Challenges to Ranking Diesel Fuels for NOx Emissions Using a Constant-Volume Combustion Chamber

This study examined the use of a constant volume combustion chamber for the measurement of fuel effects on NOx emissions, with the goal of developing a method to rapidly screen or rank fuels in a small - volume experiment. A small amount of fuel was injected into the air at 650°C and 20 bars, where it ignited and burned. The chamber was sampled post-combustion using a chemiluminescence NOx analyzer. Extensive sampling method development was required to obtain repeatable results. Seven hydrocarbon fuels and two biodiesel fuels were tested, all of which have shown difference in NOx emissions in past engine studies. When using a single injection event to deliver the same amount of fuel energy, the test method could not clearly demonstrate the difference in NOx emissions between the hydrocarbon fuels as reported in engine combustion studies. To improve this, a dual-injection strategy was used. It included a small “pilot” injection followed by a main injection, timed based on each fuel’s ignition delay. This strategy helped reduce differences in heat release caused by how quickly each fuel ignites. For hydrocarbon fuels, this approach revealed the expected relationship between NOx emissions and fuel type. However, the correlation with aromatic content was weak, while a strong correlation with cetane number explained most of the NOx variation (r2=0.91). Two soy biodiesel samples did not show higher NOx as observed in engine studies. Experiments at lower temperatures of 575° C and 500° C produced the same results. This suggests that the current test method may not fully reflect engine conditions where biodiesel tends to produce more NOx. Further improvements in test method and setup are recommended to better align constant volume chamber conditions with engine conditions under which biodiesel shows increased NOx emissions.