The NOx emissions, however, increased with biodiesel to be dominated by the thermal mechanism [12]. This may be partially attributed to the increased oxygen availability. Since biodiesel flame temperature is slightly below that of diesel, the temperature alone cannot attribute for the higher NOx. While combustion timing is also important, it may or may not support the increased NOx. In this regard,biodiesel typically advances the injection and burning thus increasing the residence time at higher temperatures close to T. D. C [17].However, its earlier and faster combustion provide lower exhaust gas temperatures. The net effect is determined by the relative contributions dictated by the operating conditions. The biodiesel small nitrogen portion may slightly contribute to NOx increase. While Frey and Kim [13] reported NOx increase with B20, others concluded that biodiesel reduces NOx via its smaller heating value and higher cetane
number (which shortens the premixed combustion and reduces the temperature gradients). The faster ignition provides more time for diffusion combustion to proceed smoothly rather than going sharply.