Nearly every vehicle on the road today is powered by some version of the four-stroke internal-combustion engine patented by Nikolaus Otto in 1876. Otto exploited the findings of French physicist Sadi Carnot, who in 1824 showed that the efficiency of an engine depends critically on the temperature differential between a hot “source” of energy and a cold “sink.” The four-stroke engine compresses an air-fuel mixture and ignites it with a spark, thus creating a fleeting but intense source of heat. Its portable efficiency has not been matched since.