Fuel, Emission Control and Alternative Fuel Systems
This domain covers petrol injection, diesel common-rail, emission formation and after-treatment, OBD monitoring and alternative fuels. Questions link a fault or gas reading to a system: misfire, catalyst, EGR, DPF, SCR or EVAP.
Core rules
- Petrol MPFI uses an electric pump, filter, pressure regulator, injectors and sensors. Diesel CRDI uses very high rail pressure; never crack a high-pressure line with the engine running.
- Exhaust gases: CO and HC rise with rich mixtures or misfire; NOx rises with high combustion temperature and lean conditions; CO2 indicates combustion efficiency; O2 shows excess air.
- Catalytic converter oxidises CO and HC and reduces NOx. P0420 means catalyst efficiency below threshold; check exhaust leaks, misfire and rear oxygen-sensor response before condemning the converter.
- EGR lowers NOx; a stuck EGR valve causes rough idle or pinging. DPF traps soot and needs regeneration; repeated short trips can block it. SCR uses AdBlue to reduce NOx.
- BS-VI vehicles monitor emissions with OBD. Alternative fuels include CNG, LPG, ethanol blends and electricity; each has specific safety and service rules.
Worked example
A car sets P0420. Freeze-frame shows normal load and speed, but the rear oxygen sensor switches almost as fast as the front sensor. The technician checks for exhaust leaks and misfire history, then tests the catalyst. Reasoning: a good catalyst stores oxygen, so the rear sensor should switch slowly; fast switching suggests reduced catalyst efficiency or an exhaust leak.
Traps
- Using leaded fuel and poisoning the catalyst and oxygen sensor.
- Forcing DPF regeneration with a high oil level and causing fuel dilution.
- Blanking an EGR valve and increasing NOx.
- Ignoring CNG cylinder inspection and hydro-test dates.