Lesson Note on Carburettors—Simple and Multi-Jet Caburettors: Operation Sketches for SS2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: Second Term
Week: 7
Age: 16 years
Duration: 45 minutes
Subject: Auto Mechanic Work
Curriculum Theme: Vehicle Systems
Previous Lesson: Properties of Fuel—Fuel Properties: Explanation and Demonstration.
Topic: CARBURETTORS—SIMPLE AND MULTI-JET CARBURETTORS.
Subject Matter: Meaning of CARBURETTORS—SIMPLE AND MULTI-JET CARBURETTORS (definition of carburettor), operation of simple carburettors (air-fuel mixing), operation of multi-jet carburettors (multiple fuel circuits).

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

• Define a carburettor.
• Explain the operation of a simple carburettor.
• Describe the operation of a multi-jet carburettor.

Affective Domain:

• Appreciate the importance of carburettors in internal combustion engines.
• Show interest in learning about the different types of carburettors.

Psychomotor Domain:

• Sketch and label the main parts of a simple carburettor.
• Identify the key components of a multi-jet carburettor from a diagram.

Social Domain:

• Collaborate with peers to discuss the functions of various carburettor components.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• Auto Mechanic Work for Senior Secondary Schools by S.A. Adebayo and F.O. Olaniyan
• https://www.autozone.com/diy/automotive-repair-how-to-guides/how-to-diagnose-and-fix-carburetor-problems
• https://www.britannica.com/technology/carburetor

Instructional Materials

The teacher will teach this lesson with the aid of:

• Live vehicle (if available)
• Charts showing simple and multi-jet carburettor diagrams
• Posters illustrating carburettor components
• Whiteboard and markers

Rationale for the Lesson

This lesson helps pupils understand how fuel and air are mixed in internal combustion engines before combustion. It enables them to grasp the fundamental principles of carburetion, which is important for diagnosing and maintaining older vehicle fuel systems.

Prerequisite/Previous Knowledge

Pupils have a basic understanding of internal combustion engines and the components of a vehicle’s fuel system.

Lesson Content/Board Summary

CARBURETTORS—SIMPLE AND MULTI-JET CARBURETTORS

Meaning of Carburettor

A carburettor is a device that mixes air and fuel in the correct ratio for combustion in an internal combustion engine. It atomizes the fuel into fine droplets and mixes them with air to create a combustible mixture.

Operation of Simple Carburettors

A simple carburettor operates on the principle of the venturi effect to create a low-pressure area, drawing fuel into the airstream.

Key components and their functions:

• Float Chamber: Maintains a constant fuel level for consistent fuel supply.
• Float and Needle Valve: Controls the fuel flow into the float chamber.
• Venturi: A constricted passage that increases air velocity and creates a low-pressure area.
• Main Jet: Controls the amount of fuel supplied to the venturi.
• Throttle Valve: Regulates the amount of air-fuel mixture entering the engine, thereby controlling engine speed.

Operating Principle: As air passes through the venturi, its velocity increases, and pressure drops. This low pressure draws fuel from the main jet, which then mixes with the air to form a combustible mixture. The throttle valve controls the quantity of this mixture supplied to the engine.

(Teacher to guide pupils in drawing a simple carburettor diagram showing these components).

Operation of Multi-Jet Carburettors

Multi-jet carburettors are designed to provide the correct air-fuel mixture across a wider range of engine speeds and loads, unlike simple carburettors which are less efficient at different operating conditions.

They incorporate multiple fuel circuits or jets that operate at different engine demands:

• Idle Circuit: Provides fuel for the engine when it is idling (low engine speed).
• Main Circuit: Supplies fuel during normal driving conditions (mid-range to high engine speed).
• Power Circuit (or Power Valve): Enriches the mixture for maximum power output during heavy acceleration.
• Accelerator Pump: Provides an extra squirt of fuel when the throttle is suddenly opened to prevent hesitation.
• Choke System: Enriches the mixture during cold starts by restricting air intake.

(Teacher to guide pupils in drawing or labeling a multi-jet carburettor diagram showing these circuits).

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher asks pupils what they remember about the fuel system in a vehicle and how fuel gets into the engine for combustion.
Pupils’ Activity: Pupils respond, recalling previous lessons on the fuel system.
Learning Point: Pupils connect prior knowledge to the new topic.

Step 2: Definition of Carburettor

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines a carburettor and explains its primary function in an internal combustion engine, writing the definition on the board.
Pupils’ Activity: Pupils listen attentively and copy the definition into their notebooks.
Learning Point: Pupils understand what a carburettor is.

Step 3: Components and Operation of a Simple Carburettor

Time: 7 minutes
Teaching Skill: Illustration/Explanation
Teacher’s Activity: The teacher uses charts and diagrams to explain the key components of a simple carburettor (float chamber, venturi, main jet, throttle valve) and demonstrates their functions.
Pupils’ Activity: Pupils observe the diagrams, listen to the explanations, and ask questions for clarity.
Learning Point: Pupils identify and understand the function of simple carburettor components.

Step 4: Sketching a Simple Carburettor

Time: 8 minutes
Teaching Skill: Demonstration/Guidance
Teacher’s Activity: The teacher sketches a simple carburettor on the board, labeling its parts, and guides pupils to copy and label the sketch correctly in their notebooks.
Pupils’ Activity: Pupils draw and label the simple carburettor in their notebooks, seeking assistance if needed.
Learning Point: Pupils can visually represent a simple carburettor and its parts.

Step 5: Operation of a Multi-Jet Carburettor

Time: 8 minutes
Teaching Skill: Explanation/Comparison
Teacher’s Activity: The teacher explains the need for multi-jet carburettors and describes how different circuits (idle, main, power, accelerator pump, choke) provide appropriate air-fuel mixtures for varying engine conditions, using charts.
Pupils’ Activity: Pupils listen to the explanation, compare it with the simple carburettor, and note down the key differences and additional circuits.
Learning Point: Pupils understand the advanced operation of multi-jet carburettors.

Step 6: Discussion on Real-World Application

Time: 5 minutes
Teaching Skill: Discussion/Application
Teacher’s Activity: The teacher facilitates a brief discussion on which types of vehicles still use carburettors and the advantages/disadvantages compared to modern fuel injection systems.
Pupils’ Activity: Pupils contribute to the discussion, sharing their observations and thoughts.
Learning Point: Pupils relate carburettor knowledge to real-world vehicle technology.

Step 7: Evaluation/Review

Time: 5 minutes
Teaching Skill: Questioning/Assessment
Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. What is a carburettor?
2. State two major components of a simple carburettor.
3. Explain briefly how a simple carburettor mixes air and fuel.
4. Mention two additional circuits found in a multi-jet carburettor that are not present in a simple one.

Pupils’ Activity: Pupils answer orally and in writing.
Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 3 minutes
Teaching Skill: Summarization/Assignment
Teacher’s Activity: The teacher summarizes the main points of the lesson, emphasizing the functions of simple and multi-jet carburettors. The teacher then assigns homework: “Draw and label a multi-jet carburettor, indicating its main circuits.”
Pupils’ Activity: Pupils listen to the summary and copy the homework assignment.
Learning Point: Pupils consolidate their learning and prepare for further study.

Lesson Keywords

• Carburettor – A device that mixes air and fuel for internal combustion engines.
• Venturi – A constricted passage in a carburettor that increases air speed and lowers pressure.
• Float Chamber – A reservoir in a carburettor that maintains a constant fuel level.
• Main Jet – A component that controls fuel flow into the carburettor’s main bore.
• Throttle Valve – A valve that regulates the amount of air-fuel mixture entering the engine.
• Multi-jet – Referring to carburettors with multiple fuel circuits for different engine conditions.

Differentiation

The teacher will provide additional diagrams and simplified explanations for pupils who may struggle with understanding the operational principles. More advanced pupils will be encouraged to research modern fuel injection systems and compare them with carburettors.

Note for teachers using this lesson plan

Emphasize the practical aspects of carburettors by using actual components or detailed visual aids. Encourage pupils to ask questions and relate the concepts to vehicles they are familiar with. Ensure clear, labeled diagrams are used and copied for better comprehension.