Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Crank Arrangement(Crankshaft Configuration)
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Crank Arrangement(Crankshaft Configuration)
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Subject: Auto Mechanical Works
Class: Senior Secondary 2
Term: 2nd Term
Week: 2
Theme: Engine System
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Sketch the linediagram of the 4 and 6cylinder in line engine Sketch the linediagram of the crankarrangement v-6 and v-8 cylinder horizontallyopposed engine Identify the mainparts of the crankshaft
The crankshaft is the backbone of the engine's rotating assembly.
Its primary function is to: Convert Reciprocating Motion to Rotary Motion: Pistons move up and down (reciprocating motion) within the cylinders. Connecting rods link the pistons to the crankshaft's crankpins. As the pistons move, they push and pull the connecting rods, which, in turn, rotate the crankshaft.
Transmit Power: The rotational motion of the crankshaft is transmitted to the gearbox (transmission) and then to the drive wheels, propelling the vehicle.
Support Other Components: The crankshaft often drives other engine components like the timing gear, oil pump, and accessory belts (via a pulley at the front end). In-line engines have all cylinders arranged in a single straight row. a) 4-Cylinder In-line Engine (I-4)
Description: This is one of the most common engine types, especially in small to medium-sized cars in Nigeria (e.g., Toyota Corolla, Honda Civic). The four cylinders are arranged in a straight line, typically perpendicular to the crankshaft axis.
Crank Arrangement: For an I-4 engine, the crankshaft typically has a 180-degree configuration. This means that two pistons are always at Top Dead Centre (TDC) or Bottom Dead Centre (BDC) while the other two are at the opposite position.
Common Arrangement: Crankpins for cylinders 1 and 4 are aligned (or 0°), and crankpins for cylinders 2 and 3 are aligned but 180° opposite to 1 and
4. Benefit: This arrangement provides good primary balance (balances forces from piston movement) but can have secondary vibrations (due to non-linear piston speed).
Diagram Explanation (for sketching): Imagine the crankshaft viewed from the front or rear. Crankpin 1 (cylinder 1) is at the 'top' (0°). Crankpin 2 (cylinder 2) is at the 'bottom' (180°). Crankpin 3 (cylinder 3) is at the 'bottom' (180°), parallel to crankpin
2. Crankpin 4 (cylinder 4) is at the 'top' (0°), parallel to crankpin
1. This gives a visual representation of 1&4 moving together, and 2&3 moving together, but opposite to 1&4. b) 6-Cylinder In-line Engine (I-6)
Description: Historically and currently used in many smooth-running vehicles and heavier-duty applications (e.g., older Mercedes-Benz, BMW, certain trucks, generator engines). Known for inherent primary and secondary balance.
Crank Arrangement: The crankpins are typically separated by 120 degrees along the crankshaft, with opposing pairs mirroring each other. Common Arrangement
Example: If cylinder 1 is at 0°, then cylinder 6 might be at 0° (or 360°), cylinder 2 at 240°, cylinder 5 at 120°, cylinder 3 at 120°, and cylinder 4 at 240°. A common and simplified view often shows 1, 6 at 0°, 2, 5 at 240°, and 3, 4 at 120° relative to an imaginary reference line. The exact arrangement ensures even firing and perfect primary and secondary balance.
Benefit: The 120-degree separation creates a perfectly balanced engine, eliminating both primary and secondary vibrations. This results in very smooth operation.
Diagram Explanation (for sketching): View the crankshaft from one end. Imagine a circle representing 360 degrees. Crankpins are positioned at angular separations of 120 degrees relative to each other. For example, if the first crankpin is at 0°, the next relevant one might be at 120°, and the next at 240°. Then the pattern repeats with specific pairing across the shaft. A common simplified view shows crankpins 1 & 6 aligned, 2 & 5 aligned, 3 & 4 aligned, but at 120° offsets from each other. V-engines have cylinders arranged in two banks, forming a 'V' shape, usually sharing a common crankshaft. They are more compact than in-line engines of the same displacement. a)
V-6 Engine Description: Widely used in mid-size to large cars, SUVs, and light trucks in Nigeria (e.g., Toyota Camry V6, certain Ford models). Offers good power in a relatively compact package.
Crank Arrangement: The design depends on the 'V' angle (e.g., 60°, 90°, 120°). 60-degree V-6: Often uses a simpler crankshaft with crankpins separated by 60 degrees to ensure even firing intervals. 90-degree V-6: To achieve even firing, these typically require offset crankpins (split pins) on the crankshaft to effectively create 60-degree firing intervals. This is more complex but allows for shared crank journals between opposing cylinders.
Diagram Explanation (for sketching): Represent the two banks of cylinders forming the 'V'. The crankshaft will have three sets of crankpins, each serving two opposing cylinders (one from each bank). For a 60° V-6, imagine three crankpin positions, each offset by 120° from the next, but the actual firing intervals are 60° due to the V-angle. For a 90° V-6 with split pins, the crankpins will appear more complex, with each pair of opposing cylinders having their connecting rods on crankpins that are offset from each other on the same journal. Simplified diagrams often show the three crank throws, each serving a pair of opposing cylinders. b)
V-8 Engine Description: Common in larger, more powerful vehicles, luxury cars, SUVs, and heavy-duty trucks (e.g., Land Cruiser, certain American muscle cars, large generator sets). Known for power and smoothness.
Crank Arrangement: Most V-8s use a cross-plane crankshaft.
Cross-plane: The crankpins are arranged at 90-degree intervals when viewed from the end, giving it a characteristic 'cross' shape. This configuration provides excellent primary and secondary balance, leading to very smooth operation. It allows for even firing pulses every 90 degrees of crankshaft rotation.
Flat-plane: Less common in road cars (found in some high-performance sports cars). Crankpins are in two planes 180° apart (like two I-4 engines sharing a crankshaft). It offers better exhaust scavenging but has higher secondary vibrations. Diagram Explanation (for sketching cross-plane V-8): View the crankshaft from the end. The four crankpins are positioned at 90-degree angles to each other, forming a cross or '+' shape. For example, if one crankpin is at 0°, the others are at 90°, 180°, and 270°.
Description: Cylinders are arranged on opposite sides of the crankshaft, 180 degrees apart, with their pistons moving towards and away from each other horizontally. Examples include older Volkswagen Beetles (common in Nigeria), Porsche, and Subaru vehicles.
Crank Arrangement: Can have a common crankpin for opposing pistons or separate, adjacent crankpins. Common Crankpin (e.g., 2-cylinder Boxer): Two opposing pistons share a single crankpin. Separate Crankpins (e.g., 4-cylinder Boxer): Each piston has its own crankpin, but the crankpins are arranged such that opposing pistons reach TDC/BDC simultaneously or at opposing points depending on the firing order. For a 4-cylinder boxer, the crankpins are usually 180° apart, resembling an I-4 crankshaft but with cylinders lying flat.
Benefit: Very low center of gravity (improving handling), and inherent primary balance due to opposing pistons.
Diagram Explanation (for sketching): Show two cylinders horizontally opposing each other, connected to the crankshaft. If it's a 2-cylinder boxer, one crankpin could be at 0° (or 180°) relative to another. For a 4-cylinder, it will resemble the I-4 configuration with crankpins at 0° and 180°, but oriented for horizontal piston travel.
Vehicle Maintenance and Repair (e.g., Engine Overhaul in Nigerian Workshops): Auto mechanics in Nigeria regularly perform engine overhauls on various vehicles, from popular Toyota models (often I-4, V-6) to commercial buses and lorries (often I-6, V-8). Knowledge of crank arrangements is critical for: Diagnosis: Identifying vibrations that might indicate a bent crankshaft or incorrect crankpin alignment after repair.
Assembly: Ensuring connecting rods are attached to the correct crankpins and that the crankshaft is correctly oriented during reassembly.
Part Sourcing: Knowing the engine configuration is vital for ordering the correct replacement crankshaft or connecting rods for a specific engine type.
Firing Order: Understanding the crank arrangement directly relates to the engine's firing order, which is crucial for proper timing and tune-ups. Engine Performance and Selection (e.g., Choosing Vehicles or Generator Sets): Smoothness: Mechanics and vehicle owners understand that I-6 engines are inherently smoother due to their perfect primary and secondary balance, making them desirable for luxury cars or long-haul trucks. V-8s, especially cross-plane, are also known for smoothness and power.
Compactness: V-engines (V-6, V-8) offer a more compact design for a given displacement, allowing them to fit into smaller engine bays of modern cars and SUVs, which is a key consideration for vehicle manufacturers and consumers in crowded Nigerian cities.
Power Output: Larger engines, often V-configurations, are chosen for heavy-duty applications like trailers or large generator sets due to their ability to produce higher power and torque. Fuel Efficiency and Emissions (e.g., Environmental Considerations): While less direct, the engine configuration, including crank arrangement, plays a role in engine efficiency and emissions. For instance, specific crank arrangements can influence valve timing and exhaust gas flow characteristics, which in turn impact fuel consumption and the amount of pollutants released. As Nigeria faces increasing environmental concerns in urban areas, understanding these underlying design principles can lead to more informed decisions about engine maintenance and technology adoption.