Combustion drives the developed world’s economy. Transportation is second only to industrial use as the largest consumer of energy in the US and accounts for about 60% of our nation’s use of petroleum—an amount equivalent to all of the oil the country imports. The numbers are staggering: Some 10 000 gallons of petroleum are burned in the US each second of every day. Although new energy sources are being developed and renewable fuels are emerging to replace crude oil, improvements in the efficiency of internal combustion engines hold the promise of increasing our energy security and mitigating climate change.

The environmental consequences and health hazards posed from urban smog and other combustion byproducts led to US regulation in the late 1970s designed to limit the emission of nitrogen oxides (NO x ), hydrocarbons, and other pollutants from internal combustion engines. Controlling those emissions while maintaining high efficiency is a continuing...

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