Southwest Research Institute (SwRI)announced on June 10, 2026, that it has successfully demonstrated a diesel engine system that allows off-road vehicles to meet proposed California Air Resources Board (CARB) Tier 5 emissions standards. The breakthrough uses exhaust gas recirculation and calibration changes to significantly reduce emissions without sacrificing performance. This matters for the off-road industry because CARB's Tier 5 standards pose a compliance challenge with no cost-effective solution for the most common off-road engine size range.What Are Tier 5 StandardsCARB's proposed Tier 5 rules target major reductions in nitrogen oxides (NOx) and particulate matter (PM) emissions from off-road diesel engines. These rules would apply to construction equipment, agricultural machinery, utility vehicles, and other off-highway applications. The standards create a split challenge based on engine size. Engines above 56 kilowatts are expected to adopt emissions control technology similar to that used by on-road trucks. Engines below 19 kilowatts are generally expected to electrify. The problem is the middle segment: engines between 19 and 56 kilowatts currently have no clear technical path to Tier 5 compliance. "CARB's Tier 5 rules aim to reduce nitrogen oxides and particulate matter 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."What SwRI DidSwRI ran extensive laboratory testing on a commercially available 55-kW, four-cylinder diesel engine. The team focused on two approaches working together: exhaust gas recirculation (EGR) to cut NOx, and calibration changes to control particulate matter. Two hardware changes were required. First, the turbocharger wastegate control was modified to increase boost pressure, thereby allowing a higher EGR rate. Second, a larger capacity EGR cooler was installed. Beyond those changes, the improvement came from calibration work, not expensive new hardware. The result was an engine that cleared both Tier 5 certification test protocols: the ramped modal cycle (RMC) and the non-road transient cycle (NRTC).ResultsThe numbers are strong. Compared to Tier 5 targets:AdvertisementAdvertisementRMC testing: NOx reduced 22%, non-methane hydrocarbons reduced 56%NRTC testing: NOx reduced 28%, non-methane hydrocarbons reduced 50%Engine performance was not degraded. This is the critical point for off-road equipment users: meeting emissions standards without sacrificing the power and durability the industry depends on. "In short, we achieved significant reductions in emissions with an innovative engine calibration and minimal hardware changes," Michlberger said.Why This Matters to Off-Road OwnersEmissions standards have historically shaped what gets built. If engine manufacturers face regulations with no viable compliance path, the result is market disruption, production gaps, or pressure to electrify segments where battery technology is not yet practical. SwRI's demonstration shows a workable path for the middle-range diesel engines that power a wide range of off-road machinery. That is good news for anyone who depends on diesel-powered off-road equipment, whether it is a UTV with a diesel option, construction equipment used for overlanding to access property, or agricultural machines used in farm and ranch environments. The research was funded through SwRI's Internal Research and Development program. In fiscal year 2025, SwRI invested more than $13 million in internally funded R&D projects. The goal is to develop technologies for both government and industry clients. For more information on SwRI's emissions research, visit swri.org.Sourceshttps://www.eurekalert.org/news-releases/1131537