Appendix 2: Engine balance

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Appendix 2: Engine balance

Emission repairs are not straightforward. The air-fuel mixture and the temperatures in the engine that tend to minimize production of CO and VOCs tend to increase production of NOx, and vice versa. Ideally, the carburetor or fuel injection system of a gasoline vehicle should provide a mixture (mass air-fuel ratio) of about 14.7:1 when the engine is at operating temperature (figure A1). In this stoichiometric condition there is just enough air to allow all the fuel to burn completely.

Click Here to View Figure A1

Excessive emissions of CO or VOCs can often be reduced by adjusting the air-fuel mixture. However, adjustments to decrease emissions of VOCs and CO can cause NOx emissions to increase dramatically as the air-fuel ratio approaches 14.7:1. As the mixture gets leaner still, all three emissions will begin to decline but conventional engines will no longer operate properly.

Besides a balance of pollutants, there is also a balance of performance. When the air-fuel mixture is slightly rich (more fuel and less air), the engine has more power; when it is slightly lean (less fuel and more air), the engine will run more economically but sometimes produce much less power (Kirk-Othmer Encyclopedia of Chemical Technology 1991: 986). Drivers, however, like power and it is this aspect of engine balance that has probably caused much of the cheating in I/M programs. In a survey done for Arizona's I/M program, 88 percent of the mechanics stated that requests by consumers to adjust their vehicles simply to pass the I/M test was ``very commonplace'' or ``somewhat commonplace'' and 78 percent stated that requests to re-adjust vehicles after they had passed the I/M test so that they would run better were also ``very commonplace'' or ``somewhat commonplace'' (Arizona, Auditor General 1988; this report was based on interviews with 201 automotive mechanics in Phoenix and Tucson.) The air/fuel ratio on an earlier model of vehicle could often be changed by a simple adjustment of the carburetor. In modern vehicles, the mixture may be changed by replacing the original controlling chip (PROM) in the on-board computer with an after-market ``high performance'' chip that gives the engine more power but may not pass an I/M test. The original chip can be reinstalled for a few hours once every year or two to pass a scheduled I/M test.

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Last Modified: Wednesday, October 20, 1999.