Assessing the Energy Efficiency of an Electric Car

Synák, František; Kučera, Matej; Skrúcaný, Tomáš · 2020 · OpenAlex-citations

DOI: 10.26552/com.c.2021.1.a1-a13

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Summary

This study investigates the overall energy efficiency of an electric vehicle (EV), specifically addressing the energy losses that occur between the electrical grid and the vehicle’s wheels. While existing literature often focuses on individual components like batteries or hybrid systems, this research aims to quantify the total transmission efficiency of a complete EV system. The motivation stems from the need to understand how efficiency impacts energy consumption, battery mass requirements, and the theoretical production of emissions associated with electricity generation, particularly in the context of road transport being a major source of greenhouse gases in the EU. The researchers conducted experiments using a prototype electric vehicle named "Edison II," designed at the University of Zilina. The vehicle features a 30 kW AC motor, LiFeYPO4 batteries, and a gross weight of 1300 kg. Measurements were performed on a MAHA MSR 1050 dynamometer, which simulates real driving resistances. The experimental design followed an amended New European Driving Cycle (NEDC) with a maximum speed of 70 km/h. The methodology involved comparing the electrical energy drawn from the socket (measured via an electricity meter) against the mechanical energy delivered to the dynamometer’s cylinders. To account for rolling resistance differences between road and cylinder driving, a coasting deceleration test was performed to calibrate the dynamometer’s simulation of road conditions. The results indicate a total transmission energy efficiency of 45% from the electric socket to the dynamometer cylinders. The analysis revealed that energy losses are predominantly electrical rather than mechanical. Specifically, 82% of the total energy loss occurred in the electrical components (between the socket and the motor shaft), including charging, conversion, and battery storage, while only 18% was lost in mechanical components (between the motor shaft and the wheels). Consequently, the average energy consumption at the wheels was 33,958 kJ/100 km, whereas the consumption at the socket was 75,373 kJ/100 km. The study also calculated theoretical emissions based on EU power plant data, finding that when accounting for real-world efficiency losses, the theoretical CO2 production associated with driving the EV increases significantly to 675 g/kWh, which is higher than the direct emissions of internal combustion engine vehicles. The significance of these findings lies in highlighting that the primary inefficiencies in electric vehicles reside in the electrical energy conversion and storage stages rather than the mechanical drivetrain. The study concludes that improving the efficiency of chargers, converters, and battery systems is crucial for reducing overall energy consumption and the associated indirect emissions. Furthermore, the results challenge the assumption that EVs are emission-free by demonstrating that their upstream energy losses contribute substantially to the theoretical production of CO2, NOx, and particulate matter, depending on the energy mix of the power grid.

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