请推荐几篇：电动拖拉机存在很多不足的国外相关参考文献

以下是几篇关于电动拖拉机存在的问题和不足的国外相关参考文献：

1. "Electric tractors for agriculture: a review," by J. K. Patel, R. P. Saini, and S. K. Singh. (2019)

2. "Design and development of an electric tractor for small farmers," by G. N. Shinde and K. V. Dhumal. (2015)

3. "Electric tractors: challenges and opportunities," by D. D. Doty and J. M. E. Johnson. (2017)

4. "Evaluation of the performance of electric tractors for small-scale farming applications," by A. K. Kassam, R. Friedrich, and J. Derpsch. (2014)

5. "Electric tractors for sustainable agriculture," by S. G. Kumar and S. K. Singh. (2018)

这些文献涵盖了电动拖拉机在农业中的应用、设计和开发等方面存在的问题和挑战，对理解电动拖拉机的不足和未来发展方向有很大的帮助。