The concept of system power for motor vehicles becomes necessary because multi energy converters and higher efficiency drivetrain are being adapted in advanced motor vehicles for the energy savings and the precise emission controls. This concept is very important in hybrid vehicles due to two different energy converters such as an internal combustion engine and an electric motor. This concept is also applicable to electric vehicles for the efficiency of energy transfer from energy sources to vehicle wheels.
The system power is measured on the powertrain test bed at the state of whole vehicle. It is possible to measure the wheel torque precisely by the direct connection between the power absorber and the end of drivetrain without tires. Two different test methods were developed for the measurement of system power. One is the acceleration at full load under road load (i.e., Method I); the other is the acceleration at full load under the constant vehicle speed (i.e., Method II). Three HEVs and two EVs were tested using the developed test methods. The different types of drivetrain such as an automatic transmission (AT), a continuously variable transmission (CVT) and reduction gear (RG) were equipped in the test vehicles respectively.
Engine speed, coolant temperature, and SOC condition were very important factors for this test in order to produce the maximum power. Method I was much better one for the measurement of system power despite of transient characteristics. The analysis of standard deviation was used for the determination of transient system power at the specific vehicle speed. The criterion for this analysis is the standard deviation of one value, which means that each power value is within +/- two percent of averaged power value.
By measuring system power at the constant vehicle speed, it is shown that the transient power by method I could be maintained during at least 30s. The repeatability of method I was within +/- 5 percent. It is found that this concept and method are applicable and reasonable for the power test of HEVs and EVs.