Emission

Engines emit CO₂, CO, HC, NOx and particulates. Learn how each forms, emission standards (Euro 6/7, BS-VI, EPA Tier 3), CO₂ from fuel consumption, three-way catalysts and λ control, diesel oxidation catalysts, DPF, SCR with urea, GPF, EGR, and real-driving emissions testing.

Modern cars emit well over 90 % less regulated pollution than 1970s cars, thanks to precise fuelling and catalytic after-treatment. CO₂, however, is directly tied to how much fuel is burned — only efficiency or low-carbon fuels reduce it.

Emission engineering combines combustion control, catalysts and sensors to meet strict limits in lab and real-world driving.

An analogy

A catalytic converter is like a chemical sieve that holds pollutant molecules long enough to react into harmless ones — but only when the mixture is balanced (λ ≈ 1) and the catalyst is hot.

Pollutants and their origins

CO and HC: incomplete combustion (rich, cold, misfire). NOx: high flame temperatures with excess oxygen. Particulates: soot from fuel-rich zones (diesel, GDI). CO₂: every kg of petrol produces ≈ 3.1 kg CO₂ — unavoidable without less fuel or low-carbon fuel.

Petrol after-treatment

The three-way catalyst oxidises CO and HC and reduces NOx simultaneously — only in a narrow window around λ = 1, maintained by upstream and downstream oxygen sensors. Cold starts produce most emissions until the catalyst lights off (~250–300 °C). GDI engines add gasoline particulate filters.

Diesel after-treatment

Diesel oxidation catalyst (CO, HC), diesel particulate filter (traps soot, periodically regenerated by burning it at ~600 °C), and selective catalytic reduction (urea → ammonia reduces NOx to N₂ and water). EGR lowers combustion temperature to cut engine-out NOx.

Regulation and testing

Euro 6/BS-VI limits (e.g. NOx 60 mg/km petrol, 80 mg/km diesel; PM 4.5 mg/km; PN 6 × 10¹¹/km) tested on WLTP plus real driving emissions (RDE) with portable measurement. Euro 7 adds brake and tyre particles and battery durability. Dieselgate showed why real-world compliance matters.

Formulas

• CO₂ from fuel use: CO₂ (g/km) = L/100 km × 23.1 (petrol) or × 26.8 (diesel)
• Required conversion: η = 1 − tailpipe / engine-out
• Urea (AdBlue) use: ≈ 3–5 % of diesel consumption

Worked example: CO₂ per km

Given: Petrol car using 6.0 L/100 km

Find: CO₂ emission.

• 6.0 × 23.1 = 138.6 g/km

Answer: ≈ 139 g/km

Worked example: SCR efficiency needed

Given: Diesel engine-out NOx 1.0 g/km; Euro 6 limit 80 mg/km

Find: Required conversion.

• η = 1 − 0.08/1.0 = 92 %

Answer: ≥ 92 % NOx conversion (plus margin for ageing)

Common mistakes

• Wrong: Believing catalysts reduce CO₂. — Right: CO₂ depends on fuel burned; catalysts convert CO and HC into more CO₂.
• Wrong: Ignoring cold start. — Right: Most emissions occur before catalyst light-off — heat it fast.

Applications

• Homologation: WLTP and RDE testing for type approval.
• On-board diagnostics: Catalyst, DPF and sensor monitors trigger MIL lamps.
• Fleet CO₂ targets: Manufacturers balance ICE, hybrid and EV sales to meet averages.

Key takeaways

• CO/HC from incomplete burn, NOx from heat, PM from rich zones.
• TWC needs λ ≈ 1 and heat.
• Diesel: DOC + DPF + SCR (+ EGR).
• CO₂ ∝ fuel burned — efficiency is the only fix.

Going further

• Electrically heated catalysts: Reach light-off in seconds for hybrids with frequent engine stops.
• Lean NOx traps: Store NOx during lean running and reduce it during brief rich purges.
• Euro 7: Adds non-exhaust particles, longer durability and battery-health requirements.