• Buying Guides

Diesel vs Gas? Differences Explained

By Autolist Editorial | June 7, 2021

More foreign automakers are offering diesel engine models in the United States, which has left many questioning whether diesel or gas engines are better and what the difference is between diesel and gas engines. One difference helping diesel engines gain their rise in demand is that they have better fuel economy, but the most important difference is the thermal efficiency of diesel engines. A diesel engine is approximately 20% more thermally efficient than its gasoline counterpart.

Since there are several differences between diesel versus gas engines, we have dedicated this article to exploring all those differences, how the two engines work, and some of the pros and cons.

The History Lesson

To better understand these two engines, let's take a look at their history and how they evolved. It started with the invention of gasoline engines. In 1876, a man named Nikolaus August Otto invented the gas engine, which used four-stroke combustion. That is the basic idea behind modern car engines. At that point, the gasoline engine wasn't all that efficient. Only 10 percent of the fuel was used to move the vehicle. The rest was merely producing heat.

Two years later, in 1878, an engineer named Rudolf Diesel was in the era's equivalent of an engineering school called Munich Polytechnic Institute. He learned about the poor efficiency of gas engines and resolved to come up with something more efficient. He eventually invented and patented the diesel engine in 1892. Because of how efficient diesel engines are, they are often found on large equipment components, like semi-trucks and container ships.

Early diesel engines earned a reputation for being noisy to operate; however, this complaint is less common now with evolving technology. The issue of noise pollution and air pollution have been mitigated, so modern diesel engines are no longer problematic.

Gasoline Engines Versus Diesel Engines

A similarity between the two engine types is that they are both internal combustion engines. This means that they use internal combustion to convert fuel into mechanical energy to move a vehicle. In an internal combustion engine, air enters the engine and combines with the fuel. The engine's cylinders work to compress the air-fuel mixture, which ignites to start the motion of the crankshaft and piston. The crankshaft activates the vehicle's transmission to turn the car's wheels. After that, the piston returns to its original position to expel the spent gas from within the engine to the tailpipe and is emitted as exhaust. Unbelievably, this entire process occurs multiple times every second.

The primary difference between the mechanical workings of these two engines is the ignition process of the internal combustion engine to start working. Gas engines have a much higher volatility point, but a lower flash point. During the compression process in a gas engine, a spark plug is responsible for igniting the fuel. However, a diesel engine does not have spark plugs because it is a compression combustion engine. Instead, they use extreme compression to generate the heat energy needed for the spontaneous ignition (also known as compression ignition).

The timing for the ignition of diesel engines is also different. Before the power stroke, the diesel engine only has a pocket full of hot air. As the piston reaches the top of the compression stroke, the air is hot enough from being compressed so small that it immediately ignites when the diesel is injected. As a result, the timing of the ignition is driven by fuel injectors for diesel engines.

Compression Ratio

Rudolf Diesel's theory turned out to be correct in that a higher compression ratio would yield higher fuel efficiency and more power. That occurs due to the concentrated air that results from the piston compressing the air inside the cylinder. The higher energy content in diesel fuel makes it more likely to react with compressed air. In other words, the air molecules are so concentrated the fuel can respond with as many of them as possible. Rudolf's theory was correct because gasoline engines have a compression ratio between 8:1 to 12:1, while diesel engines have ratios between 14:1 to 25:1. That's twice as efficient in some cases.

The Difference Between Diesel and Gas Engines

We have discussed the main differences between the mechanical workings of the two engines, but let's look further into how they influence the other differences between the two engines.

Fuel Efficiency
Both diesel and gasoline start the same way, which is in the form of crude oil mined from the earth. Once it's processed and refined, it can be separated into various types of fuels. Diesel fuel is more substantial than gas and thus evaporates more slowly. It also has more energy density. This helps explain why the fuel efficiency of diesel engines is typically better than gas engines.

An important difference between the two engines is the thermal efficiency of diesel engines. Thermal efficiency refers to the work that can be expected to be produced by the engine's fuel. A gas engine is about 20 percent less thermal efficient than a diesel engine. This means that a diesel engine has a 20 percent increase in its fuel economy. The increased thermal efficiency also contributes to higher torque, more horsepower, and greater towing capacity.

Drivers who typically drive many highway miles often favor diesel engines for their fuel efficiency. That is why diesel trucks are often the favorable choice instead of gas trucks for towing heavy loads over a long distance. It is a great combination of improved fuel economy, higher acceleration, and more power.

Because diesel engines are typically more fuel efficient, they are used in various equipment and vehicles, for example, power generators, heavy-duty construction vehicles, medium-duty trucks, work trucks, and even cruise ships. A benefit of fuel-efficient engines is that they result in lower operating costs because less fuel means fewer fill-ups with fueling stations.

Fuel Costs
For most U.S. drivers, diesel and gas fuel costs are very similar. Sometimes the pricing rises higher than the other, and other times it drops. If you want to calculate your diesel cost savings for your specific driving needs, then you can use the formula below:

Miles / (City MPG percentage of miles you drive in the city + highway MPG percentage of miles you drive on the highway) * $ per gallon = annual gas cost

If you calculate the formula yourself, then you will most likely notice that although diesel costs less per mile you drive, it takes several years to break even when comparing the up-front costs of a diesel vehicle to the cost of a gas vehicle. However, suppose you drive many highway miles annually and have a plan to keep your diesel vehicle for a long time. In that case, it may make better sense to pay the up-front costs of a diesel vehicle to enjoy a more fuel-efficient engine and fewer trips to the gas station.

Engine's Lifespan
The lifespan of the two engines is another differentiator between them. Often, when a gasoline engine reaches the 120,000 to 150,000-mile reading, the engine's cylinders will start to show some signs of wear or tear. Unfortunately, this results in decreased efficiency for the vehicle.

On the other hand, diesel vehicles are known for having a longer lifespan. Some diesel vehicles are still running with a million miles plus on the clock. Despite the engines being similarly designed, the diesel engine has a cylinder with a removable liner. So, once it reaches 200,000 to 300,000 miles, you can replace just the liner and not the entire engine; this results in lower maintenance costs.

Diesel engines have high torque but low RPM (revolutions per minute). A car's gas engine will turn twice as many revolutions per minute when driving down the highway compared to a diesel truck. This means the diesel truck will wear at half the rate of the gasoline car, giving it a higher lifespan.

Lastly, diesel engines are extremely robust because their castings and cylinder walls are built thicker, and the oiling system has a higher volume. All of this allows them to have a higher life expectancy.

A great benefit of the diesel engine's longer lifespan is that your diesel vehicle will have a higher resale value. Something to keep in mind if you plan on trading it into a dealership or privately selling it.

Lower Maintenance Costs
There are fewer components in a diesel engine than a gas engine. This means that there are fewer potential parts in a diesel engine that could malfunction, which results in lower maintenance costs in the long run.

Fewer components mean diesel engines need less repair and maintenance services compared to their gasoline counterpart. Diesel engines also need fewer oil changes.

Fuel Emissions
A big con for diesel for a long time is the misconception of it being a high-pollutant. It was once cumbersome and oily and created excessive soot. However, in the last 40 years or so, significant improvements have been made to the cleanliness of diesel. The fuel is refined better, and computer monitoring within the engine helps reduce emissions. Soot has been reduced by catalytic converters and CRT particulate filters, which burn the residue.

Today's diesel-powered vehicles are now actually some of the cleanest, most efficient choices. Now that they are no longer billowing black smoke, many are more likely to produce green benefits, with some having EPA grades north of 40 mpg.

A recent trend is biodiesel, a fuel that comes from plant oil or animal fat and has been chemically altered. Conversions for older diesel models were common. However, newer diesel models are sometimes unsuitable for biodiesel retrofits.

Diesel Vehicles Available in the U.S.

Several diesel vehicles are available in the U.S., with the numbers growing to meet the rising demand. With more dealerships stocking diesel-powered vehicles, there are not only pickup trucks making their way on the list but also cars like the Chevy Cruze and SUVs.

If you have your mind set on buying a diesel vehicle, then below is a list of some of the U.S. cars and trucks that offer a diesel engine option:

  • Chevrolet Colorado (turbo diesel)
  • Chevrolet Silverado 1500 (turbo diesel)
  • Chevrolet Suburban (turbo diesel)
  • Chevrolet Tahoe (turbo diesel)
  • Ford F-150 (turbocharged V6 diesel engine)
  • Jeep Wrangler (turbocharged V6 diesel engine)
  • Land Rover Range Rover TD6 (turbocharged V6 diesel engine)
  • Dodge Ram 1500 EcoDiesel (turbocharged V6 diesel engine)