Engine Components and Lubrication Systems
1. Engine Block and Cylinder Liners
Engine blocks are typically made of cast iron or light alloy. Cast iron, made from flake graphite, offers good mechanical strength but is heavy. Aluminum alloy, composed of silicon, is lighter. Cylinder liners, often made of cast iron, provide a wear-resistant surface within the engine block.
Types of Cylinders
- Integral Block: The cylinder is directly machined into the engine block material, without separate liners.
- Dry Liner: Thin-walled cylinders are press-fitted into the block.
- Wet Liner: These cylinders are in direct contact with the engine coolant and are detachable for easier replacement.
2. Cylinder Wear
Causes of Cylinder Wear
Several factors contribute to cylinder wear, including:
- Cold starts
- Neglecting oil and filter changes
- Operating in dusty environments
- High engine speeds
These factors can lead to ovality and taper wear in the cylinder, resulting in compression loss, reduced engine performance, and increased oil and fuel consumption.
Cylinder Wear Patterns
The top of the cylinder experiences the most wear due to the forces acting on the piston and the high temperatures and limited lubrication in this area.
3. Pistons
The piston’s primary function is to transmit the force generated by combustion gases to the connecting rod and crankshaft. It also prevents oil from entering the combustion chamber and combustion gases from leaking into the crankcase. Additionally, the piston helps transfer heat from combustion to the cooling system.
Piston Expansion
Pistons are designed with varying material thickness to accommodate thermal expansion.
Lateral Force Balance
The offset crankshaft and connecting rod mechanism can cause uneven wear on the cylinder walls. To mitigate this, the piston’s longitudinal axis may not perfectly align with the crankshaft’s axis of rotation.
4. Piston Rings
Piston rings serve several crucial functions:
- Gas Sealing: Ensuring a tight seal between the piston and cylinder wall to maintain compression.
- Heat Transfer: Conducting heat from the piston to the cylinder wall for cooling.
- Oil Control: Preventing oil from entering the combustion chamber.
There are two main types of piston rings:
- Compression Rings: Primarily responsible for sealing the cylinder and transferring heat. The top ring is exposed to the highest temperatures.
- Oil Control Rings: Scrape excess oil from the cylinder wall to prevent it from entering the combustion chamber. They allow a small amount of oil to pass for lubrication.
5. Engine Balance
Static and Dynamic Balance
Static balance is achieved when all rotating components are balanced around the axis of rotation, ensuring the system remains at rest in any position. Dynamic balance considers the centrifugal forces generated during rotation. A well-balanced system will have minimal vibration.
6. Bearing Shells
Bearing shells, typically made of a soft metal alloy, provide a low-friction surface between the crankshaft and connecting rod or engine block. They offer several key properties:
- Seizure resistance
- Good thermal conductivity
- Conformability to the shaft shape
- Ability to embed solid particles
7. Flywheel
The flywheel, a heavy cast iron disc, stores kinetic energy from the engine’s power strokes and releases it during non-power strokes. This helps smooth out engine operation. However, flywheels can resist rapid changes in engine speed.
8. Vibration Dampers
Vibration dampers, often located on the crankshaft, help reduce torsional vibrations. They typically consist of two masses connected by a spring and damping mechanism, effectively absorbing vibrations at low engine speeds.
9. Counter-Rotating Shafts
Counter-rotating shafts are used in some engines to counteract vibrations caused by crankshaft imbalance. These shafts, positioned on either side of the engine block, rotate at twice the crankshaft speed in opposite directions, canceling out the unbalanced forces.
10. Engine Oil Functions
Engine oil plays a vital role in engine operation:
- Reduces Friction: Forms a lubricating film between moving parts, minimizing friction and wear.
- Cools Components: Absorbs heat from engine components and transfers it to the crankcase for cooling.
- Improves Sealing: Enhances the seal between piston rings and cylinder walls, improving compression.
- Cushions Impacts: Absorbs shock loads and vibrations.
- Cleans Engine: Carries away dirt and wear particles.
11. Types of Lubricating Oils
- Mineral Oils: Derived from crude oil.
- Synthetic Oils: Manufactured through chemical processes, offering improved performance characteristics.
- Semi-Synthetic Oils: Blends of mineral and synthetic oils.
12. Lubricating Oil Properties
Lubricating oils should possess the following properties:
- Resistance to water contamination
- Corrosion resistance
- Resistance to fuel dilution
- High-temperature stability
13. Oil Viscosity
The SAE (Society of Automotive Engineers) classification system defines oil viscosity grades based on temperature. The first number, followed by the letter “W” (for winter), indicates the oil’s viscosity at low temperatures. The second number represents the oil’s viscosity at high temperatures.
Multigrade Oils
Multigrade oils, designated with two viscosity grades (e.g., 10W-40), offer good performance across a wide temperature range. They maintain fluidity at low temperatures for easy starting and sufficient viscosity at high temperatures for adequate lubrication.
14. Lubrication Systems
Pressure Lubrication
In a pressure lubrication system, oil is drawn from the crankcase by a pump, filtered, and then pressurized. The pressurized oil is then distributed to various engine components through oil passages.
Splash Lubrication
In a splash lubrication system, oil is splashed onto engine components by the rotating crankshaft. This method is typically used in simpler engines or for lubricating specific parts.
Mixed Lubrication (Two-Stroke Engines)
In two-stroke engines, oil is mixed with the fuel and lubricates the engine as it passes through the crankcase and combustion chamber.
15. Oil Pumps
Several types of oil pumps are used in engines:
- Gear Pumps: Two meshing gears create suction and pressure to move the oil.
- Rotor Pumps: An inner and outer rotor rotate within each other to pump the oil.
- Internal Gear Pumps: The crankshaft directly drives the inner rotor, which meshes with the outer rotor to pump the oil.
Relief Valve
The relief valve limits the maximum oil pressure in the system. If the pressure exceeds a set limit, the valve opens, allowing oil to bypass the filter and return to the crankcase.
16. Oil Filters
Oil filters remove contaminants from the oil, such as dirt, wear particles, and combustion byproducts. There are two main types:
- Full-Flow Filters: All oil pumped through the system passes through the filter.
- Bypass Filters: Only a portion of the oil is filtered at a time. These filters are more effective at removing fine particles but filter the oil more slowly.