It feels like a sci fi question but is more related to real science in present times so cannot be posted on scifi SE.
assume an engine made of a hollow solenoid with valves inside its hollow pipe part
now if we assume there are enough free moving charged particles in space we'll close the valve and collect the particles. After a certain amount of them have been accumulated open the valve and generate a momentary but extremely intense magnetic field using the solenoid which would propel the charged particles back and the engine forward.
It's very similar in principle to a rail gun. But the question is are there enough free moving charged particles of desirable polarity that can be used to drive a ion drive without having to carry it's own ion source?