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.