Fuel, Lubrication and Cooling Systems
This domain covers fuel delivery and metering, engine lubrication and temperature control. The exam tests air-fuel ratio, injector and carburettor basics, oil viscosity and pressure, coolant flow, thermostat operation and the safety rules for hot systems.
Fuel system
- Stoichiometric air-fuel ratio is about 14.7:1 by mass for petrol and about 14.5:1 for diesel. Rich mixtures have excess fuel; lean mixtures have excess air.
- Carburettors meter fuel by venturi vacuum. Modern petrol engines use electronic fuel injection, often multi-point (MPFI); common-rail diesel injection (CRDI) uses very high rail pressure and an electronic control unit.
- The fuel filter traps dirt and water; a blocked filter causes low power, hard starting and pump overload.
Lubrication and cooling
- The oil pump delivers oil through galleries to bearings and the camshaft. The filter removes particles; a bypass valve maintains flow if the filter blocks.
- Viscosity grade such as 10W-30 indicates cold and hot flow behaviour. Use the grade and API service category specified for the engine.
- Coolant circulates from the water pump through the block and head to the radiator. The thermostat opens typically around 80-90°C to control temperature. A pressure cap raises the boiling point.
- Use a 50:50 mix of antifreeze and distilled water unless the manufacturer specifies otherwise.
Worked example
A cooling system needs 4 litres of 50:50 coolant. Reasoning: half the volume is antifreeze, so 2 litres of antifreeze plus 2 litres of distilled water. Adding only water lowers the boiling point and freeze protection and increases corrosion.
Traps
Opening a hot radiator cap, using plain water as permanent coolant, mixing incompatible coolant types, fitting the wrong oil viscosity, and overfilling oil are common errors. A stuck-open thermostat causes slow warm-up and poor heater performance; a stuck-closed one causes overheating.