An impulse engine consists of one or more nuclear fusion
reactors connected to exhaust vents that generate thrust to
accelerate the ship. The thrust generated by an impulse engine
is typically tiny compared to the actual mass of the ship, but
the use of "mass lightening" technology (involving subspace
fields) reduces the effective mass of the ship enough for the
thrust to provide considerable acceleration. Because impulse
engines include fusion reactors, they are typically the ship's
secondary power source, outpowered only by the warp core.
Impulse engines are capable of powering a ship for short
journies at FTL speeds (assuming the ship has a functioning warp
drive), but doing so drains fuel rapidly. The Enterprise nearly
exhausted the fuel for its impulse engines in just 14 hours of
interstellar travel when the warp core was inoperable due to
"burning out" the ship's dilithium crystals. Trekkie
PicardAlpha's estimate of acceleration from Federation impulse
engines.