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Diesel vs. Gas: What’s the Difference?

The majority of vehicles on our nation’s roads are powered by one of two fuels: gasoline and diesel fuel. Nearly all drivers on the road know which of the two pumps to pull up to when it’s time to top off, and most know that using the wrong fuel can lead to bad things happening to their vehicle’s engine.

But what really is the difference between diesel and gasoline? Why can’t engines designed for one use the other? What happens if you mess up and put one fuel in an engine designed for the other?

This month, we’ll dive into the diesel vs. gas debate and answer these and other questions about the fuels that power the cars and trucks we drive every day.

Diesel vs. Gas Engines: What’s the Difference?

At their most basic level, diesel and gasoline engines operate on the same basic principle, with pistons moving up and down inside one or more cylinders. As the piston travels through the cylinder, the following processes occur over and over again, thousands of times a minute, to turn the engine’s crankshaft and provide power:

  • The piston travels down, drawing a fuel-and-air mixture into the cylinder (intake stroke)
  • The piston travels up, compressing the fuel-air mixture (compression stroke)
  • The fuel and air are ignited, causing them to expand and force the piston back down (power stroke)
  • The piston travels up again, forcing the gases left over from the burning fuel into the vehicle’s exhaust system (exhaust stroke).

This recurring process is known as the Otto cycle, and most vehicular internal combustion engines operate based on the four-stroke Otto cycle or a variation of it. (Small engines for things like lawnmowers and chainsaws and some very large diesels operate on a different, two-stroke cycle; the differences between gas and diesel two-stroke engines are the same as the differences between gas and diesel four-strokes.)

The fundamental difference between a gas and a diesel engine is how the fuel gets ignited:

Gasoline Engines: Spark Ignition

Gasoline engines rely on a spark to ignite the fuel-air mixture. The spark is provided by a device called an ignition coil, which sends a burst of high-energy electricity to the spark plug located at the top of each cylinder. This burst of energy must arrive at the exact-right time for the engine to give its best performance, so gasoline engines require very tightly controlled timing between the spark and the engine’s rotation.

Diesel Engines: Compression Ignition

Diesel engines don’t have spark plugs, because the heat generated by the compression of the fuel-air mixture is enough to ignite the fuel without any additional assistance. As such, diesel engines don’t just lack spark plugs, they lack the additional elements that a gasoline engine needs to maintain spark timing.

Diesel vs. Gas Fuels: What’s the Difference?

Diesel fuel and gasoline are just two of the hundreds of different fuels and lubricants that are refined from crude oil, or petroleum. Generally speaking, petroleum products fall along a continuum of “heavy,” energy-dense products to “light” volatile products:

  • Lightweight petroleum fuels include things like butane and propane. They are not very dense, and they are volatile, meaning they evaporate very easily. Lightweight products contain less energy per gallon than other products.
  • Medium weight fuels include gasoline. They are more dense than lightweight fuels but are still highly volatile. They contain more energy per gallon than light fuels, but less than heavier weight fuels.
  • Heavy fuels include diesel fuel, kerosene, and fuel oil. They are the densest refined petroleum fuels, are the least volatile, and contain the most energy per gallon.

In order for the fuel in a spark-ignition engine to burn properly, it has to be fairly volatile so that it easily becomes a flammable vapor inside the combustion cylinder. Therefore, most spark-ignition engines require a volatile fuel like gasoline or propane in order to run.

Compression engines don’t require the same volatility from their fuels, and are able to take advantage of the much higher energy density of heavy fuels, providing more power and torque at lower speed than a gasoline engine of the same size.

Diesel vs. Gas: What Happens If I Mix Them Up?

It happens almost every day: someone driving a new or borrowed vehicle isn’t paying attention and accidentally fills a gas-powered car up with diesel or a diesel truck with gasoline.

While automotive and fuel pump manufacturers have tried to reduce the likelihood of misfuelling accidents by standardizing different-sized nozzles for the two fuels that correspond to different-sized filler necks on fuel tanks, accidents still happen.  Diesel pumps use a larger nozzle that won’t fit into a gas tank’s filler, but the setup doesn’t prevent someone pumping gas into a diesel tank.  There’s also nothing to prevent someone from accidentally filling a gasoline tank with diesel from a fuel can or other dispenser.

Putting Gas Into a Diesel Tank

Three things will probably happen if you try to run a diesel engine on gasoline:

  • Compression will ignite the fuel but won’t do so completely or consistently. The engine will stutter, belch black smoke, and provide a tiny fraction of the power it should.
  • Diesel fuel also provides lubrication for the components of a diesel engine. Gasoline, lacking that lubricating quality, will cause your engine’s components to wear and fail very quickly.
  • The fuel injectors that are designed for heavy diesel fuel will burn out very quickly when they’re pumping the much lighter-weight gasoline.

To make a long story short, the damage from running gasoline into a diesel engine starts at “very expensive” and runs all the way to “complete engine replacement.” If you realize you’ve mis-fueled a diesel engine, do not start it. Immediately call for a tow, and have the vehicle taken to a qualified service center to have the tank drained and cleaned.

Putting Diesel Fuel in a Gas Tank

The results of accidentally putting diesel into a gas engine typically aren’t quite as cataclysmic as the results of pumping gas into a diesel tank, but can still be quite costly. In most cases, the engine simply won’t run with the diesel fuel because the fuel is not volatile enough to evaporate. The most likely failures include things like clogged fuel lines, damaged fuel injectors, damage to the fuel pump, and a ruined fuel filter.

As with accidentally filling a diesel tank with gas, if you realize you’ve pumped diesel into your gas tank, do not attempt to start the engine. Have the vehicle towed to a mechanic who can drain the tank, replace any damaged fuel system components, and get you back on the road.

Need Diesel? Need Gas? Let the Local Pros at Walthall Help!

Whether you need a tankful for the family car or a complete on-site fueling station for a fleet of heavy equipment, Walthall has the fuel you need. Our retail stores offer convenience and friendly service from your local oil company, and our commercial services help improve efficiencies for businesses across the Southeast. Give us a call today to find out how Walthall Oil Company can work for you! (478) 781-1234

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