I haven't really started on the engineering hull on my deck plans project, but I'm of the opinion that there are numerous compartments that look like the "engine room" sprinkled liberally throughout the hull. Three such rooms would be my minimum. But they don't necessarily have to side by side.
I've already managed to fit one such room in the saucer near the impulse engines. And
I've even made it five feet longer then the real set and it fits beautifully. Of course, I'm using a 1080' Enterprise
rather than 947'. Also I'm using the exterior proportions of the AMT model kit rather than the 11' studio miniature.