On-Road, In-Use Emissions and Fuel Usage Assessment

Characterizing in-use emissions and fuel consumption of heavy-duty vehicles

South Coast Air Quality Management District


Diamond Bar, CA

Recipient Location


Senate District


Assembly District



Amount Spent



Project Status

Project Result

As of June 2021, UCR and WVU have completed all vehicle testing phases aside from a few mobile emission laboratory tests that have been delayed due to limited goods movement vehicle availability as a result of COVID-19. The researchers are analyzing data across each test phase with the aim to submit individual draft final reports in July 2021; the draft reports will be combined for submission to CEC in October 2021. Preliminary data shows higher in-use emissions from diesel and natural gas vehicles than their certification levels; this may be caused by low loads in urban routes or accelerated deterioration of aftertreatment systems. The data has been shared with CARB to inform its EMFAC 2021 model update. CEC is also leveraging the activity data to inform future research opportunities, development of charging infrastructure projections pursuant of AB 2127, and charging demand forecasts for heavy-duty vehicles.

The Issue

Results based on U.S Environmental Protection Agency (EPA) test procedures for heavy-duty on-road engines show that oxides of nitrogen (NOx) and particulate matter emissions meet the EPA and California Air Resources Board (CARB) emissions standards. However, in-use emissions testing of the engines operating under real world load conditions on vehicles have shown deviations from certification tests, including increased ammonia emissions from natural gas vehicles and NOx from diesel vehicles. Additional studies are needed to assess in-use emissions, fuel usage, and the impact of new technology for a variety of vehicle classes and applications.

Project Innovation

This project is conducting in-use vehicle emissions and activity tests for heavy-duty vehicles used in transit, school bus, refuse, delivery, and goods movement applications. The researchers are recruiting over 200 vehicles from a variety of California fleets to represent different vehicle technology types and routes. All recruited vehicles are being tested with portable activity monitoring systems (PAMS) to gather route and activity data. Around 100 vehicles are being tested with portable emissions measurement systems (PEMS) to gather in-use emissions data, and around 60 vehicles tested with a chassis dynamometer using vocation-based drive cycles developed from analyzing the PAMS and PEMS data. Around 10 goods movement vehicles are being tested with a mobile emissions laboratory to gather real world route-based data to compare with previous tests. Results will be used to develop new drive cycles, analyze deterioration factors, identify technology shortfalls and paths for improvement, prioritize resources to support advanced engine and aftertreatment technology research, and match vehicle technologies to appropriate vocations for which technology benefits can be maximized.

Project Benefits

Identifying the shortfalls of current natural gas vehicle technology can help prioritize specific research efforts that can maximize benefits to ratepayers. Comparisons with other technologies including diesel, hybrid, and electric will also be used to match vehicle technologies to vocations for which benefits can be maximized. This project will assist fleets in alternative fuel vehicle procurement planning and inform policymakers with real world vehicle emissions data that is highly valuable for the refinement of test cycles, emissions modeling tools and inventories (e.g., the CARB Emissions Factor -- EMFAC -- model), and emissions standards development.

Lower Costs


Data collected will assist in identifying which alternative fuel technologies best match the real world drive cycle of a variety of medium- and heavy-duty vehicle types to maximize fuel economy. This can inform fleet decisions and technology advancements to reduce total cost of ownership. Deterioration effects will also be investigated through real world testing, which can better determine optimal maintenance intervals for natural gas vehicles.

Environmental & Public Health

Environmental Sustainability

This project investigates potential issues causing heavy-duty engines to consume more fuel than expected during real-world operation, leading to increased carbon emissions. High idle percentages over all vocations tested point to a need to address efficiency at low loads, especially in urban environments.

Key Project Members

Project Member

Sam Cao

Air Quality Specialist



West Virginia University


The Regents of the University of California - Riverside


Match Partners


Southern California Gas Company (SoCalGas)


South Coast Air Quality Management District


California Air Resources Board


Contact the Team