--- title: "Notes and summary: carburettor" subject: "Communication English" chapter: 1 key: "semester_5/communication_english:notes-summary-carburettor" source: https://www.bctengineer.com/q/semester_5/communication_english/notes-summary-carburettor site: BCT Engineering --- # Notes and summary: carburettor Communication English · Chapter 1: Intensive Reading - Asked in: 2068 Chaitra, 5+5 marks - Video: [CARBURETORS | How They Work](https://www.youtube.com/watch?v=1ohL6KgasjA) — Donut - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/communication_english/notes-summary-carburettor - Whole chapter: https://www.bctengineer.com/md/semester_5/communication_english/chapter-1 ## Question After taking notes, write a summary of the following passage. Since it is essential to secure rapid and complete combustion in the cylinder of an internal combustion engine, the fuel and air mixture must be thoroughly mixed; and further, it must be in the correct proportions for all running conditions of the engine. [passage of about 480 words on the working of a carburettor: jet, venturi, float chamber, throttle valve, compensating jet and idler jet] ... This jet will only function when the throttle is nearly closed. When it is opened for faster running, the suction round the edge of the throttle decreases, and the idler automatically ceases to act. ## Short answer A carburettor uses a venturi to draw petrol from a jet, with a float chamber maintaining fuel level. A compensating jet prevents over-rich mixture at high speed, and an idler jet supplies a richer mixture for starting and idling when the throttle is nearly closed. ## Answer #### Notes **Title: Working of a carburettor** 1. **Purpose**: for rapid, complete combustion, fuel and air must be well mixed in correct proportions for all running conditions 2. **Simple carburettor** - Air passes through a venturi (narrow throat) in the intake manifold; pressure falls - Suction draws petrol from a jet and sprays it into the air - Float chamber (float and needle valve) keeps petrol level constant - Throttle valve (controlled by accelerator pedal) regulates amount of mixture 3. **Problems of single jet** - At high speed, petrol drawn rises faster than air flow, so the mixture becomes too rich 4. **Compensating jet** - Second jet fed from a well open to the atmosphere - Supplies petrol quickly at first, then less as the well empties; main jet gives more as speed rises 5. **Starting / idling** - Throttle almost closed: suction at main jet too low - Idler jet near throttle valve works only when throttle is nearly closed; stops when throttle opens #### Summary A carburettor mixes petrol and air in the correct proportions for combustion. Air flows through a venturi, where reduced pressure draws petrol from a jet fed from a float chamber, and a throttle valve controls the amount of mixture. A single jet gives a mixture that becomes too rich at high speed, so a compensating jet fed from a well is added to keep the mixture correct. For starting and slow running, when the throttle is almost closed, an idler jet near the throttle valve supplies the richer mixture and stops acting when the throttle is opened. *(About 110 words.)* ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the venturi help the carburettor draw petrol? A. The narrow throat heats the air so that petrol evaporates B. It stores petrol at constant level C. It controls the amount of mixture reaching the cylinder D. Air speeds up in the narrow throat, pressure falls, and suction draws petrol from the jet Answer: D. Air flowing through the narrow throat has reduced pressure, and the suction draws petrol out of the jet. (Key point: Venturi) 2. What problem arises with a single jet at high speed? A. The mixture becomes too weak B. The mixture becomes too rich C. The petrol stops flowing D. The float chamber overflows Answer: B. At high speed the petrol drawn increases faster than the air flow, so the mixture becomes too rich. (Key point: Problem of single jet) 3. How does the compensating jet correct the mixture? A. It closes the throttle at high speed B. It sprays extra petrol as speed rises C. It keeps the petrol level constant in the float chamber D. It is fed from a well open to the atmosphere, giving petrol quickly at first and less as the well empties Answer: D. The well supplies petrol quickly at first, then less, balancing the main jet which gives more as speed rises. (Key point: Compensating jet) 4. When does the idler jet act? A. Only at full throttle B. Only when the throttle is nearly closed, as in starting and slow running C. At all speeds equally D. Only when the float chamber is empty Answer: B. Suction at the main jet is too low when the throttle is nearly closed; the idler jet near the throttle supplies fuel then and stops when the throttle opens. 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