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2 AIR QUALITY, FUEL ECONOMY
Pages 23-52

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From page 23...
... It concentrates primarily on major deficiencies and omissions in those sections of the interagency report. The committee's evaluation should not be construed as a cr~t~cism of the interagency's discussion of air quality, fuel economy, and engine performance.
From page 24...
... A major deficiency in the interagency report is that it does not give the reader information that will permit the needed assessment of the interrelationship between exhaust emissions measured during the Federal Test Procedure (FTP) and exhaust emissions produced during on-road use, especially in winter.
From page 25...
... In addition, A/F ratio has a substantial effect on power, fuel economy, cold starting, and drivability. Figure 2.3 illustrates the importance of catalyst temperature on destruction of CO and VOCs.
From page 26...
... Effect of A/F ratio on gasoline-engine pollutant emissions. '5 represents the stoichcometric A/F ratio.
From page 27...
... If on-road accelerations are greater than those used in the standard test cycle (Figure 2.4) , actual emissions may differ from emissions measured during the FTP.
From page 28...
... , and catalytic converter warm up. While not shown in Figure 2.5, a second O2 sensor, located downstream of the catalytic converter, is increasingly being added to achieve more precise control.
From page 29...
... AIR QUALITY, FUEL ECONOMY, AND ENGINE PERFORMANCE 29 0 11 Cal ~ LO ~ Q Cal Cal ~ ~ lo.
From page 30...
... 30 OXYGENATED MOTOR VEHICLE FUELS 3 . a Air _ , 1 as_ l Fuel ~ ~ b ' I ^~ 3 Exhaust _ gas 2 5 i ~-~ UA 4 Figure 2.5 Exhaust-gas emission control with the three-way catalyst and the O2 sensor.
From page 31...
... The interagency report does not provide a plausible method for
From page 32...
... of the air-quality benefits section of the interagency report presents such a plot for Denver covering the period 1980 1995. While the annual variation in CO levels has decreased, the trend of average winter CO level shows little or no change since winter oxygenated fuels were introduced.
From page 33...
... Because the CO emissions of the in-use fleet are highly skewed, it is important to understand which control strategy will produce the largest reduction in CO emissions at the lowest cost to society (Beaton et al, 1995~. For example, a relatively large percentage decrease in emissions from well-maintained vehicles with modern control technology will result in only a minor total reduction in ambient CO emissions.
From page 34...
... Fry EMISSIONS DATA FOR INDIVIDUAL VEHICLES The available CO data in the interagency report for individual vehicles used in the study were collected largely as standard FTP 75°F cycle emissions. The report covered data from emissioncontrol systems that ranged from early technology to recent technology.
From page 35...
... The interagency report discusses results from remote-sensing studies from the Denver area and North Carolina. In the first Denver study, Bishop and Stedman (1989)
From page 36...
... For all these studies, remote sensing devices measure the CO-to-CO2 ratio in the plume of vehicles that pass by the sensors. Because CO and CO2 emissions increase dramatically at lower-temperature winter conditions, it must be demonstrated that the ratio of these two pollutants from the in-use fleet is not adversely affected at cold temperatures for different fuel compositions in order for this type of experiment to be reliable.
From page 37...
... In the low-temperature regions in the United States where oxygenated fuels are required, it is clear that additional data are needed to understand whether the use of oxygenated fuels actually decreases CO emissions. The committee recommends that the air-quality chapter of the interagency report contain a summary table which presents the results of all low-temperature dynamometer studies.
From page 38...
... It states that oxygenated fuels reduce ambient CO concentrations, but it also reports that "the largest [CO] decreases were at two sites that were not using oxygenated fuels...
From page 39...
... Also, Dolislager calculated changes in CO-to-NOx ratios to infer changes in CO emissions due to oxygenated fuels by assuming that oxygenated filets do not substantially increase NOx emissions. The data from the interagency report show that oxygenated fuels do increase NOx emissions by as much as 2% per weight % oxygen.
From page 40...
... The major problem is a lack of thorough, statistically defensible analysis of ambient data where such confounding features as the lack of a control population for comparison, fleet turnover, occurrence of high emitters, I/M programs, and local economy and fleet population are properly accounted for. DISCREPANCIES BETWEEN MODEL RESULTS AND OBSERVATIONS The interagency report also suggests that there are major discrepancies between real-worId observations and the results of the MOBlLE 5a model.
From page 41...
... However, when ambient studies experience difficulties in detecting the effects of emission control programs, it suggests that the effects are not major and that additional analysis would be neecled to isolate the effect. The interagency report should include an assessment of the utility of the MOBILE 5a mode} for prediction of winter emissions from oxygenated fuels and discuss possible reasons for differences between observations and modeled predictions.
From page 42...
... However, for areas not in attainment of ozone NAAQS, potential increases in NOX from motor vehicles using oxygenated fuels should be given greater consideration in the interagency report. The interagency report adequately covers the available data on the effect of oxygenates at the standard FTP conditions (75°F)
From page 43...
... The discussion on NOx is not clearly separated in the current report, appearing as incidental comments in the discussion of CO effects, and again in the discussion of other emissions. HO EMISSIONS, MTBE, AND Tonics The interagency report correctly cites the result of dynamometer studies: enieanment by addition of oxygenates in general decreases hydrocarbon emissions (Figure I
From page 44...
... The effect of oxygenates on toxic emissions is mentioned only briefly. Clearly, very few data are available on the emissions of MTBE and aldehydes at low temperatures, even though a general decrease in total hydrocarbons is expected with the addition of oxygenated fuels, as well as a decrease (by dilution)
From page 45...
... Whereas most of the interagency report appears to be concerned with MTBE proper as a potential health hazard, it might be advisable to consider the combined effects of introduction of MTBE with the potential increase in aldehyde emissions and production of byproducts in the environment. Furthermore, the committee recommends that an assessment of the toxic emissions associated with the introduction of oxygenated fuels during the winter (or associated dynamometer tests at low temperatures)
From page 46...
... The interagency report addresses both of these issues. After new oxygenated-gasoline programs were implemented in the winter of 1992-1993 to reduce CO emissions, consumers in some areas of the country expressed concerns that these fuels had led to large reductions in fuel economy and deterioration of engine performance.
From page 47...
... As the report states, there are a number of factors other than gasoline quality and composition that contribute to engine-performance problems that may be corrected by relatively simple con sumer actions. Program Costs The draft executive summary in the interagency report states that 2Energy content per gallon is the same as the heating value per gallon.
From page 48...
... MODERN TECHNOLOGY In addition to increasingly precise adaptive control, a number of changes taking place are likely to change the outlook for CO emissions from gasoline-powered vehicles. The committee believes that the interagency group should consider these changes in its analysis.
From page 49...
... OVERALL CONC LUSIONS ON Ai R QUALITY, FUEL ECONOMY, AND ENGINE PERFORMANCE The interagency report concludes that there have been substantial reductions in ambient CO levels in the last 20 years and that vehicle emission controls have been a major factor in this reduction. However, the federal report should better characterize the uncertainty about the extent to which oxygenated fuels have contributed to this reduction.
From page 50...
... Any increase in NOx emissions could be detrimental in ozone nonattainment areas where exceedances have occurred during the period of the oxygenated fuels program. The interagency report concludes that the fuel-economy penalty associated with the use of oxygenated fuels is approximately 2% to 3% and is related to changes in energy content per gallon.
From page 51...
... . esplte uncertainties in estimating costs ant even greater uncertainties in estimating benefits, full evaluation of the oxygenated-fuel program requires that the interagency report address and document program costs and benefits at least at a broad level.


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