Page 19 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

2 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

Evolution of legislation

In general, petrol or spark-ignition engines emit oxides of nitrogen (NO and small amounts of
NO2 - collectively referred to as NO,), carbon monoxide (CO) and organic compounds, which
are unburnt or partially burnt hydrocarbons (HC). Compression-ignition or diesel engines emit
smaller amounts of CO and HC, their main problem being particulate emissions

The Clean Air Act was the first law that sought to control auto emissions throughout the
USA. This law set I975 and I976 exhaust requirements at

1.5 @mile (0.93 g/km) for HC,
15 g/mile (9.37 dkm) for CO and
3.1 @mile (1.93 g/km) for NO,.

Emission levels were measured by the Federal Test Procedure (FTP) established by the U.S.
Environmental Protection Agency (EPA).

The FTP '75 test cycle simulates an 1 I-mile driving cycle through Los Angeles at an average
speed of 34.1 Kdh. The FTP test measures CO, HC and NO, with a constant volume
sampling system and involves a cold start afler an engine sits idle for eight hours, a hot start
and a combination of urban and highway conditions ([3]).

Fig. I shows the four phases of the American FTP '75 test cycle and the behavior of the HC in
exhaust. The four phases of an engine according to this test cycle is: the cold start phase, the
stabilized phase, the engine off phase and the warm phase. The upper part of the figure shows
the variation of the velocity of the vehicle vs. time whereas the lower part of the figure shows
the variation of HC emissions vs. time. It is obvious that from all phases, the cold start phase
produces most of the HC emissions (80%) ([2]).

A change being assessed is to expand the FTP to include conditions that involve aggressive
driving behavior at high speed and high acceleration, rapid fluctuation in speed, use of air
conditioners and start-up atter an engine is turned off for intermediate periods (e.g. 30
minutes). This will result in higher space velocities and greater concentrations of pollutants,
which will place more demand on the converter.

Subsequent state and federal laws have set ever more stringent automobile emissions
standards. Amendments to the Clean Air Act in 1990 phased in together standards over a
period that extends well beyond 2000. Afler 1996, catalytic converters in new vehicles must
last 100000 miles. The standards for 2004 are:

0.125 g/mile (0.078 g/km) non-methane HC