. "Appendix E: Fuel Economy and Fuel Consumption as Metrics to Judge the Fuel Efficiency of Vehicles." Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles. Washington, DC: The National Academies Press, 2010.
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Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
The following equations hold for calculating FE and FC:.
The equations above hold from engine on to engine off in order to capture idle time. Chapter 2 refers to an “average payload” to calculate load-specific fuel consumption (LSFC) but does not indicate how to calculate it on a trip delivering cargo. The calculation for average payload is as follows:
where Px = payload in tons carried for time x when the vehicle is moving, and tx = time in hours carrying payload Px. This is a “time average payload” for a vehicle operating in the field and excludes idle time—it is the integral of payload to get the average payload.
From Figure 2.6, LSFC does not decrease significantly for a payload increase as long as the payload is greater than 70 percent of the full payload.
In the equation for payload, if any Px is zero, there is zero in the numerator for that segment, but the time is counted in the denominator, which then lowers the average payload. The FC during the no-load segment would decrease, lowering the total gallons of fuel used. If the time average payload is less than 70 percent of full load, LSFC will increase—if it is greater than 70 percent, LSFC will increase somewhat based on Figure 2.6.
For picking up cargo, the average payload equation would need a plus payload term for each segment to account for the pickup. Therefore, P2 = P1 − Pd + Pp where Pd = payload delivered and Pp = payload picked up after segment 1,
TABLE E-1 Gross Vehicle Weight Groups
Gross Vehicle Weight (lb)
NOTE: Vehicle groups used for average payloads in Tables E-2 and E-3.