Well, if the impulse engines when set to overload is able to force it's fuel together in a closed chamber with extremely high temperature and pressure to get the atoms of the fuel to fuse then you've got a fusion reaction. It would be like blasting a rocket engine at 100% but closing off the exhaust until the pressure exploded the engine. But I do think that an impulse engine is analogous to a warp engine in that it has a reactor to drive whatever field mechanism it needs to bend or warp space. Impulse engine uses a fusion reactor while a warp engine uses a matter-antimatter reactor. All IMHO. I also think that there are no separate fusion reactors though or they would have been handy in "The Naked Time".