Quantitative characterisations of spray deposited liquid films and post-injection discharge on diesel injectors

Transient injection phases have been identified as a prominent source of inefficiencies and exhaust gas constituents detrimental to both public health and the environment. The rapid reduction of in-nozzle flow rate at the end of diesel injection events inhibits spray atomisation and releases large slow-moving liquid structures into the cylinder. This uncontrolled release of fuel results in wetting of the nozzle surface through rogue droplets diverting back towards the nozzle. The resulting films create fuel-rich regions that may later get drawn into the exhaust, contributing to engine-out emissions. They also present an ideal environment for reactions with combustion products and adherence of deposit precursors. Despite recent experimental advances there is a lack of quantitative data relating the operating conditions to the quantity of fuel deposited on nozzle orifices.

To improve our understanding of the underlying near-nozzle and surface-bound processes, we applied high-speed optical microscopy under conditions relevant to passenger vehicles. Image processing techniques were used to quantify the deposition of fuel films and their spreading with time. A single component fuel (n-dodecane) was injected using an injector instrumented with a thermocouple to measure the sub-surface nozzle tip temperature. Injection duration, timing and pressure were varied to reveal their influence on the deposition and overspill of fuel onto the nozzle.

We conclude by presenting an analysis of the film behaviour as function of injection pressure, in-cylinder pressure and bulk gas temperature. Relative to the conditions investigated, spray wetting was more pronounced at the reduced load conditions.