--- title: "Catalytic converter, EGR, EVAP and DPF" subject: "Automotive Technology" chapter: 10 key: "semester_5/automotive_technology:emission-control-devices" source: https://www.bctengineer.com/q/semester_5/automotive_technology/emission-control-devices site: BCT Engineering --- # Catalytic converter, EGR, EVAP and DPF Automotive Technology · Chapter 10: Emissions and Emission Control Systems - Asked in: year not given, 8 marks - Video: [Understanding Emissions-Control Systems](https://www.youtube.com/watch?v=1560L2xTPVg) — Counterman - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/automotive_technology/emission-control-devices - Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-10 ## Question Explain the working of (a) a three-way catalytic converter, (b) exhaust gas recirculation (EGR), (c) an evaporative emission control system and (d) a diesel particulate filter. ## Short answer A three-way catalytic converter oxidizes CO/HC and reduces NOx at stoichiometric using Pt/Pd/Rh and lambda sensor. EGR recirculates exhaust to lower combustion temperature, cutting NOx. EVAP captures tank vapor in a charcoal canister, purged later. DPF traps soot and regenerates by burning via NO2 or elevated exhaust temperature. ## Answer #### (a) Three-way catalytic converter (TWC) A stainless-steel shell contains a ceramic honeycomb coated with platinum, palladium and rhodium. Three reactions take place at the same time: $$ \begin{aligned} 2CO + O_2 &\rightarrow 2CO_2 \\ C_xH_y + O_2 &\rightarrow CO_2 + H_2O \\ 2NO + 2CO &\rightarrow N_2 + 2CO_2 \end{aligned} $$ It works only close to the stoichiometric AFR (14.7:1), so a **lambda (oxygen) sensor** and the ECU control the mixture. It needs 250 to 300 degrees C to light off. Leaded fuel poisons it. #### (b) Exhaust gas recirculation (EGR) ```text Exhaust manifold -> [EGR valve + cooler] -> Intake manifold ``` A part (5 to 20 %) of the exhaust gas is returned to the intake. The inert gas dilutes the charge, absorbs heat and lowers the peak flame temperature, so **NOx formation falls** (NOx forms above about 1800 K). EGR is cut off at idle and when the engine is cold (to avoid rough running), and at full load (to avoid loss of power). #### (c) Evaporative emission control (EVAP) This system stops petrol vapour from the fuel tank escaping to the atmosphere. The vapour is led through a pipe to a **charcoal canister**, which adsorbs it. When the engine is running, the ECU opens a **purge valve**, so fresh air flows through the canister and carries the stored vapour into the intake manifold to be burnt. A vent/roll-over valve and a pressure-vacuum filler cap are also fitted to the tank. #### (d) Diesel particulate filter (DPF) A ceramic (silicon carbide or cordierite) honeycomb with alternate channels plugged, which forces the exhaust gas through the porous walls. Soot (PM) is trapped on the walls. When the pressure drop rises, the filter is **regenerated** by burning the soot. In passive regeneration, soot burns continuously at about 250 to 450 degrees C using $NO_2$ made by an upstream oxidation catalyst (or a catalyst coating). In active regeneration, the ECU raises the exhaust temperature to about 600 degrees C by post-injection of fuel. Ash remains and has to be cleaned out at long intervals. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a three-way catalyst need a lambda sensor? A. The sensor heats the catalyst B. It operates below 100 C C. It works effectively only near the stoichiometric AFR of 14.7:1 D. It needs a rich mixture Answer: C. The ECU uses the sensor to keep the mixture near stoichiometric, where the three reactions occur together. (Key point: TWC) 2. How does EGR reduce NOx? A. Inert exhaust gas dilutes the charge and lowers the peak flame temperature B. It filters particulates C. It adds oxygen D. It raises the temperature Answer: A. NOx forms above about 1800 K, so lowering the flame temperature reduces it. (Key point: EGR) 3. How is vapour from the tank disposed of? A. Released slowly outside B. Burnt in the catalyst only C. Adsorbed in a charcoal canister, then purged to the intake manifold to burn D. Condensed back through the radiator Answer: C. The ECU opens the purge valve when the engine runs so fresh air carries vapour into the intake. (Key point: EVAP) 4. How does a DPF regenerate? A. The filter is replaced after each trip B. Soot is sent to the canister C. Soot is washed out with water D. Soot is burnt at about 600 C by catalyst-assisted oxidation or post-injection of fuel Answer: D. Trapped soot is burned off when the pressure drop rises; ash remains and needs occasional cleaning. (Key point: DPF regeneration) ## Related in Automotive Technology - [Exhaust pollutants and emission norms](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aemission-norms-and-pollutants) — Automotive Technology, chapter 10 - [Automatic transmission and torque converter](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aautomatic-transmission-torque-converter) — Automotive Technology, chapter 5 ## Related in other subjects - [Recirculation ratio, filter volume 510 m³](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atrickling-recirculation-ratio-510m3) — Sanitary Engineering, chapter 7 - [Recirculation ratio, filter volume 350 m³](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atrickling-recirculation-ratio-350m3) — Sanitary Engineering, chapter 7 - [Ideal diesel cycle working principle](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Three-phase single-way thyristor rectifier: average and RMS](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3A3ph-single-way-thyristor-avg-rms) — Power Electronics, chapter 3, asked 5×