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What's the Difference Between Gasoline and Diesel?

Mia Bevacqua
September 27, 2018

Gasoline and diesel: These two unique engine designs have been fighting for automotive supremacy since the 1800s. Sure, they have different handles at the fueling station — but what really makes them different? And which is better?

The right one for you depends largely on your situation and driving habits.

Gasoline and diesel: These two unique engine designs have been fighting for automotive supremacy since the 1800s. Sure, they have different handles at the fueling station — but what really makes them different? And which is better?

Which engine type is better — gasoline or diesel?

Diesel and gasoline engines both have their pros and cons. The right one for you depends largely on your situation and driving habits.

Benefits of a gasoline engine

  • Lower upfront cost: Gasoline engines cost less because they don’t require the heavy-duty construction of diesels.
  • More power: Gas engines create more horsepower. They can also spin at a higher RPM and have a broader speed range.
  • Reduced noise: In a diesel engine, fuel burns rapidly, causing a sudden rise in pressure. This can often be heard as “diesel knock.” Gasoline-powered vehicles don’t have this issue, so they’re quieter.

Benefits of a diesel engine

  • Increased efficiency: Diesel engines create power more efficiently, leading to better fuel economy. However, gas engines have been closing the gap in recent years.
  • More torque: Diesel engines generate more torque. As a result, diesel-powered vehicles can tow more than those powered by gas.
  • Longevity: These engines are built to be sturdier, in order to withstand the pressures of higher compression. Because of this, diesel engines tend to go longer without developing major problems.

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What makes diesel and gas different?

The difference between these two petroleum-based fuels starts during the refinement process. A batch of crude oil is distilled into heavier and lighter parts. The heavier substance is used to make diesel, while the lighter is used to make gasoline. As a result, each fuel has distinct characteristics.

Gasoline

Gasoline is used in spark-ignition engines. Because it's lighter and less dense than diesel, it’s more flammable and volatile.

At the filling station, gasoline is classified by its anti-knock properties, referred to as its octane rating. You’ll typically see three options ranging from the low 80s to low 90s, in terms of octane.

Diesel 

Diesel is used in compression-ignition engines. It’s heavier than gas, which makes it more energy-dense. The downside is it’s less flammable and less volatile, making it more difficult to ignite.

At the filling station, diesel fuel is classified by its cetane rating, which indicates how quickly it can be ignited. Its grade also identifies it: Grade 1 is best for low-temperature operations, and Grade 2 is used in moderate temperatures.

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Differences between diesel and gasoline engines

Gasoline and diesel engines are both based on four-stroke, internal combustion technology.

Internal combustion means energy is created inside the engine through a series of small explosions — the fuel igniting.

Four-stroke implies the piston inside the engine takes four strokes to complete one cycle of events. The four strokes are intake, compression, power and exhaust.

The main difference between the two engine types is that, in a diesel, fuel is ignited by high compression (and subsequent heat). On the other hand, fuel in a gasoline engine is ignited by a spark from a spark plug.

Gasoline engine operation

This how the four-stroke cycle occurs in a gasoline engine:

  • Intake: In a port-injected gasoline engine, the intake valve opens and the piston moves downward. As a result, the air-fuel mixture is pulled into the engine.
  • Compression: The intake valve closes, the piston moves upward and the air-fuel mixture is compressed.
  • Power: A spark plug ignites the air-fuel mixture, which forces the piston downward.
  • Exhaust: As the engine continues to rotate, the piston goes back up. The exhaust valve opens and spent gases are pushed out.

Diesel engine operation

The four-stroke cycle happens a bit differently in a diesel engine:

  • Intake stroke: The piston moves downward, the intake valve opens, and air is pulled in.
  • Compression stroke: The intake valve closes, and the piston moves upward. Air is compressed until it reaches an extreme pressure and temperature.
  • Power stroke: Fuel is injected at a precise time. Extreme temperatures cause the fuel to auto-ignite, sending the piston downward.
  • Exhaust stroke: As the engine continues to rotate, the piston goes back up. The exhaust valve opens and spent gases are pushed out.

Whichever type of engine you go with, you can keep it running longer by following the maintenance schedule in your owner's manual. So, keep on top of those oil changes and fluid levels — your engine depends on it. 

 

Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

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