---
title: "Simple carburettor and its limitations"
subject: "Internal Combustion Engines"
chapter: 4
key: "semester_6/internal_combustion_engines:simple-carburettor-working-compensation"
source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/simple-carburettor-working-compensation
site: BCT Engineering
---

# Simple carburettor and its limitations

Internal Combustion Engines · Chapter 4: Carburetor & Fuel Injection systems

- Asked in: year not given, 8 marks
- Video: [Simple Carburetor](https://www.youtube.com/watch?v=VtPOJAqc6Lc) — BEST MECHANICAL ENGINEERING
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/simple-carburettor-working-compensation
- Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-4

## Question

With a neat sketch explain the construction and working of a simple carburettor. State why a simple carburettor cannot supply the correct mixture at all loads, and name the devices that are added to correct it.

## Short answer

A simple carburettor atomizes fuel via a venturi depression; the mixture becomes richer as throttle opens because air density changes. It cannot provide correct mixture at all loads. Compensation devices include choke, idling system, compensating jet/air bleed, economizer, and accelerator pump.

## Answer

#### Function
The carburettor atomises petrol and mixes it with air in the correct ratio (about 15:1 for cruising) for the SI engine under all running conditions.

#### Construction
```text
        air in
          |
     [ choke ]
          |
     \         /
      \ venturi/<-- fuel nozzle <-- jet <-- float chamber
       \  __  /                         (float + needle)
        |    |
     [throttle]
          |
     to inlet manifold
```

Main parts:
- **Float chamber** with float and needle valve: keeps fuel at a constant level slightly below the nozzle tip.
- **Metering jet** and **discharge nozzle** projecting into the venturi throat.
- **Venturi (choke tube):** narrowing that raises the air velocity and creates a depression at the throat.
- **Throttle valve:** butterfly valve after the venturi controlling the mixture flow.
- **Choke valve** before the venturi for cold starting.

#### Working
During suction the piston draws air through the venturi. At the throat the velocity is high and the pressure falls below atmospheric pressure. The float chamber is at atmospheric pressure, so the pressure difference forces petrol through the jet and nozzle. It is broken into fine droplets and mixed with the air. The throttle controls the quantity of mixture and therefore the power.

Flow of air and fuel (neglecting the nozzle lip):

$$
\dot{m}_a = C_{da} A_t \sqrt{2\rho_a \Delta p}, \qquad \dot{m}_f = C_{df} A_f \sqrt{2\rho_f \Delta p}
$$

Both flows depend on $\sqrt{\Delta p}$, but the air density changes with the throat pressure while the fuel is incompressible, and the discharge coefficients change with flow. As a result, the simple carburettor supplies a mixture which gets **richer as the throttle opens**.

#### Why it fails and the remedies
| Condition | Need | Device |
|---|---|---|
| Cold start | Very rich (about 9:1) | Choke valve |
| Idling | Rich, 10-12:1 | Idling jet and idling port |
| Cruising (part load) | Lean 15-17:1 | Compensating jet / air bleed / emulsion tube |
| Full load, high speed | Rich 12-13:1 | Economiser or power valve |
| Sudden acceleration | Extra fuel | Accelerator pump |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does fuel flow from the nozzle in a simple carburettor?
   A. The venturi throat pressure is below the atmospheric pressure in the float chamber
   B. The fuel pump pushes it
   C. The float chamber is below atmospheric pressure
   D. Gravity alone
   Answer: A. High velocity at the throat lowers pressure, and the pressure difference forces petrol through the jet. (Key point: Venturi action)

2. What is the function of the float chamber and needle valve?
   A. Atomise fuel
   B. Keep the fuel at a constant level just below the nozzle tip
   C. Control air flow
   D. Provide cold start enrichment
   Answer: B. A constant level ensures a steady head for metering and prevents overflow. (Key point: Float chamber)

3. What happens to the mixture of a simple carburettor as the throttle opens?
   A. It becomes leaner
   B. It stays constant
   C. It becomes richer
   D. It becomes zero
   Answer: C. Air density falls with the throat pressure while fuel is incompressible, so the fuel flow grows relatively more. (Key point: Why simple carburettor fails)

4. Which device gives extra fuel for sudden acceleration?
   A. Choke
   B. Idling jet
   C. Compensating jet
   D. Accelerator pump
   Answer: D. The accelerator pump adds extra fuel when the throttle is opened quickly; the choke is for cold start. (Key point: Corrective devices)

## Related in Internal Combustion Engines

- [Jet diameter of a simple carburettor](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Asimple-carburettor-jet-size-numerical) — Internal Combustion Engines, chapter 4

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