NOx Measurement and Characterization in a Gaseous Fueled High-Pressure Direct-Injection Engine

Heavy-duty (HD) vehicles are a crucial part of the transportation sector; however, strict governmental regulations will require future HD vehicles to meet even more rigid NOx emission standards than what already exist. The use of natural gas (NG) as the primary fuel in HD vehicles can immediately reduce the NOx emissions through lower flame temperatures as compared to traditional diesel and can serve as a precursor to even less carbon intensive fuels as they become more readily available. Pilot ignited direct injection natural gas (PIDING) engine technology is one example of how NG can be used in HD vehicles while maintaining diesel-like efficiency. However, NOx emissions still need to be mitigated to avoid negative air quality effects. Exhaust gas recirculation (EGR) is known to reduce in-cylinder temperatures and thus reduce in-cylinder NOx emissions in diesel engines, but the effects of EGR are not as well understood in PIDING engines. The intent of this study is to develop a better understanding of the sensitivity of NOx to the specific effects of EGR in PIDING engines by experimentally identifying the limits of EGR on a single cylinder research engine (SCRE). Two different equivalence ratios (φ) of 0.6 and 0.7 were used while maintaining engine load at 12 bar GIMEP, combustion phasing, and engine speed throughout an EGR sweep. The maximum EGR rate tested was ∼50% for each φ. Combustion instability (measured by the coefficient of variability (COV) of peak cylinder pressure (PCP) and GIMEP) increased by 2 and 3% at maximum EGR for φ = 0.6 and 0.7 respectively. NOx emissions were reduced ∼80% up to 25% EGR. However, NOx sensitivity to the effects of EGR diminish significantly at rates above 35%. The inverse is also true for particulate matter (PM) and methane in that these emissions significantly increase at EGR rates above 35%. Lastly, exhaust mounted electrochemical NOx sensors were found to be effective and comparable to lab-grade emissions analyzers while being more cost effective and less intrusive.