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Tech Corner-continued

hydrocarbons from individual vehicles may range from about 200 ppm to over 5000 ppm (expressed as hexane and
measured by hexane sensitized NDIR). Reports in the literature often give considerably different values for average
hydrocarbon emissions depending upon the sample of vehicle selected and measurement methods used.

Recent studies in California indicate that average hydrocarbon emissions from large numbers of vehicles represent about
six percent by weight of the fuel supplied to the engine. These studies included dynamometer and field tests representative
of the driving modes specified in the California Motor Vehicle Standards. Care should be taken in applying the data from
California to other areas, to different vehicle populations, and to other driving modes. Average carbon monoxide among
individual vehicles may range from about one percent to 10 percent and oxides of nitrogen from several hundred ppm to
over 2500 ppm. The average exhaust emission from a large number of vehicles (under conditions representing the driving
modes specified in the California Motor Vehicle Exhaust Standards) can be expected to be approximately that shown in
the table below:

Carbon monoxide 3.0-—3.5%
Hydrocarbons 800—950 ppm*
Oxides of nitrogen 900—1100 ppmf

Crankcase Emissions

The volume of emissions from this source depends upon the vehicle operating modes. In passenger cars the volume may
approach zero at deceleration and reach 10 cfm at full load in vehicles in a poor state of repair. The average blowby from
a large number of
vehicles under city driving conditions is about one cfm as compared to approximately 30 cfm for exhaust emissions.
Hydrocarbon concentrations in blow- by are high, about 10,000 to 15,000 ppm (total hydrocarbons—expressed as hexane)
and do not vary widely since this emission is mainly a carbureted fuel-air mixture. The concentration in crankcase
emissions may vary more widely depending upon the amount of ventilating air. Crankcase hydrocarbon emissions are
approximately two percent by weight of the fuel supplied to the engine.

Evaporative Losses

Evaporative losses consist of gasoline vapors and contain the most volatile hydrocarbons in the fuel. Their average
molecular weight is probably in the range of butane or pentane. Fewer data are available on these losses than for crankcase
and exhaust. Volume and concentration figures have little meaning for this emission. However, studies indicate that the
evaporative hydrocarbon losses may range from five to 15 percent of the total hydrocarbons (exhaust, crankcase, and
evaporative) emitted from motor vehicles.

Pollutant Source as Percent
-of Total of Each Pollutant Emitted
Pollutant Exhaust Crankcase Evaporative
Hydrocarbons 50-65 25-35 5-15
Carbon Monoxide 98-99 1-2 0
Nitrogen Oxides 98-99 1-2 0

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