Shares of Clean Diesel Technologies closed down 5.5 percent on Tuesday, after the company announced that Alexander “Hap” Ellis III had resigned as chairman and that board member Charles Engles had taken over the position.

Ellis resigned to focus more time on RockPort Capital, the venture capital firm of which he is a partner, Clean Diesel said.

Oxnard-based Clean Diesel has gone through a series of big changes over the last year. This summer, the company appointed executive Chris Harris as its top leader and said that it would relocate its head offices from Ventura, where the company moved in 2010 after a merger, back to its production facility in Oxnard.

At the time, Harris said the move was expected to help the company tighten up its operations by having senior managers work directly alongside middle management and junior staff.

Clean Diesel has two main divisions. One makes retrofit kits for diesel engines to bring them into compliance with regulations in California and elsewhere. The other makes coatings for the emissions cleanup gear in cars such as the Honda Accord. The coating technology is formulated to use smaller amounts of platinum and other pricey metals than its competitors, and the company is working on patenting technology to omit those metals altogether.

The company posted a $7.1 million loss on $55.2 million in 2013 revenue.

Engles, the company’s new chairman, is a veteran of the metals industry. He joined Clean Diesel’s board in October 2010, when the company merged with Catalytic Solutions. Before that, he served as a director on Catalytic Solutions’ board for 10 years.

Engles, an independent consultant, also serves on the boards of Stillwater Mining Co., the top U.S. producer of platinum group metals. He served as Stillwater’s CEO and chairman from 1994, when it went public, to 1997. Before that, he organized Stillwater’s spinout from Johns-Manville Corp. and Chevron Corp.

Engles previously served on the boards of Johns-Manville and ThermoCeramix and was the CEO of biopharmaceutical firm Cutanix Corp, which he co-founded, from 1997 to 2008.

“We owe [Ellis] a debt of gratitude for his dedication and leadership, particularly through [the company’s] difficult times,” Engles said in a statement. “As your new chairman, I will pick up where [Ellis] left off, and work with the management team to help [Clean Diesel Technologies] achieve the next level of growth. The world is facing serious shortages of platinum group metals, which are used for vehicle emissions control. Our catalyst formulation technologies promise to alleviate the need for these metals.”

Outgoing chairman Ellis was also a member of Clean Diesel’s audit and compensation and nominating committees. Bernard Cherry will fill the spot on the audit committee and and Matthew Beale will step onto the compensation and nominating committee, Clean Diesel said.

Shares of Clean Diesel fell 5.5 percent to $1.72 after the Sept. 30 announcement.