From disdain to redemption to disgrace: Rudolf Diesel's fuel-frugal invention has travelled a rocky road in North America.
"Slow, noisy and dirty" was the public perception of diesel during its disdain phase decades ago.
Redemption came with the advent of turbo-diesels – such as Volkswagen's TDI that seduced open-minded buyers with its tsunami of quiet, effortless torque, while new technologies handled the emissions challenge.
Then the disgrace: In September of 2015, Volkswagen was found to have been cheating on U.S. emissions tests.
VW abruptly pulled all its diesels from North America and shows no sign of bringing them back. Since VW was by far the market leader, it seemed diesel's long journey to acceptance had just hit the ultimate pothole.
However, diesel is recovering from the scandal, says Allen R. Schaeffer, executive director of the Maryland-based Diesel Technology Forum. The number of diesel options from brands other than Volkswagen Group is proliferating.
"Last year was a low point in choice, and the first full year without new products," Schaeffer says. "Diesel take-rate was its lowest in quite a while. But then it doubled by the end of the year."
And 2017 is "off to a good start," he adds.
Two big drivers of diesel's recovering "take-rate" are the Ram pickup diesel and the new diesel option for General Motors mid-size pickups, he said.
None, however, is more significant than a coming diesel in the 2018 Ford F-150 pickup.
"It's a very symbolic kind of vehicle," says Schaeffer. "That such a popular vehicle has a diesel option is a pretty strong statement."
European luxury brands – including BMW, Mercedes-Benz and Jaguar – offer a variety of diesels while other diesel options coming soon include GM's Chevrolet Equionox/GMC Terrain CUVs and Mazda's CX-5.
A simplistic view might be that rivals are rushing to exploit the market space vacated by Volkswagen. The reality is likely more complex.
The automobile development process is a ship that is slow to change course. The diesels coming to market this year were likely under way well before the TDI scandal broke. In turn, auto makers were motivated to develop those diesels by the imperative of evermore-stringent government fuel-economy mandates.
For individuals, diesel's primary appeal is directly to their pocket-books. Based on official government tests, the VW Passat diesel, for example, could drive 20 per cent further on a litre of fuel than the most economical gasoline version (the diesel advantage used to be greater than that, but gas engines are improving while diesels have become more powerful).
In many jurisdictions, fuel itself is also cheaper, particularly during the warmer months when supply doesn't compete with demand for heating oil.
In societal terms, a traditional attraction of diesel's efficiency – back when "peak oil" was the biggest concern – is energy conservation. Now climate change is the bogeyman, and there, diesel's benefits are more nuanced: The fuel contains more carbon than gasoline, so while diesel does reduce CO2, it's not quite in proportion with the lowered fuel consumption.
However, CO2 isn't the only cause of climate change. So is nitrous oxide (N2O, a.k.a. laughing gas), which has a global warming potential 200 to 300 times higher than CO2.
"N2O is present in vehicle exhausts," says Stelios Pesmajoglou, director of professional programs for the Greenhouse Gas Management Institute, based in Washington, D.C. "Agricultural soil management aside, combustion of liquid fuels in road vehicles is one of the biggest sources of N2O emissions."
When vehicle emission controls work as they should, the quantities of N2O in vehicle exhaust are tiny compared with CO2 and have about 1.4 per cent of CO2's global warming potential. But when real-world emissions are up to 40 times higher than the legal limits (as was alleged in VW's case), then "if this is the case, it is not 1.4 per cent, it is 55 per cent, and this is a serious problem," says Pesmajoglou.
Never mind the effect on ground-level air quality of the other nitrogen oxides.
The diesel engine used in some Ram and Jeep products has recently also come under suspicion of cheating on emissions tests. Other auto makers insist their new diesels are clean, though some have had their market launches delayed by tightened regulatory oversight by agencies such as the U.S. Environmental Protection Agency.
Which brings us back to the other big imponderable about the long-term future of diesel: U.S. President Donald Trump's hostility to regulation in general, and his denial of climate change in particular. (His budget proposals call for slashing the EPA's budget by about a third.)
On the one hand, any relaxation of traditional emissions standards (regulating HC, CO and NOx) could make it easier to engineer and certify diesel engines for the North American market.
On the other, if the Trump administration rolls back fuel-economy/CO2 emissions mandates, auto makers may have less need for diesels to help them comply in the first place. Another unknown: Canada usually adopts the same ever-tighter standards as the United States, but would it do so if the United States went the other way?
In the long term, the discussion could be moot. Saeid Habibi, an electrification researcher at McMaster University in Hamilton, says he can see the case for diesel in heavy trucks, and perhaps in light vehicles for users such as salespeople who travel long distances. Otherwise, "whether [diesel] makes sense for cars I'm not fully convinced. I think the future is more electrification, especially hybrids and battery electric vehicles."