California Air Resources Board

Off-road (or non-road) engines are vital for agriculture, construction, port operation, generation of electricity, and other activities essential to our way of life. As overall emissions have decreased from on-road engines, off-road engines have become responsible for an increasing share of total emissions. Furthermore, because off-road engines are used throughout the state, and not just on its public roadways, less is known about their activity patterns and emissions than for their on-road counterparts. Achievement of California’s air quality improvement and greenhouse gas reduction goals over the coming decades will require an increased understanding of, and reduced emissions from, its off-road engines. Below is a list of recent and current projects related to off-road engines, including activity data acquisitions, emissions characterizations, and exploratory work for future technology innovations reducing emissions.

Activity Data Collection Projects

Collection and Analysis of Agricultural Equipment Activity Data 

Agriculture employs a large fraction of the California workforce and contributes significantly to the state economy. Off-road diesel engines are widely used in agricultural goods production and supply operations: from planting to harvesting and processing. Off-road diesel engines are also important contributors to poor air quality in non-attainment areas for particulate matter (PM) and ozone (O₃). In particular, the San Joaquin Valley (SJV), which is one of only two areas in the country experiencing extreme O₃ nonattainment, contains over 50 percent of the state’s agricultural equipment. In 2012, approximately 14 percent of nitrogen oxide (NO_x) emissions in the SJV originated from farm equipment. To improve air quality and develop effective incentive strategies for the SJV, constructing an accurate agricultural emissions inventory with the latest activity data is critical.

This project is collecting Engine Control Unit (ECU) data related to engine and aftertreatment performance with dataloggers installed in up to 240 agriculture tractors (below). The results will be analyzed to better understand the activity patterns of these tractors, which will in turn be used in the development of more accurate future emission inventories and more representative engine certification standards.

Activity Data of Off-Road Engines in Construction

Construction equipment is one of the most significant sources of NO_x emissions in the heavy-duty off-road sector. Understanding the contribution of off-road diesel engines to the emissions inventory is critical to developing effective regulations for the off-road sector, and in evaluating what emissions control strategies are needed. It is important to understand the activity patterns for off-road equipment that can be used to accurately portray their in-use operation. However, the available data for off-road equipment is still considerably more limited compared to on-road mobile sources.

This study will evaluate the operational patterns of 54 pieces of off-road construction equipment, representing nine different equipment types, during typical operation in California. Equipment types were selected based on prevalence in CARB's OFFROAD model (see examples below). The current study will expand on previous University of California Riverside and CARB studies by focusing on the activity data collection that will cover a comprehensive array of equipment types and engine power ratings for construction equipment.

Preliminary results indicate that daily operational time varies from about 3 to 7 hours, depending on equipment type. More information on the project can be found in the Research Contract Catalog.

Emissions Characterization and Technology Innovation Projects

Next Tier Off-Road Diesel Aftertreatment Technology Fabrication and Optimization

Under current regulations, off-road diesel engines are expected to contribute 95 tons per day (tpd) oxides of nitrogen, or NO_x, and 3.1 tpd of PM to the California emissions inventory in 2030, making off-road diesel the single largest source of mobile emissions in California. It is imperative, therefore, that action be taken to mitigate the adverse impacts of off-road diesel emissions by adopting more stringent NO_x and PM emission standards in the near term.

The objective of this project is to design, procure, age, and optimize advanced NO_x and PM aftertreatment technologies for incorporation on a new Tier 4 off-road diesel engine with the goal of reducing NO_x emissions by 90 percent and PM emissions by 75 percent below Tier 4 final standards. The project will explore methods to help reduce any associated fuel economy penalties, providing CARB with an optimized low-NO_x and low-PM configuration to support future Tier 5 standards. Examples of investigated aftertreatment components (below) include Diesel Oxidation Catalysts (DOC), Diesel Particulate Filters (DPF), Diesel Exhaust Fluid (DEF) Dosing, and Selective Catalytic Reduction (SCR).

More information on CARB contract 19RD025 is available in the Research Contract Catalogue.

Off-Road Diesel Low-Emission Demo for Oxides of Nitrogen (NO_x), Particulate Matter (PM), and Toxics

Today, off-road diesel engines contribute 29 percent of statewide diesel PM emissions and 11 percent of statewide NO_x emissions. Because of the quick market growth for off-road diesel engines, and the continuous improvement of emission control for on-road engines and vehicles, off-road diesel engines will become increasingly important sources of engine emission-related air pollution.

The objective of this project was to measure the emission rates of several criteria and non-criteria pollutants from two off-road diesel engines (see photo below) with different aftertreatment configurations. One engine was be certified and equipped with the selective catalytic reduction (SCR) system only, and the other engine was be certified and equipped with a combined diesel particulate filter (DPF) and SCR system. The project tested both engines on an engine dynamometer over the same test cycles, and measured the emission rates of criteria pollutants including NO_x, PM, non-methane hydrocarbons (NMHC), and carbon monoxide (CO), and non-criteria pollutants such as carbon dioxide (CO₂), individual volatile organic compounds (VOCs), metals, particle number (PN), and polycyclic aromatic hydrocarbons (PAHs). In addition, the project identified and recommended potential engine and aftertreatment control strategies to maximize emission reductions over the test cycles using a numerical model.

More information on CARB contract 18RD006, including the Final Report, is available in the Research Contract Catalogue.

Hybridization and Full Electrification Potential in Off-Road Applications

Some electrified off-road equipment currently operates in the state; however, more work is needed to determine the feasibility of electrifying wider vocational applications of this sector. In this project, previously unexplored pathways for hybridization and electrification in off-road equipment that maximize climate and air quality benefits while remaining both technically and economically viable were identified. This study assessed the current trends toward electrification in the off-road equipment sector and determine the factors that have driven these trends. Additionally, this study examined the entire off-road sector with an engine horsepower greater than 50hp and classified the various vocations by energy demand and operational activity to determine which sectors have not been considered for hybridization or electrification but could be partially or fully electrified in the near future. The results of this study provided CARB with a roadmap for future incentive programs that will help meet its air quality and climate goals.

Results from this study for 17 off-road vehicles representing six equipment types were recently presented at the 100th Annual Meeting of the Transportation Research Board. They were also published in Transportation Research Record. The Final Report is also now available.

More information on CARB contract 18RD016 is available in the Research Contract Catalogue.

Evaluation of the feasibility, cost-effectiveness, and necessity of equipping small off-road diesel engines with advanced PM and/or NO_x aftertreatment

The objective of the contract was to evaluate the potential effectiveness, feasibility, and cost effectiveness of implementing more stringent emission regulations on mobile off-road diesel engines with rated powers of less than 75 hp that could be achieved using advanced emission control strategies, such as diesel particulate filters (DPFs) and selective catalytic reduction (SCR).

Commercially available aftertreatment (AT) was demonstrated on four off-road engines. DPFs were added to an 11 hp Transportation Refrigeration Unit (TRU) and a 24 hp mini-excavator; and SCRs were added to a 36 hp ride mower and a 49 hp skid steer. The emission control performance of SCR and DPF systems applied to the SORDEs were consistent over 1,000 hours of real-world or laboratory operations.

Test results demonstrate that DPFs reduced PM to at least 98% below the current standard, and SCRs reduced NOx emissions to 50-90% below the current standard (see table below). SCR adoption by 25 – 75 hp engines and DPF adoption by < 25 hp engines could reduce total statewide off-road emissions of NO_X by 8.8 – 13.6% and PM by 2.1%, with over half of these benefits occurring in the San Joaquin Valley and the South Coast Air Basin. The cost-effectiveness of the demonstrated controls was estimated to be $0.36 to 0.59/lb NO_X and $19.10/lb PM, which compares very favorably to other rulemakings adopted by CARB.

More information on CARB contract 14-300 is available in its Final Report and in the Research Contract Catalogue. This work was also described in a recent publication:

Peer-reviewed Publications:

1. "Development of an Engine Power Binning Method for Characterizing PM_2.5 and NO_x Emissions for Off-Road Construction Equipment with DPF and SCR." Qi Yao, Seungju Yoon, Yi Tan, Liang Liu, Jorn Herner, George Scora, Robert Russell, Hanwei Zhu, & Tom Durbin. Atmosphere 13(6), 975, 2022, https://doi.org/10.3390/atmos13060975.
2. "Evaluation of small off-road diesel engine emissions and aftertreatment systems."  CavanMcCaffery, JiachengYang, Georgios Karavalakis, Seungju Yoon, Kent C.Johnson, J. Wayne Miller, & Thomas D. Durbin. Energy (251) 123903, 2022, https://doi.org/10.1016/j.energy.2022.123903.
3. "Off-Road Construction and Agricultural Equipment Electrification: Review, Challenges, and Opportunities." Fuad Un-Noor, Guoyuan Wu, Harikishan Perugu, Sonya Collier, Seungju Yoon,Mathew Barth, & Kanok Boriboonsomsin. Vehicles 4(3), 780-807, 2022, https://doi.org/10.3390/vehicles4030044.
4. "Operational Feasibility Assessment of Battery Electric Construction Equipment Based on In-Use Activity Data." Fuad Un-Noor, George Scora, Guoyuan Wu, Kanok Boriboonsomsin, Harikishan Perugu, Sonya Collier, & Seungju Yoon. Transportation Research Record, 2675(9), 809-820, 2021, https://doi.org/10.1177/03611981211004581.

Conference Presentations:

1. “Evaluation of Advanced PM and/or NO_x Aftertreatment for Small Off-Road Diesel Engines (SORDE).” Thomas D. Durbin, Jiacheng Yang, Y. Jiang, K.C. Johnson, G. Karavalakis, J.W. Miller, D.R. Cocker III, C. Ruehl, S. Collier, R. Haste, N. Richards, J. Collins, & S. Yoon. CRC 28^th Real-world Emissions Workshop, poster, 2018.
2.  “Evaluation of Advanced PM and/or NO_x Aftertreatment for Small Off-Road Diesel Engines (SORDE).” Thomas D. Durbin, Jiacheng Yang, Y. Jiang, K.C. Johnson, G. Karavalakis, J.W. Miller, D.R. Cocker III, C. Ruehl, S. Collier, R. Haste, N. Richards, J. Collins, & S. Yoon. CRC 29^th Real-world Emissions Workshop, poster, 2019.