PetrolPetrol has undergone many changes over the last twenty years, both in terms of efficiency and environmental considerations. In the early 20th century, oil companies produced gasoline as a simple petroleum distillate, but the automobile engine was rapidly being improved and required a suitable fuel. The engines were modified to run on kerosene, however the kerosene-fueled engines "knocked" and broke the cylinder head and pistons. This then led to the long search for anti-knock agents, culminating in tetraethyl lead. Typical mid-1920s gasoline was 40-60 octane. The 1950s saw the start of increasing compression ratios, requiring higher octane fuels. Octane ratings, lead levels and vapor pressure increased, while sulfur content decreased. Small improvements were made to petrol formulations to improve yields and octane numbers until the 1970s, when unleaded fuels were introduced to protect the exhaust catalysts which were also introduced for environmental reasons. From 1970 to 1990, gasoline was slowly changed as lead was phased out, lead levels plummeted, and octane ratings initially dropped. With the passage of the U.S. Clean Air Act of 1990, significant changes in the composition of gasoline began to be required by the industry to minimize this major source of pollution, and these global changes are expected to continue well into the 21st century. Pre-used hydrocarbons have been used sparingly in gasoline since the 1970s for their anti-knock properties. Today they are increasingly added to petrol because research seems to indicate that they contribute to reducing the tendency of exhaust gases to form smog by reducing the reactivity of emissions; so maki...... middle of paper ......ng sulfur oxides, which damage the lungs and contribute to the formation of acid rain. The personal automobile is still considered the world's largest air polluter today, but provisions of the Clean Air Act of 1990 continually and systematically reduce vehicle emissions. The oil industry and car manufacturers are tasked with "cleaning up their act" and hopefully the world will become a healthier and less polluted place to live in the future because of all of this! We no longer burn kerosene or lead in our cars, but we still haven't found the ideal fuel we need. Emission control systems are becoming more effective and accurate at what they do. However, this is only a temporary solution and not the definitive answer. I believe perfect combustion is a realistic and achievable goal for this new century.