Diesel: How it changed Europe and how Europe might change back

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.

Modern diesels also emit lower CO₂ than 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 CO₂ levels. Developed nations that signed the Kyoto Protocol in 1997 were required to reduce CO₂ 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 CO₂ 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.