Engine emissions and Bharat Stage norms

In short

Engine emissions are the harmful products that leave an engine - carbon monoxide, unburnt hydrocarbons, oxides of nitrogen, particulate matter, plus CO₂ as a greenhouse gas - formed because real combustion is neither complete nor uniform. Bharat Stage (BS) norms are India's mass-emission standards for new vehicles and engines, notified by the Ministry of Road Transport and Highways under the Central Motor Vehicles Rules, 1989 (mainly Rule 115) and tested by ARAI, ICAT and other notified agencies. India jumped from BS-IV straight to BS-VI on 1 April 2020 and added BS-VI Phase 2 (real driving emissions and OBD-II) from 1 April 2023. The topic is examined in Automobile/IC Engines papers, in GATE (ME) under IC engines and pollution, and is asked in every interview at Maruti Suzuki, Tata Motors, Mahindra, Bajaj, TVS, Hero, Ashok Leyland, Cummins, Bosch, ARAI and at testing/homologation and calibration roles.

Explained

Pollutants: what they are and why they form

• Carbon monoxide (CO): product of incomplete combustion where oxygen is locally short. High in rich mixtures - cold start, idling, full-throttle enrichment in SI engines. Diesels run lean, so their CO is low.
• Unburnt hydrocarbons (HC): fuel that escapes burning - flame quenching at cold cylinder walls, fuel trapped in crevices (piston top land, around rings), oil-film absorption, misfire, valve overlap scavenging losses, and in two-strokes the short-circuiting of fresh charge to the exhaust.
• Oxides of nitrogen (NOx = NO + NO₂): atmospheric nitrogen oxidised at high temperature. Three routes - thermal (Zeldovich) NOx, dominant above about 1,800 K; prompt NOx in the flame front; fuel-bound nitrogen (negligible for petrol/diesel). Peaks slightly lean of stoichiometric (λ ≈ 1.05-1.1) where both temperature and oxygen are high.
• Particulate matter (PM) and particle number (PN): soot formed in fuel-rich pockets of a diffusion flame, with adsorbed hydrocarbons, sulphates and ash. Mainly a diesel and GDI problem. PM2.5 and PM10 are the health-critical sizes.
• CO₂: unavoidable product, directly proportional to fuel burnt - reduced only by burning less fuel or by lower-carbon fuel, not by after-treatment.
• Others: SO₂ (from fuel sulphur, now tiny at 10 ppm S), aldehydes (ethanol/methanol blends), ammonia slip (from SCR), and non-exhaust emissions - evaporative (fuel tank breathing, refuelling), crankcase blow-by, tyre and brake wear.
• The classic trade-off: anything that raises combustion temperature and completeness cuts CO, HC and PM but raises NOx. The NOx-PM (soot) trade-off in diesels and the NOx-fuel-economy trade-off are the central calibration problems.

Effect of operating variables

• Air-fuel ratio / lambda: CO and HC fall steeply as the mixture leans out; NOx peaks at λ ≈ 1.05-1.1 then falls; too lean causes misfire and HC rises again.
• Ignition / injection timing: advancing raises peak temperature → more NOx, less HC and CO; retarding cuts NOx but hurts fuel economy and raises exhaust temperature (useful for lighting off a catalyst at cold start).
• Load and speed: NOx rises with load; HC and CO are worst at idle and cold start (most of a test cycle's HC comes in the first 100 seconds).
• Compression ratio: higher ratio → better efficiency, more NOx, more crevice HC.
• Cooling-water temperature: cold engine → more CO and HC; over-hot → more NOx.
• Fuel quality: sulphur poisons catalysts; aromatics and olefins raise soot and HC; cetane number affects diesel ignition delay and hence noise and NOx.

Emission control - engine side, then after-treatment

In-cylinder (primary) measures

• EGR (exhaust gas recirculation): inert exhaust diluent lowers peak temperature and oxygen → big NOx cut; too much raises soot and PM. Cooled high-pressure and low-pressure EGR circuits are used on BS-VI diesels.
• Injection strategy: CRDI at 1,600-2,500 bar with pilot, main and post injections; pilot injection shortens the premixed burn and cuts noise and NOx.
• Combustion chamber design: swirl/tumble, squish, re-entrant bowl, compact chamber to shrink crevice volumes, central spark plug.
• Variable valve timing, turbocharging with intercooling, lean-burn and GDI stratified charge, and homogeneous charge compression ignition (HCCI) in research.
• PCV (positive crankcase ventilation): routes blow-by gases back to the intake - crankcase emissions must be zero.
• Evaporative control: sealed tank, carbon canister with purge valve; checked in a SHED chamber (BS-VI limit 2 g/test for cars).

After-treatment (secondary)

• Three-way catalytic converter (TWC) - petrol only: Pt/Pd oxidise CO and HC, Rh reduces NOx. Works only in a narrow window around λ = 1, so a lambda (O₂) sensor with closed-loop fuelling is mandatory. Needs light-off above ~250-300 °C; lead and sulphur poison it.
• Diesel oxidation catalyst (DOC): oxidises CO, HC and the soluble organic fraction of PM; also makes NO₂ for passive DPF regeneration.
• Diesel particulate filter (DPF): wall-flow cordierite/SiC honeycomb trapping >95% of soot by number. Regeneration - passive (NO₂, continuous) or active (post-injection raises exhaust to 550-650 °C and burns soot off). Ash accumulates and needs periodic cleaning.
• Selective catalytic reduction (SCR): 32.5% urea solution (AdBlue/DEF, to ISO 22241) injected into hot exhaust hydrolyses to ammonia, which reduces NOx to N₂ and H₂O over a vanadium or Cu/Fe-zeolite catalyst; 80-95% conversion. An ammonia slip catalyst (ASC) follows. DEF consumption is roughly 3-6% of diesel consumption; running dry triggers a legally required torque derate.
• Lean NOx trap (LNT/NSC) for small diesels, and gasoline particulate filter (GPF) for GDI engines.
• Secondary air injection and electrically heated catalysts for cold start.

The Bharat Stage framework and the current norms

• Legal basis: Motor Vehicles Act, 1988 and the Central Motor Vehicles Rules, 1989 (Rule 115 - emission standards, Rule 115A, Rule 116 - smoke); the Air (Prevention and Control of Pollution) Act, 1981 and the Environment (Protection) Act, 1986 give the wider framework. Test procedures are in the AIS (Automotive Industry Standard) series, notably AIS-137 for mass emission tests. CPCB monitors ambient air quality (NAAQS).
• History: India 2000 (≈Euro 1, 2000) → BS-II (2001 metros / 2005 nationwide) → BS-III (2005/2010) → BS-IV (2010 selected cities / 1 April 2017 nationwide) → BS-VI nationwide 1 April 2020, skipping BS-V. BS norms are broadly aligned with Euro norms, Euro 6 for BS-VI.
• BS-VI Phase 2, from 1 April 2023: on-road RDE testing with portable emission measurement systems (PEMS) and conformity factors, OBD-II with wider monitoring and in-use performance ratios, and on-board fuel-consumption/efficiency monitoring on cars.
• What BS-VI changed besides the limits: fuel sulphur cut from 50 ppm (BS-IV) to 10 ppm for both petrol and diesel nationwide - the precondition for DPF and SCR; a single national standard with no city-wise phasing; diesel and petrol NOx limits brought much closer; PN limits introduced.
• Test cycles: chassis-dynamometer type approval of cars has used the MIDC (Modified Indian Driving Cycle, NEDC-based, 1,180 s) with full-flow CVS dilution; heavy-duty engines use the WHSC/WHTC steady-state and transient cycles on an engine dynamometer. RDE supplements, not replaces, the lab cycle. A move to the WLTC has been discussed but confirm the current notification before quoting it.
• Fuel-economy/CO₂ side: CAFE (Corporate Average Fuel Efficiency) norms regulate a manufacturer's sales-weighted fleet CO₂ - CAFE-I 130 g/km (2017-18), CAFE-II 113 g/km from April 2022, normalised to a 1,037 kg reference kerb weight; CAFE-III is under finalisation.
• In-use compliance: the PUC (Pollution Under Control) certificate is compulsory under CMVR Rule 115(7) - idle CO% and HC ppm for petrol/CNG/LPG, free-acceleration smoke (light absorption coefficient, m⁻¹) for diesel. Typical notified limits are CO ≤ 0.3% vol and HC ≤ 200 ppm for a BS-IV-and-later petrol car, and smoke ≤ 1.62 m⁻¹ naturally aspirated / 2.12 m⁻¹ turbocharged for diesel; limits differ by vehicle category and BS stage and are revised, so read the current CMVR table. Also in force: the Vehicle Scrappage Policy, 2021 (mandatory fitness testing at Automated Testing Stations, scrapping at Registered Vehicle Scrapping Facilities), NGT/Supreme Court bans on 10-year diesel and 15-year petrol vehicles in Delhi-NCR, and GRAP restrictions.
• Fuels policy: E20 petrol (20% ethanol) rolled out nationwide, with E20-compatible material and calibration requirements; CNG/LPG retrofitment kits need type approval; BS-VI-compatible CNG, LNG, hydrogen and flex-fuel vehicles are being certified. A BS-VII norm has been discussed but is not notified as of now.

Measurement and instrumentation

• CO and CO₂: NDIR (non-dispersive infrared) analyser. HC: FID (flame ionisation detector), heated for diesel. NOx: chemiluminescence (CLD) or NDUV. O₂: paramagnetic or electrochemical.
• PM: gravimetric - dilute exhaust through a filter in a dilution tunnel, weigh in a conditioned room. PN: condensation particle counter after a volatile particle remover (counts solid particles >23 nm).
• Smoke: Hartridge smoke meter (opacity %), opacimeter (light absorption coefficient k, m⁻¹), Bosch smoke number (filter-paper blackening).
• CVS (constant volume sampling) full-flow dilution with critical-flow venturi collects the whole diluted exhaust in bags over the cycle, so results are mass per km regardless of flow measurement errors.
• PEMS for RDE: compact analysers plus exhaust flow meter and GPS, carried on the vehicle over urban/rural/motorway shares with ambient and altitude boundary conditions.

Key points

• Four regulated pollutants - CO, HC (THC/NMHC), NOx, PM/PN; CO₂ is regulated separately through CAFE.
• CO and HC come from incomplete combustion and quenching; NOx from high temperature; PM from fuel-rich diffusion-flame zones.
• Thermal NOx (Zeldovich) becomes significant above about 1,800 K and is extremely temperature-sensitive.
• NOx peaks at λ ≈ 1.05-1.1, not at stoichiometric - a very common exam point.
• NOx-PM trade-off: more EGR or retarded injection cuts NOx but raises soot; the DPF plus SCR combination breaks the trade-off.
• Three-way catalyst needs λ = 1, so it cannot be used on a lean diesel; diesels use DOC + DPF + SCR.
• DPF traps soot and is regenerated passively by NO₂ or actively at 550-650 °C by post-injection.
• SCR uses 32.5% urea (AdBlue/DEF), consumption about 3-6% of fuel; empty DEF forces a torque derate.
• EGR rate is measured as the ratio of intake CO₂ to exhaust CO₂.
• BS-VI from 1 April 2020, nationwide, skipping BS-V; Phase 2 from 1 April 2023 added RDE and OBD-II.
• 10 ppm sulphur fuel is what makes BS-VI after-treatment possible.
• BS-VI diesel car limits: CO 500, HC+NOx 170, NOx 80, PM 4.5 mg/km, PN 6×10¹¹ /km.
• BS-VI petrol car limits: CO 1000, THC 100, NMHC 68, NOx 60 mg/km, PM 4.5 mg/km and PN 6×10¹¹ /km for direct-injection engines.
• BS-IV → BS-VI cut diesel NOx by 68% (250 → 80 mg/km) and PM by 82% (25 → 4.5 mg/km); petrol NOx by 25% (80 → 60).
• Heavy-duty BS-VI: NOx 0.40 g/kWh (WHSC) and 0.46 g/kWh (WHTC), PM 0.01 g/kWh - with mandatory SCR, DPF and OBD.
• CAFE-II: 113 g CO₂/km fleet average from April 2022, at a 1,037 kg reference kerb weight.
• PUC certificate under CMVR Rule 115(7) is the in-use check - idle CO/HC for petrol, free-acceleration smoke for diesel.
• Cold start dominates cycle HC and CO emissions; most of the tailpipe HC appears before catalyst light-off.

Formulas and examples

1. Lambda and mixture state. An SI engine draws 320 kg/h air and 20.5 kg/h petrol.

• AFR = 320/20.5 = 15.61; λ = 15.61/14.7 = 1.062 (lean).
• Interpretation: this is almost exactly the NOx-peak region, and the three-way catalyst's NOx reduction will be poor because there is excess oxygen - the ECU must bring λ back to 1.00.

2. EGR rate and its NOx effect. Intake manifold CO₂ reads 3.2% vol, exhaust CO₂ 12.8% vol.

• EGR rate = 100 × 3.2/12.8 = 25%.
• Fresh-air flow measured 300 kg/h, so ṁ_EGR = 0.25 × total; total = 300/0.75 = 400 kg/h → EGR flow = 100 kg/h.
• At this rate a typical light-duty diesel shows roughly a 50-60% NOx reduction with a measurable rise in smoke - confirm against the engine's own map.

3. Converting a ppm reading into a specific emission (BS-VI heavy-duty check). A truck engine at 40 kW brake power has exhaust mass flow 50 kg/h and raw NOx 500 ppm by volume.

• Mass fraction = 500 × 10⁻⁶ × (46/28.9) = 500 × 10⁻⁶ × 1.5917 = 7.959 × 10⁻⁴.
• ṁ_NOx = 50 × 7.959 × 10⁻⁴ = 0.0398 kg/h = 39.8 g/h.
• BSE = 39.8/40 = 0.995 g/kWh - more than double the BS-VI limit of 0.46 g/kWh, so after-treatment is essential.
• Required SCR efficiency = (0.995 − 0.46)/0.995 = 53.8% minimum to just meet the limit; real systems are designed for 85-95% to hold margin over the cycle and in RDE.

4. Catalyst conversion efficiency. TWC inlet CO 1.20% vol, outlet 0.09%; inlet HC 1,800 ppm, outlet 120 ppm.

• η_CO = (1.20 − 0.09)/1.20 × 100 = 92.5%.
• η_HC = (1,800 − 120)/1,800 × 100 = 93.3%.

5. CO₂, CAFE and DEF consumption.

• A petrol car returns 18 km/l → CO₂ = 2,392/18 = 132.9 g/km, above the 113 g/km CAFE-II fleet target, so the manufacturer must offset it with more efficient or electric models.
• To reach 113 g/km on petrol the car needs 2,392/113 = 21.2 km/l.
• A truck burning 30 l/h of diesel with DEF dosing at 5% by volume uses 1.5 l/h; a 20-litre DEF tank lasts 20/1.5 = 13.3 hours of running.

Where it is used

• Homologation and type approval: ARAI Pune, ICAT Manesar, GARC Chennai, NATRAX - emission labs running MIDC/WHTC, SHED and PEMS tests for every new Indian model.
• Calibration and after-treatment engineering: OEMs (Maruti Suzuki, Tata Motors, Mahindra, Ashok Leyland, Bajaj, TVS, Hero, Royal Enfield) and suppliers (Bosch, Delphi-TVS, Continental, Tenneco, Eberspächer, Cummins Emission Solutions) sizing DOC/DPF/SCR and writing dosing and regeneration strategies.
• Service and diagnostics: OBD scan-tool work on P-codes for catalyst efficiency, EGR flow, DPF differential pressure and NOx sensor faults; DPF ash cleaning; DEF quality complaints.
• PUC centres and transport departments: idle CO/HC and free-acceleration smoke testing, now largely computerised and linked to VAHAN; inspection and certification centres under the scrappage policy.
• Regulators and research: MoRTH, CPCB, state pollution control boards, TERI, IITs and CSIR-IIP on fuel quality, E20/flex fuel and real-world emission inventories.
• Fuels and lubricants: refineries supplying 10 ppm sulphur BS-VI fuel and low-SAPS (low sulphated-ash) oils needed to protect DPFs.

Interview questions

• Q: Why did India skip BS-V and go from BS-IV to BS-VI? A: To cut urban pollution faster; a single leap avoided two separate investments in refineries and vehicles, and BS-VI brought 10 ppm sulphur fuel plus DPF/SCR technology in one step from 1 April 2020.
• Q: At what mixture strength is NOx maximum, and why? A: Slightly lean, around λ = 1.05-1.1 - there is still enough excess oxygen and the flame temperature is still high; richer or much leaner mixtures both lower temperature or oxygen.
• Q: Why can a three-way catalyst not be used on a diesel engine? A: It needs a stoichiometric, near-zero-oxygen exhaust to reduce NOx; a diesel always runs lean, so NOx reduction must be done chemically with urea/SCR or an LNT.
• Q: What exactly does BS-VI Phase 2 add over Phase 1? A: On-road real driving emission testing with PEMS and conformity factors, full OBD-II monitoring with in-use performance ratios, and on-board fuel-consumption monitoring - the limits themselves are largely unchanged.
• Q: How is DPF regeneration triggered and carried out? A: The ECU watches the filter's differential pressure and soot model; passive regeneration uses NO₂ continuously, while active regeneration uses post-injection or a fuel burner to raise exhaust temperature to about 550-650 °C and oxidise the soot.
• Q: What happens if the AdBlue tank runs empty? A: NOx conversion stops, so the regulation requires driver warnings followed by a torque derate and a speed limit until DEF is refilled - it is a compliance requirement, not a mechanical fault.
• Q: Why is low-sulphur fuel essential for BS-VI? A: Sulphur poisons catalyst washcoats and SCR, and forms sulphate particulates that plug DPFs; 10 ppm fuel protects them over the full durability distance.
• Q: How does EGR reduce NOx? A: Recirculated exhaust is inert and has a high heat capacity, so it dilutes oxygen and lowers peak combustion temperature, cutting thermal NOx formation.
• Q: Why do most of a test cycle's HC and CO emissions appear in the first minute? A: The engine runs rich and the mixture quenches on cold walls while the catalyst is still below its light-off temperature, so almost nothing is converted.
• Q: What is measured in a PUC test, and is it the same as a BS-VI test? A: PUC measures idle CO and HC for petrol and free-acceleration smoke for diesel on an in-use vehicle; BS-VI is a full mass-emission type-approval test in g/km on a chassis dynamometer - they are not comparable.
• Q: Can after-treatment reduce CO₂? A: No - CO₂ is proportional to the carbon burnt; only better efficiency, downsizing, hybridisation or lower-carbon fuel reduces it, which is why CAFE norms exist separately.
• Q: Why do GDI petrol engines now need a particulate filter? A: Direct injection leaves fuel-rich zones and wall wetting, producing soot like a diesel, so BS-VI applies PM and PN limits to direct-injection petrol engines and a GPF is fitted.

Common mistakes

• Saying NOx is highest at a rich mixture - it peaks slightly lean; CO and HC are the rich-mixture pollutants.
• Treating CO₂ as a regulated BS pollutant; BS limits CO, HC, NOx, PM/PN, while CO₂ is handled by CAFE.
• Claiming a three-way catalyst also works on diesel engines, or that SCR removes soot (SCR removes NOx; the DPF removes soot).
• Quoting a single "BS-VI limit" without saying petrol or diesel, vehicle category, and units - mg/km for light duty, g/kWh for heavy duty.
• Forgetting the molecular-weight ratio when converting ppm to mass, or using the HC reading in ppm directly as g/h.
• Confusing the PUC in-use test with type-approval mass-emission testing, and quoting idle CO% against a g/km limit.
• Saying BS-VI was phased in city by city like BS-IV - it took effect nationwide on the same date.
• Assuming EGR can be increased without limit; beyond the smoke limit it worsens PM, fuel consumption and combustion stability.

Related topics

IC engines · Alternative fuels · Automobile engineering (vehicle systems) · Thermodynamics and combustion · Catalysis and exhaust after-treatment · Air pollution and environmental regulation in India