Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Valve Operation Merchanism
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Valve Operation Merchanism
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Subject: Auto Mechanical Works
Class: Senior Secondary 3
Term: 2nd Term
Week: 1
Theme: Engine System
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State the functions of valvesand the ir operating mechanism Sketch the layout of the valveoperating mechanism, label itsparts and describe its operation Sketch and describe howcamshaft is driven by crankshaft Draw the valve timing diagramof multi-cylinder engine.
Valves are precision-machined components in an internal combustion engine that control the intake of fresh air-fuel mixture (or air only for diesel engines) into the combustion chamber and the expulsion of exhaust gases from it. They are critical for the four-stroke cycle of an engine.
Two primary types of valves are used: Inlet (Intake)
Valve: Larger than the exhaust valve, it allows the air-fuel mixture (petrol engine) or fresh air (diesel engine) to enter the cylinder during the intake stroke.
Exhaust Valve: Smaller than the inlet valve, it allows burnt gases (exhaust) to exit the cylinder during the exhaust stroke. Exhaust valves are subjected to higher temperatures and are often made of special heat-resistant alloys.
General Functions of Engine Valves: To open and close the intake and exhaust ports at precise times to allow for gas exchange. To seal the combustion chamber during the compression and power (combustion) strokes, ensuring maximum pressure build-up and efficient power generation. The valve operating mechanism, also known as the valve train, is a complex system designed to precisely open and close the engine valves in synchronisation with the piston's movement.
Key Components: Crankshaft: The main rotating shaft of the engine, converting the reciprocating motion of the pistons into rotary motion. It drives the camshaft.
Camshaft: A shaft with eccentrically shaped lobes (cams). There is one lobe for each intake and exhaust valve. As the camshaft rotates, these lobes push on other components to open the valves. The camshaft rotates at half the speed of the crankshaft.
Timing Gear/Chain/Belt: Connects the crankshaft to the camshaft, ensuring their rotational synchronisation.
Timing Gears: Direct gear-to-gear contact, durable but can be noisy.
Timing Chain: A roller chain connecting sprockets on the crankshaft and camshaft. More flexible than gears, quieter. Common in many modern engines.
Timing Belt: A reinforced rubber belt with teeth, connecting sprockets. Quieter and lighter than chains, but requires periodic replacement.
Tappets (or Lifters): Small cylindrical components that rest on the camshaft lobes. As the camshaft rotates, the lobe pushes the tappet upwards. Tappets can be mechanical (requiring valve clearance adjustments) or hydraulic (self-adjusting, reducing noise and maintenance). Pushrods (in Overhead Valve - OHV engines): Long, slender rods that transmit the upward motion from the tappets to the rocker arms.
Rocker Arms: Pivoting levers mounted on a shaft. One end is pushed by the pushrod (or tappet directly in OHC engines), and the other end presses down on the valve stem, forcing the valve open.
Valve Springs: Strong coil springs that keep the valves firmly closed against their seats when the cam lobe is not pushing them open. They also prevent valve float at high engine speeds.
Valve Guides: Cylindrical inserts in the cylinder head that support and guide the valve stem, ensuring smooth and accurate movement of the valve.
Valve Seats: Machined surfaces in the cylinder head or separate inserts, against which the valve head rests when closed, forming a gas-tight seal for the combustion chamber. The operation varies slightly depending on the engine's design (Overhead Valve - OHV or Overhead Camshaft - OHC), but the fundamental principle remains the same: For Overhead Valve (OHV)
Engine (Camshaft in block): Crankshaft Rotation: The crankshaft rotates, driving the camshaft through the timing gear/chain/belt at a 2:1 ratio (crankshaft rotates twice for every one camshaft rotation).
Cam Lobe Action: As the camshaft rotates, a cam lobe comes into contact with the tappet. The rising profile of the cam lobe pushes the tappet upwards.
Pushrod Movement: The upward movement of the tappet is transmitted through the pushrod.
Rocker Arm Action: The pushrod pushes one end of the rocker arm upwards. Since the rocker arm pivots on its shaft, its other end moves downwards.
Valve Opening: The downward end of the rocker arm presses against the tip of the valve stem, overcoming the force of the valve spring, and pushing the valve open from its seat.
Valve Closing: As the camshaft continues to rotate, the cam lobe's profile moves past the tappet, allowing the valve spring to push the valve back onto its seat, closing the port. The tappet, pushrod, and rocker arm follow the cam lobe's receding profile. For Overhead Camshaft (OHC)
Engine (Camshaft in cylinder head): Single Overhead Camshaft (SOHC): The camshaft is located directly above the valves in the cylinder head. The cam lobes directly act on the rocker arms, or sometimes directly on the tappets (bucket lifters), which then press on the valve stems. This eliminates pushrods.
Double Overhead Camshaft (DOHC): Two camshafts per cylinder bank (one for intake valves, one for exhaust valves). This design offers greater precision in valve timing and allows for more valves per cylinder. The cam lobes typically act directly on bucket tappets, which in turn press on the valve stems, further simplifying the valve train and reducing inertia.
Benefits of OHC over OHV: Fewer moving parts (no pushrods) results in less inertia and allows for higher engine RPMs. More precise valve timing and lift. Easier to incorporate variable valve timing systems. The camshaft is always driven by the crankshaft, ensuring that the valves open and close in precise synchronisation with the piston's movement.
Drive Mechanisms: Timing Gears: A smaller gear is mounted on the crankshaft. A larger gear (twice the diameter) is mounted on the camshaft. The gears mesh directly or via an idler gear.
Gear Ratio: The crankshaft gear has half the number of teeth of the camshaft gear.
Therefore, for every two revolutions of the crankshaft, the camshaft completes one revolution.
Reason for 2:1 Ratio: A four-stroke engine completes its full cycle (Intake, Compression, Power, Exhaust) over two complete revolutions of the crankshaft. During this cycle, each valve (intake and exhaust) opens and closes only once. Thus, the camshaft, which controls these openings and closings, needs to rotate only once for every two crankshaft rotations.
Timing Chain: A sprocket (toothed wheel) is mounted on the crankshaft. Another sprocket (twice the number of teeth) is mounted on the camshaft. A strong, link-type roller chain connects these two sprockets, wrapping around them. Tensioners and guides are used to maintain chain tension and prevent slack. Common in many durable engines found in Nigeria.
Timing Belt: Similar to timing chains, but uses a reinforced rubber belt with teeth that engage with sprockets on the crankshaft and camshaft. Tensioners are crucial to maintain correct belt tension. Quieter and lighter than chains. Requires regular replacement as recommended by the vehicle manufacturer (e.g., every 60,000-100,000 km) to prevent catastrophic engine damage if it breaks.
Timing Marks: For accurate assembly and timing, crankshaft and camshaft sprockets/gears have specific timing marks that must be aligned during engine assembly or maintenance. Misalignment can lead to incorrect valve timing, poor engine performance, or even valve-piston collision.
Automotive Repair and Maintenance (Job Creation): Understanding valve operation is crucial for Nigerian mechanics. When an engine "knocks" or shows poor performance, identifying issues like worn camshaft lobes, bent pushrods, broken valve springs, or incorrect valve timing (e.g., a jumped timing belt/chain) requires a thorough grasp of this mechanism. This knowledge directly translates into diagnostic and repair skills, enabling students to secure employment or establish their own workshops in Nigeria's vibrant informal and formal automotive sectors.
Fuel Economy and Engine Efficiency: For commercial transport operators (e.g., 'Danfo' bus drivers, 'Okada' riders) in Nigeria, fuel efficiency is a significant operational cost. Correct valve timing ensures optimal combustion and gas exchange, directly impacting fuel consumption. Students who understand valve timing can advise on or perform adjustments that improve fuel efficiency, which can lead to tangible economic benefits for vehicle owners and reduced operational costs. Engine Modifications and Performance Tuning: For enthusiasts or those involved in specialized vehicle applications (e.g., generator maintenance, agricultural machinery), advanced knowledge of valve timing, including concepts like variable valve timing (VVT), can be applied to enhance engine power output or alter torque characteristics to suit specific operational needs within the Nigerian context. This could involve modifying engines for heavy-duty applications or improving the performance of locally assembled machinery.