Analysis of cold-start NO2 and NOx emissions, and the NO2/NOx ratio in a diesel engine powered with different diesel-biodiesel blends

In the transportation sector, the share of biofuels such as biodiesel is increasing and it is known that such fuels significantly affect NOx emissions . In addition to NOx emission from diesel engines, which is a significant challenge to vehicle manufacturers in the most recent emissions regulation (Euro 6.2), this study investigates NO 2 which is a toxic emission that is currently unregulated but is a focus to be regulated in the next regulation (Euro 7). This manuscript studies how the increasing share of biofuels affects the NO 2 , NOx, and NO 2 /NOx ratio during cold-start (in which the after-treatment systems are not well-effective and mostly happens in urban areas). Using a turbocharged cummins diesel engine (with common-rail system) fueled with diesel and biofuel derived from coconut (10 and 20% blending ratio), this study divides the engine warm-up period into 7 stages and investigates official cold- and hot-operation periods in addition to some intermediate stages that are not defined as cold in the regulation and also cannot be considered as hot-operation. Engine coolant , lubricating oil and exhaust temperatures, injection timing, cylinder pressure , and rate of heat release data were used to explain the observed trends. Results showed that cold-operation NOx, NO 2 , and NO 2 /NOx ratio were 31–60%, 1.14–2.42 times, and 3–8% higher than the hot-operation, respectively. In most stages, NO 2 and the NO 2 /NOx ratio with diesel had the lowest value and they increased with an increase of biofuel in the blend. An injection strategy change significantly shifted the in-cylinder pressure and heat release diagrams, aligned with the sudden NOx drop during the engine warm-up. The adverse effect of cold-operation on NOx emissions increased with increasing biofuel share.