MOBILE SOURCE POLLUTION 703
and idle operation. “Lean surge during cruise has been largely
overcome through improvement in manifolding (better mix-
ture distribution), better carburetor fuel-metering character-
istics, higher coolant temperatures, increased heating of the
air–fuel mixture, and, in some cases, provision for heating the
incoming air to the carburetor.
Exhaust emissions of CO and HC are particularly diffi cult
to control during engine idle and closed-throttle operation
(deceleration). Considerable effort has gone into design-
ing carburetor idle systems that will provide a lean air–fuel
mixture and minimize emissions during these periods. To
ensure that idle air–fuel mixture cannot be adjusted to be too
rich (which would tend to increase CO and HC emissions
appreciably), some means of limiting idle-mixture adjust-
ment is used on most carburetors. Such devices allow idle
mixture to be adjusted leaner than a predetermined value,
but not richer.”
The effects of charge dilution on the exhaust emission of
nitric oxide (NO) from a single-cylinder engine were evalu-
ated over a range of engine design and operating parameters.^6Nitric oxide emission decreased as much as 70% as charge
dilution fraction was increased from 0.0065 to 0.164 due
to increased valve overlap, external exhaust recirculation,
and reduced compression ratio. NO emission was strongly
dependent on charge dilution fraction, but was independent
of the specifi c method used to change charge dilution. The
combined effects of increased charge dilution and 10 degree
spark retard reduced NO emission 90%. However, defi nite
limits of operation were observed on the single-cylinder
engine with high charge dilution.
The Ford Motor Company uses a system which reduces
the hydrocarbon and carbon monoxide content of exhaust
gases by continuing the combustion of unburned gases
after they leave the combustion chamber. This is achieved
by injecting fresh air into the hot exhaust stream leaving
the exhaust ports. At this point, the fresh air mixes with
hot exhaust gases to promote further oxidation of both the
hydrocarbons and carbon monoxide, thereby reducing their
concentration and converting some of them to carbon dioxide
and water.TABLE 2
Summary of results, steady state tests and federal short cycle vehicle exhaust emissions in grams per vehicle mileaCity Effluent Idle 15 mph 30 mph 45 mph 60 mph FSCb
Los Angeles HC 1.45 5.28 3.09 2.91 2.60 5.69
CO 17.02 69.11 29.50 24.60 25.51 48.53
CO 2 72.32 409.53 333.12 357.90 372.47 438.48
NOx 0.08 0.51 1.69 3.78 5.51 3.10
Denver HC 1.47 6.06 3.74 3.94 3.87 7.07
CO 17.20 86.49 52.13 56.43 71.13 94.68
CO 2 60.54 345.65 297.92 330.09 368.91 348.85
NOx 0.12 0.83 1.53 3.07 4.52 2.13
Chicago HC 1.30 5.47 3.48 3.59 3.62 5.74
CO 15.49 66.57 29.77 30.53 32.63 59.74
CO 2 63.85 358.70 318.61 357.65 417.11 392.28
NOx 0.10 0.87 2.36 4.37 6.26 3.42
Houston HC 1.50 5.68 3.02 2.83 2.47 5.52
CO 18.74 77.29 35.11 32.38 31.71 63.16
CO 2 76.13 391.06 318.01 356.13 404.79 417.03
NOx 0.14 0.83 1.49 3.47 5.70 2.83
St Louis CC 16.43 67.68 29.54 26.13 26.97 56.43
CO 2 64.28 334.24 301.64 315.26 362.26 378.28
NOx 0.08 0.53 1.57 3.38 5.43 3.10
Washington HC 1.12 4.23 2.59 2.57 2.88 4.46
CO 13.25 56.10 26.19 25.40 25.79 48.11
CO 2 65.04 380.44 343.54 390.83 452.50 417.79
NOx 0.13 1.04 2.88 6.04 8.90 4.23
All except HC 1.34 5.11 2.99 2.90 2.85 5.34
Denver CO 16.19 67.36 30.02 27.79 28.50 55.15
CO 2 68.35 374.23 323.03 355.55 401.60 408.96
NOx 0.11 0.75 2.00 4.21 6.35 3.34a Idle results in grams per minute. NO
b x not corrected for humidity.
FSC—Federal Short Cycle.C013_005_r03.indd 703C013_005_r03.indd 703 11/18/2005 10:42:27 AM11/18/2005 10:42:27 AM