17.14.
Exhaust Emissions Testing
The HC-CO tester is designed for checking exhaust emissions of automobiles in service. The probe of the tester is inserted in the exhaust pipe to sample the exhaust emissions. The amount of carbon monoxide present in exhaust is directly related to the air/fuel mixture delivered to the engine by the carburettor and manifold where as hydrocarbons are directly related to unburned or partly burned fuel.
An emission-controlled automobile running normally has HC emissions below 100 rpm (parts per million) and CO emissions between 0.5 and 1.0%. Non-emission-controlled engines have higher normal emission levels of about 400 ppm HC and 3.0% CO. HC-CO tester readings above these ranges usually indicate some depreciation in the engine emission control system or a fault in either the mechanical or electrical portion of the engine. The other exhaust pollutant, NOx, is not measured at the service gauge level. In order to maintain a low NOx level, a service technician has to make sure that all of the emission control devices are functioning properly.
Tests for HC and CO are run at idle and higher speeds. Some test equipment manufactures give specific test speeds. Emission control levels should not increase when speeds are held above idle. Generally they decrease at higher engine speeds and any increase in emission at higher speeds indicaters that a problem exists. A problem exists when either HC or CO or both exceed the limit. If the exhaust emissions are normal no further checking of the emission controls is necessary.
Carbon monoxide in the exhaust is high when there is rich mixture and it goes down with lean mixture. With too lean a mixture, as indicated by a very low CO reading, the charge does not sustain combustion and therefore, high levels of hydrocarbons are exhausted. Hydrocarbons in the exhaust may become very low but they never reach zero because there exists always some combustion chamber surface quenching. The amount of hydrocarbons in exhaut increases when the combustion charge does not completely burn, which can happen when the cylinder does not fire or when there is partial combustion or when the mixture is so rich that there is more fuel than oxygen available for combustion. In any case, high HC emission means there is wastage of fuel. The emission-control carburettor is designed to run at the leanest point in order to keep HC at its lowest level. In most emission-controlled engines this occurs when the CO in the exhaust is about 0.75% and the HC is near 100 ppm.
If the CO level is normal and the HC level is high, the problem is either electrical or mechanical. An engine scope can be used to quickly verify the condition of the ignition system (except timing). The mechanical problems can be caused by improper valve operation, low compression, incorrect timing, faulty air injection system, and unequal fuel distribution in the intake manifold. The air injection pump should be made inoperative when engine problems are checked.
The faulty cylinder can be located by shorting out one cylinder at a time. A faulty cylinder when shorted causes only a slight increase in HC level where as a normal cylinder indicates HC to a high side. Cylinder shorting should be avoided on engines equipped with a catalytic converter, as it allows excessive unburned hydrocarbons to reach the converter where they burn producing excess heat that can ruin the catalyst.
High CO is always accompanied by a high HC level which is caused due to use of rich mixture. The rich mixture may be caused by a dirty carburettor air filter, stuck choke, faulty PCV system operation, in operative heat riser valve, or a carburettor that needs service. If the HC level is high with CO abnormally low the problem is the result of a very lean fuel mixture. The lean mixture results due to incorrect functioning of the carburettor or leakage in the intake manifold. Rapid throttle opening produces an increase in CO and HC as the accelerating pump operates.
