More of the series on technology in Star Trek: the engines.
I've always thought there were both interesting and silly aspects to how Star Trek portrays the ship's engines. It makes sense that the “warp” engines are separate from the “impulse” engines (and they also have “thrusters”, which occasionally get used for very low-power propulsion). And the concept of the warp engines is a good one, which seems very consistent with the technology in general. I also like that the warp engines have limitations — and that the limitations have changed over time: in the original series, the ship's top sustained speed was around warp 6, though they could push it when they had to; Voyager can go faster, yet there's a theoretical limit that they can't achieve warp 10. “Transwarp” technology, which we've seen in Voyager's future (and in other cultures that have already developed it), can go to warp 10 and beyond.
The silly part, which was handled worse in the original series and isn't so bad in Voyager, is the whole “Scotty, I need more from the engines!” thing, usually demanded in the midst of a crisis. Scotty's response was often, “I'm giving them all I've got, Captain!”, or something similar, as though a crew were there shovelling coal (or anti-matter) into a furnace to try to get more pressure in there. It's silly because such technology would surely just be turned up to “max”, and that's that. Either you've got it turned all the way up or you haven't, and if you have, nothing else you do will change anything.
Aboard Voyager, the usual thing is to “divert more power” to a subsystem. I'm skeptical about that one... it seems sensible for some things — if there's limited power available, maybe expending more on the shields, say, makes sense when you're in danger of being blasted out of the galaxy, and maybe that really can make the shields hold longer — but it seems to be used far too often, even when the power systems aren't compromised. I'd think that the weapons systems would be self-contained, and diverting extra power to them wouldn't make them more destructive. Diverting power to the engines could go one way or the other, but I think I put it in the latter category, where it seems a bit silly.
Anyway, the limitation on the engines' performance and top speed are what make the premise in Voyager work. It's easy, in a science fiction story, to remove too many technological limitations and weaken the story. Star Trek has done that in some areas, but not with the engines... so it still makes sense that it will take them 70 years to get back home from where they wound up.
But one thing I don't get is why they'd ever travel below the maximum speed they can safely go. In the original series, I got the impression that travelling at warp 4 was as safe for the Enterprise as going warp 2. And yet Captain Kirk often said, “Get us out of here Mr Sulu. Warp 2.” Similarly, Voyager isn't always heading home at maximum warp, and there's not a clear explanation for that. Maybe it's a balance between speed and resource consumption, or something of a maintenance issue... but it seems to be handled inconsistently.