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Calculating the Fuel Economy Effects of Converting a Taurus to a Series Hybrid with Flexible Engine Operation

Shifting the drivetrain to a series hybrid configuration saves energy in several areas. First, because there is no idling of the engine, the 16 percent of fuel consumed during idling on the city cycle and 2.0 percent on the highway cycle is saved.

Second, accessory power demand is not likely to be reduced in a hybrid, as an engine running at or near its optimal brake-specific fuel consumption point rejects much more heat to the coolant, and, hence, cooling fan and water pump requirements will increase, but the engine itself is much smaller.

Accessory fuel consumption will be reduced by the improvement in efficiency. Third, the use of regenerative braking will reduce tractive energy requirements by an amount similar to that for an EV.1 Fourth, the use of an electric motor drive eliminates the transmission and improves drivetrain efficiency.

Finally, by operating at or near its optimal point, the engine brake specific fuel consumption is greatly reduced. On the negative side, a small engine (with smaller cylinders) is inherently less efficient owing to the higher surface/volume ratio of its combustion chambers.

In the Taurus example, the engine would be a 1.0 Liter four-valve four-cylinder engine, rather than the 3.0-litre two-valve V-6 used. Although some have discussed using one-or two cylinder engines, their noise and vibration characteristics are so poor that only a four-cylinder engine is thought to be acceptable in a mid-size car (even the three-cylinder Geo Metro engine is considered quite rough in automotive circles).

Detailed analysis of the efficiency without a comprehensive simulation model requires some assumptions regarding average generator and motor efficiency. To provide an optimistic view of hybrid potential, we chose a set of “2005 best” values for component efficiencies, as follows:

  • Generator efficiency at 19 kW 91 percent
  • At 45 kW 94 percent
  • Motor efficiency 82 percent
  • Urban cycle 90 percent
  • Highway cycle 94 percent
  • Drivetrain gear efficiency
  • Urban 96 percent
  • Highway 96 percent

The motor and generator efficiency values are 3 to 4 percent higher than those of the most efficient current motors and generators. Engine efficiency was assumed at slightly off-peak value of 33 percent (in reality, this is higher than the peak efficiency of small engines today). A cold-start related fuel economy loss of 5 percent was also used on the urban cycle. The calculation is detailed in table 4-11.

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