Fuel: gasoline components and their differences at the pump

Even though renewable energies are gaining ground, combustion engines continue to dominate the automotive sector, particularly in the field of motorsports. Since they run on gasoline, let's take a look at what this fuel sold at the pump actually is and what characterizes it.

This article is part of a series on fuels, which will be supplemented with additional information. See also the distinction between racing fuels and those used in road cars.

By definition, gasoline is a flammable liquid obtained by distilling petroleum and used to power internal combustion engines. (Source: Wikipedia) Gasoline, which came into being at the same time as the internal combustion engine, is a petroleum-derived mixture composed of many different substances. It is essentially a carefully balanced mixture of hydrocarbons and additives; the exact formulation may vary, but the basic components remain the same.

A sophisticated mixture of hydrocarbons and additives

• Alkanes: 20% to 30%
• Cycloalkanes: approximately 5%
• Alkenes: 30% to 45%
• Aromatic hydrocarbons: 30% to 45%

Gasoline is composed of alkanes, cycloalkanes, alkenes, and aromatic hydrocarbons. We won't delve into the chemical details, but focus on what matters for the engine. The principle of the internal combustion engine is simple: it needs a controlled explosion. This explosion is created by igniting a fuel-air mixture using a spark plug, with the compression of the cylinder facilitating the process. The details are not essential to our discussion. The fuel itself contains two particularly important hydrocarbon compounds: octane and heptane.

Octane

Octane is a well-known member of the alkane family. Its main characteristic is its high resistance to self-ignition, which means that it will not “detonate” on its own. High-octane gasoline therefore needs the spark from the spark plug to ignite, which protects the performance and longevity of the engine.

The higher the octane rating of the gasoline you use, the longer your engine will last.

Heptane

Heptane, another alkane, behaves in the opposite way: it ignites much more easily, even under the compression that occurs before the spark is fired. In short:

• Octane prevents premature ignition;
• heptane promotes it.

The octane rating tells you the proportion of each component. Unleaded 95 gasoline, for example, behaves like a mixture of approximately 95% octane (difficult to ignite) and 5% heptane (easy to ignite).

The octane rating corresponds to a fuel's tendency to resist knocking.

The trick is to mix these two components in the right proportions: enough octane for the engine to run smoothly, and just enough heptane for efficient combustion. Too much heptane, or not enough octane, can cause uncontrolled ignition, known as “knocking.” Knocking is an abnormal detonation that generates high thermal and mechanical stress, which can lead to fouling, excessive emissions, backfires, and even catastrophic engine damage, such as cracked pistons or broken connecting rods. Over time, these repeated stresses accelerate wear on the crankshaft and connecting rods. Typical ratings for French fuels:

The RON rating reflects engine performance under normal, everyday conditions, while the MON rating measures performance in more demanding situations: high revs, towing, or other high-load scenarios.

Note on diesel

Diesel fuels are not classified by octane rating because they operate on a different principle. Instead, they use a cetane rating, which measures how quickly the fuel ignites under compression. Diesel engines operate by self-ignition, not spark plugs, which is why a high cetane rating is desirable. Essentially, diesel engines are “compression ignition” engines: the air-fuel mixture is compressed to a temperature that triggers combustion without a spark. Supermarket gasoline vs. pump gasoline Romain Aubry, Technical and Partner Relations Manager at Total, explains the main challenge for fuels sold at gas stations: versatility. “Consumer fuel has to work in a wide variety of engines, from turbocharged engines and naturally aspirated cars to mopeds and scooters. And then you have a huge V12 that needs the same fuel. Gasoline sold at the pump has to be versatile, offering both performance and reliability, regardless of the engine that uses it. You can't turn a driver away because their car is too old or not powerful enough. That's simply unacceptable, so the fuel in the network must be suitable for all combustion engine vehicles on the road. When the subject of differences in gasoline quality between supermarket chains and major oil companies such as Total, Shell, or BP is raised, Romain Aubry calmly presents the reality of the market: “SP95 must be suitable for everyone and be identical at all gas stations. This means that the industry must establish clear specifications for network fuel, and everyone must comply with them. The idea that supermarkets offer a different product is just an urban myth. Networks do not own refineries, so they have to buy their fuel from refiners, which are the oil companies. As a result, the base fuel is essentially the same at most networks. The real difference lies in the additives added by each oil company, whether it's Total Excellium, Shell V-Power, BP Ultimate, etc. That's where the product differentiation lies.

Fuel is therefore the basis for the performance and reliability of a combustion engine. We strongly recommend that you use fuel with an octane rating at least equal to that recommended for your vehicle. A higher RON rating will not cause any harm, but a rating lower than that recommended may damage the engine.