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The difference between diesel- and gasoline-powered cars

Ronan Glon
Dan Baker/Digital Trends

What comes to mind when you think of a diesel engine? Dieselgate? Rumbling big rigs spewing twin plumes of black smoke? Clattering old sedans that can’t get out of their own way being driven by pipe-smoking professors wearing tweed jackets? Big dually pickups hauling horse trailers in a remote part of the nation?

None of those images are what we’d call flattering. They’re not entirely accurate, either. It’s true, diesel has gotten a terrible reputation in the United States over the past few years, but the rumors of its imminent demise are greatly exaggerated. It’s not dead yet, and the current crop of diesel-powered models demonstrate it’s possible to build a clean, efficient diesel engine without sacrificing performance.

Here’s what you need to know about modern turbodiesel engines.

Diesel vs. gasoline

Diesel engines, like their close cousins, gasoline engines, are internal combustion engines (ICE). That means 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 up in the cylinder, pushing the burnt gases out of the engine and out the tailpipe as exhaust. This 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 occur very close together. The more cylinders an engine has, however, 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.

Where diesel and gas engines diverge is how the fuel is ignited inside the engine. In a gas engine, the air and fuel are compressed and, at a critical point in the timing of the cycle, a spark plug ignites the mixture. 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 spontaneously combusts. This is known as “compression ignition,” and it’s the basis of how a diesel engine works. When a gasoline engine has compression ignition, we often call it “knock,” and it can ruin the engine. But diesel engines are built to take advantage of it.

The price difference

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 for using either fuel over time is going to be nearly 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 — especially if it’s older — but it depends on several factors, including what percentage of your fuel is biodiesel, which can be blended with regular diesel. 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 power, 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, 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.

What car manufacturers have found is that drivers love torque, both in pickups and 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.

Reliability

2018 Subaru Crosstrek engine

Because diesel engines use compression ignition, which can quickly ruin a gas engine, they’re built tough. And because they come from a workhorse heritage, they tend to be reliable and require little care. This used to translate into a ton of 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 simpler than a gas engine since it has no spark plugs and the associated electrical system needed for them. Now, cars and SUVs can run on diesel and have performance more akin to 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 is one reason diesels have traditionally had a hard time making inroads in the new car market — especially in the wake of Dieselgate — but technology has caught up and now diesels run cleaner than ever before. This applies to big rigs, pickups, and cars, though, diesels can still be dirtier than cars because the fuel 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 sulfur, which is present in diesel fuel. Most of the sulfur, however, 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.

Noise

Let’s face it, early diesel cars sound just like… trucks. It didn’t matter if it had a Mercedes-Benz star on the hood and got 50 mpg, it still sounded like a small semi when you started it. Once again, however, modern technology has largely mitigated this complaint. The noise pollution often associated with old diesels is gone, along with the cloud of dark smoke that appeared when the vehicle 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.

Which cars are available with a diesel?

2019 Chevrolet Silverado Custom Trailboss

In America, the number of cars offered with a diesel engine is growing at an unprecedented rate. Chevrolet openly tried luring former Volkswagen owners into showrooms with its own range of clean turbodiesel models. The family now includes the Cruze, the Equinox, and the Colorado. The all-new 2019 Silverado will get a diesel option, too. Sister company GMC currently offers a diesel version of both the Terrain and the Canyon, and it will later offer one in the 2019 Sierra.

Jaguar Land Rover is also a big proponent of clean diesel technology. The company’s 2.0-liter turbodiesel engine appears in the XE, the XF, the F-Pace, the Discovery, and the Range Rover Velar. Range Rover and Range Rover Sport buyers can select a 3.0-liter V6 diesel. To add context, the diesel-powered Range Rover returns 22 mpg in the city, 28 mpg on the highway, and 24 in a combined cycle, according to the EPA. Order the SUV with a six-cylinder gasoline engine and you’ll see those figures drop to 17, 23, and 19, respectively.

Ford recently introduced a diesel-powered F-150. Ram’s diesel-powered 1500 is expected to return soon, Jeep continues to offer a diesel in the Grand Cherokee, and the 2019 Hyundai Santa Fe will get a turbodiesel option soon. If it’s a German car you’re after, however, your options are limited to BMW’s 3 Series, 5 Series, and X5 models. Audi, Volkswagen, and Porsche all confirmed they won’t sell another diesel in the United States, and Mercedes-Benz has given up on the segment entirely.