--- title: "Air-fuel mixture needs at different operating conditions" subject: "Automotive Technology" chapter: 2 key: "semester_5/automotive_technology:air-fuel-mixture-requirements" source: https://www.bctengineer.com/q/semester_5/automotive_technology/air-fuel-mixture-requirements site: BCT Engineering --- # Air-fuel mixture needs at different operating conditions Automotive Technology · Chapter 2: Fuel Supply Systems - Asked in: year not given, 6 marks - Video: [Mixture Requirements at Different Condition (Loads and Speeds) Thermodynamics-2 Chapter-2 4 sem. ME](https://www.youtube.com/watch?v=2RpYPP1w7Fc) — Study With Promila Mam - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/automotive_technology/air-fuel-mixture-requirements - Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-2 ## Question Explain why an SI engine needs different air-fuel mixture strengths for cold starting, idling, cruising and full-load (acceleration). State the approximate air-fuel ratio in each case and how a carburettor or injection system provides it. ## Short answer Cold start and idling require very rich mixtures (2:1 to 12:1) to overcome weak vaporisation and residual gas. Cruising uses a lean mixture (15:1-17:1) for economy; full load and acceleration need a rich mixture (~12.5:1) for maximum power. A carburettor uses choke, idle and main circuits plus a power valve and accelerator pump; EFI varies injector pulse width. ## Answer The **stoichiometric** air-fuel ratio (AFR) of petrol is about 14.7:1 by mass ($\lambda = 1$). The engine's demand varies, so the mixture strength must change with the operating condition. #### Requirements | Condition | AFR (approx.) | Mixture | Reason | |---|---|---|---| | Cold start | 2:1 to 5:1 (supplied) | Very rich | Little fuel vaporises on cold walls; only the light ends burn | | Idling | 10:1 to 12:1 | Rich | Throttle nearly closed, high residual gas fraction, poor mixing | | Part-load cruising | 15:1 to 17:1 | Lean | Best fuel economy, throttle partly open | | Full load / maximum power | 12:1 to 13.5:1 (about 12.5:1) | Rich | Gives maximum power; excess fuel cools the charge | | Acceleration | Momentarily rich | Rich | Throttle opens suddenly and the fuel lags behind the air | Maximum **economy** occurs near 16:1 to 17:1 (lean) and maximum **power** near 12.5:1 (rich). Engines with a three-way catalyst are held at 14.7:1 during cruise. #### How the mixture is provided **Carburettor** - The *choke* enriches the mixture for starting. - The *idle jet* supplies a rich mixture below the throttle plate. - The *main jet with compensating jet* gives a nearly constant lean mixture for cruising. - The *power (economiser) valve* enriches the mixture at full load. - The *accelerator pump* gives an extra squirt of fuel when the throttle is opened suddenly. **Electronic fuel injection** - The ECU reads the coolant temperature, throttle position, mass airflow and lambda sensor signals. - It lengthens the injector pulse width for cold start, idle and acceleration, and trims the mixture to 14.7:1 in closed loop at cruise. - At full load it switches to open loop and gives a rich mixture. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the stoichiometric air-fuel ratio of petrol? A. About 10:1 B. About 12.5:1 C. About 14.7:1 D. About 17:1 Answer: C. Stoichiometric AFR (lambda = 1) is about 14.7:1 by mass. (Key point: Stoichiometric AFR) 2. Which mixture gives best fuel economy at part-load cruising? A. Rich, 10:1 to 12:1 B. Very rich, 2:1 to 5:1 C. Lean, 15:1 to 17:1 D. Rich, 12:1 to 13.5:1 Answer: C. Economy is best with a lean mixture near 15:1 to 17:1 at part throttle. (Key point: Cruising mixture) 3. Why does a cold engine need a very rich mixture supplied? A. Air is denser when cold so extra fuel is needed to match B. Little fuel vaporises on cold walls and only light ends burn C. The throttle is wide open D. The catalyst needs it Answer: B. Cold walls prevent vaporisation, so a very rich mixture is supplied for enough vapour to ignite. (Key point: Cold start enrichment) 4. What does the accelerator pump of a carburettor do? A. Weakens the mixture at cruise B. Gives an extra squirt of fuel when the throttle opens suddenly C. Enriches the mixture for cold starting D. Meters fuel at idle Answer: B. On sudden throttle opening, air rushes in while fuel lags; the accelerator pump supplies the extra fuel. (Key point: Providing the mixture) 5. How does EFI trim the mixture to 14.7:1 at cruise? A. Choke plate B. Closed-loop control using the lambda sensor C. Power valve D. Fixed jet size Answer: B. The ECU adjusts injector pulse width using the lambda sensor feedback in closed loop. 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