Trends Driving System Evolution

25.7.1 Trends Driving System Evolution

The conventional 12-V automotive electrical power system has many defects, including a widely varying steady-state system voltage and large transients, which force the electrical func- tions to be over-designed. However, these limitations alone have not been a strong enough driver for automotive compa- nies to seriously evaluate advanced alternatives. Now a number of new factors are changing this situation. The most important of these are future load requirements that cannot be met by the present 12-V architecture.

25.7.1.1 Future Load Requirements

Table 25.6 gives a list of electrical loads expected to be intro- duced into automobiles in the next ten years [53]. Some of these loads (electrohydraulic power steering, electric engine fan, electric water pump, and electromechanical valves) will replace existing mechanically or hydraulically driven loads. The remaining are new loads introduced to either meet government mandates or satisfy customer needs.

TABLE 25.6 Electrical loads expected to be introduced into automobiles in the next decade [53]

Load

Peak power (W) Average power (W)

Exhaust air pump

Electrohydraulic power

Electric engine fan

Heated catalytic converter

Electric water pump

Heated windshield

Electromechanical engine

valves (6 cylinders at 6000 rpm)

Active suspension

Total

25 Automotive Applications of Power Electronics 661 The average electrical power requirement of a present- plans to increase the average fleet fuel efficiency to 39.9 mpg

day automobile is in the range of 500–900 W depending on by 2005 – compared to 31.4 mpg in 1990 [58]. whether it is an entry-level or a luxury vehicle. When the

Another driver behind the development of fuel-efficient loads of Table 25.6 are introduced, the average electrical power vehicles is the partnership for a new generation of vehi- requirement will increase by 1.8 kW. Furthermore, if the air- cles (PNGV). This ten-year research program, launched in conditioning (A/C) pump were to ever become electrically September 1993, is a collaboration between the US Federal driven, the peak and average power demands would increase Government and the big three US automakers (General by an additional 3.5 kW and 1.5 kW, respectively. Distributing Motors, Ford, and DaimlerChrysler) that aims to strengthen such high power at a relatively low voltage will result in unac- national competitiveness in the automotive industry and ceptably bulky wiring harnesses and large distribution losses. reduce dependence on foreign oil. The PNGV has set a goal to Since the alternator has to generate both the power consumed develop an 80 mpg midsize vehicle by 2004 [59]. The German by the loads and the power dissipated in the distribution Automotive Industry Association is pursuing similar targets. network, its output rating (and hence size and power con- The VDA has undertaken a pledge to introduce a 3 L/100 km sumption) will be greater than in an architecture with lower (78 mpg) vehicle by the year 2000. This is complemented by distribution losses. With the large premium attached to the the introduction of highly fuel-efficient vehicles (in excess of size of the alternator (due to space constraints in the engine

50 mpg) in both the Japanese and American markets. compartment), an architectural change in the distribution and

With the present alternator, 800 W of electrical power con- generation systems is essential before many of the future loads sumes 1.33 L of gasoline for every 100 km driven when the can be introduced.

vehicle has an average speed of 33.7 km/h. This represents a There is also an increasing disparity in the voltage require- 45% increase in fuel consumption for a 3 L/100 km vehicle. ments of future electrical loads. High pulse-power loads, such Hence, if future high fuel economy vehicles are going to as the heated windshield and electrically heated catalytic con- have comfort, convenience, and safety features comparable to verter, become feasible only at voltages greater than the current present-day vehicles, the efficiency of the electrical generation

14 V [57]. On the other hand, incandescent lamps and elec- and distribution system will have to be substantially improved. tronic control units (ECUs) will continue to require low Furthermore, as discussed in Section 25.8, one widely con- voltages. For example, present day ECUs have linear regula- sidered means of achieving high fuel economy is the use tors which convert the 14 V distribution voltage to the 5 V of a hybrid vehicle architecture. In practice, this approach needed by the integrated circuits. The efficiency of these reg- necessitates the introduction of a higher voltage in the vehicle. ulators is equal to the ratio of output to input voltage, i.e. 35%. Furthermore, the next generation of higher speed lower