Roush CleanTech has received California Air Resources Board certification for 2023 model year propane engines at 0.02 grams per brake horsepower-hour. Previously available as an added-cost option, the ultra-low nitrogen oxide (NOx) emissions package is now standard on every propane engine sold by Roush CleanTech, the company notes.

According to the company’s findings, the engines emit 90% less NOx emissions than allowed under 2023 regulations, and are 60% cleaner than California’s 2024 low-NOx standard, without, Roush CleanTech says, compromising performance or efficiency.

Roush CleanTech first released low NOx engines in 2018 and in 2021, Roush CleanTech received CARB’s ultra-low 0.02 grams per brake horsepower-hour certification for Blue Bird school buses and commercial vehicles, according to the company.

There are more than 37,000 advanced clean vehicles equipped with Roush CleanTech’s engines on the road today, including 18,500 school buses in 1,000 U.S. school districts, according to the company.