Difference Between Leaded Petrol and Unleaded Petrol

Petrol is a product of petroleum. It is manufactured by a process called fractional distillation. Apart from petrol, petroleum is also used to produce kerosene and other plastic products. Petrol plays a pivotal role as a fuel in the automobile industry.


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Petrol is ignited in the engine through a spark plug. There are two types of petrol available – leaded petrol and unleaded petrol.

Leaded Petrol vs Unleaded Petrol

The difference between leaded petrol and unleaded petrol is that leaded petrol consists of tetraethyl lead additives while unleaded petrol does not contain any additives like tetraethyl lead. Leaded petrol does not consist of any aromatic additives while unleaded petrol consists of about 50% aromatic additives.

Leaded Petrol vs Unleaded Petrol 1

Leaded petrol is an obsolete option of fuel. It is no longer available in the fuel station. Leaded petrol was introduced into gasoline as it could raise the content of octane on the base gasoline. It could reduce the knocking of the engine.

The leaded petrol releases lead as a pollutant in the air. It also releases other harmful gases and causes pollution.

On the other hand, unleaded petrol was introduced as an alternative to leaded petrol. It contains aromatic additives. It is an accepted fuel and is used all over the world. It is expensive than unleaded petrol.

Comparison Table

Parameters of ComparisonLeaded petrolUnleaded petrol
Constituents Tetraethyl lead Benzene, naphthalene, trimethylbenzene, MTBB, and other compounds
Current usage It is not used currently It is used currently
Emissions It releases heavy metals like lead and other harmful gases It releases toxic gases like carbon monoxide, carbon dioxide, and others
Advantages It is inexpensive, can reduce knocking in the engine, and can increase the octane rating of the engine It does not release any harmful heavy metal like lead and does not pose serious health risks.
Disadvantages The heavy metals like lead which are emitted from the combustion of leaded petrol can be fatal for mankind It is expensive and causes pollution in the environment due to the aromatic additives

What is Leaded Petrol?

Leaded petrol is tetraethyl lead or TEL. It is an organolead compound. It was used in huge amounts in the 1920s, as it was found to increase the octane rating and allowed compression of the engine to rise substantially.

Leaded petrol could reduce knocking in the engine and thus gained popularity due to its antiknock effectiveness. The antiknock characteristics were found by research by General Motors.

The unique characteristics of leaded petrol made it an inexpensive yet effective fuel. The leaded petrol has a sweet pleasant odor. It has a density of 1.653 g per cubic centimeter.

The melting point of the fuel is -136°C while the boiling point is 84 – 85°C. The refractive index of the fuel is 1.5198 and has a structure of tetrahedral with a dipole moment of 0 D.

Leaded petrol has several advantages but numerous disadvantages too. Leaded petrol release leads into the air which is harmful to humans and animals. The lead released can poison the catalytic converters and this causes spark plug fouling.

There have been researches conducted on automotive fuel to evaluate its parameters.

From the 1970s, the awareness about leaded petrol grew in number and resulted in the decrease in usage of the same. Eventually, it was banned.

Later, in a study of 2011, it was found that after the ban on leaded petrol, there were 1.2 million fewer deaths. The UNEP (United Nations Environment Programme) has declared an official end to the usage of the same.

What is Unleaded Petrol?

Unleaded petrol refers to the fuel that does not contain or release any lead compounds. Most of the vehicles run on unleaded petrol. Unleaded petrol is a volatile flammable mixture of hydrocarbons.

The main constituents of hydrocarbons are heptane, hexane, octane, and others. Unleaded petrol is the basis of most modern internal combustion engines.

Unleaded petrol is also known as regular gas. It was introduced as an alternative to leaded gas as leaded gas was producing and emitting harmful and toxic substances.

People were discouraged from the use of leaded petrol, and later different tax rates were imposed to stop the mass from the usage of leaded petrol.

Eventually, leaded petrol was completely banned and unleaded petrol was used all over the world commercially.

The engines that run on unleaded petrol combine gas and air to compress and then ignite the engine. The ignition is usually done with a spark. The vehicles with unleaded petrol give moderate to high mileage.

The price of unleaded petrol fluctuates every day. Unleaded petrol does not contain any additive tetraethyl lead.

Unleaded petrol does not cause much harm to the environment and does not release any toxic heavy metal like lead. It does not pose any serious health risk and is widely available and accepted for fuel all over.

The unleaded petrol is also available with 10% bioethanol and is called E10 petrol, which has the least risk to the environment.

Main Differences Between Leaded Petrol and Unleaded Petrol

  1. Leaded petrol consists of lead additives while unleaded petrol consists of aromatic additives.
  2. Leaded petrol is banned from usage while unleaded petrol is accepted worldwide consumed as a fuel by the mass.
  3. Leaded petrol is hazardous for humans and animals while unleaded petrol does not pose any serious health problem to mankind.
  4. Leaded petrol release harmful gases and heavy metals and cause pollution while unleaded petrol does not release any major harmful gas comparatively.
  5. Leaded petrol releases lead into the air which can cause lead poisoning while unleaded petrol does not release any lead in the air.


  1. https://www.sciencedirect.com/science/article/pii/S0048969705004900
  2. https://www.scielosp.org/article/ssm/content/raw/?resource_ssm_path=/media/assets/bwho/v80n10/8010a02.pdf
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