As with so many other things, it all started in California in the 1960s. People got tired of palls of brownish smog hanging over otherwise lovely vistas--and breathing air they could see.

State and federal agencies studied the problem and concluded that auto exhaust was primarily to blame. The first hardware-based partial fix, a positive crankcase ventilation valve that routed crankcase fumes into the engine for combustion, was required on new cars sold in California in 1961, then on most vehicles nationwide by 1962.

The initial tailpipe emission standards legislated by California for the 1966 model year were followed by national standards for 1968. The regulatory California Air Resources Board was founded in 1967 and the U.S. Environmental Protection Agency in 1970, with congruent missions to tighten tailpipe emissions standards progressively year by year.

The regulated emissions were: (1) hydrocarbons, unburned or partially burned fuel, toxic to humans and a major contributor to smog; (2) carbon monoxide, which replaces oxygen in the blood and can be fatal in large quantities; and (3) oxides of nitrogen, a smog contributor generated when nitrogen in the intake air reacts with oxygen during combustion. Added later were particulate emissions, tiny bits of soot or smoke that can cause respiratory problems in humans and animals, and sulfur oxides from high-sulfur fuel.

By the early 1970s, increasingly tough emissions requirements and add-on technology necessary to meet them had dramatically reduced engine power, smoothness, and efficiency, and still-tougher new ones for 1975 (along with federal fuel-economy standards) seemed all but impossible. Then a team at General Motors (led by future GM CEO Bob Stempel) saved the day with their just-in-time development of the emissions-eating catalytic converter.

Since then, increasingly sophisticated catalysts, closed-loop exhaust sensing, electronic engine controls and fuel injection, and an abundance of additional technologies have managed to meet ever-tightening requirements to where today's modern vehicle exhausts are essentially 99 percent pristine. Yet, while CARB and EPA continue to ratchet down tailpipe standards almost annually, the notion that vehicle-emitted carbon dioxide is a factor in climate change has led to a rapid and immensely challenging ramp-up in corporate average fuel economy requirements over the next few years.

Because fuel-economy ratings are derived from the same battery of tests that ensures emissions compliance, we set out to learn what it takes for automakers to accomplish both.