Fuel, Ignition, Cooling and Lubrication Systems
This domain covers the four circuits that keep an engine running and alive: fuel supply, ignition, cooling and lubrication. The paper asks for component names and functions, carburettor versus MPFI, petrol versus diesel injection, ignition advance, thermostat operation, and oil and coolant specifications.
Fuel and ignition
- Petrol path: tank, filter, pump, injectors or carburettor, inlet manifold. MPFI meters fuel electronically from sensor inputs such as MAP, TPS and lambda.
- Diesel path: tank, filter, high-pressure pump, injector. Common rail holds rail pressure up to roughly 2000 bar and the ECU controls injection timing.
- Ignition: battery, switch, coil, distributor or ECU, spark plug. The spark must advance with engine speed (centrifugal) and with light load (vacuum).
- Spark plug heat range: a hot plug suits a cold, slow engine; a cold plug suits a hot, high-speed engine.
Cooling and lubrication
- Water cooling: pump, block, head, thermostat, radiator, fan. The thermostat stays closed when cold so the engine warms quickly and opens near 80-90 degrees Celsius.
- Lubrication is pressure-fed from the oil pump through the filter to main and big-end bearings, camshaft and rockers; cylinder walls and pins receive splash.
- SAE 15W-40: the W number is the cold (winter) viscosity, 40 is the hot viscosity. Oil colour alone does not prove the oil is worn out.
Worked example
An engine receives 30 kW of chemical energy from fuel; 30 percent becomes brake power and 30 percent passes to the coolant. Heat to coolant = 0.30 x 30 = 9 kW. With a coolant temperature rise of 8 degrees C and cp = 4.18 kJ/kg-K, flow = 9 / (4.18 x 8) = 0.269 kg/s, about 0.27 litre/s or 16 litre/min. Reasoning: Q = m x cp x delta T, so a larger permitted temperature rise or a larger pump reduces radiator size.
Traps that cost marks
Saying the thermostat controls the water pump. Mixing OAT and conventional coolant. Assuming a higher octane number adds power to a low-compression engine. Treating AdBlue as an engine-oil or fuel additive.