Which of the following types of jet engines has the highest efficiency at low speeds?

The turbofan engine is recognized for its high efficiency at low speeds primarily due to its design and operational characteristics. Unlike turbojets, which expel all the airflow at high speed and are optimized for high-speed performance, turbofans incorporate a large bypass ratio. This means that a significant portion of the thrust is produced by the large volume of cooler, slower-moving air bypassing the engine core, rather than just the high-velocity exhaust.

This configuration allows turbofans to achieve better fuel efficiency, particularly during takeoff and climbing phases of flight, where speeds are lower. The additional thrust generated by the bypass air not only enhances performance at these speeds but also reduces noise, which is a significant advantage for commercial aircraft operating near populated areas.

In contrast, while turboprops also demonstrate efficiency at lower speeds by using propellers for thrust, turbofans are generally considered more efficient at the higher airspeeds typical of modern jet aircraft operations. Ramjets, designed for supersonic speeds, and turbojets, focused on high-speed operations, do not provide the same low-speed efficiency as turbofans, which are tailored for a balance of speed and fuel economy across a range of flight profiles.