Challenges to Ranking Diesel Fuels for NO _x Emissions Using a Constant-Volume Combustion Chamber

The market is witnessing an unprecedented proliferation of low-emission fuel components. To effectively evaluate the suitability of these novel fuels for engine applications, fuel blenders and original equipment manufacturers require rapid and reliable assessment methodologies. Traditionally, such evaluations rely on comprehensive engine testing, which, while thorough, is both time-intensive and costly. In response to the growing diversity of emerging fuel options, this work aims to establish a streamlined screening approach capable of effectively replicating the outcomes of full-scale engine testing. We 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 air at 650°C and 20 bar, 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. Heat release rate analysis suggested this was caused by large differences in ignition delay and premixed burn fraction for the fuels tested. 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 to coincide with the pilot heat release. 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. Two soy biodiesel samples did not show higher NOx as observed in engine studies. 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.