Turn Paticle Render On

I have this script on my spacecraft causing it to move. Parented to my spacecraft I have an Engine Left and An Engine Right that have a paricle effect on them. I want to start with the rendering of these particle effects off and when you press any of the keys to move it turns them on.

This is my code.

var Speed = 200;
var TorqueSpeed = 20.0;
var warpSpeed = 400;
var portal : Transform;

 
function FixedUpdate ()  
{

if(Input.GetKey(KeyCode.UpArrow))
	{
		
		constantForce.relativeForce.z = Speed;
	}
else
	{
if(Input.GetKey(KeyCode.DownArrow))
	{
		constantForce.relativeForce.z = -Speed * Time.deltaTime;
	}
else
	{
constantForce.relativeForce.z = 0;
	}
}
//Turn right and left
if(Input.GetKey(KeyCode.D)){constantForce.relativeTorque.y = TorqueSpeed * Time.deltaTime;}else{
if(Input.GetKey(KeyCode.A)){constantForce.relativeTorque.y = -TorqueSpeed * Time.deltaTime;}else{
constantForce.relativeTorque.y = 0;}}
//Rotate Up and Down
if(Input.GetKey(KeyCode.E)){constantForce.relativeTorque.x = TorqueSpeed * Time.deltaTime;}else{
if(Input.GetKey(KeyCode.Q)){constantForce.relativeTorque.x = -TorqueSpeed * Time.deltaTime;}else{
constantForce.relativeTorque.x = 0;}}
//Rotate right and left
if(Input.GetKey(KeyCode.Z)){constantForce.relativeTorque.z = TorqueSpeed * Time.deltaTime;}else{
if(Input.GetKey(KeyCode.X)){constantForce.relativeTorque.z = -TorqueSpeed * Time.deltaTime;}else{
constantForce.relativeTorque.z = 0;}}

//if(Input.GetKey(KeyCode.W)){constantForce.relativeForce.z = warpSpeed;}

 if( Input.GetKeyDown(KeyCode.P) ) 
    {
       Instantiate( portal, transform.position + (transform.forward * 2.0), Quaternion.identity );
    }

}

Call ParticleSystem.Play(). Unity - Scripting API: ParticleSystem.Play