Since the news of Volkswagen's illegal "defeat device" software in TDI diesel cars broke almost a year ago, the future of diesel cars has been the topic of much discussion.
But the makings of diesel's demise may have been building even before the VW diesel emissions scandal began.
Carmakers have known about tightening emissions standards for some time, but now the threat of on-road testing in real-world conditions appears to be boosting the costs of getting diesel engines to meet them.
That prospect is leading one of the carmakers most heavily invested in diesels to predict that the technology will fade from the mass market.
Renault believes the costs involved with meeting stricter emissions standards will cause diesel to gradually disappear from the European market, according to Reuters.
Europe is a major market for diesel cars, and automakers have known about the implementation of stricter Euro 6b emissions standards for years.
Set to take effect on January 1, 2017, these are roughly equivalent to standards that have been in place in the U.S. since January 1, 2008.
But the Volkswagen emissions scandal has vastly increased public scrutiny on the question of whether cars can meet these standards in the real world—not just in laboratory tests.
Carmakers expect much tougher real-world testing to be mandatory in Europe by 2019, and for emissions standards to get even more stringent beyond that, according to Reuters.
Thus far, the real-world tests conducted have not produced results at all favorable to carmakers.
In a recent test order by a French commission on diesel emissions, cars from Renault and partner Nissan averaged emissions of nitrogen oxides fully eight times as high as the legal limit.
The majority of cars were closer to 10 times over the legal limit, despite being certified as compliant with the current first part of the Euro 6 standards.
Renault Twingo GT
Diesel cars have been popular in Europe for decades, as their greater fuel efficiency proved attractive to consumers facing higher fuel prices than in other regions.
In France, Renault's home country, diesel fuel has typically been taxed at a lower rate than gasoline, and the same applies in certain other European countries as well.
The combination of cheaper fuel and greater fuel efficiency has brought diesels to roughly 50 percent of the European new-vehicle market over the past several years.
But a better understanding of the environmental impact of diesel exhaust has led lawmakers to change policies, which is starting to cloud diesel's future even in its European stronghold.
Diesel will likely stick around for luxury SUVs and—in North America—full-size pickup trucks.
Yet the fact that a carmaker as heavily invested in diesel as Renault is ready to give up indicates that the days of mass-market diesel cars may be coming to a close, perhaps sooner rather than later.
[hat tip: Brian Henderson]