The first thing every up-and-coming trucker should know is that their semi-truck takes diesel fuel, not gasoline. What even some experienced truckers may not know are the nuances between diesel and gasoline, and why there is a distinction. Here, we go over the differences between the two.
Back in the 19th century, when automobiles being a standard part of American life was as much of a fantasy as fully autonomous vehicles are today, gasoline was considered a waste product. People would refine petroleum to create kerosene for lighting, and discard the gasoline made from the process as worthless. It was John D. Rockefeller, one of the richest people in human history, who made a good part of his fortune on using this abundant “waste” to create a plethora of products. While Rockefeller proved the usefulness of gasoline to the public, it was not until 1895 that Charles Duryea patented the gasoline-powered automobile.
Diesel has a very similar history to gasoline, it was considered a waste byproduct of producing kerosene by refining petroleum, but it was not used for other products like gasoline was. In the early 1890s, an inventor named Rudolf Diesel invented the diesel engine. Originally used to fuel ships, it was not until 1923 that it was first used in automobiles.
Gasoline and diesel fuel come from the same crude oil! When refining a 42-gallon barrel of crude oil, refineries can expect to get about 19 gallons of gasoline and 11 gallons of diesel (the rest is other petroleum products such as kerosene and butane).
Refineries boil crude oil to extract the different fuel based on the temperatures at which they separate from the crude mixture. Gasoline hydrocarbons separate from the crude at ranges from 85 to 185 degrees Fahrenheit. In contrast, diesel hydrocarbons separate from the crude mixture (which is now free of gasoline, kerosene, and butane, to name a few) at a range of 350 to 450 degrees Fahrenheit.
Diesel fuel separated from crude oil at a much higher temperature because the hydrocarbons that form diesel are much larger. This increased molecular size also makes it more energy-dense. A gallon of diesel fuel has 40.7 kilowatt-hours (kWh) of energy, whereas gasoline has 36.6 kilowatt-hours of energy.
If gasoline has less energy per gallon than diesel, why don’t all vehicles just use diesel to obtain a better MPG? While diesel has 8.8% more energy per gallon, as stated above, crude oil tends to have 72% more gasoline than diesel. Doing the math, a 42-gallon barrel of crude oil, on average, will have 695.4 kWh of gasoline and 448.8 kWh of diesel.
At the same time, gasoline is lighter than diesel. Gasoline weighs about six pounds per gallon, whereas diesel weighs about seven and a half pounds per gallon. This makes gasoline on a more energy-dense than diesel on a weight basis, whereas diesel is more energy-dense on a volume basis.
This is part of the reason why smaller cars such as family sedans are better off using gasoline, as there is simply not as much room for a fuel tank than there is on a massive semi-truck. With big rigs, they can pile on dozens of gallons of diesel with almost no problem, barring going over the weight limit of DOT regulations. In any case, volume is hardly an issue for an 18-wheeler.
Both gasoline and diesel engines use combustion to turn fuel into motion. There are some slight nuances in differences that make the two different, however.
Gasoline engines use spark plugs. As gasoline and oxygen mix in the chamber, the spark plug sends an electric current to the mixture to make it ignite and kickstart combustion.
Diesel engines use Boyle’s Law to get a similar effect without a spark plug ignition. Boyle’s Law essentially states that temperature, pressure, and volume are all linked. When the diesel engine takes in air and then decreases its volume by compressing it, its pressure and temperature both rise. The diesel engine compresses the air so much that when the fuel is introduced into the system, it ignites from the heat of the compressed air. Diesel #2, the type used in semi-trucks, has an autoignition temperature of 489 to 545 degrees Fahrenheit. At this high heat, no sparks are needed to create combustion.
Diesel and gasoline emit the same types of pollution, in different quantities.
Both produce similar amounts of carbon dioxide, the most common greenhouse gas, but diesel produces less carbon monoxide than gasoline engines while also producing more sulfur dioxide. Carbon monoxide is directly lethal to humans when inhaled in increased concentrations, while sulfur dioxide is the primary cause of acid rain.
As a result, gasoline engines are more harmful to humans, whereas diesel exhaust can cause more damage to the environment as a whole, as sulfates can increase the acidity of water and more directly impact other organisms such as trees and fish.
Sulfur content in diesel fuel has made great strides in reduction during the past three decades; thanks to the EPA phasing in regulations over time, the sulfur content in diesel fuel has been reduced to 0.3% of what it was before 1993, 15 parts per million compared to 5,000 parts per million.
While diesel and gasoline have a lot in common, there are specific nuances to each.
- Both were considered worthless byproducts of refining crude oil into kerosene, but the gasoline-powered car came into existence almost thirty years before the diesel engine, originally used in marine machines, was put into an automobile.
- Both come from the same crude oil but separate at different temperatures.
- Diesel has more energy per gallon; gasoline has more energy per pound.
- Both engines use combustion, but gasoline engines use sparks while diesel engines use high temperatures to start the combustion.
- Gasoline produces more carbon monoxide, diesel produces more sulfur dioxide.
If you are still confused about the difference between the two, you can always take solace in knowing that a diesel nozzle will not fit into a gasoline tank’s entrance. Just make sure not to make the opposite mistake.
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