The day is coming when Californians won’t be able to buy new passenger cars with gasoline engines.

Even so, leaders at Sorrento Valley-based Achates Power believe there will still be a need for fossil fuel engines, in certain situations and geographies. Even past 2035, when California’s all-electric mandate goes into effect. Whatever fossil fuel engines remain will have to be a lot cleaner than what is offered now.

Achates Power has been at work on a cleaner, next-generation engine since its founding by inventor James Lemke in 2004. The business is developing several variations using a unique opposed-piston design.

Walmart plans to test a big rig truck that uses an Achates Power diesel engine in California for at least three months, beginning this summer. There is a chance commercial truck makers could offer the engine in their products by the end of the decade.