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Posted in: 16/07/2019

The difference between Regular Gas, Premium Gas and Diesel.

When it comes to cars and trucks, “premium” differences are pretty obvious. They’re the contrasts between cloth upholstery and leather, or between halogen headlights and LEDs. Under the hood, that’d be like comparing a mass-market four-cylinder engine to a hand-built V8. In the gas tank? The difference between “regular” and “premium” fuel is a lot less noticeable for most people.

That’s because for the 2018 model year, only about 25 vehicles require premium gasoline, according to their manufacturers. All other modern-day, gas-powered automobiles, even the ones that “recommend” premium fuel, can do just fine with the regular stuff.

We’ll find out why today, as well as look at how both types of gasoline are different from diesel fuel.

Service Station

What’s the Difference Between Regular Gas and Premium Gas?

You’re probably familiar with the buttons on a gas pump that select the grade of gasoline you want. The buttons generally show both the fuel type and an octane number. The octane number represents the only real difference between the grades.

The octane rating indicates how well a blend of gasoline resists “knocking.” Also known as detonation, knocking occurs during the compression cycle of an internal-combustion engine. It begins when a mixture of air and fuel enters an engine’s cylinder. The piston then moves up in the cylinder to squeeze that mixture together during compression. During the power stroke, the spark plug ignites the fuel-air mixture. This releases the energy needed to push the piston back down in the cylinder. Finally, the piston rotates around the crankshaft to come back up the cylinder, pushing out the exhaust gases.

The issue with regular gasoline is that the fuel-air mixture will combust from pressure alone if it’s being squeezed too much. When that happens, the combustion takes place when the piston is still rising, and the collision between the chemical reaction and the upward-moving piston makes the tell-tale “knocking” sound. It also can lead to serious engine damage. The key advantage for premium fuel is that it can withstand higher pressures in the cylinder before knocking occurs.

What Does an Octane Rating Really Mean?

Before we look at why you’d want a fuel that can stand up to those pressures, here’s a quick explanation of an octane rating.

To evaluate a specific gasoline, the industry runs it through a standard test engine to find out when it begins to start knocking. Scientists then compare that to the knocking properties of a reference fuel that’s made of two other chemical compounds, iso-octane and heptane. Regular 87 octane gas has the same knock-resisting properties as a mixture of 87 percent iso-octane and 13 percent heptane. Premium gasoline, with an octane rating of 91 for example, has the same anti-knock properties as a mix of 93 percent iso-octane and 7 percent heptane.

Gas Pump

Why Do Some Cars Require Premium Fuel?

Given the potential problems with higher compression ratios, you might wonder why some automakers would be using them. Well, the experts know that, in the right situation, raising the compression ratio can boost both performance and efficiency. A higher initial pressure just before combustion also increases the pressure driving the piston afterward. Further, because the piston moves up an extra amount to create the higher pressure, it can travel a little longer in the other direction. This delivers more power as well.

The bottom line is that all grades of gasoline contain the same amount of chemical energy. Using premium fuel, in an engine with a higher compression ratio, lets you turn more of that chemical energy into mechanical energy.

What’s the Difference Between Gasoline and Diesel Fuel?

It’s also worth noting that both gasoline and diesel fuel have the same amount of energy for the same amount of mass. This is partly because diesel and gasoline have the same chemical foundation. They’re both hydrocarbons made from hydrogen and carbon atoms connected in different combinations. A typical molecule of gasoline will have 20 hydrogen atoms and nine carbon atoms. For diesel, an expected recipe would have 23 hydrogen atoms and 12 carbon atoms.

The big difference for us is that diesel fuel is denser than gasoline. That means there’s more mass in the same volume. In the real world, you can think of it this way: One gallon of gasoline and one gallon of diesel will both fill the same amount of space. But the denser diesel fuel is about 14 percent heavier for the same gallon. As a result, one gallon of diesel has roughly 14 percent more energy than a gallon of gasoline.

Gas pump

Can I Use Different Types of Fuel for My Car?

You shouldn’t, at least not for most vehicles. The worst-case scenario is putting gasoline into a diesel engine or the other way around. Despite the chemical similarities between diesel fuel and gasoline, there are vital differences in the engines they power. Running the wrong fuel in the wrong one can quickly turn into an expensive repair bill. In fact, most experts say you shouldn’t even start your car in these situations. Get it towed to the repair shop instead. That can limit contamination to the fuel tank.

Mixing premium and regular gasoline is less of a concern. If your car requires premium gasoline, an accidental, one-time use of regular may cause some knocking. However, the advanced engine control units in today’s vehicles rely on high-tech anti-knock systems that can minimize problems. When the system’s sensors detect an issue, the spark-plug timing automatically is adjusted to reduce the chance of early combustion. Then, switching back to premium the next time should resolve the problem.

If an automaker only recommends premium fuel, filling up with higher-octane gas will generally have no effect at all. The same is true if your car is intended to run on regular gasoline. Remember, the single major benefit of premium gas is that it doesn’t knock under high pressure.

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