Overview
Heavy-duty vehicles represent one of the most important contributions to the emissions inventory for both nitrogen oxides (NOx) and particulate matter (PM) emissions. Heavy-duty engines have been subjected to increasingly more stringent standards over the years. The latest round of standards essentially requiring the use of diesel particulate filters (DPFs) and selective catalytic reduction (SCR) to meet the PM and NOx emissions standards, respectively. While these aftertreatment systems have provided significant reductions in PM and NOx emissions, it is important to verify that these systems are operating optimally under the full range of in-use conditions to ensure that air quality standards can be met. This is particularly true in the South Coast Air Quality Management District (SCAQMD), where it is projected that mobile source NOx emission levels will need to be reduced significantly to meet the 2023 and 2031 ozone standards. The heavy-duty diesel SCR equipped engine has reduced NOx emissions, by 90%, from previous levels when tested on the certification cycle, but not during in-use operation. Compliant 2010 vehicles were found to have NOx emissions ten times higher than the certification limit on low power test cycles, but meeting the limit on certification like cycles, (Dixit et al, 2017, Misra et al., 2013, Quiros et al., 2017, and Lee et al., 2018). Studies with ultra-low NOx natural gas engines have demonstrated emissions well below the current standard, as much as 100 times lower, even during low power test cycles (Johnson et al 2017 and 2018). Zero emission and hybrid trucks show the ability for zero emissions, but unfortunately their wide spread use is low. A dynamic standard, such as a zero emissions or ultra-low NOx zone, could be utilized which would encourage different technologies in urban areas where in-use NOx emissions are much higher than the standards. The Air Resource Board is proposing a low-NOx standard of 0.02 g/bhp-hr, a low power test cycle, and revisions to the Not-To-Exceed compliance test to try and close the gap between certification and in-use. A different solution is to measure all the trucks all the time with on board sensing and validate compliance from the in-use fleet under the conditions where they produce emissions. It is suggested, that there will always be a gap between our standards (policy) and the in-use emissions (real exposure) until we focus on the in-use for our compliance and certification methods. Agencies, industry and academia are in agreement that “in-use” is where we need to focus, so it is time to start thinking this way in our standards. Complying with in-use conditions will be a challenge and will require a change from laboratory or portable emissions measurement system (PEMS) testing to an on-board continuous measurement and reporting system. The system proposed here is called Onboard Sensing, Analysis, and Reporting (OSAR). OSAR utilizes sensors technology that is borrowed from the vehicles embedded control system. These control sensors have been studied and are relatively accurate, repeatable, and very durable. Tan et al., (2018) evaluated the in-use NOx emission rates from the on-board diagnostic (OBD) sensors of 72 HDDVs. They found that high NOx emissions were still a common problem in from in-use heavy-duty diesel fleets, primarily due to low SCR conversion efficiencies during low temperature operation and potentially from malfunctioning SCRs. Montes (2018) compared these same OBD sensors with the laboratory and found that the sensors on average were with-in 15% (with a range from -5% to +50%) of the laboratory measurement. The laboratory NOx emissions ranged from 2.5 to 0.046 g/bhp-hr and the OBD NOx sensor emissions ranged from 2.6 to 0.061 g/bhp-hr. The wide range of data between vehicles is significantly more important than the slight difference between the measurements. One significant concern with the OBD NOx sensor is that they are disabled below 250 C to prevent humidity damage to the ceramic sensing element. NGK-sparkplugs, a large manufacturer of OBD NOx sensors, developed prototypes that operate at lower conditions than the OBD sensors and also improved their accuracy. The low temperature conditions are important because this is where the highest NOx emissions are generated for SCR equipped diesel engines. Yang et al. (2018) utilized this prototype sensor and found NOx measurements were within approximately ±10% of those the full 1065 compliance PEMS system over a range of driving conditions and emissions sources from 15 g/bhp-hr to 0.2 g/bhp-hr including cold start conditions. The overall goal of this research is to create an on-board reporting system to guide the industry into a sustainable path of emissions control for their vehicles using the real world as the design platform. The funds provided by AQMD will leverage larger dollars from industry as we demonstrate and consider in-use design methods for regulations. It is believed this seed funding will spur the industry into a solution that includes instrumenting all new heavy-duty trucks with ideas for retrofitting older ones depending on feedback from the Agencies. Also it is believed this proposal will be supported by the industry and fleets owners as it benefits everyone with a fair and practical solution for emissions regulations. Eventually other mobile sources will follow this patter including non-road and light duty passenger cars. The goal of this Phase 1 research proposal is to develop and demonstrate a low cost NOx and PM sensor-based emissions measurement designed for heavy duty engines. The low cost system would be designed to include future capabilities such as dynamic engine calibration control, in-use policy enforcement with a fine based method, and a data driven exposure model specific to the South Coast Air Basin.
Lead Researcher: Dr. Kent Johnson Co-researcher: Dr. Thomas Durbin