what are the most practical solutions for the fluctuation of diesel-powered generator?

Answer 1

If your installation isn't limited by weight, you might consider a heavy flywheel. The momentum of the rotating wheel would tend to smooth the output energy, even with some fluctuation in the diesel's input power. If you are generating AC, then you still need to carefully "tune" the flywheel speed to the synchronous speed of the generator.

Of course you'll lose some of your efficiency by requiring more fuel to get it up to speed, plus you'd need to support the extra weight with bearings, which would reduce your running efficiency a tad.