shaft
轅 refers specifically to the shaft of a carriage or cart—the long pole to which horses or oxen are attached.
轅 combines 車 (vehicle) with 袁, which likely provides the sound. The character originally denoted the shaft of a carriage.
The left side is a vehicle 車, and the right side 袁 looks like a long robe. Imagine a long robe-like shaft extending from the carriage to harness the horses.
For エン, picture the end of the shaft: end -> エン. The shaft ends where it connects to the carriage.