--- title: "Fuel requirements and structure of petroleum" subject: "Internal Combustion Engines" chapter: 3 key: "semester_6/internal_combustion_engines:fuel-requirements-petroleum-structure" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/fuel-requirements-petroleum-structure site: BCT Engineering --- # Fuel requirements and structure of petroleum Internal Combustion Engines · Chapter 3: Engine fuels - Asked in: year not given, 3+4 marks - Video: [Gasoline (Motor Spirit explanation) Fuels for Internal Combustion Engine](https://www.youtube.com/watch?v=kyKL_TRLDaQ) — Jaspreet Hira Engineering Topics - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/fuel-requirements-petroleum-structure - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-3 ## Question 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. ## 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. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which anti-knock quality is required for SI and CI fuels? A. High octane for SI, high cetane for CI B. High cetane for SI, high octane for CI C. High octane for both D. High cetane for both Answer: A. SI fuels must resist auto-ignition, while CI fuels must self-ignite readily. (Key point: Anti-knock rating) 2. How does the structure of paraffins affect octane and cetane? A. Straight chains give high octane B. Branched chains give high octane; straight chains give high cetane C. Branched chains give high cetane D. No effect Answer: B. Branched molecules resist auto-ignition, while long straight chains break down easily. (Key point: Paraffin structure) 3. What is the effect of aromatics on engine performance? A. Low octane and high cetane B. High cetane and clean burning C. High octane but very low cetane D. Unstable gum formation Answer: C. Aromatics suit SI engines but ignite poorly in CI engines and tend to smoke. (Key point: Aromatics) 4. What is the drawback of olefins? A. They have very low octane B. They have no double bond C. They are saturated rings D. They are unstable and form gum in storage Answer: D. Olefins have good octane but their double bond makes them chemically unstable. (Key point: Olefin drawback) ## Related in Internal Combustion Engines - [Requirements and types of SI combustion chambers](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Acombustion-chamber-requirements-types) — Internal Combustion Engines, chapter 5 - [Octane number and rating of SI fuels](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aoctane-number-rating-si-fuels) — Internal Combustion Engines, chapter 3 - [Engine performance parameters and efficiencies](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aengine-performance-parameters-definitions) — Internal Combustion Engines, chapter 8 - [Cetane number and rating of CI fuels](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Acetane-number-diesel-fuel-rating) — Internal Combustion Engines, chapter 3 - [Efficiency ratio and specific fuel consumption](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Arelative-efficiency-sfc-numerical) — Internal Combustion Engines, chapter 8 - [Combustion of octane with excess air](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aoctane-combustion-air-fuel-numerical) — Internal Combustion Engines, chapter 3 ## Related in other subjects - [Requirements of SI and CI engine fuels](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Asi-engine-fuel-requirements) — Automotive Technology, chapter 2 - [Phases of combustion in a CI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Acombustion-ci-engine-phases) — Automotive Technology, chapter 3 - [Gas turbine combustion chamber: requirements and types](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Acombustion-chamber-requirements-types) — Turbo Machines, chapter 3 - [Engine test: fuel measured in litres](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aengine-test-fuel-litres) — Power Plant Equipment, chapter 5, asked 2×