--- title: "Octane number and rating of SI fuels" subject: "Internal Combustion Engines" chapter: 3 key: "semester_6/internal_combustion_engines:octane-number-rating-si-fuels" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/octane-number-rating-si-fuels site: BCT Engineering --- # Octane number and rating of SI fuels Internal Combustion Engines · Chapter 3: Engine fuels - Asked in: year not given, 4+4 marks - Video: [Knocking in engine | Octane and Cetane Number, Antiknocking agents](https://www.youtube.com/watch?v=4xVxhK7acic) — Chemistry Scholars - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/octane-number-rating-si-fuels - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-3 ## Question What is meant by the octane number of a fuel? Explain how SI engine fuels are rated and describe the methods used to improve the anti-knock quality of petrol. Write short notes on the Highest Useful Compression Ratio (HUCR) and performance number. ## Short answer Octane number measures knock resistance as the volume % of iso-octane in a reference blend. SI fuels are rated in a CFR engine by RON (600 rpm) and MON (900 rpm) methods. Anti-knock quality is improved by adding aromatics, branched paraffins, oxygenates, or by refining. HUCR is the maximum usable compression ratio; performance number indicates knock-limited power above 100 ON. ## Answer #### Octane number The octane number (ON) is the percentage by volume of iso-octane (2,2,4-trimethylpentane) in a blend with n-heptane that gives the same knocking behaviour as the fuel under test in a standard engine. Iso-octane (very knock-resistant) is given ON = 100 and n-heptane (very knock-prone) ON = 0. A fuel of ON 90 behaves like a mixture of 90% iso-octane and 10% n-heptane. #### Rating method The fuel is run in a standard variable-compression-ratio **CFR engine**. The compression ratio is raised until a standard knock intensity is shown by a knockmeter. A blend of reference fuels is then prepared that gives the same knock at the same compression ratio. The blend percentage is the ON. | Method | Speed | Inlet temp | Use | |---|---|---|---| | Research (RON, ASTM D2699) | 600 rpm | 52 degrees C | Mild conditions, low speed | | Motor (MON, ASTM D2700) | 900 rpm | 149 degrees C | Severe conditions, high speed | MON is usually 8-10 units lower than RON. The anti-knock index is $(\text{RON} + \text{MON})/2$. #### Improving anti-knock quality - Blend high-octane components: aromatics, iso-paraffins, ethanol, MTBE. - Catalytic reforming and alkylation in the refinery. - Anti-knock additives: tetra-ethyl lead (TEL) with ethylene dibromide scavenger (now banned in most countries because of lead pollution); modern fuels use oxygenates and aromatics. #### Highest useful compression ratio (HUCR) It is the highest compression ratio at which a fuel can be used in a given engine, at the given speed and with the mixture and spark timing set for best power, without detectable knock. A higher HUCR gives higher efficiency. #### Performance number (PN) Used for fuels better than iso-octane (ON above 100). It is the ratio of the knock-limited power of the fuel to that of iso-octane, multiplied by 100. For example, a fuel with PN 130 allows 30% more knock-limited power than iso-octane. Approximately, $\text{ON} = 100 + (\text{PN} - 100)/3$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does an octane number of 90 mean? A. The fuel knocks like a blend of 90% iso-octane and 10% n-heptane B. The fuel contains 90% octane C. The fuel has 90% efficiency D. The fuel knocks like 90% n-heptane Answer: A. ON is the volume percent of iso-octane in a blend with n-heptane matching the fuel's knock. (Key point: Definition of octane number) 2. How do the RON and MON tests differ? A. RON is more severe than MON B. MON is run at higher speed and inlet temperature, and is usually lower C. They use the same conditions D. MON uses lower speed Answer: B. MON (900 rpm, 149 C) is severe, and about 8-10 units below RON (600 rpm, 52 C). (Key point: RON versus MON) 3. In the CFR engine method, what is adjusted to compare the fuel with the reference blends? A. Fuel injection timing only B. Engine bore C. Compression ratio until standard knock intensity is reached D. Valve lift Answer: C. The compression ratio is raised to a standard knock intensity, then a blend with the same knock is found. (Key point: Rating procedure) 4. What is the performance number used for? A. Diesel fuels B. Fuels worse than n-heptane C. Measuring ignition delay D. Fuels better than iso-octane (ON above 100) Answer: D. PN is the knock-limited power relative to iso-octane times 100; PN 130 means 30% more power than iso-octane. (Key point: Performance number) ## Related in Internal Combustion Engines - [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 - [IP, BP and efficiencies of a petrol engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aindicated-brake-power-efficiency-numerical) — Internal Combustion Engines, chapter 8 - [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 - [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 - [Stages of CI combustion and diesel knock](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Adiesel-combustion-stages-knock) — Internal Combustion Engines, chapter 5 - [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 - [Air flow and fuel flow of an SI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Afuel-consumption-air-flow-numerical) — Automotive Technology, chapter 2 - [Engine displacement, compression ratio and power](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aengine-displacement-bmep-numerical) — Automotive Technology, chapter 1 - [Knock and pre-ignition in SI engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aknock-and-pre-ignition-si-engine) — Automotive Technology, chapter 3