As to the plural by default, how so?
I meant that the Enterprise obviously has two engines (the big pods on the outside) - but that this provides no information on how many sources of dangerous explosion there are in the ship. Engine damage or malfunction could cause one explosion in one central location, or sixteen ones in a cascade, depending on the technological details, but not necessarily depending on the number of engines. If the m/am engines of the TOS ship got hurt, there could be a single explosion at the site of Scotty's "That Which Survives" heroics. If the same happened to the STXI ship, eight separate warp cores might blow up. With the TMP ship, perhaps the reputed vertical and horizontal intermix shafts would be the ones to go. And so forth.
The multiple engines could be fueled by antimatter, just like multiple engines on an aircraft could be fueled by gasoline. Worrying about the antimatter or the gasoline by saying "if the engines..." would be a valid approach if one treats the engine&power system as an integrated whole and blames the engines for the presence of the antimatter or the gasoline.
However, "fueling" need not mean the actual pumping of fuel to the engine. The TOS engines might instead receive the power benefits from the use of antimatter fuel the same way the TNG and ENT ones do, via plasma-based power leads. They'd still very much be matter-antimatter engines...
...As possibly opposed to the impulse ones, whose malfunction is described as a fusion explosion in "DDM", and might leave a wholly different radiation signature. Sulu would do right to speculate on the exact nature of the explosion, then, as a radiation signature would be their only means of deducing what had happened to the ship. Kirk then proceeds to use that means to exclude the possibility of either sort of engine explosion.
Also, Sulu could have another very good reason to worry about whether one or the other blew: an impulse explosion might be survivable for the landing party, while a warp explosion might already have killed them, too, via delta ray poisoning or somesuch. Kirk's swift waving of a tricorder preempts those concerns, too.
Timo Saloniemi