Despite the dramatic progress year on year with reducing emissions in the USA, the U.S. EPA says air pollution continues to threaten Americans’ health and welfare. Furthermore, emissions from the transportation sector are comparatively on the rise, now claiming first position amongst contributing sectors. A new study prepared by UC Riverside’s College of Engineering and published by CARB last week suggests SCR technology is less effective, vocation-specific and requires further research to determine the frequency of good SCR function needed to obtain real-world NOx emissions data.
UC Riverside : Collection of Activity Data from On-Road Heavy-Duty Diesel Vehicles – FINAL REPORT
The study, prepared for California Air Resources Board (CARB) and California Environmental Protection Agency by an investigative team from University of California at Riverside’s College of Engineering – Center for Environmental Research and Technology, says the effectiveness of SCR emissions reduction is highly dependent on engine temperature and recommends a new measurement methodology.
The research was led by principal investigator Kanok Boriboonsomsin, with support from co-researchers Kent Johnson, George Scora, Daniel Sandez, Alexander Vu, Tom Durbin, and Yu (Jade) Jiang; also Andrew Burnette of carbonBLU.
The following abstract for the report is published by CARB:
“A selective catalytic reduction (SCR) aftertreatment system on 2010-technology heavy-duty diesel vehicles reduces oxides of nitrogen (NOX) emission in the exhaust stream, but requires adequate temperature, typically at least 200°C, for the reduction to take place. However, there are times when this temperature requirement is not met, such as initial engine start and during low load operations experienced when the vehicle is idling or moving slowly to meet various vocational demands. To better quantify real-world NOx emissions achieved by SCR it is thus critical to characterize heavy-duty diesel vehicle vocation-specific activity profiles including duty cycles, number of engine starts, SCR temperature profiles, and engine soak time distributions. The project team recruited and collected instantaneous location and engine activity data for a minimum of 4 weeks for each of 90 heavy-duty vehicles with gross vehicle weight ratings (GVWR) of 26,000lbs or higher. Eight of the 90 vehicles were not equipped SCR aftertreatment systems: five express buses powered by natural gas and three diesel trucks older than 2010 model year. A total of 19 vocation types were identified covering a range of vocational uses, GVWR, and geographic regions. Activity profiles of idling, diurnal distribution of activity, trips per day, soak time distribution, SCR temperature profiles, and characteristic of driving cycles were characterized for each vocation type. These activity profiles are extremely important for improving the heavy-duty NOx emission inventory and estimating the frequency of good SCR function.”
The authors of the report point out that, while these results provide a significant insight into vehicle activity patterns of heavy duty vehicles of a variety of vocations, the data were collected from a limited number of vehicles (no more than 8 vehicles in a vocation) from a very limited number of fleets (no more than 3 fleets in a vocation). They recommend care be taken when interpreting the results as they may not be generalized to the entire heavy-duty vehicle population of that particular vocation in the state.
Natural Gas Engines
Natural gas engines such as those manufactured by Cummins Westport Inc. (CWI) do not require active aftertreatment such as a Diesel Particulate Filter (DPF) or SCR.
CWI’s 2018 ISX12N and L9N engines will be the lowest certified NOx emission engines available in North America. NOx exhaust emissions are 90% lower than the current EPA NOx limit of 0.2 g/bhp-hr, and the engines also meet or exceed the 2017 EPA greenhouse gas (GHG) emission requirements.
CWI natural gas engines have met the 2010 EPA standard for particulate matter (0.01 g/bhp-hr) since 2001.
CWI engines provide operators a choice of compressed natural gas (CNG), liquefied natural gas (LNG) or renewable natural gas (RNG) as a fuel. According to California ARB Low Carbon Fuel Standard (LCFS) studies, RNG can reach subzero GHG carbon intensity levels.
“Natural gas medium and heavy-duty engines offer unmatched reductions of NOx. Because natural gas engines do not require SCR, the duty cycle (i.e. speed, idling, traffic, etc) does not negatively impact the emissions from natural gas engines. The study completed by UC-Riverside shows that there is a significant environmental advantage to using natural gas vehicles including in urban transport applications. We look forward to continued research from UC-Riverside on this topic,” commented Dan Bowerson, Director, Technology & Development for NGVAmerica.
US Emissions Profile
The U.S. Energy Information Administration (EIA) reported in January this year (1) that CO2 emissions from the transportation sector have now surpassed electricity as the biggest air polluter in the country. Transport-related emissions reached 1,893 million metric tons (MMmt) from October 2015 through September 2016, exceeding electric power sector CO2 emissions of 1,803 MMmt over the same time period. On a 12-month rolling total basis, electric power sector CO2 emissions are now regularly below transportation sector CO2 emissions for the first time since the late 1970s. CO2 emissions from electric power have been trending lower since 2007; amongst the main reasons for this is the increased use of natural gas for power generation.
In fact, the U.S. transportation sector was the only consumption sector where CO2 emissions increased in 2016. CO2 emissions increased by 1.9%, largely reflecting emissions from motor gasoline, which increased 1.8% in 2016. Transport sector emissions look set to stay above electricity in a trend that persists through to “at least 2040 in the Reference case projections in EIA’s 2017 Annual Energy Outlook“, says EIA. (2)
The UC Riverside report is available here.
(Sources: US EPA, US EIA, Cummins Westport, CARB, NGVAmerica)