Gasoline and diesel engines are actually quite similar. They are both internal combustion engines designed to convert fuel into energy. This energy is what moves pistons up and down inside cylinders and powers the vehicle. The main difference is how this energy is ignited.
The Difference in How They Work
Car engines, both gasoline, and diesel, function on four simple strokes: intake, compression, power, and exhaust. The difference between the two types of engines is how these strokes are executed. In gasoline engines, the intake stroke pulls air and fuel into the combustion chamber. A diesel engine only pulls in air. In the compression phase, both engines squeeze down the air into a small pocket.
Both engines then convert fuel into energy through a series of small explosions or combustions. The difference is how the explosions happen. In gasoline engines, the mix of fuel and air is compressed by pistons and ignited by the spark plugs. In diesel engines, only the air is compressed, and then the fuel is injected directly into the cylinder. This is called direct fuel injection. Because the air compression heats the air to a high temperature and pressure, when the fuel is injected, it immediately ignites. This aggressive compression results in a more violent vibration and is what causes the typical diesel rumble.
Gasoline Fuel vs Diesel Fuel
For those who have owned both gasoline and diesel vehicles, you’ll know the two types of fuel are different. They look different and even smell different.
Both diesel and gasoline originate from crude oil. Oil refineries turn crude oil into different types of fuel with different consistencies such as gasoline, jet fuel, kerosene, and diesel.
Diesel fuel is less refined and therefore thicker and a lot more oily. This is why it is often called diesel oil. Because of its denseness, diesel evaporates much more slowly than gasoline. Gasoline is refined to a lighter consistency, making it more flammable, and is the reason it needs less heat to ignite.
Image via Flickr by Cliff Johnson
Which Is More Efficient?
Diesel engines have a bad reputation for higher emissions often seen in the black smoke coming out of exhaust pipes. This is changing, though, as diesel manufacturers make advances in engine technologies, use ultra-low sulfur diesel fuel, and improve exhaust treatments to make diesel vehicles cleaner on the roads.
In addition, diesel cars are actually more fuel-efficient than gasoline cars and emit less carbon dioxide and carbon monoxide. Diesel vehicles almost always beat their gasoline counterparts on miles per gallon, delivering up to 30 percent better fuel economy. This is largely due to the more energy-dense fuel and higher compression ratio. On the flip side, gasoline vehicles are constantly striving for better fuel efficiency and hybrid vehicles are strides ahead in the zero emissions game.
As the automotive industry moves toward cleaner fuel and lower emissions, the gap between gasoline and diesel is narrowing. Both are constantly improving their technology, and there are benefits to driving both types of vehicles. In the end, deciding which to choose may simply come down to which is cheaper at the pumps.