2.7 opposed piston 2 cycle diesel engine, apparently it's good for 270 hp and 480 lb feet and their test truck got 37mpg combined.
Really! A Super and Turbocharged Boxer Truck Diesel/Gas Engine with No Spark Plugs - YouTube
Opposed Piston Diesel Engines Are Crazy Efficient - YouTube
Really! A Super and Turbocharged Boxer Truck Diesel/Gas Engine with No Spark Plugs - YouTube
Opposed Piston Diesel Engines Are Crazy Efficient - YouTube