- Practice · 6 marks
Engine lubrication systems
Short answer
Lubrication minimizes friction, wear, and power loss, cools, seals, cleans, and prevents corrosion. In a wet sump pressure system, oil is pumped from the sump through a filter to the main gallery, then to bearings via drilled passages; splash lubricates cylinder walls and a relief valve limits pressure. Wet sump stores oil in the crankcase using one pump, is cheap but can surge; dry sump uses a separate tank and scavenge pump, giving reliable oil supply and better cooling at higher cost.
Why is lubrication necessary in an IC engine? Describe the wet sump pressure lubrication system with a sketch. Differentiate between wet sump and dry sump systems.
Need for lubrication
Lubrication of the sliding parts (piston and rings against liner, bearings, cams and valve gear) is needed to:
- Reduce friction and wear, and so power loss.
- Cool the parts by carrying heat away to the sump.
- Seal the piston ring gap, preventing gas leakage.
- Clean the engine by carrying away carbon and metal particles to the filter.
- Prevent corrosion and reduce noise.
Wet sump pressure (forced feed) system
Sump --> [Strainer] --> [Gear pump] --> [Relief valve]
|
[Oil filter]
|
Main gallery
+---------+-------+--------+
Main Big-end Cam Valve gear
bearings (via bearings (rocker)
drilled
crank)
Return by gravity to sump.
The oil is in the sump at the bottom of the crankcase. A gear (or rotor) pump draws it through a strainer and delivers it at 2-4 bar through a filter into the main gallery. Drilled passages in the block and crankshaft carry it to main bearings, big-end bearings, camshaft bearings and rocker arms. The cylinder walls and gudgeon pins are lubricated by splash and oil mist. A relief valve limits the pressure. The oil returns by gravity to the sump.
Wet versus dry sump
| Basis | Wet sump | Dry sump |
|---|---|---|
| Oil storage | In the crankcase sump | In a separate external tank |
| Pumps | One pressure pump | Scavenge pump plus pressure pump |
| Sump depth | Deep | Shallow, so lower engine height |
| Oil cooling | Poor | Better (separate tank/cooler) |
| Operation on slopes, high g | Oil surge may starve pump | Reliable |
| Cost and complexity | Low | High |
| Use | Cars, bikes, common engines | Aircraft, racing and off-road engines |
✓ Answer checked by Claude Opus 5.5 (high effort)
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