Ultra-Low NOx Near-Zero Natural Gas Vehicle Evaluation ISX12N 400

Heavy-duty on-road vehicles represent one of the largest sources of NOx emissions and fuel consumption in North America. Heavy-duty vehicles are predominantly diesels, with the recent penetration of natural gas (NG) engines in refuse collection, transit, and local delivery where vehicles are centrally garaged and fueled. As emissions and greenhouse gas regulations continue to tighten, new opportunities to use advanced fleet specific heavy-duty vehicles with improved fuel economy are becoming available. NOx emissions have dropped 90% for heavy-duty vehicles with the recent 2010 certification limit. Additional NOx reductions of another 90% are desired for the South Coast Air basin to meet its 2023 NOx inventory requirements. Although the 2010 certification standards were designed to reduce NOx emissions, their in-use NOx emissions are actually much higher than certification standards. The main reason is a result of the poor performance of aftertreatment systems for diesel vehicles during low duty cycle operation. Recent studies by UCR suggest 99% of the operation within 10 miles of the ports are up to 1 g/bhp-hr NOx. Stoichiometric natural gas engines with three-way catalysts tend to have better low duty cycle NOx emissions than diesel engines with SCR aftertreatment systems. Thus, a real NOx success will not only be providing a solution that is independent of duty cycle, but one that also reduces the emissions an additional 90% from the current 2010 standard. Goals: The goals of this project was to evaluate Cummins West Ports (CWI) ISX12N (Near-zero) 11.9 liter ultra-low NOx natural gas (NG) truck. The evaluation included regulated and nonregulated emissions, ultrafines, global warming potential, and fuel economy during in-use testing. This report presents a summary of the results and conclusions for the CWI ultra-low NOx NG 11.9L truck (ISX12N). Approach: The testing was performed on UC Riverside’s chassis dynamometer with their Mobile Emissions Laboratory (MEL) located in Riverside CA just east of the South Coast Air Quality Management District (AQMD). The cycles selected for this study are representative of operation in the South Coast Air Basin and included drayage port cycles (near dock, local, and regional), the urban dynamometer driving schedule, and three cycles designed by CARB (called HHDDT cycles). Measuring NOx at 90% of the 2010 certification level (~ 0.02 g/bhp-hr is approaching the detection limit of the dilute CVS method. Previously, advanced NOx measurement methods were evaluated by UCR and the raw measurement method was recommended and utilized (Johnson et al 2016). The raw NOx chemiluminescence measurement method was also used for this study with the addition of a new spectroscopy method not susceptible to interferences from NH3 emissions. In addition to the regulated emissions, the laboratory was equipped to measure particle size distribution, particle number (both solid and total), equivalent black carbon, ammonia, and nitrous oxide emissions. The measurements were collected to investigate the benefit of the ISX12N engine and aftertreatment system compared to other approaches. Results: The ISX12N NG engine showed NOx emissions below the CARB optional low NOx standard (0.02 g/bhp-hr) and averaged between 0.0012 and 0.02 g/bhp-hr for the various hot start tests, see Figure ES-1. The NOx emissions were well controlled at low loads (Creep and Near Dock cycles) as well as during cruise conditions (Regional and HHDDT Cruise) where diesel vehicles tend to have much higher emissions at light loads but perform well at cruise conditions. This suggests stoichiometric NG engines are a good choice for regional NOx mitigation strategies where light loads are common. More than 90% of the NOx emissions resulted from these transient de-accelerations. The variability in the emissions is a result of the magnitude of the NOx spike. This suggests possible driver behavior may impact the overall NOx in-use performance of the vehicle where more gradual de-accelerations are desired, such as with hybrid applications. Cold start NOx emissions represent a significant part of the total NOx emissions reported. The cold start emissions averaged 0.130 g/bhp-hr (around ten times higher than the hot UDDS) where the hot/cold weighted emissions was 0.028 g/bhp-hr which is above the certified 0.02 g/bhp-hr emission factor. More than 90% of the NOx emissions occurred in the first 50 seconds of the cold UDDS test. Once the catalyst warmed up, the remaining portions of the cold UDDS test showed low NOx emissions similar to the hot UDDS test. It is expected the real impact of the cold start emissions is much lower than 1/7 weighting factor required by the regulations and would be represented by 50 seconds divided by the actual shift time (typically more than 3600 seconds). More research is needed to understand cold start emissions and their impact regionally. The cold start emissions suggest hybrid stop-start technology may need electrically heated catalyst to minimize potential warm-start emissions during long periods of electric only operation. The other emissions such as carbon monoxide, particulate matter, nitrous oxide, and ammonia also showed some differences compared to similar stoichiometric 2010 certified and NZ certified NG vehicles tested by UCR. For example, the PM for the ISX12N was slightly higher than the NZ and 2010 certified NG engine (0.002 g/bhp-hr vs 0.001 g/bhp-hr), the ammonia was slightly lower ~50 ppm vs ~200 ppm, and N2O was about the same.

To continue reading the executive summary, download the full report.