ity and stability of American blend stock and the variety of feedstocks, including soy, recycled used oils, fats, etc.

Efficiency Improvements from Transmissions

The transmission technology utilized in the FSS results shown in Table 5.1 was a dual-clutch 6-speed (automated manual) transmission (DCT), which is a very efficient design concept. Transmissions used for CI diesels must be designed to handle their larger torque, which may reduce their efficiencies slightly due to larger gears, bearings, and seals. DCTs are already in production for smaller displacement CI engines (e.g., 2009 VW Jetta). The most challenging aspect of designing DCTs with the higher torque capacities needed for larger displacement CI engines is providing adequate cooling for their wet clutches (i.e., oil-cooled clutches). Dual-mass flywheels, which reduce drive train vibration, thus reducing heat-generating clutch slippage, will be used. Nonetheless, it is not presently known when such DCT units will be available with 500-650 N-m torque capacities for larger CI engines.

Expected transmission-based CI vehicle efficiency improvements beyond those already comprehended by the use of the DCT6 transmissions are estimated at 1 to 2 percent for downspeeding the engine by increasing the number of discrete speed ratios beyond six. The increased number of ratios allows keeping the average engine speed lower while still maintaining equal performance, which is why this approach is called “downspeeding.” Another 2 to 3 percent is expected from reduced transmission internal losses.

Overall Fuel Consumption Reduction Potential

The FC reduction potential via replacement of SI gasoline power trains by base-level CI power trains is illustrated by Table 5.1 (i.e., ~33 percent) for CI engines with advanced transmissions (plus EACC, HEA, and EPS) and by Figure 5.4 for engine replacement alone (i.e., ~25 percent). Additional technical improvements, as noted earlier, from downsizing, thermodynamic improvements, friction reduction, and engine accessory improvements, are being developed and will be implemented. CI engines with these technologies implemented are termed advanced-level CI engines. Transmission improvements are also possible.

Based on interactions with OEMs, consulting companies, review of the technical literature, and the judgment of the committee, estimates of the overall FC reduction potential from these advanced-level technology areas are presented in Table 5.2. For the ranges shown, the 10 percent for engine technologies alone and 13 percent for vehicles applies to larger vehicles with automatic transmissions. For smaller vehicles with manual transmissions and engine displacements less than 1.5 L, cost constraints are likely to reduce the extent of downsizing and the potential would be about 6 percent for engine alone and 7 percent for vehicle due to elimination of not only the gain from automatic transmission efficiency

TABLE 5.2 Estimated Fuel Consumption Reduction Potential for Advanced-Level CI Power Trains Compared to Base-Level CI Power Trains

Item

Average Reduction (%)

Min

Max

Large Vehicles

 

 

 

Downsizing

4

3

5

Downspeeding

1.5

1

2

Friction reduction

1.5

1

2

Combustion improvement

3

2

4

Total engine improvement

10

 

 

Accessory improvement

1

0.5

1.5

Transmission loss reduction

2

1.5

2.5

Combined engine and transmission potential

13

 

 

Item

(%) Reduction

Min

Max

Small Vehicles (<1.5 L)

 

 

 

Downsizing

1

0

2

Downspeeding

0.5

0

1

Friction reduction

1.5

1

2

Combustion improvement

3

2

4

Total engine improvement

6

 

 

Accessory improvement

1

0.5

1.5

Thermal management

0

0

0

Transmission loss reduction

0

0

0

Combined potential

7

 

 

NOTE: The values shown for the combined potential do not show a range. It is tempting to use the sum of the minimum values for the lower limit of the range and the sum of the maximum values for the upper end of the range. However, this would be inappropriate because no original equipment manufacturer is likely to simultaneously achieve either the minimum or the maximum for all items. Therefore, a realistic range for the combined potentials is about ±1 percent.



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