The days of black smoke and diesel ruckus are behind us. Because the level of refinement is so much better than it was in the past, these engines are right at home in luxury sport/utilities. In the case of trucks and sport/utilities, diesel power gives you the fuel efficiency of a V-6 and lets you enjoy the effortless torque -- its peak at low rpm -- that is the same or better as that of a V-8. Diesel engines are tough. They are reliable. They are clean-burning.

Which brings us to this comparison story. We assembled four two-row midsize luxury sport/utilities: the Mercedes-Benz M-Class, the Porsche Cayenne, the 2014 Jeep Grand Cherokee, and the Volkswagen Touareg.

All four are powered by 240-hp, 3.0-liter V-6 turbodiesels that use common-rail fuel injection and SCR with DEF. All four have four- or all-wheel drive. All four have base prices in the $50,000s for the trim levels we tested. And three of the four have won our Sport/Utility of the Year honors. We took this group on the highway, on twisting side roads through spectacular countryside, and off-road through Central California to see which of the new breed of oil-burner SUVs is best.

4th Place: Volkswagen Touareg TDI

Don't let the Touareg's fourth-place finish make you think any less of it. That it came in fourth should give you an idea of how tough this comparison was, and how good all four SUVs are. The Touareg proved competent and capable in everything we asked of it during our evaluation. It has the lowest as-tested price and tied for first in towing capacity, and its 26.7 mpg combined was the second-best fuel economy of test.

But the Touareg didn't rise above being competent. Associate online editor Benson Kong noted, "The coil-spring suspension does not have the level of body- and wheel-control adaptability of the other three, and the effects are obvious on-road," and it felt tippy and tall through the curves.

Though antiseptic in the traditional VW way, it's replete with everything you'll need.

It was the only one here that didn't come with air suspension -- in fact, air suspension isn't even available -- so handling suffered on twisty roads. We also noticed plenty of tire, wind, diesel, and road noise on the drive. No air meant we couldn't increase the VW's ground clearance off-road. However, it still wasn't bad on the trail.

Judges also felt the vague steering feel was the worst of test. The transmission was good, but in Sport mode, it shifted hard and at times seemed confused as to what gear it should be in. One judge noted that the chassis bounces off bumps rather than absorbing them.

The Touareg's interior impressed us. Said associate online editor Nate Martinez, "The interior's airiness is the Touareg's strong point. Though antiseptic in the traditional VW way, it's replete with everything you'd need. The materials are nice and nothing is off-putting." The navigation system is fairly intuitive, and cabin controls are easier to learn than in past Touaregs. But there were issues with the seats. The seat bottom is long, and anyone who is on the shorter side has to scooch forward. They are also hard, flat, and extremely unsupportive after a couple hours of straight driving.

Another thing we took into consideration here is value. Sure, the Touareg was the least expensive of the group, but our tester didn't come as well-equipped as the others. If you factor in the cost of air suspension in the other three, the VW is the least expensive, but not the best value. It is competent, but forgettable.

3RD PLACE: Mercedes-Benz ML350 BlueTec 4Matic

The diesel-powered SUV for those who aren't diesel people, Kong said, "It probably delivers the most civilized approach to the diesel-powered midsize SUV. The least obvious diesel of the bunch, the ML350 sounds like a quiet gasoline engine throughout the rev range."

Everyone came away thoroughly impressed with the M-Class. It was without question the most refined of the group, combining a smoothly shifting transmission with excellent power from the diesel engine, and the most torque. Associate online editor Karla Sanchez wrote, "It was my favorite to drive on the curvy roads and highways. Felt light on its feet, and the most effortless to maneuver of the bunch. Easy to toss around turns without feeling body roll." She continued, "I was amazed at how quiet the cabin was. You wouldn't even know you were driving a diesel." We agreed the Benz's diesel exhibited the least turbo lag. That plus the whisper-quiet engine earned raves, and after the drive loops, placed this SUV at the top of the list for many judges. One key highlight was the size of the cargo area. With the rear seats down, you can fit 80.3 cubic feet of gear. That was the most, by far, of this test.

The Mercedes probably delivers the most civilized approach to the diesel-powered midsize SUV.

As impressed as the judges were with the fun of driving the Mercedes, doubts crept in. The ML's fuel economy was the worst of the four.

We know most people who buy luxury SUVs aren't going to take them off-road. However, there'll likely be times when you'll want to go to the cabin or drive through snow, and there is a presumption that an SUV will be able to tackle those tasks better than a car will. That's why we took these vehicles on trails, which was where we discovered the Mercedes was just not as confident in mild ruts and dirt as the others. Kong: "Off-road, even in its raised position, the ML350 felt indifferent yet fragile. It didn't have much trouble with any of the tasks, but felt more stretched out on the harder stuff than the others. Experienced a couple of odd ABS pulses when using the hill descent control."

That shortcoming is probably fine for most SUV drivers, but that, the worst fuel economy, and the lower value compared with some of the others here kept the ML350 in third place.