What's the difference between diesel vs. gas engines?

How do engines work?

Motor vehicles are powered when fuel is converted into energy. Most engines ignite fuel in a combustion chamber, creating the pressure that moves the pistons and consequently propelling the wheels and vehicle.

Both gas and diesel engine types use a four-stroke combustion cycle: intake, compression, power, and exhaust. However, the details of these steps differ in gas and diesel engines.

How do diesel engines work?

A diesel engine draws in only air on the intake stroke. It compresses that air at a much higher ratio — often 14 to 23 times — and then injects diesel fuel into the chamber of super-compressed air. Unlike a gas engine, a diesel engine doesn't need a spark plug because the compression ratio is so high that the fuel ignites on its own. After the fuel ignites, there is a power stroke and an exhaust stroke similar to those in a gas engine.

How do gas engines work?

In a gasoline engine, self-ignition of the fuel is a problem — sometimes referred to as engine knock — that causes a loss of efficiency and power. In fact, premium fuel is required in gas-powered vehicles with higher compression ratios to avoid engine knock.

Gas vs diesel engines: What are the key differences?

• Fuel efficiency: Diesel engines are typically much more fuel-efficient than gas engines, even those with very high compression ratios. Diesel fuel is also more energy-dense than gasoline, meaning more energy per gallon. As a result, it's not uncommon for a diesel-powered car to get 50 mpg or higher.
• Life expectancy: Diesel engines need to be built more sturdily than gas engines to handle the extremely high compression forces of the diesel engine cycle. This, combined with the fact that diesel fuel is a natural lubricant, can result in astonishing engine longevity. Many diesel vehicles can achieve 200,000 miles before they need major repairs — double the mileage at which gas engines may start requiring more frequent work.
• Cost: While diesel fuel is generally more efficient than gas, it is typically more expensive. Find out the average price difference for diesel compared to regular, mid-grade, and premium gas in your state.

Is diesel better than the gas?

Diesel and gas appeal to different drivers. While diesel offers reliability and some fuel economy advantages, it isn't appropriate for all situations. Diesel performs best over long distances at highway speed but not as well in stop-and-start traffic, for instance.

Get more information about how to decide whether you should buy a diesel car. Alternatively, if you're looking for the most fuel-efficient vehicles for city driving, you might consider a plug-in hybrid or electric vehicle. Learn more about the differences between hybrid vs. electric cars.