Which anti-knock quality is required for SI and CI fuels?

High cetane for SI, high octane for CI

How does the structure of paraffins affect octane and cetane?

Branched chains give high octane; straight chains give high cetane

Straight chains give high octane

Branched chains give high cetane

What is the effect of aromatics on engine performance?

High octane but very low cetane

What is the drawback of olefins?

They are unstable and form gum in storage

Practice · 3+4 marks

Fuel requirements and structure of petroleum

Short answer

Good fuel must have high calorific value, proper volatility, anti-knock quality (high octane for SI, high cetane for CI), clean burning, and stability. Petroleum contains paraffins (straight chains give low octane/high cetane; branched give high octane), olefins, naphthenes, and aromatics (high octane, low cetane). Gasoline blends branched and aromatic hydrocarbons; diesel uses straight-chain paraffins.

List the basic requirements of a good fuel for internal combustion engines. Describe the chemical structure of petroleum fuels, naming the hydrocarbon families and their effect on engine performance.

Requirements of an engine fuel

High calorific value per unit mass and volume.
Easy vaporisation (SI fuel) or good atomisation and self-ignition (CI fuel).
Good anti-knock quality: high octane number for SI engines, high cetane number for CI engines.
Complete combustion with little deposit and low pollutant formation.
Chemically stable in storage, no gum formation.
Non-corrosive (low sulphur) and non-toxic as far as possible.
Low freezing point, suitable viscosity (diesel), safe flash point.
Cheap and easily available.

Chemical structure of petroleum

Petroleum is a mixture of hydrocarbons (C and H), with small amounts of S, N and O compounds. The main families are:

Family	General formula	Structure	Example	Effect on engine
Paraffins (alkanes)	C $_n$ H $_{2n+2}$	Straight or branched chain, saturated	n-heptane, iso-octane	Straight chains: low octane, high cetane. Branched chains: high octane
Olefins (alkenes)	C $_n$ H $_{2n}$	Chain with one double bond	Hexene	Good octane, but unstable (gum formation)
Naphthenes (cycloparaffins)	C $_n$ H $_{2n}$	Saturated ring	Cyclohexane	Medium octane and cetane
Aromatics	C $_n$ H $_{2n-6}$	Benzene ring	Benzene, toluene	High octane, very low cetane, smoky

Gasoline is mostly C $_5$ -C $_{12}$ paraffins, naphthenes and aromatics. Diesel is C $_{12}$ -C $_{20}$ , rich in straight-chain paraffins, which is why it ignites easily. Compact, branched and aromatic molecules resist auto-ignition, so they suit SI engines. Long straight chains break down easily, so they suit CI engines.

✓ Answer checked by Claude Opus 5.5 (high effort)

Requirements and types of SI combustion chambers · Internal Combustion Engines, chapter 5
Octane number and rating of SI fuels · Internal Combustion Engines, chapter 3
Engine performance parameters and efficiencies · Internal Combustion Engines, chapter 8
Cetane number and rating of CI fuels · Internal Combustion Engines, chapter 3
Efficiency ratio and specific fuel consumption · Internal Combustion Engines, chapter 8
Combustion of octane with excess air · Internal Combustion Engines, chapter 3
Requirements of SI and CI engine fuels · Automotive Technology, chapter 2
Phases of combustion in a CI engine · Automotive Technology, chapter 3
Gas turbine combustion chamber: requirements and types · Turbo Machines, chapter 3
Engine test: fuel measured in litres · Power Plant Equipment, chapter 5 · asked 2×

All Internal Combustion Engines chapter 3 questions →

Fuel requirements and structure of petroleum

Requirements of an engine fuel

Chemical structure of petroleum

Related questions