An OP can be configured and packaged to fit the vehicle it’s intended for. For trucks, the cylinders are likely to be vertical or slanted. Photo: Achates Power Security’s tight at Achates Power Inc.

Converting the ignition of a fuel-air mixture into usable mechanical energy lies at the core of a dizzying number of internal combustion engines developed over the course of more than century.

Flat engines are horizontally-opposed piston engines, and this group includes Boxer engines, but not opposed-piston engines like the Cummins 14.3L Advanced Combat Engine (ACE) designed to power tanks.

The plastic cover of a Subaru Boxer engine showing the Subaru constellation logo - Topuria Design/Shutterstock Flat engines are horizontally-opposed piston engines, and this group includes Boxer ...