SAN ANTONIO — June 10, 2026 —Southwest Research Institute (SwRI) has successfully demonstrated a diesel engine system designed to help off-highway diesel vehicles meet Tier 5 emissions standards proposed by the California Air Resources Board (CARB). SwRI-developed hardware modifications and optimized calibration strategies allowed an off-road vehicle with a commercially available diesel engine to meet the standards without negatively affecting performance.
“CARB’s Tier 5 rules aim to reduce nitrogen oxides (NOx) and particulate matter (PM) emissions by a significant margin,” said Alex Michlberger, a lead engineer in SwRI’s Powertrain Engineering Division. “However, the industry doesn’t currently have a cost-effective solution to meet Tier 5. Our project is meant to help provide that solution.”
An engine’s power level determines the allowable emissions for off-road vehicles. Most off-road engines operating between 19 and 56-kilowatts (kW) do not have a clear technical path for compliance with the new standards. Engines above the 56-kW threshold are expected to use emissions control technologies resembling those used by on-road vehicles, while applications currently using engines below 19 kW are expected to electrify.
As part of an internally funded research project, SwRI conducted extensive laboratory testing on a commercially available 55-kW, four-cylinder diesel engine to identify potential Tier 5 solutions. The project utilized exhaust gas recirculation (EGR) to mitigate NO X and other calibration strategies to control PM. This approach required both hardware and calibration modifications. To enable a higher EGR rate, the team modified the turbocharger waste gate control to increase boost, and they installed a higher capacity EGR cooler. The hardware and calibration changes together demonstrated low emissions while maintaining engine performance.
SwRI’s improved system achieved the Tier 5 targets in both ramped modal cycle (RMC) and non-road transient cycle (NRTC) certification tests. The researchers compared NOx, non-methane hydrocarbon (NMHC) and PM emissions with the Tier 5 targets.
“In the RMC demonstrations, we saw NOx emissions reduced by 22%and NMHC by 56%, compared to the Tier 5 target. For the NRTC, we found NOx emissions reduced by 28%and NMHC by 50%, compared to the Tier 5 target.,” Michlberger said. “In short, we achieved significant reductions in emissions with an innovative engine calibration and minimal hardware changes.”
During the engine mapping exercise, an automated testing system was used to efficiently collect data at hundreds of points in a short period of time. SwRI’s researchers were able to identify the optimal engine calibrations across their operational map.
The project was funded through SwRI’s Internal Research and Development program, which provides resources for future-focused, unproven concepts to advance technology for government and industry clients. In fiscal year 2025, SwRI invested more than $13 million to fund IR&D projects to pioneer new technologies, expand institutional knowledge and capabilities, and encourage the professional growth of its staff. Learn more .
For more information, visit https://www.swri.org/markets/automotive-transportation/automotive/emissions .