Cooling and Lubrication Systems
This domain asks why an engine must be cooled and lubricated, how each system works, and how to diagnose the common failures - overheating, running cold, low oil pressure and oil consumption.
Why cooling is needed
Only about a quarter to a third of the heat released by the fuel becomes useful work. Some heat is carried away by the exhaust and some by the coolant; without control, the pistons, valves and bearings would overheat, the oil film would break down and the metal would expand out of tolerance.
Air and water cooling
Air cooling uses fins on the cylinder and head to increase surface area, with a fan and shrouds; it is simple and has no coolant to leak, but cooling is less even. Water cooling uses a water jacket around the cylinder and head, a pump (usually a centrifugal impeller type), a thermostat, a radiator with a pressure cap, a fan and hoses. The pressure cap raises the boiling point above 100 degrees C, and antifreeze (ethylene glycol) lowers the freezing point and raises the boiling point further. A thermostat that sticks closed causes overheating; one that sticks open makes the engine run cold, wasting fuel and giving poor heater performance.
Why lubrication is needed
Oil reduces friction and wear, carries heat away from bearings, helps seal the rings, keeps the engine clean in suspension, cushions shock loads and protects against corrosion. Systems include splash, pressure feed with a gear or rotor pump, dry sump, and the petroil method for small two-strokes where oil is mixed with petrol (typically 3 to 5 percent). Vital parts are the strainer, filter, pressure relief valve, oil galleries and dipstick. Oil is graded by SAE viscosity, and multigrade oils such as 15W-40 behave as a thin oil when cold and a thicker one when hot.
Worked example
An engine overheats only at speed on the road while the temperature is normal at idle. Reasoning: the fan and thermostat are probably working, but coolant flow or heat rejection is inadequate at high load - check for a blocked radiator core, collapsed hose, slipping or loose fan belt, or a failed water pump.
Traps that cost marks
Topping up with plain water permanently; overfilling the sump; confusing low oil pressure with low oil level; condemning the thermostat when the fault is a slipping belt.