A visualization of the ignition process of n–dodecane under multiple injections: An optical study in a heavy–duty diesel engine

Abstract

Multiple-injection is a widely adopted operating strategy applied in modern compression ignition engines, which involves various combinations of small pre-injections and post-injections of fuel before and after the main injection and splitting the main injection into multiple smaller injections. This strategy has been conclusively shown to improve fuel economy in diesel engines while achieving simultaneous NOx, soot, and combustion noise reduction – in addition to a reduction in the emissions of unburned hydrocarbons (UHC) and CO by preventing fuel wetting and flame quenching at the piston wall. Despite the widespread adoption and an extensive literature documenting the effects of multiple-injection strategies in engines, the complex interplay between the underlying flow physics and combustion chemistry involved in such flows, which ultimately governs the ignition and subsequent combustion processes thereby dictating the effectiveness of this strategy is not fully understood. To this end, experiments were performed in a heavy-duty, optical, single-cylinder engine to study the ignition and combustion behavior of n-dodecane under various multiple-injection schedules and intake conditions. Optical diagnostics involving simultaneous, planar laser-induced fluorescence (PLIF) imaging of formaldehyde (HCHO) and hydroxyl (OH) radical are used as an indicator of low-temperature heat release (LTHR) and high-temperature heat release (HTHR) respectively. To complement the PLIF imaging diagnostics with time-resolved images, high-speed OH* chemiluminescence imaging and infrared (IR) imaging are acquired simultaneously to probe the ignition and subsequent combustion behavior of n-dodecane under various multiple-injection schedules. Parametric sweeps include variations of the intake temperature, EGR dilution rates, and multiple-injection schedules by varying the mass of pilot-fuel injected and the associated dwell time that influences the ignition and combustion characteristics. Based on the imaging results and the thermodynamic analysis, conclusions are drawn on the effectiveness and optimization of multiple-injection schedules and the minimal required pilot-fuel injection quantity.