The committee estimated that the emission control technologies identified by EPA in Figure 2.17 for meeting Tier 3 emission standards for a large, light-duty truck are likely to result in less than 0.31 percent increase in fuel consumption. The increases in fuel consumption in other vehicles are expected to be less. An analysis of the reasons for this conclusion, based on three emissions control technologies that EPA identified in the Tier 3 rule, is provided below.
Technology 1: Secondary air injection with a 100 W electric air pump operating for the first 60 seconds of the cold start FTP cycle may be added.
A continuous 100 W load was estimated by EPA/NHTSA to result in an average increase of 2.5 g/mi CO2 on the 2 cycle CAFE test (EPA/NHTSA 2012, 5-66, Table 5-18). Converting the CO2 increase to fuel consumption yields the following:
2.5 g CO2/mi × (gal/8887 g CO2) = 0.00028 gal/mi
For the 2025 MY CAFE 2 cycle fleet average fuel economy requirement of 49.6 mpg, the 100 W load, operating full time, would provide the following percentage increase in fuel consumption:
(0.00028 gal/mi)/((1/49.6) gal/mi) = 1.4% increase in fuel consumption if the 100 W load were operating full time.
The 60 seconds of operation during the 505 seconds of the FTP cold start Bag 1 results in an 11.9 percent weighting. The 11.9 percent for the FTP cold start Bag 1 is weighted 43 percent when combined with the FTP hot start Bag 1. Bag 1 is weighted 36.8 percent (505 seconds/1371 seconds) over the complete 1,371 seconds of the FTP cycle. The FTP cycle is weighted 55 percent over the two CAFE test cycles. Applying these weightings to the 1.4 percent reduction in fuel consumption for a 100 W load operating full time yields the following result:
1.4% × (0.119) × (0.43) × (505/1371) × (0.55) = 0.015% increase in fuel consumption
Technology 2: An HC adsorber may be added.
A recent study of an HC adsorber showed that catalyst volume for the HC adsorber had to be added since the existing TWC catalyst volume was required for NOx control. In this study, an adsorber with a volume of 0.67L was added to an existing 2.2L TWC, resulting in a 30 percent increase in overall catalyst volume (Gao et al. 2012). A 30 percent increase in catalyst volume due to the HC adsorber would result in the following increase in fuel consumption:
At ¼ maximum load (approximately 3 bar BMEP) and 1,500 rpm, which is a typical condition in the CAFE test cycles, an average catalyst results in an exhaust gas pressure drop of 6 mbar (Persoons 2006). This back pressure on the engine would result in a 0.2 percent increase (0.006 bar/3 bar × 100) in fuel consumption. Adding an adsorber to increase catalyst volume by 30 percent would increase fuel consumption by 0.06 percent (0.30 × 0.2%).
Technology 3: Calibration changes may consist of spark retard and increased idle speed for 30 seconds for faster catalyst warm-up.
Spark retard is likely to be required beyond the first 20-second idle period and may be needed during the next idle period for a total of 30 seconds.
By assuming that half of the maximum acceptable spark retard was used for Tier 2 emissions, the remaining half of the maximum spark retard would result in approximately a 15 percent increase in fuel consumption for 30 seconds during the FTP cold start Bag 1 (Zareei and Kakaee 2013). Applying this to the CAFE test cycle would result in 0.08 percent increase in fuel consumption as shown below.
15% × (30/505) × (0.43) × (505/1371) × (0.55) = 0.08% increase in fuel consumption
In addition to retarding spark timing, heat flux to the catalyst at idle may be increased by raising idle speed. Raising idle speed by 30 percent (from 600 rpm to 780 rpm) would result in a 30 percent increase in idle fuel consumption for the 30 seconds during the FTP cold start Bag 1. Applying this over the CAFE test cycle would result in 0.16 percent increase in fuel consumption as shown below.
30% × (30/505) × (0.43) × (505/1371) × (0.55) = 0.16% increase in fuel consumption
Therefore, spark retard and increased idle speed could result in approximately a 0.24 percent increase in fuel consumption.
Combining the above three reasons yields the following estimated increase in fuel consumption for the Tier 3 emission standards:
Percent Increases in Fuel Consumption
|100 W air pump for 60 seconds||0.01|
|Calibration changes for 30 seconds||0.24|
EPA/NHTSA (Environmental Protection Agency/National Highway Traffic Safety Administration). 2012. Joint Technical Support Document, Final Rulemaking 2017-2025 Light-Duty Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards. EPA-420-R-12-901.
Gao, Z., C. Daw, and L. Slezak. 2012. Advanced Light-Duty Engine Systems and Emissions Control Modeling and Analysis. DOE Annual Merit Review, May 16.
Persoons, T. 2006. Experimental Flow Dynamics in Automotive Exhaust Systems with Close Coupled Catalyst. Katholieke Universiteit Leuven, August.
Zareei, J., and A. H. Kakaee. 2013. Study of the effects of ignition timing on gasoline engine performance and emissions. Eur. Transp. Res. Rev. 5:109-116.