Diesel vs gasoline engine

You can learn about the processes of the individual engines by clicking these links: Diesel engine, Gasoline engine.

The two main types of engines used in cars right now burn either diesel fuel, or gasoline. While the engines share many of the same parts, including the engine block, the engines have a few distinct differences, namely, ignition, starter motors, and outputs.


The most significant difference in a diesel and a gasoline 4 stroke engine is the method of ignition. A gasoline engine relies on a timed spark plug, while a diesel engine relies on spontaneous combustion. Spontaneous combustion is a condition - temperature and pressure - at which a material, in this case, diesel - combusts without a spark. The diesel engine’s efficiency can be attributed to its higher compression ratio; i.e. the ratio of the highest volume to the lowest volume in the compression chamber in a diesel engine is much higher.

A diesel engine achieves spontaneous combustion through high pressure and temperature. The temperature of the fuel air is raised as by its compression in the cylinder. The pressure is also achieved during the compression. Diesel engines actually require high compression ratios. If the same high compression ratios were to be applied to a gasoline engine, the fuel air mixture would ignite too early in its compression. This would force the engine to change its direction nearly instantaneously. The compression ratio of the gasoline engine is typically much lower than that of a diesel engine.

Starter motors

If you have ever owned a diesel-powered car, and had your battery go flat, you'll know that it is a nightmare to get it going again. This is because the batteries used in diesel engines are much more powerful than those used in gasoline engines. Because diesel engines don't have spark plugs, the starter motor must compress the piston, allowing for spontaneous combustion. Doing so requires a lot more energy than simply lighting a spark plug.


As a general rule, diesel engines have a higher specific torque than gasoline engines. This is a ratio of torque output to engine size. For example, the four-cylinder diesel engine on the 2015 Golf TDI produces 236 pound-feet of torque, compared to just 185 for its gasoline counterpart[1]. Also, because diesel engines are able to run higher compression ratios, they tend to be more efficient. For example, the 2014 Golf TDI achieved 8 mpg better on a combined cycle, and 12 mpg better highway.[2]

On the flip side of the coin, diesel engines have the potential to pollute more, as diesel fuel can contain harmful chemicals and tend to produce more particulate matter[3].


  1. www.volkswagen.ca
  2. http://www.fueleconomy.gov/
  3. R. Wolfson, "High-Energy Society," in Energy, Environment and Climate, 2nd ed. New York: Norton, 2012, ch. 2, pp. 20–21