Gasoline is a complex liquid mixture of hydrocarbons derived from crude oil refining. Its purpose is to act as a dense store of chemical energy for use in internal combustion engines. The process of burning gasoline, known as combustion, is a rapid, high-temperature chemical reaction. This reaction converts the fuel’s stored chemical energy into thermal energy and mechanical work.
What Initiates the Burning Process
Combustion requires the simultaneous presence of three components: the fuel source, an oxidizer, and a source of activation energy. In a gasoline engine, the fuel is the vaporized hydrocarbon mixture, and the oxidizer is oxygen (O2) drawn from the surrounding air. These two components must be mixed in a precise ratio to support the reaction effectively.
The reaction will not begin on its own at standard temperatures because the chemical bonds in the fuel molecules are relatively stable. An input of energy, known as the activation energy, is required to break these initial molecular bonds and start the process. This energy is provided by an intense heat source, such as the spark from a spark plug or the high temperature resulting from rapid compression.
Once the initial molecules break apart, the released heat energy acts as a catalyst, breaking the bonds of neighboring fuel molecules. This creates a self-sustaining chain reaction that rapidly spreads through the entire fuel-air mixture. The process is highly exothermic, meaning it releases a significant amount of energy, which manifests as heat and a massive expansion of gases.
The Chemistry of Complete Combustion
Complete combustion occurs when there is a sufficient supply of oxygen for the fuel to react fully. Every carbon atom bonds with two oxygen atoms, and every hydrogen atom bonds with one oxygen atom. The primary products of this chemical transformation are carbon dioxide (CO2) and water (H2O).
Using octane (C8H18) to represent gasoline, the reaction shows how components rearrange: 2C8H18 + 25O2 -> 16CO2 + 18H2O. The water is produced as a vapor due to the high temperatures generated during the burn.
The weak chemical bonds in the hydrocarbon molecules and the oxygen molecules are broken, requiring an initial energy input. However, the new bonds formed in the product molecules, CO2 and H2O, are much stronger. This difference in bond strength means that far more energy is released during the formation of the products than was absorbed to break the reactants, resulting in a net surplus of energy released as heat and pressure.
Why Real-World Burning Creates Pollution
Real-world conditions inside a car engine are rarely perfect, leading to incomplete combustion and the formation of unintended byproducts. The most common cause of incomplete burning is an insufficient supply of oxygen relative to the amount of fuel. When oxygen is deprived, the carbon atoms cannot fully oxidize to CO2.
Instead, the carbon forms Carbon Monoxide (CO), a colorless and odorless gas where each carbon atom bonds to only one oxygen atom. Under extreme oxygen deprivation, some carbon atoms fail to bond with oxygen and are emitted as pure carbon particulates, known as soot. Additionally, some fuel molecules may escape the process entirely, leading to the emission of unburnt hydrocarbons.
A separate pollution mechanism involves the air itself, which is approximately 78% nitrogen (N2). While nitrogen is inert at normal temperatures, the intense heat and pressure inside the combustion chamber, often exceeding \(1300^\circ\text{C}\), cause the atmospheric nitrogen and oxygen to react. This high-temperature reaction creates a family of compounds collectively referred to as nitrogen oxides (NOx), which includes Nitric Oxide (NO) and Nitrogen Dioxide (NO2).