Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Factors Affecting Engine Performance
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Factors Affecting Engine Performance
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Subject: Auto Mechanical Works
Class: Senior Secondary 3
Term: 3rd Term
Week: 1
Theme: Safety And Maintenace
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List the factorsaffecting engineperformance Carry out simplepreventivemaintenance e.g.valve clearanceadjustment and engine timing
Concept: Valve clearance (or tappet clearance) is the gap between the valve stem tip and the rocker arm (or cam follower) when the valve is fully closed. This small gap is necessary to accommodate thermal expansion of the valve components when the engine heats up. Without this clearance, the valves would remain slightly open when hot, leading to compression loss, overheating of the valve (and potential burning), and reduced power.
Tools Required: Feeler gauge set Spanner set (usually 10mm, 12mm, 14mm, 17mm depending on engine) Screwdriver (flat head) Socket wrench with extension and appropriate sockets (for turning crankshaft and removing valve cover) Torque wrench (for valve cover bolts) New valve cover gasket (optional, but recommended if old one is brittle) Clean rag Procedure for Valve Clearance Adjustment (Overhead Valve (OHV) or Overhead Camshaft (OHC) with Rocker Arms - Cold Engine):
1. Preparation: Ensure the engine is cold. Valve clearance specifications are almost always for a cold engine. Disconnect the negative terminal of the battery. Remove the engine's plastic cover (if present) and carefully remove the valve cover (rocker cover). This may require disconnecting PCV hoses, spark plug wires, or other components. Place the valve cover and bolts aside safely. Inspect the valve cover gasket. Replace if cracked or hard. Identify the engine's firing order (e.g., 1-3-4-2 for a 4-cylinder engine). This is usually found in the service manual or on an engine sticker.
2. Locate Top Dead Center (TDC) for Cylinder 1 Compression Stroke: Using a socket wrench on the crankshaft pulley bolt, slowly rotate the crankshaft in the normal direction of rotation (clockwise for most engines). Observe the valves on cylinder #
1. As you rotate, watch the exhaust valve on cylinder #1 close, then the intake valve on cylinder #1 open and close. Continue rotating until both valves for cylinder #1 are closed and the piston is at the top of its stroke. This is the TDC of the compression stroke for cylinder #
1. Alternatively, align the timing marks on the crankshaft pulley with the mark on the timing cover (usually '0' or 'TDC') while ensuring the distributor rotor (if present) is pointing to cylinder #1 spark plug wire terminal.
3. Check and Adjust Valves for Cylinder #1 (and associated cylinders based on firing order): With cylinder #1 at TDC on its compression stroke, its valves are fully closed. Insert the specified feeler gauge between the valve stem end and the rocker arm/adjuster screw for both the intake and exhaust valves of cylinder #
1. The feeler gauge should slide in with slight drag. If it's too loose, too tight, or won't fit, adjustment is needed.
To Adjust: Loosen the lock nut on the rocker arm adjuster screw. Turn the adjuster screw with a screwdriver until the feeler gauge slides in with the correct slight drag. Hold the adjuster screw in position with the screwdriver and tighten the lock nut firmly. Re-check the clearance with the feeler gauge after tightening to ensure it hasn't changed.
Rule of 9 (for 4-cylinder engines): When cylinder X is at TDC compression stroke, its valves (intake and exhaust) can be adjusted. Also, the valve opposite in the firing order (9 minus cylinder number) will be at TDC overlap. For other cylinders, if cylinder 1 is at TDC compression, then you can adjust valves on cylinder 1 and cylinder 4 (if 1-3-4-2 firing order) where cylinder 4 is at TDC Exhaust. A simpler method is "valve overlap" method.
4. The "Valve Overlap" or "Opposite Cylinder" Method (Common for 4-cylinder engines): This method allows adjustment of multiple valves without constantly checking TDC compression for each cylinder.
Step 1: Rotate crankshaft until cylinder 1 is at TDC on the compression stroke (both valves closed, rocker arms loose). Adjust Intake and Exhaust valves for Cylinder
1. Adjust Intake valve for Cylinder 2 (if applicable based on specific engine manual instructions, often only exhaust for #2). Adjust Exhaust valve for Cylinder 3 (if applicable). Adjust Intake and Exhaust valves for Cylinder 4 (if or "Opposite Cylinder" Method (Common for 4-cylinder engines): This method allows adjustment of multiple valves without constantly checking TDC compression for each cylinder.
Step 1: Rotate crankshaft until cylinder 1 is at TDC on the compression stroke (both valves closed, rocker arms loose). Adjust Intake and Exhaust valves for Cylinder
1. Adjust Intake valve for Cylinder 2 (if applicable based on specific engine manual instructions, often only exhaust for #2). Adjust Exhaust valve for Cylinder 3 (if applicable). Adjust Intake and Exhaust valves for Cylinder 4 (if 1-3-4-2 firing order and cylinder 4 is at TDC Exhaust).
Simpler rule: When the exhaust valve of any cylinder begins to open, and its intake valve is fully closed (or just starting to close from exhaust stroke), that cylinder is at "valve overlap". At this point, the opposite cylinder in the firing order will be at TDC compression stroke, and its valves can be adjusted.
Example (Firing Order 1-3-4-2): When cylinder 4 exhaust valve is fully open and its intake valve is just closing (overlap), adjust cylinder
1. When cylinder 2 exhaust valve is fully open and its intake valve is just closing (overlap), adjust cylinder
3. When cylinder 1 exhaust valve is fully open and its intake valve is just closing (overlap), adjust cylinder
4. When cylinder 3 exhaust valve is fully open and its intake valve is just closing (overlap), adjust cylinder
2. Step 2: Rotate crankshaft 180 degrees.
Step 3: Rotate crankshaft another 180 degrees (total 360 degrees from Step 1). Now cylinder 4 is at TDC on the compression stroke (for 1-3-4-2 firing order). Adjust Intake and Exhaust valves for Cylinder
4. Adjust Intake valve for Cylinder
3. Adjust Exhaust valve for Cylinder
2. Adjust Intake and Exhaust valves for Cylinder 1 (if at TDC Exhaust).
5. Final Steps:** Double-check all clearances. Install the valve cover with a new gasket (if replaced) and tighten bolts to the specified torque. Reconnect all hoses, wires, and the battery terminal. * Start the engine and listen for proper operation. Slight tappet noise on startup is normal until oil pressure builds, but excessive or persistent noise indicates an issue.
Concept: Ignition timing is the precise moment the spark plug fires relative to the piston's position in the cylinder during the compression stroke. It is critical for efficient combustion. The spark must occur slightly before the piston reaches Top Dead Center (BTDC) to allow time for the fuel-air mixture to ignite and burn, reaching maximum pressure just after TDC for optimal power delivery.
Advanced Timing (Spark Too Early): Can cause knocking (detonation), leading to engine damage, reduced power, and overheating.
Retarded Timing (Spark Too Late): Results in incomplete combustion, loss of power, reduced fuel economy, increased emissions, and overheating.
Types of Ignition Timing Systems: Distributor-based Systems (Older Engines): Timing is mechanically set by rotating the distributor housing. Electronic Ignition Systems (Modern Engines): Timing is controlled by the Engine Control Unit (ECU) based on various sensor inputs (crankshaft position sensor, camshaft position sensor, knock sensor, etc.). Manual adjustment is typically not possible, or only for base timing using diagnostic tools. Tools Required (for Distributor-based systems): Timing light (strobe light type) Spanner set (for loosening distributor clamp bolt) Tachometer (to set engine idle speed if necessary) Procedure for Checking and Adjusting Ignition Timing (Distributor-based, Older Engines): Preparation: Ensure the engine is at normal operating temperature. Verify battery voltage is adequate. Locate the timing marks on the crankshaft pulley (harmonic balancer) and the timing indicator on the engine block/timing cover. These marks typically include TDC and marks for degrees Before Top Dead Center (BTDC) or After Top Dead Center (ATDC). Locate the distributor. Consult the vehicle's service manual for the specified ignition timing (e.g., 8° BTDC at idle) and any specific instructions (e.g., disconnecting a vacuum advance hose, shorting a diagnostic terminal).
Connect Timing Light: Connect the timing light's power clips to the vehicle's battery (red to positive, black to negative). Clip the inductive pick-up clamp of the timing light onto the spark plug wire for Cylinder #
1. Ensure it's pointing in the correct direction (usually indicated on the clamp).
Start Engine: Start the engine and allow it to idle at the specified RPM (if applicable). If instructed by the manual, disconnect the vacuum advance hose from the distributor and plug it.
Check Timing: Aim the timing light at the crankshaft pulley and timing indicator marks. When the light flashes, it illuminates the timing marks, making them appear stationary. Note the position of the mark on the crankshaft pulley relative to the indicator on the timing cover. This indicates the current ignition timing. Compare this reading to the manufacturer's specification.
Adjust Timing (if necessary): If adjustment is needed, loosen the distributor clamp bolt (just enough to allow the distributor to rotate, but not too loose).
To Advance Timing (spark earlier): Slowly rotate the distributor opposite to the direction of rotor rotation.
To Retard Timing (spark later): Slowly rotate the distributor in the direction of rotor rotation. Make small adjustments, re-tighten the clamp bolt, and re-check with the timing light until the marks align with the specified timing.
Final Steps: Once the timing is correctly set, securely tighten the distributor clamp bolt. If a vacuum hose was disconnected, reconnect it. Switch off the engine and disconnect the timing light. Test drive the vehicle to ensure smooth operation and no knocking. --- Teacher Activities: Introduction (10 minutes): Teacher initiates a discussion on common engine problems observed in Nigeria (e.g., keke napep struggling uphill, generator consuming too much fuel, a car smoking heavily). Teacher explains that these issues are often linked to factors affecting engine performance. Teacher introduces the lesson objectives.
Explanation of Factors (20 minutes): Teacher systematically explains each factor affecting engine performance (fuel, ignition, air intake, exhaust, cooling, lubrication, mechanical). Teacher uses visual aids (diagrams of engine systems, real components if available) to illustrate points. Teacher draws connections to local Nigerian contexts, e.g., effects of poor fuel quality, dusty roads affecting air filters. Detailed Explanation and Demonstration of Valve Clearance Adjustment (30 minutes): Teacher explains the concept of valve clearance and its importance clearly. Teacher demonstrates, using a mock-up engine head or a large, clear diagram/video, the step-by-step procedure for checking and adjusting valve clearance. Teacher shows the correct use of a feeler gauge, spanners, and screwdriver for adjustment. Teacher emphasizes the 'cold engine' requirement and the "valve overlap" method or "opposite cylinder" method for efficient adjustment. Teacher discusses common mistakes and how to avoid them. Detailed Explanation and Demonstration of Engine Timing (Ignition Timing) (25 minutes): Teacher explains the concept of ignition timing and its significance. Teacher demonstrates, using diagrams or a video of a distributor-based system, how to use a timing light to check and adjust ignition timing. Teacher highlights the timing marks on the crankshaft pulley and the engine block. Teacher explains the effects of advanced and retarded timing. Teacher briefly mentions modern ECU-controlled timing and its advantages. Activity Guidance and Support (15 minutes): Teacher divides students into groups for discussion and practical simulation activities. Teacher moves around groups, providing support, clarification, and correcting misconceptions.
Student Activities: Brainstorming and Discussion (10 minutes): Students participate in the introductory discussion, sharing observations of engine performance issues. Students discuss in small groups what they think might cause an engine to perform poorly.
Note-taking and Q&A (45 minutes): Students actively listen, take notes, and ask questions during the teacher's explanations of engine performance factors, valve clearance adjustment, and engine timing. Practical Simulation/Observation (40 minutes): If an actual engine or engine head is available, students observe the teacher's demonstration of valve clearance adjustment and engine timing. They can practice identifying timing marks and using the feeler gauge under supervision. If no physical engine, students work in groups to draw and label the components involved in valve clearance and ignition timing. They can simulate the adjustment process using hand gestures or role-play, explaining each step to their peers. Students discuss the implications of incorrect adjustments, relating it to scenarios like a commercial bus driver experiencing poor fuel economy or loss of power when carrying passengers.
Group Reporting and Recap (10 minutes): Each group briefly reports on their understanding of one key concept or a step in the adjustment process. Students collectively review the factors affecting engine performance and the importance of preventive maintenance. ---
Fuel Efficiency for Commercial Transport: In Nigeria, commercial vehicles like okada, keke napep, and mini-buses (danfo) are the backbone of urban transport. Proper engine maintenance, particularly valve clearance and ignition timing adjustments, directly leads to better fuel efficiency. This translates into significant cost savings for operators, impacting their daily earnings and the affordability of transport for the general public, especially with fluctuating fuel prices. Students can understand how their skills can directly support livelihoods. Generator Maintenance in Homes and Businesses: Due to inconsistent power supply, many Nigerian homes and businesses rely heavily on petrol or diesel generators. These generators often use simpler engines where manual valve clearance and ignition timing adjustments are critical for optimal performance, fuel economy, and longevity. Students can apply their knowledge to maintain these essential power sources, potentially saving families and small businesses money on fuel and repairs. Local Mechanic Workshops and Entrepreneurship: The skills learned in this topic (diagnosing engine performance issues and performing basic preventive maintenance like valve clearance and timing) are foundational for working in local mechanic workshops across Nigeria. This knowledge empowers students with practical skills that are immediately employable, fostering entrepreneurship and contributing to the informal and formal automotive repair sector. They can start their own repair businesses or work as skilled technicians, serving the community. ---