Emission control: EGR, catalytic converters, DPF and SCR

Why it matters

No modern petrol or diesel vehicle can meet BS-VI limits on engine-out emissions alone; every one of them carries an after-treatment train and, usually, exhaust gas recirculation. Knowing what each device does, which chemical reaction it relies on and under what conditions it works lets you size it, diagnose it (a blocked DPF, an empty AdBlue tank, a dead lambda sensor) and answer the "which device removes which pollutant" questions that appear in university exams and interviews.

Key ideas

Two levers: in-cylinder and after-treatment. In-cylinder measures (EGR, injection timing, combustion chamber design) reduce what the engine produces. After-treatment devices clean what leaves the engine. Most strategies trade one pollutant against another: retarding injection or adding EGR lowers NOx but raises soot and fuel consumption, which is why a diesel typically needs both a NOx device and a particulate device.

Exhaust gas recirculation (EGR). A metered fraction of exhaust is returned to the intake. The recirculated CO₂ and H₂O have higher molar heat capacity than air and displace oxygen, so the peak flame temperature falls and thermal (Zeldovich) NO formation, which is strongly temperature-dependent, drops sharply. Cooled EGR (passed through an EGR cooler) is more effective because it also keeps charge density up.

• Costs: lower oxygen availability raises soot and HC in a diesel (the NOx–PM trade-off), deposits and oil contamination, and slightly worse combustion stability. At full load EGR is cut back because the engine needs all the oxygen for power.
• In SI engines EGR (or internal EGR through valve overlap) also reduces throttling (pumping) losses at part load and helps suppress knock.

Three-way catalytic converter (TWC) — petrol engines. A ceramic (cordierite) or metal honeycomb is coated with a high-surface-area washcoat (alumina plus ceria for oxygen storage) carrying Pt/Pd (oxidation) and Rh (NOx reduction). It simultaneously oxidises CO and HC and reduces NO, using CO, H₂ and HC as the reducing agents.

• It works only in a narrow window around stoichiometric mixture (λ ≈ 1, within about ±1%). Rich of this window NO conversion is high but CO/HC conversion falls; lean of it CO/HC are oxidised but NO is not reduced because excess O₂ consumes the reductants. A closed-loop lambda (O₂) sensor upstream holds the mixture in the window; a downstream sensor monitors the catalyst (OBD).
• The catalyst must reach its light-off temperature (the temperature for 50% conversion, typically a few hundred °C, depending on formulation) before it works; most of the emission in a test cycle comes from the cold start. Close-coupled catalysts and fast-warm-up strategies address this.
• Lead, and to a lesser extent sulphur and phosphorus from oil, poison the catalyst; overheating (misfire, rich operation) sinters it.

Diesel oxidation catalyst (DOC). A diesel runs lean, so a TWC cannot reduce NOx there. The DOC (Pt/Pd) oxidises CO and HC (including the soluble organic fraction of PM) and converts part of the NO to NO₂, which helps both passive DPF regeneration and the "fast" SCR reaction.

Diesel particulate filter (DPF). A wall-flow honeycomb (cordierite or silicon carbide) with alternate channels plugged at opposite ends forces exhaust through porous walls; filtration efficiency for soot mass is typically above 90%. Soot accumulates and raises back pressure, so the filter is regenerated:

• passive regeneration — continuous oxidation by NO₂ at moderate temperature (roughly 250–450 °C);
• active regeneration — late post-injection or a dosing injector burns fuel over the DOC to raise exhaust temperature to about 550–650 °C, where soot burns in O₂. Incombustible ash from oil additives is not removed by regeneration, which is why the filter needs eventual cleaning and low-ash oils are specified. Short city trips that never reach regeneration conditions are the usual cause of DPF blockage.

Selective catalytic reduction (SCR). An aqueous urea solution (AdBlue/DEF, 32.5% urea by mass) is sprayed into the hot exhaust, hydrolyses to ammonia, and NH₃ reduces NOx to N₂ and H₂O over a vanadium, Cu-zeolite or Fe-zeolite catalyst. "Selective" means NH₃ reacts preferentially with NOx rather than with the excess O₂. Conversion above 90% is possible once the catalyst is warm. Over-dosing causes ammonia slip, which is removed by an ammonia slip catalyst (ASC) downstream. Urea deposits form if the exhaust is too cold for complete decomposition.

Lean NOx trap (LNT). An alternative for small diesels: NOx is stored as barium nitrate during lean running and released and reduced during short rich purges. It is sensitive to sulphur.

Typical layout. BS-VI diesel car: EGR + DOC → DPF → urea injection → SCR → ASC. BS-VI petrol car: close-coupled TWC (plus a gasoline particulate filter on some direct-injection engines).

Formulas

η_conv = (C_in − C_out) / C_in × 100

• η_conv = conversion efficiency of a device (%); C_in, C_out = pollutant concentration (ppm) or mass emission (g/kWh, g/km) entering and leaving. Applies to any device when flow is the same at inlet and outlet.

η_overall = 1 − (1 − η₁)(1 − η₂)…

• Overall fractional reduction of devices in series, each η as a fraction.

EGR rate = ṁ_EGR / (ṁ_air + ṁ_EGR) × 100

• ṁ_EGR = recirculated exhaust mass flow (kg/s), ṁ_air = fresh-air mass flow (kg/s). Some texts define it on a volume or CO₂-concentration basis; state which you use.

TWC reactions: 2CO + O₂ → 2CO₂ ; CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O ; 2NO + 2CO → N₂ + 2CO₂

SCR reactions: CO(NH₂)₂ + H₂O → 2NH₃ + CO₂ (urea hydrolysis) ; 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O (standard SCR) ; NO + NO₂ + 2NH₃ → 2N₂ + 3H₂O (fast SCR)

DPF regeneration: C + O₂ → CO₂ (active) ; C + 2NO₂ → CO₂ + 2NO (passive)

Molar masses used below: urea 60.06 g/mol, C 12 g/mol, O₂ 32 g/mol; exhaust taken as about 29 g/mol.

Worked examples

Example 1 (standard): EGR plus SCR in series. Given: engine-out NOx without EGR = 5.0 g/kWh; EGR reduces engine-out NOx by 40%; SCR conversion = 90%. Find the tailpipe NOx and the overall reduction.

1. After EGR: NOx₁ = 5.0 × (1 − 0.40) = 3.0 g/kWh.
2. After SCR: NOx₂ = 3.0 × (1 − 0.90) = 0.30 g/kWh.
3. Overall: η_overall = 1 − (1 − 0.40)(1 − 0.90) = 1 − 0.06 = 0.94. Answer: tailpipe NOx = 0.30 g/kWh, overall reduction = 94% (not 40% + 90% = 130%).

Example 2 (GATE level): urea consumption of an SCR system. Given: exhaust mass flow 0.10 kg/s with molar mass 29 g/mol; NOx at SCR inlet = 600 ppm (molar), all as NO; complete conversion by standard SCR; AdBlue is 32.5% urea by mass. Find the AdBlue consumption in kg/h.

1. Exhaust molar flow: ṅ = 0.10 / 0.029 = 3.448 mol/s.
2. NO flow: ṅ_NO = 600 × 10⁻⁶ × 3.448 = 2.069 × 10⁻³ mol/s.
3. Standard SCR uses NH₃ : NO = 1 : 1, so ṅ_NH₃ = 2.069 × 10⁻³ mol/s.
4. Each mole of urea gives 2 mol NH₃: ṅ_urea = 1.034 × 10⁻³ mol/s; mass = 1.034 × 10⁻³ × 60.06 = 0.0621 g/s.
5. AdBlue: 0.0621 / 0.325 = 0.191 g/s = 0.191 × 3600 / 1000 = 0.688 kg/h. Answer: AdBlue consumption ≈ 0.69 kg/h.

Example 3: EGR rate and DPF regeneration oxygen. (a) Fresh air 0.050 kg/s, recirculated exhaust 0.010 kg/s: EGR rate = 0.010 / (0.050 + 0.010) × 100 = 16.7%. (b) A DPF holds 10 g of soot (taken as carbon). Oxygen needed for active regeneration, C + O₂ → CO₂: m_O₂ = 10 × 32 / 12 = 26.7 g. Answers: 16.7%; 26.7 g of O₂.

Common mistakes

• Adding efficiencies of devices in series instead of multiplying the pass-through fractions (1 − η).
• Saying a TWC works on a diesel: it needs λ ≈ 1; diesel exhaust is lean, so NOx needs SCR or an LNT.
• Writing that the catalyst splits NO into N₂ and O₂ by itself; NO is reduced by CO, H₂ or HC (or NH₃ in SCR).
• Thinking EGR reduces all emissions: it cuts NOx but tends to raise PM, HC and fuel consumption.
• Confusing DPF regeneration (burning soot) with SCR (no regeneration, but needs urea refills) and forgetting that ash is not burned off.
• Forgetting that urea gives two NH₃ per molecule, or using AdBlue mass as urea mass.
• Ignoring light-off: a cold catalyst converts almost nothing.

For GATE ME

Expect conceptual MCQ/MSQ matching device to pollutant (TWC: CO, HC, NOx at λ ≈ 1; DOC: CO, HC; DPF: PM; SCR and EGR: NOx), questions on why TWCs fail on lean mixtures, the NOx–PM trade-off, and short numericals on conversion efficiency, devices in series, EGR rate and reagent quantity from stoichiometry. Practise balancing the SCR and oxidation reactions and carrying ppm to mol/s to kg/h conversions.

Quick check

1. Why can a three-way catalyst not reduce NOx in a lean-burn diesel exhaust?
2. Two devices remove 50% and 80% of NOx in series. What is the overall reduction?
3. Which reaction makes passive DPF regeneration possible at around 300 °C?
4. How many moles of NH₃ does one mole of urea give?
5. Why does EGR raise soot in a diesel engine?

Answers: 1. Excess O₂ consumes the reductants (CO, H₂, HC), so none are left to reduce NO. 2. 1 − 0.5 × 0.2 = 90%. 3. C + 2NO₂ → CO₂ + 2NO (NO₂ made by the DOC). 4. Two. 5. Less oxygen and lower flame temperature reduce soot oxidation.