Diesel 101: What's the Difference Between Diesel and Gas

Vince Lombardi, the famed Green Bay Packers coach, began each year's training camp with the same opening line. "Gentlemen, this is a football."

The point he made was that you are never too old or experienced to be reminded of the fundamentals now and then. How to hold a football, how to run with it, how to recover a loose ball... it's all pretty basic, but Lombardi believed this annual ritual was essential for success.

That anecdote is what came to mind when I saw the Florida News Times story this past weekend, What Is the Difference Between Diesel and Gas? It's not a clickbait title, just a straight up presentation of basic but useful information. "Just the facts, ma'am."

The story has a great opening line: "Did you know that diesel can give you 12% more energy per gallon in comparison to gasoline?" 

There are probably a lot of people who don't know that, so it serves as a nice attention getter. The question suggests that if you read this article maybe there will be other things you didn't know. Here's an overview of this piece, along with a link at the end so you can read it in its entirety.

The first decision most people make is deciding what type of vehicle they want, whether a car, sedan or truck. The next question ought to be whether to go diesel or gas power. To make this decision it helps to know how internal combustion engines work, whether gasoline powered or diesel.

Air enters the engine and mixes with fuel where it is compressed in the combustion chamber (cylinders) and ignited. The explosion pushes the piston and crankshaft which turns the gearbox. When the piston returns to its starting position the spent gases get pushed out into the exhaust system.

The difference between an engine powered by gasoline and a diesel engine takes place at the point of ignition. In a gasoline engine a spark ignites the fuel. In a diesel engine there are no spark plugs. Rather, ignition occurs by means of compression of the air-fuel mixture. High compression produces heat which generates spontaneous ignition.

Because diesel fuel produces more energy per gallon, it is more cost efficient. For this reason the engine of choice for people who do a lot of highway travel is often diesel. They also have more torque, so they accelerate faster.

The article notes that not all diesel fuel is created equal, however. Black diesel, biodiesel and other diesel fuels may detract from performance.

Some people are unaware of the differences between the two kinds of engines. Diesel engines are more durable and last longer. They used to be quite a bit heavier, but new manufacturing techniques and materials have addressed this. Because they have fewer parts, diesel engines have fewer components that can fail, which translates into fewer repairs.

Two primary drawbacks for diesel engines used to be how noisy they were and the black smoke they belched. These issues have also been addressed and are no longer the issues they once were.

Fuel Efficiency
Diesel tend to offer better fuel efficiency. This is due, in part, to the manner in which diesel engines work. Gasoline engines have a lower compression ratio in order to avoid self-ignition. In diesels, compression stroke only compresses the air because there is no fuel injected during the intake stroke. The greater compression ratio enables greater fuel efficiency.

Because of the different systems for combustion, the fuels are different. Both originate as crude oil extracted from the ground. The difference is in the refining. Diesel fuel is heavier and has a higher energy density. It also takes longer to evaporate.

Biodiesel is different from the diesel fuel refined from crude. It's important to know that diesel and biodiesel are not interchangeable. If you plan to use biodiesel, there are some changes necessary. The author doesn't detail these, so you will have to consult your mechanic.

The author ends his story by repeating the earlier assertion that diesel engines are more reliable. He adds that when things do go wrong they are usually not as expensive to fix. The article ends with a question: Are you ready to make the shift?

Ironically, right after reading this article I saw another about a diesel owner named Carl who did make a mistake and it proved very expensive. The title of the story, which you can read here, is Anyone Can Make A Mistake.

And what was the mistake Carl made? On a camping vacation, after a long day's drive, he pulled into a filling station before heading to the campground. Whether from being tired, or just the habit of filling the tank without thinking about much, he absent-mindedly filled up with 87 octane gasoline instead of diesel fuel. Oops.

An expensive mistake indeed. According to the article the final tab to fix this fiasco was $22,000. Fortunately he had good insurance, which covered most of the cost. You can be sure that this was not the kind of drama he needed for his vacation.

You can read the Florida News Times story here.