The impulse drive may actually be a misnomer. Some sources hold it is a contraction of "I.M.Pulse Drive", standing for Inertial Magnetronic Pulse drive. It operates not through expulsion of energy or mass but through less-controlled distortions of local space-time. Star Trek starships do, in fact, have a propulsion system based on a more conventional reaction drive; usually, they are referred to (on the series) as 'thrusters,' occasionally with an adjective such as 'docking thrusters,' 'maneuvering thrusters' or 'maneuvering jets'.

The impulse drive works (supposedly) through the following sequence of events. A fusion fuel is fused to produce energy. However, the actual fusion of the fuel pellet takes place inside a magnetronic field. The field is used to amplify or modulate the burst of energy into the gravitic range; as a result, each miniature fusion explosion creates a very small gravity wave. Since the impulse drive units are offset from the ship's center of mass, these waves serve to 'push' or 'pull' the vessel through space.

Impulse fusion reactions are likely to be D-D, or deuterium-deuterium reactions. Although D-T, or deuterium-tritium reactions are easier to start, tritium is radioactive and has a very short half-life as well. Technical sources for Star Trek: The Next Generation finally admit to fuel tanks aboard the Enterprise-D: they contain 'slush deuterium' for use both as a fusion fuel and to serve as the matter for the dilithium-focussed matter/antimatter reaction that powers the warp drive and the rest of the ship. Oh, and as 'base mass' for the replicators aboard.

Impulse engines, in Trek, do have an 'exhaust' where the energy and plasma byproducts of the fusion reactions are vented. On the Enterprise, these vents are at the aft of the Primary Hull, just to either side of the connecting dorsal, and glow orange when in use.

In a way, the impulse drive is just a super-high-tech version of Project Daedalus, without the mess.

My sources for this are both my own hazy memory of the original Star Trek Technical Manual, as well as the (slightly better) source of Diane Carey's Trek Novel 'The Final Frontier.' Since Diane Carey has been claimed to be a pen name for D.C. Fontana, one of the original Star Trek scriptwriters, this would offer some confirmation of my own whole-cloth reconstruction above. Note that in 'The Final Frontier' the I.M. Pulse drive is in fact the 'Internally Metered Pulse' drive, not Inertial Magnetronic; I specifically recall "Magnetronic' being in there, but bow to her as the Authority on par with the Great Bird of the Galaxy on this one.