Catalytic converter, EGR, EVAP and DPF

(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)

 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.