Fuel, Lubrication and Cooling Systems
This domain tests fuel delivery, fuel ratings, lubrication and engine cooling.
Fuel systems
- Petrol engines are spark-ignition; diesel engines are compression-ignition.
- The stoichiometric air-fuel ratio for petrol is about 14.7:1 by mass.
- Octane rating measures petrol's resistance to knock; cetane rating measures diesel ignition quality.
- Carburettors mix fuel and air mechanically; MPFI and CRDI systems use electronic injectors.
- A clogged air filter richens the mixture and raises fuel consumption.
Lubrication and cooling
- Oil lubricates, cools, cleans and seals. Viscosity is its resistance to flow.
- In 15W-40, W means winter; 15 is the cold-temperature grade and 40 the high-temperature grade.
- The oil pump circulates oil; the filter removes particles; the relief valve limits pressure.
- The thermostat controls coolant flow and typically opens around 82–88 °C.
- A pressure cap raises the boiling point of the coolant.
- Coolant is usually a 50:50 mix of antifreeze and water.
Worked example
Explain 15W-40. The 15W grade means the oil behaves like a 15-grade oil when cold, aiding starting; the 40 grade means it behaves like a 40-grade oil at operating temperature. It is a multigrade oil, not a mixture with a 15–40 range. The reasoning is that the W rating is measured cold and the second number hot.
Traps
- Thinking higher octane always gives more power; it mainly resists knock.
- Opening a hot radiator cap, risking scalding.
- Overfilling engine oil, which can cause foaming and seal leaks.
- Confusing octane and cetane ratings.