NOx emission of biodiesel compared to diesel: Higher or lower?

A little decrease in NOx emission of biodiesel compared to diesel is observed at low loads under low and medium engine speeds. An overall experimental investigation on NOx emission based on combustion characteristics of biodiesel has been carried out. Negative effects of high viscosity and distillation temperature on the spray quality and homogeneity of air fuel mixture are clear at low loads under low and medium speeds. Brake thermal efficiency (BTE) and combustion temperature of biodiesel are lower than those of diesel, resulting in a lower level of NOx emission. With the increase of load, in-cylinder thermal condition and the injection pressure are improved, and with the increase of speed, air swirl movement is strengthened. Oxygenated biodiesel produces more reactive radicals than diesel, and these radicals will surely accelerate the combustion speed, improves the intensity of diffusion combustion, shortens the combustion duration and increase the BTE. Consequently, the diffusion combustion temperature, especially the peak combustion temperature of biodiesel is higher than that of diesel. On the whole, NOx emission of biodiesel is higher than that of diesel in most cases, except in the condition of low loads under low and medium speeds.