The USS Sutherland has 8 Main Impulse Engines, 4 clustered on the dorsal side of the Secondary Hull facing aft, and 2 paired in the port and starboard aft sides of the Primary Hull. It also has 4 Backup Fusion Generators which supply power and may be used to provide auxiliary energy to the Impulse Engines.
The USS Explorer has 2 Main Impulse Engines the port and starboard sides of Main Engineering. It also has 2 Backup Fusion Generators which supply power and may be used to provide auxiliary energy to the Impulse Engines.
Maximum Impulse speeds are presently restricted to ¼ C. This is not due to any inability of the Impulse Engines which still function at an 85% efficiency at speeds of ½ C. However, since near light speeds decrease perceived time on the space ship it is important to limit this effect. The alternative is to have a crew travel the 22hrs from Earth to Pluto to stop a Romulan vessel, only to find on their arrival that the vessel had left a week ago. Or even worse, having their reactions so slowed that a Romulan can decloak and open fire before they can react.