What Is the Difference Between Gasoline and Diesel?

By Jim Taddei, December 6, 2011

As a follow up to last week’s blog describing how a four stroke piston engine works, this week I would like to talk a bit about the different fuels used in cars today. Although an internal combustion engine can run on just about any liquid or gaseous fuel, the two most common are gasoline and diesel.

How Gasoline and Diesel Are Created
In the last few years, much work has been done creating gasoline and diesel from biomass, but for all intents and purposes, the fuel we purchase at our local gas station is distilled from crude oil. When crude oil is refined, the yield is about 45 percent gasoline and 20 percent diesel fuel and home heating oil (which is processed as necessary depending on market conditions).

The remaining crude oil is refined and processed into other items including jet fuel, heavy fuel oil (“bunker fuel” used by very large ships and other industrial applications), road asphalt, and many other products. Very little is lost during the refining process. In fact, due to the increased volume of the lighter fuels like gasoline, 42 gallons of product is produced from each 40 gallon barrel of crude oil.

Gasoline is a much lighter, more volatile fuel—it evaporates quickly when spilled on the ground or left in an open container—with fewer carbon atoms than its heavier, more oil-like diesel cousin. Because gasoline has less carbon atoms per gallon, it has less available energy (about 10 percent less) than diesel fuel. This is one of the main reasons why diesel engines have better fuel economy.

How Gasoline and Diesel Engines Work
Over the years, gasoline had been delivered to the cylinders in many different ways. Starting with mechanical carburetors and fuel injection, and evolving to electronically controlled carburetors and fuel injection—these systems all mixed gasoline with the incoming air before it entered the combustion chamber. Now, the latest fuel injection systems inject gasoline directly into the combustion chamber. All of these advances in fuel delivery have resulted in improved gas mileage, reduced exhaust emissions, and improved drivability.

Diesel engines are “compression ignition” engines. As air is compressed, heat is generated. Diesel engines compress the air in the cylinders to the point where it is hot enough to ignite the diesel fuel when it is injected into the combustion chamber. Older diesel engines use a mechanical fuel injection system, while most modern diesel engines use electronically controlled fuel injection, which achieves many of the same benefits seen with gasoline engines.

Our current "spark ignition" gasoline engines have lower compression than diesel engines (about 1/2) and do not create enough heat from compression to ignite the gasoline when it is injected directly to the combustion chamber as diesel engines do. However, work is currently being done on compression ignition gasoline engines and we may see this technology sooner than later.

The Future of Fuel
As the price of crude oil continues to rise, we will see more and more research being done to find a way to produce gasoline and diesel from biomass, rather than crude oil. Production is still on a limited scale, but progress is being made, and we will hear more about it as advancements are made. A few Fleet operators may be using these biofuels already, but until we figure out a way to deliver them on a wide scale, you won’t be seeing them at your local gas station … but that is another story for another day.

Do you know anyone who is currently using biofuel in his or her everyday vehicle?

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