Emissions and Emission Control Systems

Pollutants

Petrol engines mainly emit CO, HC and NOx; diesel engines mainly NOx and PM.

Emission norms

Norms are legal limits on the mass of pollutants per km (cars) or per kWh (heavy engines), measured in a standard test cycle.

• Euro norms (Europe): Euro 1 (1992) to Euro 6. Limits are tightened at each step; for example for diesel cars NOx fell from 0.50 g/km (Euro 3) to 0.08 g/km (Euro 6).
• Bharat Stage (BS) norms (India): based on Euro norms; BS-VI is equivalent to Euro 6 and was introduced in April 2020, skipping BS-V.
• EPA (USA): Tier 3 and California LEV standards, in g/mile, with a fleet average.

BS-IV vs BS-VI

• Sulphur in fuel falls from 50 ppm to 10 ppm.
• Petrol car NOx limit falls from 0.08 to 0.06 g/km; diesel NOx falls by about 70 % (0.25 to 0.08 g/km), and PM by 80 % (0.025 to 0.0045 g/km).
• Needs DPF, SCR for diesel, GPF for GDI engines, on-board diagnostics (OBD-II) and real-driving-emission (RDE) tests.

(a) Three-way catalytic converter (TWC)

A stainless-steel shell contains a ceramic honeycomb coated with platinum, palladium and rhodium. Three reactions happen at once:

$$\begin{aligned} 2CO + O_2 &\rightarrow 2CO_2 \\ C_xH_y + O_2 &\rightarrow CO_2 + H_2O \\ 2NO + 2CO &\rightarrow N_2 + 2CO_2 \end{aligned}$$

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)

 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). It is cut off at idle, cold, and full load because of power loss.

(c) Evaporative emission control (EVAP)

Petrol vapour from the fuel tank is stopped from going to the atmosphere. Vapour is led through a pipe to a charcoal canister that adsorbs it. When the engine is running, the ECU opens a purge valve, so that fresh air flows through the canister and carries the stored vapour to the intake manifold to be burnt. A tank pressure valve is fitted.

(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 pressure drop rises, the filter is regenerated: soot is burnt at about 600 degrees C by (passive) catalyst-assisted oxidation or (active) post-injection of fuel. Ash remains and has to be cleaned at long intervals.