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Class-8 Heavy Truck Duty Cycle Project Final Report - Center for ...

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fuel sensor could not be calibrated; however, the literature indicates that the databus in<strong>for</strong>mation<br />

regarding fuel consumption is accurate, with some researchers reporting errors of just 1%. ORNL’s<br />

limited analysis involving one of the participating trucks and using just a few short segments of data<br />

<strong>for</strong> which complete fuel ticket in<strong>for</strong>mation was available indicates that the databus <strong>for</strong> the type of<br />

truck and engine used in this study underestimates fuel consumption by less than 8% (note: in these<br />

computations it was not possible to determine with 100% accuracy if the fuel tank was full at both the<br />

starting and ending point of the data segment used). Assuming that this is a systematic bias, even if<br />

the actual fuel efficiencies are erroneous, the percent differences are still accurate as was discussed in<br />

Section 6.2.2. Nevertheless, future sensitivity analyses will be per<strong>for</strong>med that will study the<br />

differences in fuel efficiencies when instead of systematic biases in the databus readings, random<br />

errors (within a certain range) are assumed to be present in those readings.<br />

These results were statistically analyzed by per<strong>for</strong>ming a test of hypothesis. The null hypothesis<br />

stating that there were no significant differences among any of the four distributions (i.e., one <strong>for</strong><br />

each truck-tire combination studied) of observed fuel efficiencies was tested against the alternative<br />

hypothesis indicating that the average fuel efficiency of any combination that had NGSWBTs was<br />

larger than the average fuel efficiency of the base case (Duals-Duals). In any case, it was possible to<br />

reject the null hypothesis with 100% confidence in favor of the alternative hypothesis. This strongly<br />

suggests that fuel efficiencies in these cases involving NGSWBTs are higher than in the case of a<br />

truck equipped with all dual tires. In other words, the percent improvements in fuel efficiencies that<br />

are achieved when using all NGSWBTs or combinations of duals and NGSWBTs are statistically<br />

significant.<br />

The search engine of the DCGenT Prototype was used to parse the data by truck-tire combination and<br />

vehicle load level. The latter was divided into four categories: 1) Tractor Only (trips made without<br />

any trailer), 2) Light Load (total vehicle weight between 24,000lbs and 44,000lbs); 3) Medium Load<br />

(total vehicle weight between 44,000lbs and 62,000lbs); and 4) <strong>Heavy</strong> Load (total vehicle weight<br />

between 62,000lbs and 80,000lbs). As in the previous case, 100-mile segments were considered <strong>for</strong><br />

which the fuel efficiency was computed. Once again, <strong>for</strong> any load level considered, the results of the<br />

analysis show that there is always an improvement in fuel efficiency with respect to the base case<br />

(i.e., duals-duals) when NGSWBTs are used. In fact, <strong>for</strong> the particular case in which all tires are<br />

NGSWBTs, there was considerable improvement in fuel efficiency with respect to the base case (i.e.,<br />

improvements that were above 10%), and those improvements were more significant as the load level<br />

increased. They were also statistically significant at the 99% level of confidence.<br />

8.3 FUEL EFFICIENCY AND TRUCK SPEED<br />

An analysis of the fuel efficiency of <strong>Class</strong>-8 trucks as a function of vehicle speed was also conducted.<br />

The results of this analysis showed that under congested conditions (i.e., speeds between 0+ and 20<br />

mph) any combination of tires that includes NGSWBTs per<strong>for</strong>ms better than the base case. However,<br />

in this case the percentage differences with respect to the base case were less than 5%. This result<br />

needs to be studied further by controlling <strong>for</strong> the type of class of road (i.e., freeways vs. surface<br />

streets) since what can be classified as congestion on a given facility (e.g., freeways) may be normal<br />

operations under another one (e.g., arterials and collectors).<br />

For speeds above 70 mph, there was a noticeable increase in fuel efficiency. It is very likely that this<br />

increase was due to the vehicle achieving those high speeds while traveling down slope, in which case<br />

the fuel consumption would be comparatively small, thus reflecting higher fuel efficiency. This<br />

needs to be studied further by controlling <strong>for</strong> the type of terrain (mainly, grade) to isolate its effect on<br />

achieved fuel efficiency.<br />

110

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