Can gliding your plane save fuel?
This approach of using an engine is called pulse and glide. It generally works because each engine has an optimal power setting at which it converts fuel into power most efficiently.
If the most efficient power is higher than is required, something should be done to accumulate and later use the excess energy. Raising the vehicle up looks like a solution, better than accelerating.
This method can only work if the power to efficiency curve of the machine is known. Otherwise, it is easy to do worse.