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Black exhaust smoke, tractor-like engine noises, and sluggish performance: all things that were associated with diesel engines. And I say “were" because they’re mostly a thing of the past. Modern diesel engines, for the most part, are as refined as gasoline engines and have more than enough performance to tackle urban and highway driving.
Europeans caught on to this a few decades ago, and in 2015 48% of new cars sold in Europe were diesel. But Euro 6 emissions have been strangling diesel engines since 2015, making them increasingly more expensive to produce. And, since the new emission standard was introduced, diesel sales in Europe have been steadily declining.
Although diesel sales rose in the USA between 2021 and 2022, gasoline-powered vehicles still make up the majority of sales. So, what are the main differences between the two most popular fuel sources in the world, and is it time for you to make the switch?
Let’s find out.
What’s the Difference Between Diesel and Gasoline?
Gasoline and diesel are both refined from crude oil, but it’s easier to refine diesel than gasoline. Diesel contains more pollutants that need to be extracted before it can meet emission standards for road use.
Gasoline and diesel are extracted from the same substance, but the fact that gasoline is more refined makes a huge difference to how these fuels produce energy, how efficient they are, how reliable their engines are, and how much it costs to own one of these engines in the long run.
Different Gasoline and Diesel Types
These are the three most common gasoline types you’re likely to find at the pumps: regular, mid-grade (sometimes called “plus” or “special”), and premium (sometimes called “ultra” or “special”). The only thing that differentiates these grades of gasoline is their octane levels. The higher the octane level, the higher the possible compression ratio.
Regular gas usually has an octane rating of 87, while mid-grade gas is around 89 to 90. Some automakers recommend that certain cars use premium gas, particularly high-performance and turbocharged models, which normally has an octane rating of 91 to 94.
There are two main types of diesel to be aware of: clear diesel and dyed diesel. Clear diesel is intended for on-road use and is sometimes referred to as “auto diesel”. This fuel is clear with a light-green hue, which becomes darker as the diesel degrades.
Dyed diesel is, as its name suggests, dyed. It’s often dyed with a solvent and can be red, blue, or purple. Dyed diesel has a higher sulfur content than clear diesel and is illegal to use on public roads. The most common uses for dyed diesel are in construction, farming, and aviation vehicles.
Diesel and gasoline are used in combustion engines, but how each fuel ignites is one of the major differences between how gasoline and diesel engines work.
Gasoline is mixed with air, compressed by a piston, and then a spark created by a spark plug ignites the air/fuel mixture and causes a contained explosion. This pushes the piston and ultimately moves your vehicle’s wheels.
Diesel engines don’t have any spark plugs, as the combustion of the air/fuel mixture is caused by compression. The fact that diesel engines have much higher compression ratios than gasoline engines is what makes this possible. The pistons in diesel engines compress air, which raises the temperature of the combustion chamber.
Before diesel is sent to the combustion chamber, it’s vaporized. When vaporized diesel is injected into this hot-air environment, it combusts. This is known as compression-ignited injection. Although diesel engines don’t use spark plugs to cause ignition, sometimes they feature glow plugs to increase the temperature.
Gasoline is more refined than diesel, which makes it thinner and more volatile. Practically, this means that it burns faster and can produce more horsepower. Diesel is thicker and burns more slowly, but it has a higher density. This means that diesel engines generally produce around 20% more energy than gasoline engines while using the same amount of fuel.
Diesel engines produce more torque at lower rpm, which makes them better suited to large, heavy vehicles or vehicles that are carrying or towing a heavy load. Since gasoline engines produce more horsepower than diesel engines at higher rpm, they’re better suited to lightweight vehicles.
To get an idea of how diesel and gasoline perform in the same vehicle, let's take a look at the Chevrolet Silverado 1500. You can pick up a Silverado with the 3.0-liter Duramax inline-six diesel engine, which produces 495 lb-ft of torque and 305 horsepower. The closest gas engine you can get, in terms of displacement, is a 2.7-liter turbo inline-four powerplant. This pumps out 430 lb-ft of torque and 310 horsepower.
As mentioned before, diesel burns more slowly than gasoline and produces about 20% more energy, meaning it can make the same amount of energy with less fuel. The low-rpm performance and higher compression ratio mean that diesel engines are generally around 20% more efficient than gasoline engines.
The higher-compression ratio of diesel engines leads to better thermal efficiency and directly affects a vehicle’s fuel efficiency. The higher the compression ratio, the more energy is produced in each explosion. Gasoline engines can’t operate at such a high compression ratio because gasoline ignites at a lower temperature than diesel.
If a gasoline engine operated at the same compression ratio as a diesel engine, the excess heat would likely result in uncontrolled explosions, which would damage the engine.
The efficiency benefits of diesel engines are more noticeable at highway speeds, where traveling at high speed while keeping the rpm low is a huge advantage. However, the benefits are less noticeable in stop-and-go city traffic.
Again, we’ll use the Chevrolet Silverado 1500 to highlight the differences between gas and diesel engines. A Regular Cab, Standard Bed 4WD with the 2.7-liter turbo gasoline engine will put out an EPA-estimated 18/21/19 MPG (city/highway/combined). The same model with the 3.0-liter turbo diesel engine will put out an EPA-estimated 22/27/24 MPG (city/highway/combined).
Diesel engines are considered to be more reliable than gasoline engines. Diesel engines have fewer moving parts and are generally more robust, meaning there’s limited movement while the engine is running, which results in less wear and tear.
It’s not just the mechanical design that makes diesel engines more reliable, as the fuel itself plays a part. Gasoline is more acidic than diesel. It acts as a solvent on an engine’s internals, which causes corrosion over time and increases wear and tear.
As mentioned, diesel is denser than gasoline, which gives it the consistency of light oil. It lubricates the cylinders and other engine components as it runs through the system, which reduces wear and tear.
Gasoline engines tend to last for up to 200,000 miles before needing a major overhaul, while it’s not uncommon to see diesel engines running well after 300,000 to 500,000 miles, and sometimes much more. Of course, this is mainly down to how well-maintained the vehicle is, just check out this Honda Accord with nearly 1,000,000 miles.
Cost of Ownership
Considering the reliability and efficiency of diesel engines, it might seem like a no-brainer that they’re also cheaper to own than gasoline engines. Well, they might be, but that depends on the type of driving you do and how much of it.
Firstly, diesel-powered vehicles tend to cost more than their gasoline counterparts. A Chevrolet Silverado 1500 with a 3.0-liter Duramax turbo diesel engine costs around $2,340 to $3,160 more than an equally equipped model fitted with a 2.7-liter turbo gas engine.
Yes, diesel engines generally last considerably longer than gasoline engines, but they’re more expensive to maintain. For example, it costs considerably more to change and maintain the oil in a diesel engine than a gas engine. This is mainly because diesel engines require much more oil and need more frequent oil changes.
Diesel engines are more efficient, but unfortunately, diesel often costs more than gasoline in the U.S. So, fuel savings can be negligible. At the time of writing this post, the average cost of premium gasoline in California costs $5.266 per gallon, and diesel costs $5.612 per gallon.
If you do a lot of highway driving and travel more than 15,000 miles per year, then a diesel engine could save you money at the pumps. But if you mainly drive in stop-start traffic and travel 10,000 miles or less per year, then you could save more money by using a gasoline car.
What To Choose
If you’ve read this entire article, you’ll see there’s no clear winner between gasoline and diesel, as each has its unique pros and cons.
Gasoline engines are likely the best option financially for people who drive less than 10,000 miles per year. These engines will also be the ones to choose for those who enjoy a rev-happy engine that’s peak horsepower lies high in the rev range.
If you rack up a lot of miles each year, especially on the highway, you could save money by choosing a vehicle with a diesel engine. Anyone who regularly hauls heavy cargo will find that the extra low-down grunt provided by diesel engines makes day-to-day driving easier.
To get a more visual idea of how the mechanical operation of gasoline and diesel engines differ, check out Car Throttle’s video below.