Very much so! All engines are designed to provide a specific power output efficiently, operating 'off-design' will result in very poor efficiency indeed.
This is true of all types of engine and indeed all kinds of machines. It's easier to illustrate using a reciprocating engine as an example.
If you wanted to generate 15hp, such as a 125cc motorcycle, then the single cylinder lightweight engine would efficiently deliver this power level. The internal losses due to piston inertia, friction and thermal losses minimised by virtue of the system operating at a point where the output power is significant in comparison to the internal power consumed in operating the engine. (I.e: Compressing the air, overcoming frictions, pumping coolant and lubricant etc etc).
If we then use a 20 litre, 10 cylinder truck engine (normally rated at say 450hp) to generate our 15 hp then clearly the power required to operate the engine, (which will be much greater than 15hp!) is extremely significant in comparison to the output power and hence a very poor efficiency and hence fuel consumption is obtained.
This is an extreme case but same is true with turboshaft engines.
By operating two engines in place of one you are making the output of each engine less significant in comparison to the power required to operate that engine. Hence the poorer efficiency.
If you operate both engines at 50% output, they are both clearly 'off-design' if you operate one at full power and the other at idle you can see that you will have to provide fuel to idle the second engine in addition to fuel for the driving engine.
Hope this helps.
CRAN
