Particle system to delay start

If you want particle system to start after certain time period from an event, you can do

 int TRIGGER_TIME = 20;
 
 //increase the timer per frame
 update()
 {
   timer++;
 
   if(timer > TRIGGER_TIME && !emitter.emit)
     emitter.emit = true
 }

 // Initialize the timer when the event occurs
 onWhateverEvent()
 {
   timer= 0;
 }


 if you want emitter to slowly increase emitting the particles, 
 play around with emitter.maxEnergy

 int MAX_ENERGY = 5;

 // in update do
 if(emitter.maxEnergy < MAXENERGY)
   emitter.maxEnergy += 0.01f;

Hope it helps !!