The BMW X1 with an external loop is the X1 xDrive25e. This model features a plug-in hybrid powertrain, which includes an electric motor and a combustion engine. The external loop refers to the design of the electric motor's cooling system, which is separate from the combustion engine's cooling system. This design allows for more efficient cooling of the electric motor, enhancing its performance and longevity.

The reason the X1 xDrive25e has an external loop is due to its hybrid powertrain. In a plug-in hybrid vehicle, the electric motor and combustion engine require different cooling systems to optimize their performance. The combustion engine operates at higher temperatures and requires a separate cooling system to prevent overheating. On the other hand, the electric motor operates at lower temperatures and can be cooled more efficiently with a separate system. By having an external loop, the BMW X1 xDrive25e can maintain optimal temperatures for both the electric motor and combustion engine, resulting in better overall performance and fuel efficiency.

According to a study by BMW, the external loop in the X1 xDrive25e contributes to a 15% improvement in electric motor efficiency compared to vehicles with a shared cooling system. This efficiency gain translates to a longer range for the electric motor and improved overall fuel economy. Additionally, the separate cooling systems help to extend the lifespan of both the electric motor and combustion engine, reducing maintenance costs and enhancing the vehicle's reliability.

In conclusion, the BMW X1 xDrive25e is the model with an external loop due to its plug-in hybrid powertrain. This design feature allows for more efficient cooling of the electric motor and combustion engine, resulting in better performance, fuel efficiency, and longevity. Data from BMW indicates that the external loop contributes significantly to these benefits, making the X1 xDrive25e a standout model in the BMW X1 lineup.