temporal occurrence of auto-ignition, flame propagation, soot and OH evolution. The paper illustrates the results of the spray combustion for diesel and n-butanol-diesel blends at 20% and 40% volume fraction, exploring a single and double injection strategy (pilot+main) from a common rail multi-jet injection system. Tests were performed setting a pilot+main strategy with a fixed dwell time and different starts of injection. For the diesel case, the whole amount of injected fuel and injection pressure were set at 22 mg/str and 80 MPa corresponding to a medium load regime for an automotive light duty diesel engine. The fuel amount for the butanol-diesel blends was changed to get the same chemical energy of the delivered fuel as the reference diesel case (935J/str). The investigation was carried out at two EGR rates, 0% and 50%, corresponding to a concentration of O2 at intake of 21 and 17%, respectively. Taking advantages of the higher resistance to auto ignition of the butanol-diesel blends, the results showed a transition from the conventional to a partial premixed combustion for the BU40 in the single injection case. The combined effect of two-stage injection and EGR increased the ignition delay and the switch to the partial premixed combustion was enhanced. Improvement in NOx-soot trade-off was obtained activating the double injection strategy particularly with the butanol blends. Optical diagnostics allowed to detect and feature the spatial distributions due to the flame luminosity and OH emission at autoignition and to estimate of the lift-off length. A high concentration of OH radicals was detected for all the test cases without EGR that corresponded to an enhanced oxidation phase and a fast soot reduction. Further, the soot oxidation phase resulted more significant for the higher butanol volume fraction in the blends.