Rudolf Diesel's engine came a little late to the internal combustion party. Patents were first issued in the mid-1890s. But after commercial and shipping use, diesel engines were first dropped into a passenger car in 1933 by Citroen (as an engine option) and in 1936 by Mercedes-Benz.

The modern wave of diesel cars took root 60 years later, and it required several planets aligning properly: highly precise fuel metering, sophisticated turbocharging, better machining of tiny but durable fuel injector pintles—the injector's business end—and very high, yet manageable fuel pressures (up to 29,000 psi or 2,000 bar). But even with those technical hurdles passed, a foundation was laid in the 1990s that allowed the modern diesel engine to ascend in popularity in Europe, its home ground.

So what exactly caused this rapid transformation from gasoline engines to diesels in an enormous market?

To start, the modern diesel engine represents very good science and highly developed technology. At its core, the diesel combustion cycle yields a leaner fuel-air mixture to operate at maximum efficiency than gasoline engines. Measured by volume, diesel fuel is more energy-dense than gasoline. The combustion cycle itself works best at leaner mixtures, and diesels—even earlier, less-advanced naturally aspirated examples—deliver a torque curve that works well for cars and trucks.

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Modern diesels also emit lower COthan gasoline engines. And this very fact played a pivotal role in the ascension of diesels in Europe.

Several European environmental and government studies in the 1990s argued that air quality could be improved by lowering CO 2 levels. Developed nations that signed the Kyoto Protocol in 1997 were required to reduce CO 2 by eight percent over the following 15 years. The big European car manufacturers, dominated by those from Germany and followed closely by the French and Italian companies, vigorously lobbied European regulators and politicians to further the diesel cause, citing diesel's inherently low CO 2 output relative to the gasoline engine. The car manufacturers' position was that diesels would be a quick, effective way to lower overall carbon emissions, which was already the primary pollution concern.

Behind the scene, European manufacturers had already gained a big advantage in diesel engine development and saw the opportunity to bring a very favorable product to market. To complete the large incentive toward diesels, EU governments also manipulated the price of diesel fuel itself, keeping it below gasoline. They also taxed the registration of diesel-powered passenger cars at a much lower rate.

So one by one, impediments to diesel domination were removed. In addition to the inherent technical merit of the modern diesel engine, manufactured market forces like fuel price, tax, and emissions regulations all paved a smooth road toward cars powered by diesel.

Of course, the nature of diesels to emit higher levels of nitrogen oxides (NOx) and particulates was not of great concern at the time. So rapidly, the European automotive landscape switched. While diesel had a 10 percent market share in the mid-1990s, by 2000 it hit 31 percent. By 2012, it was in the majority at 55 percent. As of 2014, the EU diesel market share is only slightly lower—at 53 percent—due to small inroads by hybrids and electric vehicles (EVs).

Now Volkswagen's scandal throws a shadow over all diesels, not just VWs. That's only the start of the problems. Europe recently adopted much more restrictive NOx emissions regulations for passenger cars. As of last fall, the European NOx emissions specification is 0.08 g/km; the prior limit was 0.18 g/km—56 percent more restrictive in plain language. This makes diesels a far harder technical challenge, virtually requiring some sort of exhaust after-treatment system like selective catalytic reduction ("SCR" or urea injection). This is a costly addition to a passenger car, especially to smaller and cheaper cars like many of VW's products.

Meanwhile, despite more attention and stricter emissions regulations, some cities around the world—like England's Bristol, Brazil's Sao Paulo, and Belgium's Brussels—have started experimenting with car-free days in city centers. European cities are beginning to face EU fines for air quality violations, forcing them to find ways of reducing smog elements like NOx.

Paris saw an alarming spike in air pollution this past March, surpassing Beijing to become—temporarily—the most polluted city in the world, according to environmentalists. In tangible terms, landmarks like the Arc de Triomphe and the Eiffel Tower grew invisible from a short distance. Particulate levels were 1.5 times higher in 2014 than the city's target. Paris responded initially by banning diesel vehicles and half its overall car volume on alternate days (a similar approach was used in Beijing in the run up to the Olympic games). The city also launched a bike-sharing service, Vélib, with a similar service promoting EVs.

As host to this December's climate change conference, Paris decided to institute a car-free day this past Sunday, September 27, where cars were banned from the city's central areas. The only exceptions were vehicles belonging to residents, busses, and taxis, all of which were restricted to 12mph (20 kph) to see if there would be a difference in air quality.

While no hard numbers yet exist, Paris' experiment was shocking. People on city streets reveled in how clear and smog-free—and quiet—their Sunday was. With their passenger-car diesel fleet at roughly 80 percent, the French public is going to have a real challenge if France's larger plan of banning diesels entirely for light-duty transportation becomes reality.

These days, things aren't looking good for diesel. It faces tougher regulations, pressure from environmental lobbyists, and smog-filled city experiments. Plus, the VW scandal is easily the biggest auto industry scandal in years thanks to its deceit of governmental regulations, customers, dealers, and partners, not to mention the sheer number of vehicles involved. Add to that the high cost of componentry required to make modern turbodiesels comply with regulations, (especially for price-sensitive car segments where those costs cannot easily be spread over a larger profit margin per unit), and diesel faces more potential hurdles than ever.

This all adds up to a legitimate threat to the future of Rudolf Diesel's engine. The conditions for diesel's success over the past 20 years are evaporating. This is particularly true in the US, where common perceptions of diesel engines were formed in the late '70s and early '80s with sub-par engines from Detroit. Today the most popular perceptions of diesel in the US may be heavy commercial trucking's dirty diesel reputation and modified pickup trucks that "roll coal," spewing antisocial clouds of black smoke on-demand.

But even in Europe, where one out of every two passenger cars is diesel-powered, the headwinds of change are especially strong. Rudolf Diesel's invention may simply be a threatened species. And that's a bit ironic, as Germany and France truly gave birth to Rudolf's compression-ignition engine.

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