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ALL ABOARD THE CLEAN ENGINE

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

MISFIRE DETECTION

Misfire is the term used to denote when the air/fuel mixture doesnýt combust or partially combusts in a given cylinder. Misfire, which occurs most often during startup or transients, can severely damage the catalytic converter. Because of the high catalytic converter temperatures (over 800ýF, or 426.7ýC), the air/fuel mixture often burns in the converter, which causes eventual failure in the device.

Misfire detection is a challenging task. First, because it is a random and intermittent event. Second, normal variations in steady-state engine speed can be as much as 200 rpm. These variations can be caused by mass imbalances, torsion vibrations, and flexing of the drive shaft.

One method of misfire detection is analyzing the rotational velocity of the engine from the crank sensor data. A cylinder misfire is correlated to variations in the characteristic acceleration of the firing of each cylinder. External conditions like a rough road can cause the engine velocity profile to appear as if misfire events are occurring. These conditions and the normal variations we mentioned lead to sophisticated hardware and software filtering and analysis of the velocity data. Newer techniques, like the analysis of cylinder pressure waves, promise better misfire detection but probably will be equally compute-intensive.

OBD II specifies misfire detection from idle to 55 mph with moderate acceleration. OBD III requires misfire detection over the full operating range of the engine, which will further increase the central processing unitýs (CPU) burden.

SECONDARY AIR SYSTEM

To prevent the hydrocarbons that are not consumed in the combustion process from burning in the catalytic converter, secondary air systems feed oxygen into the exhaust gas. The hydrocarbons burn when oxygen is introduced into the high-temperature exhaust gas. This technique also reduces carbon monoxide in the exhaust gas. Secondary airflow is regulated by a solenoid under control of the EMS. OBD II requires that the secondary air system be monitored for proper operation. This monitoring is typically done by correlating oxygen sensor readings before and after the catalytic converter.

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