Fuel and Lubrication Systems
This domain tests the petrol and diesel fuel circuits, the meaning of octane and cetane numbers, and the job of engine oil, its viscosity grades and its service intervals.
Fuel systems
- Petrol: tank → electric fuel pump → filter → carburettor or MPFI injectors → inlet manifold; a three-way catalytic converter needs closed-loop mixture control to work.
- Diesel: tank → feed pump → diesel filter → injection pump or common rail → injectors; injection pressures are far higher than in petrol systems.
- Octane number rates petrol's resistance to knock, with iso-octane as 100; higher octane suits higher compression. Cetane number rates diesel ignition quality, with hexadecane as 100; higher cetane means easier cold starting.
- A restricted air filter richens the mixture and raises fuel consumption.
Lubrication
Oil reduces friction, carries heat away, holds dirt in suspension, seals the rings and protects against corrosion. SAE 20W-50 means a winter (W) low-temperature grade of 20 and a hot viscosity of 50 - a multigrade. API or ACEA letters grade service performance, not thickness.
Worked example
A petrol car specified for 91 RON is filled with 87 RON in summer and knocks under load. Reasoning: the lower-octane fuel auto-ignites before the spark arrives, and the knock sensor retards timing, cutting power. Remedy: use the specified octane and check for carbon deposits and over-advanced timing, not add extra oil.
Traps
Swapping octane and cetane; believing a thicker oil is always better, though over-thick oil starves the top end when cold; ignoring an oil-pressure warning instead of stopping the engine; checking oil level with the car on sloping ground; overfilling, which aerates the oil.