Think of “diesel” and what comes to mind? Rumbling big rigs spewing twin plumes of black smoke? Clattering old sedans that can’t get out of their own way being driven by professors smoking pipes while wearing tweed jackets? Big dually pickups hauling horse trailers? If that’s what you think of and you live in the U.S., then it’s no wonder that America was one of the slowest adopters of diesel-powered cars in the world. Outside U.S. borders, diesel-powered vehicles have reach nearly 50 percent parity with gas-powered cars and trucks. So what happened to suddenly make dirty old diesel the current darling of the liquid fuel world?

One of the key players in the diesel car market is Mercedes Benz, famous for its “turbodiesel” sedans of yore. Mercedes’ involvement with diesel engines dates back many decades. 

“The high compression ratio of diesel engines allows for better efficiency compared to gasoline engines”

Mercedes Benz is well aware of diesels’s problematic past image. “The stigma from diesels of the past being smelly or smoky is still a perception that needs to be overcome, but positive changes have been seen in consumer sentiment that will only continue to improve,” Robert Angner, Department Manager in Advanced Product Planning with Mercedes Benz told Digital Trends. Today, Mercedes continues to modernize the diesel engine and their SUVs and cars with diesel powerplants can be hard to spot because they don’t really sound or even drive like a diesel vehicle from past decades. 

I recently spent a week driving a Mercedes Benz GLK250 BlueTEC diesel SUV (above) and am happy to report no cowboy hats, ball caps or tweed jackets were required to enjoy its prodigious power, long range and quiet. Yes, quiet. The smelly, tell-tale black clouds of diesel smoke have largely disappeared from diesel cars (and even semis) as better diesel fuel, improved engine technology and cleaner exhaust systems have come into play. All of those advances were apparent while driving the GLK250.

What has Mercedes done to make diesels more accepted by American buyers? First, it’s important to understand the key differences between gas and diesel-powered engines.

How a diesel engine is the same – and different – from a gasoline engine

Diesel engines, like their close cousins, gasoline engines, are Internal Combustion Engines (ICE). Fuel is mixed with air as it goes into the engine and that mixture is compressed internally, inside the engine’s cylinders. At some point, the fuel ignites (combusts), driving a piston down and turning the crankshaft, which is connected to the vehicle’s transmission and ultimately turns the wheels. The piston then moves moves up in the cylinder, pushing the burnt gases out of the engine and out the tailpipe as exhaust. Then the cycle repeats, several times per second.

The more cylinders an engine has, the smoother it tends to run and the more power it can make since the combustion events come very close together in time. However, the more cylinders an engine has, the more complex and mechanically inefficient the engine becomes. How the cylinders are configured also has a bearing on performance, vibration and other factors. That’s true for both gas and diesel engines.

Watch how the major parts of a diesel engine come together and operate in this video:

Where diesel and gas engines diverge is how the fuel is ignited inside the engine. In a gas engine, the air and fuel is compressed and at a critical point in the timing of the cycle, a spark plug ignites the gas. But in a diesel engine, there are no spark plugs. When diesel fuel and air are squeezed enough, the extreme compression generates enough heat that the mixture essentially spontaneously combusts. This is known as “compression ignition,” and it’s the basis of how a diesel engine works. When a gasoline engine has combustion ignition, we often call it “knock” and it can ruin the engine. But diesel engines are built to take advantage of it.

Here are some of the advantages and differences diesel has with gas engines:

Diesel costs more than gas, but…

“Many diesel offerings in the U.S. market are more expensive than their respective gasoline models and combined with the higher cost of diesel fuel vs. premium gasoline, some customers may have a hard time justifying the price difference,” Angner said. However, it’s not an apples-to-apples comparison when it comes to the price of diesel and gasoline.

Diesel fuel, which is syrup-like compared to watery gasoline, packs more of a punch in terms of energy per unit of weight than gasoline (say, per gallon or liter). So even though it often costs more than gasoline, it contains more potential energy, so less diesel is required in terms of fuel to accomplish the same amount of work (in this case, driving distance) as gasoline. That’s why diesel cars tend to get mileage in the 30s, 40s or 50s – or more – depending on the engine and vehicle type. In terms of fuel comparative costs, it’s either a wash or a slight advantage for either fuel, depending on the price difference, vehicle and driving style. But overall, the final cost of use for either fuel over time is going to be pretty close to the same.

Diesel owners also have another fueling option: biodiesel. Biodiesel is made from non-petroleum sources such as fry oil or vegetable oil. Indeed, the first diesel engines ran on peanut oil-derived diesel fuel. And while making your own gasoline at home is very complex, dangerous, and ill-advised, it is possible to safely make your own biodiesel using ready-made kits.

Getting your diesel vehicle to run on biodiesel may require some small modifications (or it may not), but it depends on several factors, including what percentage of your fuel is biodiesel, which can be blended with regular diesel. Mercedes-Benz says its BlueTEC engines are warrantied for up to a 5 percent biodiesel blend. There is a large biodiesel community online that can answer questions about the popular alt-fuel. The per-gallon cost of bio-diesel may also be higher than pump fuel, but since you can make it yourself, it really depends on aggregating the cost of the equipment, the oil source (which can be free) and your time. 

Power output: horsepower vs. torque

Both gasoline and diesel engines are measured in terms of horsepower and torque. Horsepower is a measure of how much work is being down, while torque is a measure of an engine’s twisting force on the driveline. Big amounts of both are great, but if you have a lot of horsepower with only a little torque, the acceleration of your vehicle is going to be slow to get going. It’s torque that gets things moving, which is why diesel engines are used in big trucks – they are great for moving heavy loads because they have so much torque. But diesel engines don’t rev up very high, so they make less horsepower on average than gas engines, which is why they aren’t many sports cars with diesel engines, although that is beginning to change.

What car makers have found is that drivers love torque – in pickups and in cars. Step on the gas and a diesel-powered car has brisk acceleration. Coupled with a capable transmission, modern diesels can get going fast in a hurry. While they don’t scream to a high redline like a gas engine, that’s not a big deal for most people, who would rather have real-world power (torque), acceleration and good gas mileage.

While driving the Mercedes GLK250, I was impressed by the stout acceleration, even though we had the SUV filled with people, vacation gear and even a large dog. Going up hills, I had no problem passing other vehicles as the GLK rode a broad wave of torque as it climbed the grade. It was like driving a very powerful gas-powered car, except we still got nearly 40mpg over the course of our journey.

Reliability

“The high compression ratio of diesel engines allows for better efficiency compared to gasoline engines,” Angner said, “but this high compression ratio also requires more robust parts, which in general are considered to add longevity to the life span of the engine.”

Diesel engines… are great for moving heavy loads because they have so much torque.

Because diesel engines use compression ignition, which can quickly ruin a gas engine, they must be made very tough. And because they come form a workhorse heritage, they tend to be very reliable and need a minimum of care. That all used to mean a lot more weight, but with modern manufacturing methods, the weight penalty for a diesel engine has been greatly reduced and the engine’s weights are now generally on par with gas engines.

Also, a diesel engine is more simple than a gas engine since it has no spark plugs and the associated electrical system needed for them. So now, cars and SUVs can run on diesel and have performance more like a gas engine, but with the added gas mileage and reliability of a diesel. In general, a diesel engine will outlast a gas engine in terms of how many miles or hours it can run before it needs major service, so repair bills are smaller and typically further apart for a diesel engine.

Pollution

This was one reason diesels have traditionally had a hard time making inroads in the car market, but technology has caught up and now diesels are much more clean-running that ever before. That goes for big rigs, pickups and cars, but still, diesels can be dirtier that cars because diesel still does not burn as cleanly as gasoline. All the black stuff coming out semi exhausts for decades included a lot of soot, caused in part by the burning of sulphur, which is present in diesel fuel. However, most of the sulphur has been removed from modern diesel fuel, and further filtration of diesel exhaust in some cars has actually pushed them to the forefront of clean vehicles. 

Many diesel cars now use a separate additive called DEF, also know by the name AdBlue. AdBlue is a urea-based liquid additive that is held in a small tank onboard the vehicle, and is used to treat diesel exhaust to make it cleaner. Mercedes’ BlueTEC system uses this additive in its Selective Catalytic Reduction system, which includes several other tech tricks and filters to clean up emissions and allows ”BlueTEC to be among the cleanest and most advanced diesel engine technologies,” according to Angner. 

In 2007, the Mercedes-Benz E320 BlueTEC sedan was voted World Green Car for its low emissions, according to Mercedes-Benz.

Noise

Let’s face it, early diesel cars sounds just like … trucks. It didn’t matter if it had a Mercedes Benz star on the hood and got 50mpg, it still sounded like a small semi when you started it. Once again, however, modern technology has largely mitigated this complaint. The GLK250 I drove sounded a bit different from a car when I started it, but not in a disagreeable way. The noise pollution of old diesels is gone, along with the cloud of dark smoke that appeared when the vehicle was started  up. And from the inside of most diesel passenger vehicles, the driving experience is essentially the same as a gas-powered car. For most people, if you didn’t tell them they were in a diesel-powered vehicle, they probably wouldn’t know.

Additionally, I was surprised at how very quiet the GLK250 was as it motored down the road. Stepping on the gas just brought more speed, not more engine noise. The days of the clattering diesel seem to be over.

Diesel tech tricks, new and old (and new again)

New technologies are being applied to diesel engines to increase efficiency. One of the latest is Direct Fuel Injection, also know as DFI. Direct Injection is an advanced version of Electronic Fuel Injection (EFI). Instead of using fuel injectors to mix precise amounts of gas and air before it goes into the engine, DFI squirts fuel directly into the combustion chamber a fraction of a second before the combustion cycle. It’s even more precise than fuel injection and has made marked improvements in engine efficiency and emissions cleanliness for both diesel and gasoline engines.

Another tech trick is an oldie but a goodie: turbocharging. Turbocharging has been around for decades but it is especially effective in increasing the power output of diesel engines. Stuffing more air (and fuel) into the combustion chamber significantly raises power output to the point that current models are topping 200 horsepower and nearly 400 foot-pounds of torque. In the 1980s, Mercedes diesel cars often made half that much power.

On the horizon: Diesel hybrids

What’s the next step? Hybrids. Mercedes has recently introduced a hybrid diesel model in Europe and other carmakers are also looking into combining diesel engines with electric motors. That combination should give drivers more of what they crave: both powerplants are great for making torque. Adding in electric motorization to a diesel powertrain will up the fuel efficiency even more, especially if the vehicle can drive in electric-only mode for some distance.

“Through improvements in turbocharging and other technologies; we’ll continue to develop smaller, more powerful engines that will deliver better gas mileage while reducing emissions,” was Angner’s final thought on diesel tech. Judging by the rapid adoption outside the U.S. and growing interest in America, the timing might be right for diesel to overpower gasoline in the marketplace.

What do you think about driving a diesel? The time has come or still on the fence? Why? Tell us in comments.