Internal combustion engines are all about volumetric efficiency. Like all heat engines, higher combustion temperatures mean greater efficiency, but those high combustion chamber temperatures and pressures form an ideal process for creating oxides of nitrogen in the exhaust gas stream. NOx has long been known as a major contributor to photochemical smog, and the state of California has had uniquely stringent NOx emission standards for decades. Upcoming standards set for 2027 appeared impossible to meet, but a test of opposed piston diesel built by Achates Power as part of a CALSTART research project has met the 90 percent reduction requirement without novel exhaust gas aftertreatment technologies. The engine is running in a test truck and promises better fuel efficiency and lower CO2 emissions as a bonus.Access all episodes of This Week in Engineering on engineering.com TV along with all of our other series.