Diesel-powered cars may have fallen out of fashion, but it’s a different story in the truck segment. With two more recently joining the club, five of the six U.S.-branded light-duty pickups will offer diesel engines for 2018. By 2019, that very well could reach seven out of seven. And all of the Chevrolet, Ford, GMC and Ram heavy-duty pickups already are available with diesel powerplants. So is the 2018 Nissan Titan XD, which splits the difference between the light- and heavy-duty ranks.
Today, we’ll look at some of the reasons behind the growing popularity of diesel-engine pickups.
What’s the Difference between a Diesel Engine and a Gasoline Engine?
Typically, gasoline engines and diesel engines are very similar. They’re both piston-based internal combustion engines that run on the traditional four-stroke power cycle. What sets them apart is how they manage the first two parts of that cycle. In a gasoline engine, fuel and air first an engine cylinder as the piston moves downward. During the second stroke, compression, the piston rises up to squeeze the fuel-air mixture. When the piston reaches its highest point, the spark plug ignites the fuel-air mixture.
Compare that to a modern diesel engine. Here, the intake and compression strokes involve only air. Fuel is directly injected into the cylinder right at the end of compression, when pressure is highest. That high pressure heats the air so much that the diesel fuel spontaneously ignites when it’s squeezed into the cylinder. With both engines, the combustion drives the piston downward. When it returns upward for the exhaust stroke, the piston pushes out any waste gases.
This means the primary mechanical differences between the engine types have to do with their fuel-injection systems and the fact that a gas engine has spark plugs. A diesel engine doesn’t. Also, beneath their similarities, diesel engines are built to be more durable. This actually meets two needs. First, it lets diesel engines hold up to increased compression pressures. But it also allows these units to take advantage of the unique properties of diesel fuel. That’s the real advantage for pickups.
(Note: Most diesel engines for the U.S. automotive market have what are known as glow plugs. These can provide some extra help to the combustion process, especially during cold starts and to ensure as much fuel as possible is burned off for emissions purposes.)
What’s the Difference between Gasoline and Diesel Fuel?
Regardless of how they’re used in the real world, pickup trucks are engineered primarily for towing and hauling. For those tasks, the chemical makeup of diesel fuel is a major benefit. Both are made from oil, yet one gallon of diesel fuel contains about 18 percent more energy than a gallon of gasoline. As a result, a diesel engine’s power advantage comes during combustion. It’s from the stronger “explosions” in the cylinder that push harder against the face of the piston. The pistons tend to have smaller faces but travel further in their cylinders, in diesel engines. That again boosts how much power is captured from combustion.
The flip-side of this situation is that diesel engines usually can’t operate as fast as gasoline engines. When you put it all together, diesel powerplants are able to produce more torque because of the strength of the combustion stroke, which allows trucks to get heavy loads moving. But because diesel engines don’t spin as fast as gasoline units, they can’t deliver as much horsepower for acceleration and top speed.
There’s also the matter of emissions. The same chemical composition that makes diesel fuel so energy-dense leads to more of two kinds of emissions. Because it’s less refined, any unburned diesel fuel is released into the atmosphere as particulate matter. This is packed with potentially dangerous chemicals that can include heavy metals and carcinogens. Additionally, diesel fuel creates more nitrogen oxides, which create more ozone. None of that is particularly good for living things.
Yet modern-day diesels are able to deal with the problem by trapping those emissions before they reach a truck’s exhaust outlets. These strategies can be effective, but they require a little extra maintenance cost for new filters and fluids.
A Quick Comparison
The 2018 Ram 1500 is available with a standard 3.6-liter V6 gasoline engine and an optional 3.0-liter V6 that runs on diesel fuel. The gas engine features cylinders that are 92 mm in diameter and allow 83 mm of piston travel. The diesel engine has nearly the exact opposite dimensions, with a 96-mm “bore” and an 83-mm “stroke.” In terms of output, the gas-powered V6 produces 305 horsepower and 269 pound-feet of torque, while the diesel unit makes 240 horsepower and 420 pound-feet of torque.
The tow ratings also show the differences between the two. Backed by the diesel engine, the Ram 1500 can tow 9,210 pounds. The same truck with the gas V6 is rated to pull up to 7,600 pounds. It’s a more than 20 percent advantage for the diesel truck. Looking at fuel economy, the energy-dense diesel fuel enables EPA grades of 20/27 mpg city/highway. The top marks with the gasoline V6 are 17/25 mpg city/highway. Included is a 15 percent advantage in combined fuel economy with the diesel engine.
As for the disadvantages of a diesel-powered pickup, it’s not as quick as a truck with a gasoline engine. Consider a Car and Driver comparison of Ram 1500 acceleration times from 2014. They indicated that the diesel truck takes about 1.2 seconds longer to reach 60 mph than a similar pickup with the gas V6. Cost is a factor, too. Because diesel engines have to be built strong enough for the higher compression and combustion pressures, they’re more expensive. In the case of the Ram, the price of the diesel works out to be roughly $4,500 higher than for the gas-fueled V6.
Beyond efficiency, durability and pulling power, that’s another reason so many pickups offer diesel engines. Since trucks tend to be more expensive than cars in the first place, pickup customers tend to be comfortable spending more money in general.