Decades before carbon emissions dominated today’s headlines, the U.S. was already deep into a far‑reaching effort to clean up tailpipe pollution. This push targeted the familiar culprits of urban smog: carbon monoxide, nitrogen oxides, unburned hydrocarbons and other VOCs, particulate matter, and a mix of toxic compounds. These weren’t abstract concerns. They were pollutants people could see and feel in real time.
Smog‑choked skylines and the unmistakable symptoms that came with them – coughing, wheezing, tightness in the chest, irritated eyes and throat, and worsening asthma – made the problem impossible to ignore. That direct connection between what came out of a tailpipe and how people felt walking down the street is what drove implementation of the Clean Air Act and the early and sustained campaign to cut criteria emissions, a campaign that continues shaping cleaner vehicles today.
EVs Rise, Gas Still Matters
Electric cars are an obvious option since they emit zero localized emissions. But as we know, electric cars are an unfolding story with a growing, though still minority, percentage of a light-duty U.S. car population that now numbers some 290 million vehicles. The vast majority are gasoline internal combustion…thus the importance of cutting their emissions as much as possible.
This thought takes us back several decades to a feature we published detailing just how clean internal combustion vehicles could be. In the decade before that time, conventional wisdom was that alternative fuel vehicles would be the most likely path to achieving environmental harmony on the highway. If you wanted to achieve zero emissions – or more realistically, near-zero emissions since even an electric car has recharging emissions from the powerplant – then you were looking at a battery electric vehicle. Another likely option seemed to be a natural gas–fueled car, something that was an option at the time with vehicles like Honda’s ultra-clean natural gas Civic GX.
A Shift in Expectations
Things change. Today the focus is again on electrified vehicles. But as our report noted back then, an increasingly cleaner‑running generation of gasoline internal combustion vehicles still deserves a place in any strategy aimed at reducing emissions and delivering cleaner cities and a healthier environment.
Supporting this perspective were the conclusions of a 2002 University of California, Riverside program – the Study of Extremely Low Emission Vehicles (SELEV) – which illustrated how new advances in internal combustion engine technology were reducing vehicle emissions to levels considered impossible just a few years earlier. This technology wasn’t just in the concept stage. A handful of 2003 and 2004 model vehicles with conventional internal combustion engines were certified by California as PZEVs (Partial Zero Emission Vehicles)…cars that ran so clean they were awarded partial credits toward that state’s Zero Emission Vehicle mandate at the time. PZEVs not only achieved incredibly low tailpipe emissions but zero evaporative emissions from their fuel system as well.
PZEV Models Emerge
Cars meeting this emissions milestone at the time of the study included mainstream models like the BMW 325i, Ford Focus, Honda Accord, Nissan Sentra, Toyota Camry, Volkswagen Jetta, and Volvo S60 and S70, when equipped with specific engines. Some of these PZEV models were sold only in California, while others were available there and in other like-minded states such as New York and Massachusetts. This signaled the beginning of an important trend in ever-cleaner-running gasoline models.
Ford’s early‑2000s Focus PZEV served as an example of just how far internal combustion could be pushed. Building a gasoline car that edges toward zero emissions isn’t about one breakthrough, but rather a rethinking of the entire combustion and fuel‑handling system so every component contributes to cleaner operation. Ford engineered the Focus PZEV to meet SULEV limits and virtually eliminate evaporative emissions, refining airflow, fuel delivery, combustion stability, and sealing throughout the powertrain.
Engineering Toward Clean Air
The 2.3‑liter engine at the heart of the program demonstrated the payoff. Lightweight materials, improved intake airflow, better fuel atomization, tighter valvetrain control, and a redesigned exhaust and catalyst system all worked together to deliver cleaner tailpipe output. Ford also re‑engineered more than a hundred components in the evaporative‑emissions pathway to keep fuel vapors contained. The result was an engine that ran cleaner, made more power, and used fuel more efficiently, a clear demonstration of how much internal combustion can achieve when every detail is aimed at cleaner air.
Dr. Joseph Norbeck, then-director of UCR’s Bourns College of Engineering – Center for Environmental Research and Technology (CE-CERT), identified the importance of the university’s SELEV findings at the time: "Ten years ago, nobody thought gasoline ULEVs (Ultra Low Emission Vehicles) and SULEVs (Super Ultra Low Emission Vehicles) would be possible," pointed out Norbeck. "Now they're becoming common, and it's clear the emissions reductions they offer are significant.”
Research That Moved the Needle
The SELEV program launched in 2000 at CE-CERT in partnership with Honda, Chevron, the U.S. Environmental Protection Agency (EPA), the California Air Resources Board (CARB), and the Manufacturers of Emission Controls Association. It represented one of many programs at CE-CERT, an innovative research and education center founded at the university in 1992 with the aid of a $10 million endowment from Ford.
CE‑CERT developed the technology to measure emissions at far lower levels, a milestone in an era when scant emissions posed real measurement challenges. This capability allowed researchers to accurately quantify the performance of ULEVs and SULEVs. Beyond the SELEV program, the center was also involved in developing an integrated research effort examining the economic, environmental, and social implications of evolving automobiles, fuels, and transportation.
A Trend That Never Stopped
The significance of the SELEV program is that the emissions achievements it documented prompted a broader reappraisal of what’s possible with gasoline internal combustion vehicles. Just as important, the ultra‑clean gasoline models studied were not a one‑time anomaly. They marked the beginning of a trend that continues today. Most major automakers selling vehicles in the U.S. now offer several, and in many cases many, gasoline models that run this cleanly.
Of course, continuing to run on fossil fuels doesn’t speak to energy diversity, as some pointed out at the time, and that’s a good point. And today it could be added that combustion vehicles don’t eliminate carbon emissions like electric vehicles, though today’s significantly more fuel‑efficient gas models do produce far fewer carbon emissions than in the past.
A Role for Internal Combustion
If extremely low emissions and cleaner air in our cities is the goal, then the progress already achieved, and still being achieved, with the internal combustion vehicles we’ve driven for more than a century deserves recognition. With continued advancements in combustion technology and the emerging potential of carbon‑neutral synthetic fuels, advanced internal combustion engines may well remain an important and strategic part of our driving future for decades to come.