What is "power-egg"

The power-egg, a literal translation into English of the German term Kraftei, was a design feature in German Second World War era aircraft. The power-egg was a completely "unitized" modular engine installation, consisting of engine and all ancillary equipment, which could be swapped between suitably designed aircraft, with standardised quick-changing attachment points and connectors. In aircraft so designed the power-egg was removed before mean time to failure was reached and a fresh one installed, the removed engine would then be sent for maintenance. In ideal conditions spare engines were placed in sealed containers protected from damage and the elements, to be opened when needed. These existed in two differing formats — the initial Motoranlage format which used some specialized added components depending on what airframe it was meant for use on, and the Triebwerksanlage format, a more complete unitization format usually including exhaust and oil cooling systems.