Diesel vs. Gasoline: Which Is Better?

Diesel vs. Gasoline: Which Is Better?
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Long-haul trucking, smelly old pickup trucks and industrial generators—this is what comes to mind for most people when diesel fuel is mentioned. But you might be surprised to learn modern vehicles occasionally offer a diesel-powered option, igniting the diesel versus gas debate.

How do the two different fuels compare, and which one has the better advantages? Can you still get a diesel vehicle that fits your needs? Here’s the lowdown on the diesel versus gasoline topic.

How do car engines work?

All engines fundamentally work toward the same goal: turning the crankshaft to transfer horsepower and torque into the transmission, then onward to the wheels. Gas or diesel, it’s the same:

  1. Fuel and air mix and are drawn into the combustion chamber through intake valves as the piston moves downward in what’s called the intake stroke.
  2. On an upstroke with the valves closed, known as the compression stroke, the air-fuel mixture is compressed to a fraction of its volume at normal atmospheric pressure.
  3. The air-fuel mixture ignites and burns fast and hot, flinging the piston downward in what’s known as the power stroke.
  4. When the piston reaches the bottom of its travel, exhaust valves open and, on its upstroke, the burnt gases in the combustion chamber are pushed out into the exhaust system.
  5. Then, the exhaust valves close, the intake valves open, and the process starts all over again.

Because the moving pistons are connected to the crankshaft, every motion causes the crankshaft to rotate.

Where the systems diverge is in the details. They might sound minor but there’s a huge difference in certain important functions.

Gasoline engines

Gasoline is refined from crude oil and is energy-dense yet easily combustible. To harness the energy unleashed in the combustion chamber, the air-fuel mixture is compressed anywhere from 10 to 14 times, shown as a ratio (10:1). The compressed mixture is ignited with a powerful spark from a spark plug to initiate the power stroke. Without the source of ignition, the fuel wouldn’t burn and the engine wouldn’t run.

The earliest gasoline engines were made more than 150 years ago by German engineers Eugen Langen and Nikolaus August Otto, with the first four-stroke car with four cylinders coming in 1890 from Wilhelm Maybach. Engineers have been refining them over the years. Today’s gas engine still performs in the same general way.

Diesel engines

Diesel fuel is refined from crude oil but contains different properties that make it more like oil than gasoline. It’s less volatile than gasoline, so it can be safely put under pressure without risk of detonation, unless it’s also heated.

Rudolf Diesel invented the diesel engine in 1890. Although it goes through the same four strokes as a gas engine, there are some major differences in how Mr. Diesel got it to function.

Diesel engines don’t have a spark plug to ignite the air-fuel mixture inside the combustion chamber. Instead, they rely on heat and compression to cause the controlled explosion for the power stroke. When the two factors are properly calibrated, the diesel fuel ignites without needing a spark. A glow plug heats the cylinder so the engine can start, then the engine creates its own heat to keep going. Compared with gasoline, diesel engines have much higher compression ratios, ranging from around 14:1 to 25:1.

Diesel vs. gas: Which one is better?

If you’re shopping for a vehicle, you probably look at its features to determine if it fits your needs. The same rings true for the engine selection. Compared to diesel, gas engines are less expensive and produce more horsepower. But diesel engines make more torque, which is necessary for hauling or towing, and they’re more fuel efficient in general. Is diesel better than gas? You need to make that decision according to the following criteria.

Diesel vs. gas: engine power

Let’s quickly define the two components of performance: torque and horsepower. Torque is the rotating force that an engine’s crankshaft produces, measured as the capacity to perform a certain amount of work in one direction. Horsepower is measurement of how much force is required to move 550 pounds a distance of one foot in one second.

Diesel engines generally have more torque and less horsepower than a gas engine of comparable size. Higher torque gives diesel engines the ability to move more mass, which is why they’re great at towing heavy loads. Because they have less horsepower, they tend to be slower to accelerate.

Gasoline engines typically have more horsepower and less torque. So, the opposite is true from diesel engines. They’re great at accelerating, but they don’t compare to diesel for hauling because the torque isn’t there.

If you never test the limits of performance, you might not notice any difference. That’s certainly true for people who mostly use their vehicle for daily commuting, but gas engines are still more than capable of towing things like boats and small trailers. If you regularly tow more, diesel may be the better option.

Diesel vs. gas: fuel cost

Is diesel cheaper than gas? That’s no longer the case. Since 2007, diesel fuel emissions standards have been much stricter, and the cost to produce diesel has increased as a result. It’s also taxed at a higher rate than gasoline, and now diesel fuel is consistently more expensive.

Like any product, supply and demand are also an influence on diesel prices. When the trucking industry is booming and demand is high, diesel prices inch higher and higher.

Because diesel vehicles are more fuel efficient, weighing the higher fuel prices against how much less you consume is going to be a key factor.

Diesel vs. gas: noise

If you recall Volkswagen turbo-diesel engine noises from the 1990s or the clatter from a Ford 7.3 Power Stroke engine, you may believe diesel engines all make a racket. But modern engines, particularly those with direct, high-pressure diesel injection pumps and common-rail designs, are much quieter.

Modern diesel engines are still slightly louder than the quiet purr of a gas engine, but from the cabin, you likely won’t even notice the difference.

Diesel vs. gas: fuel economy

Diesel fuel can unleash more energy from the same volume compared with gasoline. While they have the same energy density, diesel has a higher mass, so less fuel is required to achieve the same power production.

What does that mean in real life? When all else is the same, diesel is more fuel efficient. Say you have a two-liter gas engine and a two-liter diesel engine. Installed in the same vehicle and operating under the same conditions, diesel-powered engines typically burn less fuel.

Diesel vs. gas: environmental impact

In recent years, both diesel and gas vehicles have become much cleaner in terms of their emissions. Researchers in the UK (where diesel cars are more common) found that diesel vehicles still produce particulate matter and nitrous oxides at higher rates than gasoline engines—the particulates make up the black smoke from the exhaust pipe.

This isn’t an open-and-shut case. While gas-powered cars have fewer visible emissions, they still produce higher levels of hydrocarbons and carbon monoxide. They also produce more carbon dioxide. Essentially, a case can be made for both types of fuel.

Diesel vs. gas: engine life

While gas engines often need major repairs to run for more than 200,000 miles, diesel engines can run for a whopping one million miles and beyond. The disparity comes down to the design of the engine and its components, the nature of the fuel and the way diesel engines operate.

Compared to a gas engine, which uses belts and chains to sync components, diesel engines are gear-driven. That means fewer fragile parts, and shared parts (such as the cylinder heads) tend to be larger and sturdier in a diesel engine. Diesel fuel’s spontaneous combustion also puts less stress on the engine. Finally, diesel engines typically run at about half the RPMs of a gas engine—they are doing less work per minute, prolonging their longevity.

What would happen if I put diesel into a gas car?

There are stories of people putting diesel fuel into a gas-powered car. It takes effort to make that happen, especially because diesel fuel nozzles are larger than the standard gas pump handle size to avoid this very mistake. However, when it does happen, the result isn’t good.

To begin, the diesel fuel contaminates and clogs the fuel filter because of its dense, oily composition. The fuel entering the engine plugs the fine-misting tips of each fuel injector. If this isn’t corrected soon after, engines can fail.

Putting gasoline in a diesel vehicle is even worse. At a high enough concentration, the gasoline dilutes the diesel and causes pre-ignition in the combustion chamber. With the explosion happening before it’s supposed to in the cylinder, the piston is forced downward while still travelling up. A catastrophic failure is possible.

If you find yourself in either situation, don’t run the engine. A repair shop can clean the injectors, drain the fuel, change the fuel filter and fix whatever else is wrong.

Where can I find diesel cars?

Are you thinking about making the switch to diesel? Although it’s not for everyone, diesel could be the right choice for you. Not all carmakers have the option for diesel, but these do:

  • BMW
  • Chevrolet
  • GMC
  • Land Rover
  • Ford
  • Dodge
  • Cadillac
  • Jeep

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At Bumper, we are on a mission to bring vehicle history reports and ownership up to speed with modern times. A vehicle is one of the most expensive purchases you'll likely make, and you deserve to have access to the same tools and information the pros use to make the right decisions.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.