Officially, there are no useful schematics or power specifications mentioned for the SHIELD Helicarriers.
This schematic is for one of the Insight Carriers taken from a terminal inside SHIELD. Note the lack of useful information internal information.
Unofficially, since we are left with nothing but speculation, we can make only some assumptions about the potential power sources of the Helicarriers.
The Helicarriers are likely to be nuclear powered: In a fashion similar to nuclear-powered supercarriers that can't fly, like the USS Nimitz (CV-68), the helicarriers likely use the same types of nuclear powerplants. It would be at least two nuclear-powered General Electric pressurised water reactors driving four turbines of 260,000hp (194MW) and four shafts. There are four emergency diesels of 10,720hp (8MW).
Given we are unable to lift the mass of the Nimitz out of the water with its nuclear-reactors (it weighs conservatively 101,600 tons) and can only drive it at 30+ knots, there must be other legerdemain and handwavium being performed. We should assume then the nuclear engines and secondary diesel engines are there supporting energy needs and perhaps are capable enough to land the vehicle during emergencies or offer propulsion when the vehicle is at sea.
We are shown Howard Stark had repulsor technology used to levitate vehicles (and we suspect that same technology is used by Stark to keep his suit in the air.) Stark's company is listed as one of the contractors for the Helicarrier so it shouldn't be a stretch to assume SHIELD has licensed the anti-gravity aspects of Howard Stark's repulsor design to keep the helicarrier in the air. This floating car was demonstrated at Stark's expo in 1943.
Stark's floating car did not have a nuclear reactor inside of it (as far as we know) thus we can assume the technology could have been run on relatively non-exotic energy capable of being powered by electro-dynamic forces.
Images of the Helicarrier show both flight turbines and later repulsor-flight generators on the second generation carriers, indicating there is the possibility of Stark-tech or other Marvel super-technologies onboard, designed by Howard Stark, Reed Richards or Tony Stark. Given what technologies SHIELD has worked on in the past, it is also possible they are utilizing alien technology either in the flight or propulsion capacity of the Helicarrier.
Unofficially, there is one blueprint for the early comic versions of the Helicarrier. This schematic implied there are nuclear-powered engines onboard, which makes sense since the mass of the Helicarrier would make it impossible to power with any form of commonly used liquid propulsion: