Experimental investigations on the increase in nitric oxide emissions using biodiesels and their mitigation

One of the major shortcomings to be addressed in the widespread applications of biodiesel fuel for compression ignition engines is the formation of higher nitric oxide emissions. It is well established in the literature that thermal nitric oxide is a dominant source for nitric oxide formation in engines. Thermal nitric oxide formation increases by any in-cylinder combustion strategy that alters the in-cylinder temperatures, the oxygen fraction or the residence time of high-temperature post-flame burned gases. The differences between the properties of biodiesel in terms of a higher bulk modulus, a higher cetane number and the presence of a fuel-bound oxygen fraction and the properties of diesel are found to affect the in-cylinder charge conditions and thus the nitric oxide formation. The present work aims to understand the major contributor to the higher nitric oxide formation with biodiesel based on experimental investigations in two different engine configurations: one with a conventional mechanical-type injection system and the other with a modern common-rail direct-injection system. The experimental results highlight that the dynamic injection timing advanced up to a maximum of 2.6° crank angle owing to the higher bulk modulus of biodiesel. This factor contributes to specific nitric oxide emissions which are 7.5% higher in an engine having a mechanical-type injection system. The increase in the nitric oxide is neutralized on restoring the injection timing to that of the diesel injection time setting. In the case of an engine with a modern common-rail direct-injection system, the injection timings remain unaltered, and the nitric oxide concentrations for diesel and for biodiesel–diesel blends also remains the same.