I know this is 'just' a computer game which takes some liberties toward the laws of physics, but here goes....
Who said that the 'engines' in GalCiv II propel starships through the exhaust of some particle?
A jet engine has a flame because of the 'hot and compressed air' which flows through it. It it this flow of air particles that propels the aircraft.
A rocket engine has a flame because of the solid or liquid propellant is burns. It it this exhaust of propellant particles which propels the rocket.
Do you have any idea how much particles you would need in your starships' 'gas tank' to accelerate a 500 meter tall, 1000,000,000,000 + ton starship to near light speed? Suffice it to say: too much!
For example: the warp drive (from star Trek) 'bends' space between point A and B, so that these points in space itself become 'closer'
For a nice gaming experience: just suppose that the engines have no exhaust! Instead they have nice colourfull surface which they use to manipulate space itself....
If you can figure out how, there might be a Nobel Prize for you!
