On a smoggy day in Paris, police enforce an anti-pollution measure temporarily banning cars with even-numbered license plates. Michael Euler/AP

The highly efficient fuel was supposed to be the future. Is it now doomed?

Once upon a time, diesel fuel was going to be the future. It was seen as more efficient, on a mileage-per-gallon basis, than other fossil fuels, and for that reason was also thought to be less polluting. About two decades ago, acting on those beliefs, policy makers in Europe—where high energy prices already made mileage a more-pressing issue than in the U.S.—made a number of rules that incentivized the growth of diesel over gasoline for use in passenger cars, moving past its traditional role in trucking and construction.  

These policies were remarkably successful at meeting their goals, and diesel-powered cars soon accounted for half of the cars sold on the continent. Car companies poured resources into developing diesel-related technology. But the result of this success has been not greener, friendlier, cheaper motoring, but the creation of toxic clouds over major European cities. At the end of 2016, Paris was choked by its worst episode of smog in more than a decade, lasting longer than two weeks, according to the city’s pollution-watching agency Airparif, and prompting the city to enact emergency measures that included restricting car use. It was not the first time. During a March 2015 pollution event, Paris was briefly the most polluted city in the world, surpassing famously smoggy Beijing. London shared in the ignominy when it too beat out Beijing for the first time in January of this year.

Diesels have played the main role in this. Since the 1960s, advances in technology that treats and filters gasoline engines’ exhaust, like the widespread use of catalytic converters, have cut down on the amount of dirty, unhealthy, and smog-producing emissions these engines spew out into the surrounding environment. But while diesels get better mileage and so contribute less to global climate change, the local effects of diesel pollution are much worse than those of gasoline. Diesel is a less refined fuel, and so it contains more of the particulate matter that can have deadly health effects when spewed into the surrounding environment. And burning diesel produces, among other noxious gases, nitrogen dioxide, the main cause of smog.

In many cases the same regulatory bodies that were trying to get citizens into diesels only a few years ago are now working to get the engines off the road entirely, instituting additional, diesel-specific congestion-charging and other disincentives in cities, in recognition of the fact that their green-friendliness was mistaken. During particularly bad bouts of smog, several European cities have temporarily banned driving outright, or instituted restriction schemes where, for example, cars with odd and even number plates are allowed in on alternate days. The mayors of Athens, Mexico City, and Madrid have committed to ridding their cities of diesel cars altogether by 2025, and Paris Mayor Anne Hidalgo said “there will be no diesel vehicles in Paris in 2020.” Other cities around the continent and world are implementing smaller-scale efforts to discourage diesel too.

But lately, the biggest story when it comes to diesel remains Volkswagen’s ongoing “Dieselgate” scandal, in which the company installed “defeat devices” that allowed its diesel cars to put out dramatically higher levels of toxic emissions on the road than show up during regulators’ lab tests. A year and a half after the cheating was discovered, the full results of the scandal are still uncertain: The U.S. has levied more than $22 billion in fines against the company, the world’s largest automaker, and more may still be coming. Meanwhile, millions of the deceptive VWs are still on roads around the world, with consequential EU action on the matter minimal.

Much worse for diesel at large, rumors that the offending practice was not limited to Volkswagen Group, which have been floating around since the scandal first broke, seem to be turning out to be true. The Environmental Protection Agency recently moved against the trans-Atlantic auto giant Fiat Chrysler for using similar devices, and General Motors is being sued in a class action for cheating in its diesel pickup trucks, which outnumber the offending VWs on American roads by several hundred thousand. The EU also started legal action against Italy for failing to meet its obligations as a member state to enforce regulations on Fiat Chrysler.

Even if Volkswagen hadn’t been cheating, there are enough counts against diesel that the wisdom of using it would likely still be being questioned. Is diesel, the workhorse fuel that was supposed to save the world, doomed? To evaluate its future, it’s important to understand how diesel came to be hailed as the fuel of the future in the first place.

Before a German inventor named Rudolph Diesel created the compression engine in 1895, the earliest American oil companies used to simply throw away the fuel that would come to bear his name. Firms had started drilling and extracting oil in the U.S. in 1859, but they distilled it, separating out its parts by density, mainly in an effort to get the very light kerosene that was burned in the lamps that lit the middle of the 19th century. For the purposes of illumination and most other needs of the era, heavier fuels like gasoline were seen as byproducts of little use—except sometimes as a particularly pungent and flammable cleaning agent. Diesel was unpopular because as fuel oils get heavier, they get harder to ignite, and diesel fuel is even heavier than gasoline.

It is actually this property of diesel—its density—that has driven the story of its use all the way through the present day. The great innovation of Rudolph Diesel’s engine was that instead of a regular gasoline engine’s method of igniting the mixture of fuel and air that combusts and drives the cylinders using a spark, in Diesel’s invention the force of the pistons themselves would compress the air until it got so hot it ignited, squeezing out almost the maximum thermodynamically possible amount of energy. There were a number of benefits to engines like this, the primary one being that they provided a lot of oomph at low rpms, which is why they came to be used widely in trucks, which need to get heavy loads up to speed. Another benefit was that the hotter-burning process makes better use of the fuel, combusting it more completely so that the engines could have better mileage and range.

Almost a century later, an ingenious engineer at one of Volkswagen Group’s brands named Ferdinand Piëch, a grandson of the company’s founder, would be among those who noticed that these characteristics had a lot of unexploited potential in the passenger-car sector, particularly in Europe, where fuel prices are high and every liter at the pump is precious. Piëch, who went on to run VW and then sit on its governing board from the early ’90s until 2015, was heavily involved in the development of turbocharged direct injection, or TDI, technology. This technology, which allowed small diesel engines with great mileage to provide more power at a wider range of speeds, was and is an impressive development, and it has continued to be refined since.

Many of the traditional problems consumers associated with the diesel engines that were already common in trucks, school buses, and construction equipment were solved during this time: Previously, diesels loudly clattered, whereas gasoline engines purred and burbled and growled. Diesels also smelled acrid and belched visible black smoke. Modern diesel-engine technology is still dirtier for the direct local environment—contributing to poor air quality and some health problems—but it has become relatively quiet, and it took on an aura of environmental friendliness, a complete inversion of its previous reputation and the key change that made it possible for VW and others to use diesel power as a positive selling point for peppy, cheap-to-run cars.

One more thing added to the proliferation of diesels in the mass passenger-car market: concerns about climate change, which made the efficient physics of the diesel engine even more of an asset. Public policy started (haltingly) to acknowledge that the harmful environmental impact of emissions was not just local, through pollution of the surrounding air, but also global, through the release of gases that contribute to climate change. For some consumers, this changed the moral status of cars and car ownership: Having good fuel economy isn’t just a consumer good, but a civic good. As recently as five years ago, diesel was an environmental priority among many politicians, car companies, scientists, and even environmental activists.

Policy makers, particularly in Europe, moved to incentivize diesel in the ‘90s and ‘00s with tax breaks at the pump, and they tightened regulations that focused on mileage per gallon (or the equivalent European measurement unit, liters per 100 kilometers), which inherently favor Rudolph Diesel’s thermodynamically efficient engines over gasoline-powered ones that use more gallons but spew fewer toxins. In some countries, including France and Spain, taxes were lower on diesel cars than gas ones. The market share of diesel cars in Western Europe went from less than 14 percent in 1990 to half today, after touching a high of 56 percent in 2011. As Bloomberg’s Leonid Bershidsky points out, Japan had a similar rate of diesels at the turn of the millennium, but absent similar policies, those in Japan more or less disappeared over the period that diesel sales in Europe tripled, as Japanese manufacturers pushed into hybrids.  

Diesel’s market share in the U.S., where fuel prices are traditionally much cheaper and thus exert less pressure on demand for efficient engines, is puny by comparison and pretty much always has been. While there are real differences between the North American and European markets, some might argue that the market for diesel in the U.S. was irrationally underserved, and this seems to have been Volkswagen’s assessment when it decided to roll out its big “Clean Diesel” campaign in 2008 to try to get Americans buying TDIs at closer to the rate of Europeans. Americans, however, never came around, and diesel is so tainted now by the various scandals that they likely never will.

In Europe, meanwhile, the push for diesel has turned out to be a disaster. “They're these relatively powerful and efficient engines that are kind of deadly in terms of their emissions,” said Timothy Lipman, the co-director of the Transportation Sustainability Research Center at the University of California, Berkeley. The “deadly” effects Lipman refers to are extremely serious. Both the nitrogen oxides that come from diesel engines’ high-compression combustion and the fine, sooty particulate matter that can get into human tissue carry serious health effects that cost lives. In addition to lower respiratory diseases like chronically obstructive pulmonary disease and asthma, which they can cause especially in children and worsen among people who are already asthmatic, particulate matter in the engines’ fumes is associated with heart attacks, lung cancer, and strokes, among other things.

“I'm sure the levels of particulate matter and nitrogen oxide as well would be much, much lower in European cities if they didn't have the incentives they had for diesel cars,” said Helotonio Carvalho, a molecular biologist at Brazil’s Federal University of Pernambuco, Recife. He added, “We are going in a direction where diesel cars probably won't have a place.” Carvalho has written in the medical journal The Lancet that 400,000 people died prematurely from all the various sources of air pollution in Europe in 2011. I asked him how many deaths can be attributed specifically to Europe’s elevated rate of diesels, relative to, for example, North America or to his native Brazil, where diesel passenger cars haven’t been allowed as a matter of policy. He told me that the number is in the “hundreds of thousands.”

Policy makers in Europe seem to be adjusting to the reality. Sir David King, the chief science adviser to the U.K. government and the public face of its “Dash for Diesel” policy, who once famously caused a controversy by declaring carbon dioxide more threatening than terrorism, said in April that “it turns out we were wrong” to think automakers would make diesels that emitted within regulatory limits. Former French Prime Minister Manuel Valls used similar language. And Elzbieta Bienkowska, an EU commissioner who oversees the auto industry, made a bold prediction in some recent comments to policy makers: "Diesel will not disappear from one day to another. But after this year of work ... I am quite sure they will disappear much faster than we can imagine.”

The economic effects of that will ripple. For one thing, there are lag times that come with ratcheting up the production of regular gasoline, so while the energy industry adapts its supply to the change, increased European demand for gasoline will push up prices in the U.S. and elsewhere. Then there’s the car industry, in which, Lipman told me, the companies that have focused on batteries and hybrid technology are now positioned better than the European companies that made big bets on diesel, partly as a response to pro-diesel policies.

Finally, there is the most urgent issue with the “Dieselgate” scandal, that of the millions of Volkswagen and other diesels still on the road, emitting far outside of acceptable limits. For the good of the environment and public health, they cannot be left noncompliant. But the reason companies like Volkswagen would want to cheat in the first place is because few would want to own and drive a diesel car that met regulations, and performed like it. Will they now just be left to rot in automotive graveyards? Or will they be made compliant but unloveable?

These are the kinds of questions that thinking about the future of diesel raises. They’re grim, for car lovers, for car companies, and for people whose health is on the line. In a quite real sense they are just a different version of the same dilemma engineers and managers at Volkswagen must have faced, and crucially bungled at the expense of other people’s lives, when and public-health regulations and the limits of the engines’ chemistry came into conflict. It does nothing to excuse these companies to observe that the pressures they felt to cheat on behalf of diesel existed for the very same reasons that made engineers start using the dirty, dense stuff in the first place: Diesel engines’ pep, efficiency, and cheapness to run offered a solution to the challenge of making cars that met customers’ expectations about performance as regulators who were focused on global climate change pushed for tighter fuel efficiency standards—without anticipating the fateful tradeoffs in toxic air pollution. And so the story of diesel seems to have turned out to be a tragedy, because its good qualities are inexorably linked to its bad ones. And as with any good tragedy, everything seems to be building towards the death of the main character. In the modern world, diesel just doesn’t fit.

This story originally appeared on The Atlantic.

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