WOOD DALE, Ill. — Navistar has extended its engine supply agreement with alternative-fuel power systems supplier Power Solutions International Inc. (PSI) to run through 2021.

The agreement, originally set to run through 2018, covers PSI's supply of 8.8-liter propane and gasoline engines for Navistar's subsidiary, IC Bus.

"We're very excited that Navistar has chosen to extend our supply agreement well into the future," said Gary Winemaster, PSI's chairman and CEO. "We look forward to a long and successful partnership as bus fleets and local school districts continue to embrace the growing market trend towards alternatives to diesel engines."

Navistar began partnering with PSI in 2014 in an effort to expand its IC Bus product line with alternative-fuel offerings to meet growing market demand. In November 2014, IC Bus launched its PSI-powered, CE Series Type C propane school bus at the National Association for Pupil Transportation (NAPT) Summit in Kansas City, Missouri.

Since production started, IC Bus has over 450 in-service units powered by PSI's propane engine, according to PSI. Deliveries of note include 149 buses to Waterbury (Conn.) Public Schools and 100 buses to Indianapolis Public Schools. Additionally, 63 buses have been operating in Tuscaloosa (Ala.) City Schools since early 2016.

"We are mindful of the many operational needs of our school bus customers and we take pride in offering them powertrain options that meet those needs while delivering on our promise to build reliable, safe, and efficient buses," said Persio Lisboa, president of operations at Navistar. "We are excited to continue our partnership with PSI in providing high-performance, alternative-fuel engines in our buses."

In July, IC Bus showcased a PSI-powered, CE Series Type C gasoline school bus at the School Transportation News (STN) Expo in Reno, Nevada. The gasoline-powered CE will also be on display at the upcoming NAPT trade show in Kansas City, Missouri, on Nov. 8. Full-scale production of the CE gasoline school bus will be announced at a later date.

Both school bus models are powered by customized PSI 8.8-liter engines, designed to deliver diesel-like performance and drivability with savings on fuel and maintenance, according to PSI.