MANUFACTURING STANDARDS
Federal and state governments have established air quality standard during the past 20 years. Automobile manufacturers design their vehicles to conform to standards where the vehicle will be sold and operated. These standards cover carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx).
Federal and California Standards which must be met by manufacturers are specified in units easily measured in a testing laboratory. Since 1970, these standards have been in "grams per mile". This means no vehicle, whether 2-cylinder or V8, may emit more than a set weight (in grams) of pollutants for each mile travelled. Since large engines burn more fuel per mile than do small engines, they must be "cleaner" per gallon burned if they are to meet these standards.
When manufacturers certify vehicle models prior to sale, the vehicles are placed on a dynamometer and the exhaust gases are collected in a bag. After the vehicle runs for a specified time, the gases are analyzed and weighed. Engines and emission systems are designed so the weight of emissions will be less than the specified grams per mile standard.
Infrared exhaust analyzers are commonly used in vehicle test stations. The analyzer uses a test probe placed in the exhaust stream to sample the exhaust gases, and measure the percentage of CO and the parts per million of HC. These are not the same units used by the manufacturer when the vehicle is certified. The NOx emissions cannot be measured by an infrared exhaust analyzer. Laboratory equipment must be used to determine NOx emissions.
TUNE-UP STANDARDS
The technician must use the proper specifications when adjusting the vehicle during a tune-up. The first few years of emission-regulated vehicles were adjusted using an exhaust gas analyzer which measured CO and HC.
In the past few years, manufacturers have produced much cleaner running vehicles. The CO (percentage) and HC (ppm) have become very low, especially when measured downstream of catalytic converter. It has become difficult to measure the effect of fuel and ignition adjustments.
One solution to this problem for vehicles using carburetors requires the use of artificially-enriched propane adjustments. The added propane increases or decreases engine RPM for evaluation of carburetor rich/lean setting. This allows the technician to check carburetor setting quickly and accurately.
As computer-controlled systems were developed, it became possible for the vehicles to adjust the air/fuel ratio, ignition timing and emission control device operation throughout the entire driving range. These computer control systems use a variety of sensors that provide the electronic control unit with information on vehicle speed, altitude of vehicle operation and transmission gear position, along with engine operating conditions.
Fuel delivery to achieve a lean air/fuel ratio is controlled by the computer. The computer controls the on/off (duty cycle) time of the fuel injector(s) or carburetor mixture control solenoid to achieve leanest possible air/fuel ratio while maintaining good driveability.
Although most repair shops have exhaust gas analyzers, computer-controlled vehicles normally do not have a CO and HC specification for tuning. An abnormal exhaust gas reading on an exhaust analyzer may be helpful in diagnosing a problem, but should not be used as a basis for adjustments.
These procedures and specifications are supplied by the manufacturer and may not list CO or HC specifications.