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THE CHEMISTRY OF PETROL & DIESELThere's a lot behind the fuel we put in our cars – in this graphic, we take a look at the differences between diesel, leaded petrol, and unleaded petrol.

LEADED UNLEADED DIESEL

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PETROL & DIESEL – THE DIFFERENCE

DIESEL

PETROL

250 – 300 ˚C

35 – 200 ˚C

OCTANE RATINGS & KNOCKING

HOW ENGINES WORK LEADED & UNLEADED PETROL

Petrol and diesel are both obtained by fractional distillation of crude oil. However, they differ in their

composition. Diesel is a fraction of crude oil that is removed at a higher boiling point, and contains a larger amount of energy per litre, meaning more

miles can be covered with the same volume of fuel.

Knocking occurs when fuel's combustion doesn't occur in sync with the engine cycle. This causes lower engine efficiency and engine damage. Octane ratings gauge how well fuel avoids this problem, with higher values indicating less knocking. Isooctane (100) and

n-heptane (0) are used as references.

n-HEPTANE ISOOCTANE

OCTANE RATING: 0 OCTANE RATING: 100

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Compounds can be added to petrol to boost its octane rating. Tetraethyl lead was one of these, but is now banned in most countries as it releases toxic

lead fumes. Alternative anti-knocking agents used in unleaded petrol include methyl tertiary-butyl ether

(MTBE), ethanol, benzene, and toluene.

OCH3

TETRAETHYL LEAD MTBE BENZENE

10-15 CARBONS

5-12 CARBONS

36.9 MJPER LITRE

33.7 MJPER LITRE

INTAKE1

COMPRESSION2

COMBUSTION3

EXHAUST4

AIR, FUEL INJECTED (PETROL ENGINES)

FUEL INJECTED (DIESEL ENGINES)

HIGHER COMPRESSION WITH DIESEL

WASTE GASES FROM COMBUSTION

1

SPARK PLUG(petrol engine only)

AIR IN

FUEL IN(petrol only)

2 3 4

FUEL IN(diesel)