Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Type of starter motors
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Type of starter motors
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Subject: Auto Electrical Works
Class: Senior Secondary 1
Term: 2nd Term
Week: 3
Theme: Starting System
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This topic introduces students to the various designs of starter motors commonly found in vehicles and generator sets in Nigeria. Understanding the different types and their operational principles is fundamental for auto electrical technicians, enabling them to diagnose faults, perform maintenance, and replace components effectively. This knowledge is crucial for vocational competence and contributes to safe and efficient vehicle operation, directly impacting the transportation sector and power generation in local communities. Performance Objectives (Learner-friendly language):
(e.g., when the engine starts), preventing the engine from driving the starter motor.
Method of Operation (Axial Engagement):
1. Activation: When the ignition key is turned to the "start" position, a small current flows to the starter solenoid's pull-in and hold-in coils.
2. Solenoid Engagement (First Stage): The energized solenoid pulls its plunger inwards. This plunger, via the shift fork, pushes the pinion gear forward along the armature shaft, ensuring it moves axially towards the flywheel.
3. Pinion Meshing: The pinion gear fully meshes with the flywheel ring gear before full current is supplied to the main starter motor. This axial movement ensures smooth and positive engagement.
4. Main Contact Closure (Second Stage): As the solenoid plunger reaches the end of its travel, it closes heavy-duty contacts, connecting the high current from the battery directly to the starter motor windings.
5. Cranking: The starter motor begins to rotate, cranking the engine.
6. Disengagement: Once the engine starts, the overrunning clutch allows the engine's faster rotation to decouple from the starter motor. When the ignition key is released, the solenoid is de-energized, and a return spring pulls the plunger back, retracting the pinion gear from the flywheel.
Advantages: Positive and quiet engagement, prevents gear clashing, better performance with varying battery voltages, longer lifespan for gears.
Disadvantages: More complex design than Bendix, repair can be more intricate. Nigerian Context
Example: Most modern vehicles, including commercial buses, SUVs, and passenger cars popular in Nigeria (e.g., Toyota Camry, Honda Accord, various Ford models). --- 2.
3. Gear Reduction Starter Principle: This type of starter motor incorporates a set of reduction gears (usually planetary gears) between the armature and the pinion gear. This allows the electric motor (armature) to spin at a very high speed, while the pinion gear spins at a much slower speed but with significantly increased torque.
Components: High-Speed Motor: A smaller, high-speed electric motor.
Planetary Gear Set: Consists of a sun gear, planet gears, and a ring gear. This mechanism provides the torque multiplication.
Shift Fork and Solenoid: Similar to the pre-engaged starter, responsible for engaging the pinion.
Pinion Gear and Overrunning Clutch: Similar to the pre-engaged starter.
Method of Operation:
1. Activation: Similar to the pre-engaged starter, the solenoid is energized.
2. Engagement: The solenoid pushes the pinion gear forward to engage with the flywheel.
3. Motor Cranking & Gear Reduction: The high-speed motor rotates. This rotation is transmitted through the planetary gear set. The gears reduce the speed but multiply the torque.
4. High Torque Cranking: The pinion then rotates the flywheel with greatly increased torque, making it easier to crank stiff engines or engines in cold weather.
5. Disengagement: The overrunning clutch and solenoid retraction mechanism work similarly to the pre-engaged type.
Advantages: Higher Torque Output: Excellent for cranking larger engines, diesel engines, or engines in challenging conditions (e.g., cold start).
Smaller and Lighter: The motor itself can be smaller due to the torque multiplication.
More Efficient: Draws less current for the same cranking power compared to direct-drive starters.
Disadvantages: More complex internally due to the gear train, potentially more expensive to manufacture and repair. * Nigerian Context
Example: Commonly found in diesel trucks (e.g., heavy-duty lorries, mass transit buses), SUVs, modern petrol engines, and generator sets that require significant cranking power. --- Summary of Key Differences: | Feature | Inertia Drive (Bendix) | Pre-engaged Drive (Solenoid) | Gear Reduction Starter | | :-------------------- | :----------------------------------- | :----------------------------------- | :---------------------------------- | | Engagement Method | Inertia of pinion during motor spin | Solenoid pushes pinion axially | Solenoid pushes pinion axially | | Engagement Timing | After motor starts spinning | Before main motor power applied | Before main motor power applied | | Pinion Protection | Relies on quick disengagement | Overrunning clutch | Overrunning clutch | | Noise/Smoothness | Can be noisy, harsh engagement | Smooth, positive engagement | Smooth, positive engagement | | Torque Output | Moderate | Moderate | High (due to gear multiplication) | | Size/Weight | Varies, direct drive | Varies, direct drive | Smaller, lighter (for given torque) This section details the primary types of starter motors and their operational mechanisms.
Definition of a Starter Motor: A starter motor is an electric motor used to rotate (crank) an internal combustion engine to initiate the engine's operation under its own power. It converts electrical energy from the vehicle battery into mechanical energy to turn the engine flywheel.
Main Types of Starter Motors: The three primary types of starter motors widely discussed and encountered are:
1. Inertia Drive (Bendix Drive) Starter
2. Pre-engaged Drive Starter (Solenoid-operated)
3. Gear Reduction Starter --- 2.
1. Inertia Drive (Bendix Drive)
Starter Principle: This type relies on the inertia of a weighted pinion gear to engage with the engine's flywheel ring gear. It is an older design, relatively simple, and commonly found in older vehicles or smaller engines.
Components: Armature: The rotating part of the motor.
Field Windings: Stationary coils that produce a magnetic field.
Commutator and Brushes: For directing current to the armature windings.
Bendix Drive Assembly: This is the key component and consists of: Pinion Gear: A small gear that meshes with the flywheel.
Pinion Sleeve: A threaded sleeve on which the pinion gear rides.
Drive Head (Weight): Provides inertia.
Return Spring: Pulls the pinion back once the engine starts.
Method of Operation (Inertial Effect):
1. Activation: When the ignition key is turned to the "start" position, current flows from the battery to the starter motor.
2. Armature Rotation: The armature begins to rotate rapidly.
3. Pinion Engagement: The Bendix pinion gear is mounted on a threaded sleeve on the armature shaft. Due to its inertia (resistance to change in motion), as the armature shaft spins, the pinion, initially stationary, lags behind. This inertia causes the pinion to screw itself along the threaded sleeve, moving forward towards the flywheel.
4. Meshing: The pinion eventually meshes with the larger teeth of the engine's flywheel ring gear.
5. Cranking: Once engaged, the rotating starter motor then cranks the engine.
6. Disengagement: As soon as the engine starts and reaches a speed faster than the starter motor, the flywheel forces the pinion to rotate faster. This increased speed causes the pinion to screw back along the threaded sleeve, disengaging from the flywheel. The return spring also assists in pulling the pinion back.
Advantages: Simple design, robust.
Disadvantages: Can be noisy, engagement can be harsh (leading to wear on gear teeth), less effective at low battery voltage compared to modern types. Nigerian Context
Example: May be found in older taxis (e.g., Peugeot 504), older generators, or some agricultural equipment in rural areas. --- 2.
2. Pre-engaged Drive Starter (Solenoid-operated)
Principle: This is the most common type of starter motor in modern vehicles. It ensures positive and smooth engagement of the pinion with the flywheel before the main motor current is fully applied. This prevents clashing of gears.
Components: Starter Motor Assembly: Contains the armature, field windings, commutator, and brushes.
Solenoid: An electromagnet that serves two main functions: Engaging Mechanism: It pulls a plunger, which moves a shift fork.
Main Switch: It acts as a heavy-duty relay, connecting the battery's high current directly to the starter motor windings.
Shift Fork (Lever): A lever connected to the solenoid plunger on one side and the pinion assembly on the other.
Pinion Gear: Engages with the flywheel.
Overrunning Clutch (One-way Clutch): Located behind the pinion gear, it allows the pinion to transmit torque from the starter to the flywheel but free-wheels when the flywheel rotates faster than the pinion (e.g., when the engine starts), preventing the engine from driving the starter motor.
Method of Operation (Axial Engagement):
1. Activation: When the ignition key is turned to the "start" position, a small current flows to the starter solenoid's pull-in and hold-in coils.
2. Solenoid Engagement (First Stage): The energized solenoid pulls its plunger inwards. This plunger, via the shift fork, pushes the pinion gear forward along the armature shaft, ensuring it moves axially towards the flywheel.
3. Pinion Meshing: The pinion gear fully meshes with the flywheel ring gear before full pushes pinion axially | Solenoid pushes pinion axially | | Engagement Timing | After motor starts spinning | Before main motor power applied | Before main motor power applied | | Pinion Protection | Relies on quick disengagement | Overrunning clutch | Overrunning clutch | | Noise/Smoothness | Can be noisy, harsh engagement | Smooth, positive engagement | Smooth, positive engagement | | Torque Output | Moderate | Moderate | High (due to gear multiplication) | | Size/Weight | Varies, direct drive | Varies, direct drive | Smaller, lighter (for given torque) | | Current Draw | High | High | Lower (for given cranking power) | | Application | Older vehicles, smaller engines | Most modern petrol/diesel engines | Heavy-duty, diesel, larger engines | --- This section outlines the step-by-step activities for delivering the lesson. 3.
1. Introduction (10 minutes)
Teacher Activity: Begin by asking students to recall how a car or generator starts. Facilitate a brief discussion on the importance of the starting system. Introduce the topic "Types of Starter Motors" and state the learning objectives clearly. Display visual aids (charts, diagrams, or actual starter motors if available) of different types of starter motors.
Student Activity: Students brainstorm and share their initial understanding of how engines start. Students observe the displayed starter motors and try to identify any visible differences. 3.
2. Key Concept Explanation and Discussion (25 minutes)
Teacher Activity: Inertia Drive (Bendix): Explain its principle, components, and detailed operation, emphasizing the "inertial effect." Use diagrams to illustrate the threaded sleeve and pinion movement. Provide examples of older vehicles.
Pre-engaged Drive (Solenoid): Explain its principle, components (solenoid, shift fork, overrunning clutch), and detailed two-stage operation, emphasizing "axial engagement." Use diagrams to show the solenoid's role in moving the pinion and closing contacts. Highlight its prevalence in modern cars.
Gear Reduction Starter: Explain its principle (torque multiplication), components (planetary gears), and how it combines with the pre-engaged mechanism. Discuss its advantages for heavy-duty applications. Facilitate questions and answers throughout the explanations to check for understanding.
Student Activity: Students take notes, observe diagrams, and ask clarifying questions. Students identify key components of each starter type from the visual aids. Students participate in discussions about the operational differences. 3.
3. Practical Observation/Demonstration (15 minutes)
Teacher Activity: If possible, have actual dismantled starter motors (Bendix, Pre-engaged, Gear Reduction) available. Point out the specific components (Bendix spring, solenoid, shift fork, overrunning clutch, planetary gears) on each physical unit. Demonstrate (manually, if safe and possible) the movement of the pinion in a pre-engaged starter (pushing the fork) and explain the Bendix mechanism. Emphasize the robust nature and common wear points for each type.
Student Activity: Students closely observe the physical starter motors, identifying the components discussed. Students handle the parts (if allowed and safe), feeling the weight of the Bendix drive, observing the solenoid plunger movement, or examining the gear sets. Students compare the physical differences between the types. 3.
4. Group Activity: Comparative Analysis (10 minutes)
Teacher Activity: Divide students into small groups. Provide each group with a handout or ask them to draw a comparison table (similar to the summary table in Key Concepts) for the three types of starter motors, focusing on engagement method, advantages, and disadvantages. Circulate, provide guidance, and address misconceptions.
Student Activity: Groups collaborate to complete the comparison table based on their notes and discussions. Groups present their findings to the class, fostering peer learning.
Vehicle Diagnostics and Repair (Local Workshops): Students will be able to apply this knowledge in actual auto repair shops (e.g., "Igbo-Eze Auto Mechanic Workshop" or similar local garages). When a vehicle won't start, the ability to differentiate between a Bendix type starter and a pre-engaged type allows for correct troubleshooting. For instance, if a pre-engaged starter clicks but doesn't crank, it points to a faulty solenoid or battery, whereas a Bendix starter failing to engage might suggest a stiff Bendix mechanism. This directly informs the repair process for common Nigerian vehicles like commercial buses (danfo, molue), taxis, and personal cars.
Generator Set Maintenance: Many homes and businesses in Nigeria rely on petrol or diesel generator sets due to power supply challenges. These generators often use starter motors. Understanding the types helps in diagnosing why a generator might not start (e.g., a "hard to start" diesel generator might benefit from a gear reduction starter if currently using a direct drive). This knowledge empowers students to offer servicing for these vital home and business appliances, potentially creating self-employment opportunities. Entrepreneurship in Auto Parts Sales and Servicing: With a strong grasp of different starter motor types, students can advise customers on appropriate replacement parts. Knowing the advantages of a gear reduction starter over a direct drive for specific applications (e.g., a farmer's tractor or a large lorry) can lead to better service provision and potential business ventures in selling or rebuilding specialized starter motors. This contributes to the local economy by providing essential technical services.