This is an idea put forward at the Duxford Creative Workshop of June 2009. It was put forward with the accompanying text "Power units are retracted during periods out of use." This article is an extension of the idea.
The concept of somehow dispensing with services that are not needed is applied in many walks of life. Large trucks sometimes hydraulically raise some of their wheels when lightly loaded to reduce rolling friction. Luxury cruising boats may have sail and diesle power and can retract the propelller when under sail. Motor gliders often have foldable engines for use during assisted take-offs and then fold them away in flight.
This idea is that concept applied to aircraft. The maximum power demand is generally at take off at which time all the available power of the aircraft should be accessible to use in an emergency. At other times in the mission the engine load may be very much lighter and the aircraft may also have clear skies and be at a high altitude i.e. in a low power and low risk situation.
It would be possible to address this concept along two main avenues (a) having multiple engines so that they are smaller and can be applied or not in a very flexible manner or (b) to have a small number of engines that can be retracted into the body of the aircraft when not needed to aid drag reduction. Of course, the two approaches can be linked and merged to some extent.
A large number of small engines will probably imply efficiency penalties as the usual rule is that large units can deliver better performance more efficiently. Folding the unused engines into the aircraft body is also likely to carry with it an overhead made up of the weight of the mechanism to control the stowage of a large unit. The measure of any such scheme will be (a) the fuel savings of the operable system (b) the cost of the installation (c) any safety, accessibility or operating issues that may emerge.