Gas vs. Diesel: What’s The Difference?

The only time most people think about “gas versus diesel” is likely at the fuel pumps, where there is typically several types of regular gas available and (maybe) one diesel option. While most people are probably aware that diesel vehicles exist, chances are they’ve never actually used diesel fuel themselves. They also might not know the real differences between a gasoline and diesel engine, other than to not use the wrong fuel for their own car. It turns out that the differences are significant. Here’s what you need to know concerning gas and diesel engines.

Gas vs. Diesel

In some respects, diesel and gasoline engines are the same. They are both internal combustion engines that convert the chemical energy available in fuel to mechanical energy that powers a vehicle. This is done through a series of small explosions or “combustion.” The mechanical energy created by diesel and gasoline engines moves pistons up and down inside cylinders. The pistons are attached to a crankshaft, and the up-and-down motion of the pistons creates the rotary motion needed to turn the wheels and move a vehicle forward.

However, while they are both classified as internal combustion engines, diesel and gasoline engines are very different in the way they convert fuel into mechanical energy. In gasoline engines, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. This is how gasoline engines create the tiny explosions needed to convert chemical energy into mechanical energy. In a diesel engine, the air is compressed first and then the fuel is injected. Because air heats up when it’s compressed, the fuel in a diesel engine ignites and causes the combustions needed to power a vehicle.

So the main difference is that diesel engines do not use spark plugs. Rather, a diesel engine takes in air and compresses it, and then injects the fuel directly into the combustion chamber. The heat of the compressed air lights the fuel in a diesel engine rather than a spark plug.

Is one more efficient?

Diesel fuel is heavier and oilier than gasoline. Diesel also evaporates more slowly than gasoline. Some people refer to diesel fuel as “diesel oil” because it is so slick and oily. It takes less refining to create diesel fuel, which is why it has historically been cheaper than gasoline. However, since 2000, demand for diesel has risen due to increased industrialization and construction in China and other developing countries.

Diesel fuel has a higher energy density than gasoline, meaning it creates more energy than gasoline. Typically, diesel engines get a bit better mileage than equivalent gasoline engines. Due to its cost and efficiency, diesel fuel is used to power large, industrial vehicles such as trucks, boats, school and city buses, trains, cranes and farm equipment. Many large power generators also run on diesel rather than gasoline. However, gasoline engines have been found to be better for consumer vehicles as they are quieter and emit less soot than diesel engines.

Cost and the Environment

In terms of the environment, diesel engines have traditionally emitted less carbon dioxide than gasoline engines. However, improvements made to gasoline fuel and standard gasoline engines over the past decade mean that, today, there is really no difference between the two types of fuel when it comes to carbon dioxide emissions. Still, many car manufacturers charge more for diesel vehicles under the claim that they are better for the environment.

The Transport and Environment lobbying group, for example, compared the price of a Volkswagen Golf powered by a 150-horsepower gasoline engine and an identical car powered by a 150-horsepower diesel engine. It found that both versions of the Golf produced the same carbon dioxide emissions, but Volkswagen charged $3,500 more for the diesel engine model. Cars do travel further on a liter of diesel than on a liter of gasoline, so your mpg will improve, but diesel produces more soot than gasoline. The bottom line is that most analysts today find diesel and gasoline engines about the same in terms of their environmental impacts.

At the pumps, diesel usually costs more than gas – especially if you use the lowest grade gasoline that’s available. As of 2018, the national average for a gallon of gasoline in the U.S. was $2.88. Diesel averaged $3.17, according to statistics from the petroleum industry. The same statistics show that, on average, diesel fuel prices in America have been higher than regular grade gasoline prices since 2004.

The price of diesel tends to be higher for many reasons, including strong foreign demand (more European vehicles run on diesel), high taxes (the U.S. federal tax on diesel fuel is six cents higher than gasoline per gallon), and seasonality (when demand for heating oil rises in the winter, diesel prices tend to rise too).