Tom Wenzel

SOME ISSUES IN THE STATISTICAL ANALYSIS OF VEHICLE EMISSIONS

Some of the issues complicating the statistical analysis of vehicle emissions and the effectiveness of emissions control programs are presented in this article. Issues discussed include: the variability of inter- and intra-vehicle emissions; the skewness of emission distribution from in use vehicles; the difficulty of obtaining statistically representative vehicle samples; the effect of repeated testing on a subset of the vehicle fleet; and, differences among common test methods and pollutant measurement devices. The article discusses the relevance of these issues in light of the following regulatory purposes: compliance testing of in-use vehicles that have certification standards; effectiveness evaluation of inspection and maintenance programs of vehicles; and emissions inventory estimates for air quality modeling and compliance planning. The article also provides a brief history and description of common vehicle emission tests.

Analysis of Modal Emissions From Diverse In-Use Vehicle Fleet

The initial phase of a long-term project with national implications for the improvement of transportation and air quality is described. The overall objective of the research is to develop and verify a computer model that accurately estimates the impacts of a vehicle’s operating mode on emissions. This model improves on current emission models by allowing for the prediction of how traffic changes affect vehicle emissions. Results are presented that address the following points: vehicle recruitment, preliminary estimates of reproducibility, preliminary estimates of air conditioner effects, and preliminary estimates of changes in emissions relative to speed. As part of the development of a comprehensive modal emission model for light-duty vehicles, 28 distinct vehicle/technology categories have been identified based on vehicle class, emission control technology, fuel system, emission standard level, power-to-weight ratio, and emitter level (i.e., normal versus high emitter). These categories and the sampling proportions in a large-scale emissions testing program (over 300 vehicles to be tested) have been chosen in part based on emissions contribution. As part of the initial model development, a specific modal emissions testing protocol has been developed that reflects both real-world and specific modal events associated with different levels of emissions. This testing protocol has thus far been applied to an initial fleet of 30 vehicles, where at least 1 vehicle falls into each defined vehicle/technology category. The different vehicle/technology categories, the emissions testing protocol, and preliminary analysis that has been performed on the initial vehicle fleet are described.

Real-World Emissions from Conventional Passenger Cars

It has long been recognized that vehicles emit more pollutants than allowed under the new car emission standards. Further tightening of the certification standards based on existing test procedures does not directly address the largest sources of emissions. This study attempts to quantify vehicle emissions by source, in order to prioritize future policymaking. Several new sets of data are used in conjunction with regulatory emission models to characterize the lifetime emissions from the average Model Year (MY)93 vehicle. Special attention is paid to two of the largest sources of real-world emissions: (1) high-power driving by cars with properly functioning emissions controls, and (2) cars with malfunctioning emissions controls. Emissions are projected to MY2000 and 2010, based on estimates of the effectiveness of recently adopted and proposed regulatory policies. These new policies are projected to reduce total emissions substantially.

Vehicle Design and the Physics of Traffic Safety

Light trucks cannot safely coexist with passenger cars under existing conditions. The problem becomes particularly urgent as more and more light trucks are used simply as car substitutes.

Characterization of Recent-Model High-Emitting Automobiles

In-use vehicles which are high emitters make a large contribution to the emissions inventory. It is not known, however, whether high-emitting vehicles share common emissions characteristics. We study this by first examining laboratory measurements of second-by-second engine-out and tailpipe emissions from a small number of MY90-97 highemitting vehicles. We distinguish high-emitter types by the behavior of six ratios in low- and moderate-power driving: the engine-out emissions indices (engine-out pollutant to fuel-rate ratios) and the catalyst pass fractions (tailpipe to engine-out ratios) for CO, HC, and NOx. Four general types of high emitter are observed: 1) fuel-air ratio excessively lean, 2) fuelair ratio excessively rich, 3) partial combustion such as misfire, and 4) severe deterioration in catalyst performance in vehicles where malfunctions of Types 1, 2 or 3 are not predominant. We also find that these behaviors may be chronic, or may only occur transiently. The second step is to determine the prevalence of the four different types of high emitter in the on-road fleet. For this we analyze IM240 tailpipe emissions from a large sample of cars measured in the Arizona inspection and maintenance program. We find that all four types of failure are observed with roughly comparable probabilities.

Reducing emissions from in-use vehicles: an evaluation of the Phoenix inspection and maintenance program using test results and independent emissions measurements☆

Abstract Various federal regulations require states to evaluate the effectiveness of their vehicle inspection and maintenance (I/M) programs in reducing in-use emissions. One method to evaluate program effectiveness is to compare initial and final program test results of individual vehicles. Unscheduled emissions measurements, from remote sensing measurement or roadside pullover testing, can also be used to provide an independent assessment of program effectiveness. We compared emissions reductions from the Arizona IM240 program measured by program data and a large set of remote sensing measurements. Remote sensing measurements indicate smaller emission reductions from the program than those calculated directly from program test results. We discuss some possible causes of the differences obtained from the two sets of measurements.

Increasing the Fuel Economy and Safety of New Light-DutyVehicles

One impediment to increasing the fuel economy standards forlight-duty vehicles is the long-standing argument that reducing vehiclemass to improve fuel economy will inherently make vehicles less safe.This technical paper summarizes and examines the research that is citedin support of this argument, and presents more recent research thatchallenges it. We conclude that the research claiming that lightervehicles are inherently less safe than heavier vehicles is flawed, andthat other aspects of vehicle design are more important to the on-roadsafety record of vehicles. This paper was prepared for a workshop onexperts in vehicle safety and fuel economy, organized by the William andFlora Hewlett Foundation, to discuss technologies and designs that can betaken to simultaneously improve vehicle safety and fuel economy; theworkshop was held in Washington DC on October 3, 2006.

The effect of recent trends in vehicle design on U.S. societal fatality risk per vehicle mile traveled, and their projected future relationship with vehicle mass

The National Highway Traffic Safety Administration (NHTSA) recently updated its 2003 and 2010 logistic regression analyses of the effect of a reduction in light-duty vehicle mass on US fatality risk per vehicle mile traveled (VMT). The current NHTSA analysis is the most thorough investigation of this issue to date. LBNLs assessment of the analysis indicates that the estimated effect of mass reduction on risk is smaller than in the previous studies, and statistically non-significant for all but the lightest cars. The effects three recent trends in vehicle designs and technologies have on societal fatality risk per VMT are estimated, and whether these changes might affect the relationship between vehicle mass and fatality risk in the future. Side airbags are found to reduce fatality risk in cars, but not necessarily light trucks or CUVs/minivans, struck in the side by another light-duty vehicle; reducing the number of fatalities in cars struck in the side is predicted to reduce the estimated detrimental effect of footprint reduction, but increase the detrimental effect of mass reduction, in cars on societal fatality risk. Better alignment of light truck bumpers with those of other vehicles appears to result in a statistically significant reduction in risk imposed on car occupants; however, reducing this type of fatality will likely have little impact on the estimated effect of mass or footprint reduction on risk. Finally, shifting light truck drivers into safer, car-based vehicles, such as sedans, CUVs, and minivans, would result in larger reductions in societal fatalities than expected from even substantial reductions in the masses of light trucks. A strategy of shifting drivers from truck-based to car-based vehicles would reduce fuel use and greenhouse gas emissions, while improving societal safety.

Evaluating the long-term effectiveness of the Phoenix IM240 program

Abstract In an earlier analysis of the Phoenix Enhanced I/M program, we found that the effectiveness of the program decreased in the few months following final I/M testing. In this analysis we tracked individual cars over two successive I/M cycles. We find that 60% of cars that failed their initial I/M test and eventually passed a retest (presumably after repairs) passed their next biennial I/M test. However, 40% failed their next biennial test. Half of these repeat failures failed for the same combination of pollutants in each I/M cycle, suggesting that repairs were never made or were somehow deficient. More importantly, emissions of cars that passed their initial test in the first cycle increased dramatically over the two years between I/M tests; most of this increase came from older cars. This suggests that more frequent testing of older vehicles could make I/M programs more effective. The net result is that the overall emission reductions from a given fleet of cars over two biennial I/M cycles appear to have been rather small, less than 10% for each pollutant. These emission reductions are the minimum attributable to the program; reductions from what emissions would have been without the I/M program would likely have been greater. In addition, large numbers of failing cars did not appear to be completing program requirements, further limiting program effectiveness. And cars immigrating into the area from other states appeared to have slightly higher emissions than the native car fleet. I/M program managers should conduct similar analyses to determine how effective their programs are in reducing emissions, and how to address identified shortcomings.

Sipping Fuel and Saving Lives: Increasing Fuel Economy Without Sacrificing Safety

Sipping Fuel and Saving Lives: Increasing Fuel Economy Without Sacrificing Safety A Report Informed by an October 3, 2006, Experts Workshop on Simultaneously Improving Vehicle Safety and Fuel Economy Through Improvements in Vehicle Design and Materials