The success of the internal-combustion engine is mainly a thermodynamic one. As Vaclav Smil has argued, the key metric is grams per watt (g/W): how much mass it takes to generate a certain amount of energy. Human beings and draught animals operate at about 1,000 g/W. Steam engines got that down to about 100 g/W. The Mercedes 35hp was more like 8.5 g/W and the Model T Ford just 5 g/W. And the cost just kept on falling. In 1913 somebody earning the average American wage would have had to work 2,625 hours to earn enough to buy a Model T. In 2013, on the average wage, he or she would have needed
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