Is a engine essential to keeping blimp in the air?

No, an engine is not essential for keeping a blimp in the air. Blimps are lighter-than-air craft, meaning they are buoyant due to the lifting gas (usually helium) inside the envelope. The engine is used for propulsion and maneuvering, not for maintaining altitude. A blimp could, theoretically, remain aloft without an engine, though it would be at the mercy of the wind.