Electric Vehicle Real-Drive Tests
As the automotive industry advances power conversion and battery technologies. Dewesoft has been actively working to provide flexible and powerful measurement systems, specially designed for electric and hybrid drivetrains. Measurement systems need to fulfil more and more requirements. In the case of electric vehicles, the measurement systems have to suit High-Voltage environments (up to 1000V or even higher) to ensure safe and reliable operation. The most challenging application is Real-Drive tests of vehicles. Harsh environments, temperatures from -30°C to +60°C and all kind of terrains requires adequate measurement instruments. And that’s exactly where Dewesoft is perfect – ruggedised and powerful measurement systems.
Efficiency Analysis of Real-Drive Tests
Nowadays the determination of energy consumption and CO2 emissions is done at test benches by means of standardised driving cycles (NEFZ, WMTC etc.). However, these driving cycles are not suitable for determining the energy consumption of electric vehicles as they do not consider fundamental influencing factors such as energy recuperation, weather conditions or the tremendous effects of auxiliary loads. Simply put, the ideal test bench conditions do not even come close to the real driving conditions. As a result, the real energy consumption of electric vehicles can be up to 60% higher.
The electrification of vehicle drivetrains changes the requirements for testing vehicles apart from analysing the combustion process. Electric and Hybrid vehicles can have multiple motors, inverters and battery packs. For comprehensive energy and efficiency analysis, all energy sources and loads have to be considered. Figure 1. gives an overview of some of the possible drivetrains of electric and hybrid vehicles.
Figure 1. Drivetrains of electric and hybrid vehicles
Analysis of essential data
To analyse all necessary data there are several requirements. The measurement system must be able to measure the power at different points inside the vehicle completely synchronous. Using conventional measurement equipment would require the use of several power analysers, combustion analysers, data loggers, GPS loggers etc. Challenges like data merging, data synchronisation, the power supply of the instruments (independent of the vehicle battery), etc. make it nearly impossible to get reliable measurement results. Another important point that must be considered, especially for real driving tests, is that the power supply of the measurement instruments must be independent of the vehicle battery to carry out a comprehensive and clear statement about the energy efficiency.
Figure 2. Measurement system
To tackle all these problems in a convenient way the Dewesoft R2DB Power Analyser allows to measure multiple 3-phase systems at the same time and combines all the functionality of a power analyser, an oscilloscope, a data logger, an FFT spectrum analyser and a transient recorder in one instrument. The system also has a built-in battery pack which allows supplying the measurement device and all sensors (current clamps, GPS, video, etc.) directly out of the instrument itself. The high accuracy (0.03%) and high sampling rate (up to 15 MS/s) of Dewesoft Sirius high-voltage and low-voltage input amplifiers ensure accurate analysis for electric vehicles (see figure 2.).
In the measurement software, all data (electrical, mechanical, video, GPS, CAN etc.) can be viewed together and individual screens can be generated and customised. This allows state-of-the-art analysis at Real-Drive tests.
Figure 3. Screenshot of measurement software, DEWESoft®
Grid to Wheel efficiency
In this test-case, the grid-to-wheel efficiency of a battery electric vehicle (BEV) was determined. The routes for the road tests were chosen to consist of different representative characteristics (city, freeway, uphill, downhill, etc.) to further underline their influence on the energy consumption and the performance of BEVs under real driving conditions.
Figure 4. shows the results via a Sankey Chart, giving an expressive statement about the energy consumption including the influencing factors.
Figure. 4. Grid-to-Wheel efficiency of an electric vehicle as a Sankey chart
In this case, the power was measured at 6 different points including battery power, motor power and power of major loads. The averaged energy consumption over the test track was 24.6 kWh/100km.
As you can see, the charging/discharging process, in this case, is already responsible for a major energy loss of 15%.
Due to this loss, only 60% of the grid energy arrive at the motor. This is again a 15% loss, but it is not wasted, as it is used by auxiliary loads, like heating, air conditioning etc. Finally, 54% of the grid energy arrives at the wheel. The motor loss is 7%. The recuperation rate of the whole test-cycle was 20%, which is quite high.
This chart underlines the importance of efficiency determination of the overall system from the grid to the wheel. For instance, it emphasises the main factors for energy loss and thus efficiency improvements. For detailed energy consumption analysis, it is substantial to include video, log GPS, and measure also mechanical data besides the electrical parameters. In that way, each energy source and load can be analysed in detail for any operating and driving situation.
Conclusion of Electric Vehicle Real Drive Tests
Finally, the electrification of vehicles changes the requirements for measurement systems. Firstly, on the hardware, side Instruments have to suit the High Voltage environments and have to withstand harsh environments. Secondly, on the software side the synchronous data acquisition of different parameters. Such as electrical and mechanical, adding GPS, video and others is a substantial need. These features need to be providing powerful data processing and visualisation possibilities.