How to make a particle emitter's direction to follow a parts velocity?

So if I wanted to make a particle emitter's direction to follow a parts velocity, how do I do it, I did this, but it didn't work:

local p = script.Parent
local emitter = script.Parent.ParticleEmitter

emitter.EmissionDirection = p.Velocity.X --or Y or Z

Possible, but not that way. Remember Emission Direction is "Top", "Bottom" or the such. Not a Number.

ParticleEmitter is possible to do ways of direction. But you need to use 'top' 'bottom' 'left' 'right' 'back' and 'front'. You cannot use numbers.