Fuel, Emission Control and Alternative Fuels
Fuel, Emission Control and Alternative Fuels
This domain tests fuel delivery, mixture formation, emission formation and after-treatment, plus CNG, LPG, ethanol, biodiesel and electric vehicles. Expect air-fuel ratio, lambda, catalytic converter, EGR, PCV, EVAP, DPF, SCR and BS-VI norms.
Core rules and concepts
- Petrol stoichiometric ratio is about 14.7:1 by mass. Rich mixtures raise CO and HC; lean mixtures raise NOx and can misfire.
- A three-way catalytic converter converts CO, HC and NOx near stoichiometry. Lead and sulphur poison it, and a misfire can overheat it.
- EGR lowers peak combustion temperature and reduces NOx. PCV returns crankcase vapours to the intake. EVAP captures fuel vapour from the tank.
- Diesel runs lean and produces NOx and particulates. A DPF traps soot; SCR uses AdBlue urea solution to reduce NOx.
- BS-VI limits are tighter than BS-IV and rely on advanced after-treatment and OBD monitoring.
- CNG has a high octane rating and is stored at high pressure. LPG, ethanol blends, biodiesel and electric vehicles each need specific safety procedures.
Worked example
A scan tool shows P0171 with long-term fuel trim +25% at idle and +5% at 2500 rpm. Reasoning: a vacuum leak adds unmetered air mainly at idle, when manifold vacuum is high. The effect falls at higher speed. Check intake hoses, the PCV system and manifold gaskets, repair the leak, then verify trims return near zero.
Traps that cost marks
Putting AdBlue into the diesel tank; using leaded fuel in a catalyst-equipped car; ignoring a misfire that overheats the catalyst; skipping CNG cylinder inspection; and assuming a blocked EGR reduces NOx when it actually tends to increase it.
Key points
- Petrol stoichiometry is about 14.7:1.
- Rich mixture raises CO/HC; lean raises NOx and misfire risk.
- Three-way catalysts need closed-loop stoichiometry.
- DPF handles soot; SCR with AdBlue handles NOx.
- CNG is high-pressure and needs cylinder inspection.