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Automotive Emissions and Onboard Diagnostics

by John West & Mark Stachew

Start ý Why Onboard Diagnostics? ý Major Components ý Catalytic Converter ý Oxygen Sensor ý EGR ý Fuel System Adaptive Learning ý Misfire Detection ý The Need for Speed ý New Regulations ý 0 to 100 KB in 6 Seconds ý Sources and PDF

FUEL SYSTEM ADAPTIVE LEARNING

OBD II requires the engine management system to compensate for changes in the operation of the engine and emissions over the lifetime of the vehicle. The engine control software monitors actual engine and emissions performance relative to expected performance based on the control parameters. For example, for an engine that always runs lean, the EMS will compensate by learning to command a rich mixture.

EVAPORATIVE EMISSIONS SYSTEMS

Evaporative emissions systems are used to prevent raw fuel vapor from escaping into the air. The primary component in the system is a charcoal canister that attracts the hydrocarbon vapors to its surface. When the engine is running, intake air is passed over the canister, purging the fuel into the air/fuel mixture. This process ends up being somewhat complex because the purged vapor can significantly alter the air/fuel ratio and, therefore, engine performance. A PWM-driven vent solenoid controlled by the EMS adjusts the amount of flow from the canister and the precise periods of delivery.

Another component of evaporative emissions systems is the leak detection system. To perform a diagnostic of leaks in the fuel system, the vent solenoid is closed and the engine is used to create a specific level of vacuum in the fuel system. A fuel pressure sensor is then used to monitor the vacuum level in the system. If the monitored vacuum drops to a specific level faster than normal, a leak is present. If the vacuum does not fall quick enough, the system is blocked. In either case, an OBD II fault is issued.

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