Fuel, Lubrication and Cooling Systems
Fuel, Lubrication and Cooling Systems
This domain tests petrol and diesel fuel properties, fuel delivery, lubrication and cooling. Questions ask about octane and cetane numbers, oil grades, thermostat operation and overheating causes.
Core rules
- Petrol: octane number resists knock; higher octane suits high compression. Diesel: cetane number measures ignition quality; higher cetane gives easier cold starting.
- Fuel systems: carburettor (venturi and float), MPFI (port injectors, fuel pump, filter, pressure regulator), CRDi (common rail, high-pressure pump, injectors, ECU).
- Lubrication: sump → strainer → oil pump → filter → galleries → bearings and head → drain back. Oil grade such as 15W-40: 15 is the cold (W = winter) viscosity grade, 40 the hot grade. API/ACEA ratings define service category.
- Cooling: water pump, thermostat, radiator, pressure cap, fan, coolant. A 50:50 coolant concentrate and water mix raises the boiling point and prevents corrosion. The thermostat restricts flow until the engine reaches operating temperature.
- Overheating causes: low coolant, failed thermostat, blocked radiator, faulty fan, slipping belt, blown head gasket.
Worked example
A car overheats only at highway speed, while the temperature is normal in traffic. Reasoning: at speed, airflow through the radiator is high, so the fan is not the likely cause. A collapsed radiator hose, a blocked radiator core, a faulty thermostat or a slipping water-pump belt are more likely. Check coolant level and condition, feel the top and bottom hoses for a temperature difference (a large difference suggests a restricted radiator or closed thermostat), pressure-test the system and check the cap. A failed head gasket would usually show bubbles in the coolant or white exhaust smoke.
Traps
- Using only water as coolant, causing boiling and corrosion.
- Opening the radiator cap when hot.
- Assuming a higher oil grade is always better; follow the manufacturer's specification.
- Confusing octane rating with fuel energy content.