Three components of the emission control system

PRIORITY: What are the three main emission-control systems/components?

Evaporative emission control (EVAP), exhaust gas recirculation (EGR) and the catalytic converter.

If asked for “charcoal canister, EGR and catalytic converter,” which system contains the charcoal canister?

The evaporative emission control system (EVAP).

What two main sources of vehicle emissions are identified in the notes?

Exhaust gases and evaporative fuel vapours.

PRIORITY: Match each emission-control system to its main job.

EVAP captures fuel vapours; EGR reduces NOx formation; the catalytic converter treats exhaust CO, HC and NOx.

What does EVAP do with gasoline vapours?

It stores them in a charcoal canister and later sends them into the engine to burn.

What is adsorption in the charcoal canister?

Fuel-vapour molecules adhere to the charcoal’s surface.

What is EVAP purging?

Airflow draws stored fuel vapours from the charcoal canister into the engine intake.

Trace fuel vapours through EVAP.

Tank → charcoal canister → purge valve → intake manifold → combustion.

Why should you avoid overfilling the fuel tank?

Liquid gasoline can enter and interfere with a canister designed for vapours.

What is the normal state of the EVAP purge valve?

Normally closed; the PCM pulses it open during purging.

What is the normal state of the EVAP vent valve?

Normally open; the PCM closes it during a system leak test.

What two functions does enhanced EVAP monitoring check?

Purge flow and system leakage.

When does the lecture say EVAP purging is permitted?

During closed-loop operation at cruising speeds.

What is the purpose of onboard refuelling vapour recovery (ORVR)?

To capture fuel vapours displaced from the tank during refuelling.

PRIORITY: Explain how EGR reduces NOx.

It recirculates some exhaust into the intake, diluting the fresh charge and lowering peak combustion temperature, which reduces thermal NOx formation.

Does EGR mainly remove NOx after combustion?

No. It reduces how much NOx forms during combustion.

When is EGR usually not required in the lecture model?

At idle, when the engine is cold and at wide-open throttle.

Why is EGR usually disabled at wide-open throttle in the lecture model?

To avoid diluting the fresh charge when maximum power is required.

How does a digital EGR valve control flow?

It uses combinations of three independently controlled solenoids with different-sized orifices.

How many nonzero combinations are possible with three digital EGR solenoids?

Seven; the eighth combination is all off.

How does a linear EGR valve regulate flow?

A pulse-width-modulated solenoid controls opening, with a potentiometer reporting valve position.

PRIORITY: Which pollutants does a three-way catalytic converter treat?

Carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx).

Which pollutants undergo oxidation in the converter?

CO and HC.

What are the products of CO and complete HC oxidation?

CO becomes CO2; HC becomes CO2 and water.

Which pollutant undergoes reduction in the converter?

NOx, producing nitrogen.

Which catalyst metals are listed in the notes?

Platinum, palladium and rhodium.

What is cerium’s role in the converter?

It stores oxygen during lean periods and supplies oxygen during rich periods.

What gasoline air–fuel ratio is used as the converter’s stoichiometric reference?

Approximately 14.7:1 by mass.

Why is operation near stoichiometry important for a three-way converter?

It balances the conditions needed for NOx reduction and HC/CO oxidation.

What is catalytic-converter light-off?

The temperature at which approximately 50% conversion efficiency is achieved under the stated conditions.

What approximate light-off temperature is given in the notes?

260°C.

What is the purpose of the converter’s porous washcoat?

To increase surface area for catalyst material and reactions.

What is catalyst poisoning?

Compounds attach to and deactivate active catalyst sites.

What is catalyst fouling?

Deposits accumulate on the surface and reduce access to active sites.

What is catalyst sintering?

High temperatures cause catalyst particles to coarsen or fuse, reducing usable surface area.

What is washcoat sintering?

High temperatures densify the washcoat structure and reduce access to catalytic material.