Combustion
The alkane components of gasoline are kinetically stable in the presence of oxygen, however, they are unstable with respect to their oxidation products. Therefore, they will react with oxygen when the sufficient activation energy is supplied, usually in the form of an ignition spark.
The combustion process occurs via a free-radical mechanism, yielding water and carbon dioxide. However, if the oxygen supply is limited then carbon monoxide or soot may be produced.
As the chain length increases, more energy is required to break the carbon-hydrogen bonds, however, the formation of the strong carbonyl bonds in carbon dioxide and hydrogen-oxygen bonds in water releases even more energy hence the reaction is exothermic overall. As the chain length increases, the amount of energy released and hence enthalpy increases steadily.
The common petrol engine used in cars and other domestic machines works entirely on the principle of the combustion process.
Photo
courtesy
DaimlerChrysler
A mixture of gasoline vapour and air is compressed into a small volume inside each of the cylinders and then ignited by an electrical spark from a spark plug allowing combustion to occur. The explosion obviously creates carbon dioxide increasing the pressure within the cylinder hence driving the piston down.
The sequence of operations for a single four-stroke combustion cycle are outlined below:
1) The intake valves open and the piston moves down allowing for the intake of air and gasoline known as the intake stroke.
2) Compression of the air/fuel mixture when the piston moves back up increases the efficiency of the combustion and hence makes the explosion more effective.
3) At the top of the intake stroke a spark is emitted by the spark plug causing the explosion driving the piston down causing the exhaust values to open allowing the exhaust gases to be released.
This cycle repeats itself and the linear motion created by the piston is converted into rotational motion by the crank shaft as is shown in the animation below.
Animation
courtesy of How Stuff Works Inc.
Knocking:
This is a problem associated with premature ignition of the gasoline-air mixture, which is common among gasoline mixtures rich in straight-chain alkanes since these ignite rapidly, therefore, a solution to this is to use branched hydrocarbons such as 2,3,4-trimethylpentane: