Experimental Evaluation of the Impact of ETF on System Efficiency

The electric transmission fluid (ETF) can improve the system efficiency of the electric drive unit (EDU). However, there is no unified method to evaluate the corresponding performance of ETF in the industry, which leads to misdirection in production development. This paper analyzes the factors affecting system efficiency, redefines the scope of system efficiency, innovatively introduces the power consumption of the electronic oil pump into the calculation of system efficiency, and redefines the calculation formula for system efficiency. The paper also explains the influence of ETF viscosity and heat transfer performance in the stator cooling process and suggests the need to set reasonable boundary conditions and operating conditions to comprehensively evaluate ETF performance. The paper examines the reasons for the low repeatability and discrimination in ETF efficiency testing and suggests a remedy by continuously gauging steady-state conditions and calculating the system efficiency in terms of work. The paper develops a test procedure for evaluating ETF and validates it by putting 12 ETFs to the test on the same EDU. The results show that ETF can have a significant impact on the system efficiency of the EDU under different operating conditions, cycles, and boundary conditions, especially in the common speed range of the vehicle and at low temperatures. Low-viscosity ETF can increase system efficiency by more than 1.5% compared to high viscosity reference ETF. The verification results demonstrate that ETF viscosity is not the only element affecting system efficiency and that comparatively low-viscosity ETF does not always perform better.