ARB Research Seminar

This page updated March 19, 2020

Evaluation of the Feasibility, Cost Effectiveness, and Necessity of Equipping Small Off-Road Diesel Engines with Advanced PM and/or NOX Aftertreatment

Photo of Tom Durbin, Ph.D

Tom Durbin, Ph.D, University of Denver

March 26, 2020
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA

Research Project


Off-road emissions represent one of the largest sources of NOx and particulate matter (PM) emissions in California. The existing standards for tier 4 off-road engines were developed based on a Regulatory Impact Analysis (RIA) conducted back in 2004, and do not require aftertreatment for NOx below 75 horsepower (hp) or PM below 25 hp. Since aftertreatment control devices for diesel vehicles and equipment are considerably more common now, the use of these strategies for small off-road diesel engines (SORDEs) may be considerably more viable than when the standards were last updated, which could warrant renewed consideration for adopting more stringent exhaust standards for these engines.

The objective of this study is 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 and selective catalytic reduction. This project included a comprehensive review of available aftertreatment and other technologies, demonstration of selected aftertreatment technologies on actual engines and verification of the emissions performance of these devices through a series of emissions and durability tests, evaluation of the cost implications of the added emissions control strategies, evaluation of the potential impacts of additional emissions controls on the emissions inventory, and evaluation of the potential impact on the small engine marketplace and consumer choice in that area. The demonstrations included two DPF applications for the under 25 hp category on a transportation refrigeration unit and a mini-excavator, and two SCR/DPF applications for the 25 to 50 hp category on a ride mower and skid steer.

The results showed that the application of aftertreatment systems for PM for under 25 hp engines and for NOx for 25 to 75 hp was technically feasible. Preliminary estimates suggest that reductions in the off-road equipment emissions inventory of 2.1% in PM and 8.8-13.6% in NOx emissions could be achieved through additional regulations on emissions for the under 25 hp category for PM and for the 25 to 75 hp category for NOx. Cost-effectiveness of the demonstrated controls was estimated to be $0.36 to 0.59/lb NOx and $19.10/lb PM, which compares very favorably to other rulemakings adopted by CARB.

Speaker Biography

Dr. Durbin is a Research Engineer in the emissions from advanced vehicles and fuels research group of CE-CERT and an adjunct professor in the Chemical and Environmental Engineering Department. He is conducting research in the area of vehicle emissions with an emphasis on studying particle and in-use emissions, fuels, and advanced technology vehicles. Prior to joining the vehicle emissions group, Dr. Durbin was involved in several other areas of research at CE-CERT including renewable energy and fuel sources and advanced vehicle technologies. Tom Durbin received his doctorate degree in Physics from the University of California, Riverside, in 1994.

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