Mechanical differences of diesel and gas engines
In most cars, both gas and diesel engines use a four-stroke cycle, but the way the engines work is fundamentally different. Both 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.
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.
Diesel vs. gas: miles per gallon (mpg)
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.
Diesel vs. gas engine: 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.
Diesel vs. gas trucks and cars: how to choose
With their advantages in reliability and fuel economy, why aren't diesel engines used in all cars? Diesel and gas appeal to different drivers, and diesel 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.
Alternatively, if you're looking for the most energy/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.