Fuel, Ignition, Cooling and Lubrication Systems
Fuel, Ignition, Cooling and Lubrication Systems
This domain tests the systems that prepare the charge, ignite it, remove heat and reduce friction. Expect questions on petrol octane rating, diesel cetane rating, stoichiometric air-fuel ratio, ignition timing, coolant circulation, thermostat operation, oil pumps and viscosity.
Core rules
- Octane number measures petrol's resistance to knock; cetane number measures diesel's ignition quality. High octane suits high-compression SI engines; high cetane gives easier cold starting.
- Stoichiometric AFR is about 14.7:1 by mass for petrol and about 14.5:1 for diesel. A rich mixture has excess fuel; a lean mixture has excess air.
- The ignition system must produce a high-voltage spark at the correct time; advancing timing too far causes knock, retarding it loses power and raises exhaust temperature.
- Cooling system: water pump circulates coolant, radiator rejects heat, thermostat keeps the engine near operating temperature, and the pressure cap raises boiling point.
- Lubrication: oil pump supplies pressurised oil to bearings; viscosity grade must match ambient temperature and engine design. Splash lubrication is common in small engines.
Worked example
A petrol engine burns 4 kg of fuel per hour at an actual air flow of 70 kg/h. Stoichiometric air needed = 4 × 14.7 = 58.8 kg/h. The excess air is 70 − 58.8 = 11.2 kg/h, so the mixture is lean. Reasoning: compare actual air with the stoichiometric requirement; a higher actual air figure means excess air, not excess fuel.
Traps
- Swapping octane and cetane ratings.
- Treating rich and lean as opposites of 'more power' without linking to air-fuel mass.
- Assuming a thermostat stuck open causes overheating; it usually causes slow warm-up and overcooling.
- Using too thick an oil to protect a modern engine and restricting cold flow.