Diesel engines tend to be more efficient at converting fuel into motion, but only at certain RPMs. For something like a tractor trailer hauling freight, a diesel engine will easily outperform a similar gasoline engine. Similarly, a diesel car driven mostly on the highway will outperform a gasoline car.
Around town though, it’s generally the opposite. Since you aren’t using the diesel engine in its “sweet spot”, the gasoline engine tends to outperform it.
With modern diesel and gasoline engines (and transmissions), the difference isn’t as pronounced as it used to be, and in some cases the diesel can even beat the gasoline engine around town.
Diesel engines also have a disadvantage that they don’t like to start when they are cold. I have to plug my diesel pickup truck in during the winter.
Diesel engines have lower gaseous emissions than gasoline engines, but much higher particulate emissions. If you look at an old diesel engine’s exhaust, it looks like it pollutes more, even though overall it is polluting less, because you can easily see the diesel’s soot but you don’t easily see the gases that the gasoline engine emits.
Diesel trucks are the standard in the U.S. because of the cost/mileage difference, but diesel cars were a rarity until recently. Part of the problem is the narrower power range of diesel. Not only are diesels less efficient around town, but they tended to have much more sluggish acceleration with older transmissions. Also, even though their overall emissions are less, the higher particulate emissions make them seem “sooty”. Earlier diesel cars also tended to be noisier than their gasoline equivalent. This gave diesel cars a reputation for being noisy, sluggish machines that spewed black smoke wherever they went.
With better technology, most of that isn’t an issue any more. With a modern transmission, a diesel car is no longer sluggish, and the fancier transmission is able to keep the diesel engine in its power range a lot better, which increases diesel’s efficiency around town. All of the diesel cars that I have driven have been downright peppy, weren’t any noisier than their gasoline equivalents, and did not spew clouds of black smoke wherever they went. And since younger drivers don’t remember the sluggish, smoke-spewing beasts of the 70s, diesels are starting to lose their bad reputation and are becoming more popular.
VW didn’t do diesel cars any favors though with their fraudulent emissions tinkering. A lot of folks don’t trust diesels because of that mess.
As for Europe, there are a lot of reasons that Europeans prefer diesels for cars. Europeans have smaller roads and therefore drive smaller cars, so Europe never had those big clunky smoke-spewing diesel cars of the 1970s that gave U.S. diesel cars such a bad reputation. A major part of it though is taxes. A lot of countries in Europe tax vehicles based on engine displacement, and you get the most bang for your buck (or the most bang for your cubic centimeters) with a turbodiesel.
As for reliability, it’s hard to get an apples to apples comparison. Even if you compare the old Oldsmobile 350 diesel to the old Chevy V8 in cars that used both engines, it’s not an apples to apples comparison because the diesel engine had some design flaws that made it much less reliable for early models. Once they got the kinks out of it, the Olds diesel was actually very reliable. But that’s a classic example of a diesel being less reliable than its gasoline equivalent. On the other hand, there are numerous examples of diesel engines being more reliable than gas engines in other cars.
Part of a diesel engine’s reliability though often comes from the fact that they use thicker cylinder walls, heavier crank shafts, sturdier valves, and just generally more robust parts. If you designed a gasoline engine with similarly stronger parts, it would be more reliable too.
Often, a diesel is used in an application where ruggedness and high torque is needed, where a gasoline engine will be used in applications where the emphasis is on performance. This makes the diesels more reliable, because that’s what they were designed to be.
Diesels don’t like to start in cold weather, but it’s not as much of an issue as it used to be. I have to plug my pickup truck in during the winter so that the block heater keeps the engine warm, but if I forget there’s no fiddling with anything on the dash. When you turn the key it automatically activates the glow plugs, and you just have to wait a couple of seconds for the light on the dash to go out before starting the engine. If I have forgotten to plug the truck in during cold weather, I just turn the key, wait for the light to go out, then turn the key back off, turn it on again, wait for the light to go out a second time, then start the engine. The truck has never failed to start on me. While it’s true that a gas engine would just start up no matter what and also never needs to be plugged in at night, the diesel is nowhere near as fiddly as it used to be on older vehicles.