Particle dont dissapear and leave trail behind ship :(

I have big problem with my particles :/

http://imgur.com/o34Vs

When I move my ship with "wsad", particle leave trail behind the ship - they should disapear I think but they dont :/. Could anyone can help me with this? Without destroying particles, everything sucks...

Here is my whole script :
I got thir error and meyby this is the problem:
Assets/Moje_Skrypty/LookAtMouse.cs(73,28): warning CS0219: The variable `showEngine' is assigned but its value is never used

Code (csharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class LookAtMouse : MonoBehaviour
5.    
6. {
7.  
8.     // speed is the rate at which the object will rotate
9.     public float speed;
10.     public float shipSpeed;
11.     public GameObject engine;
12.     public GameObject enginePrefab;
13.     public bool engineStarted;
14.     public GameObject  showEngine;
15.    
16.    
17.  
18.     void FixedUpdate ()
19.     {
20.        
21.         Plane playerPlane = new Plane(Vector3.up, transform.position);
22.  
23.        
24.         Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
25.  
26.        
27.         float hitdist = 0.0f;
28.        
29.         if (playerPlane.Raycast (ray, out hitdist))
30.         {
31.            
32.             Vector3 targetPoint = ray.GetPoint(hitdist);
33.        
34.        
35.        
36.        
37.            
38.             Quaternion targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
39.  
40.            
41.             transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, speed * Time.time);
42.            
43.            
44.             if (Input.GetKey("w") || Input.GetKey("s") || Input.GetKey("a") || Input.GetKey("d"))
45.             //if(silnikiodpalone == false)
46.             {
47.                 startEngine(); 
48.             }
49.             else
50.                
51.             {
52.                 stopEngine();
53.                
54.             }
55.         float movingShipAxisX = Input.GetAxis("Horizontal") * shipSpeed * Time.fixedDeltaTime;
56.         transform.Translate(Vector3.right * movingShipAxisX, Space.World);
57.        
58.        
59.         float movingShipAxisY = Input.GetAxis("Vertical") * shipSpeed * Time.fixedDeltaTime;
60.         transform.Translate(Vector3.forward * movingShipAxisY, Space.World);
61.         }
62.        
63.        
64.        
65.        
66.        
67.     }
68.    
69.     void startEngine()
70.     {
71.         //if(engineStarted == false)
72.         {
73.         GameObject showEngine = (GameObject)Instantiate(enginePrefab, engine.transform.position, engine.transform.rotation);
74.         }
75.         //engineStarted = true;
76.     }
77.    
78.     void stopEngine()
79.     {
80.         Destroy(showEngine);
81.        
82.     }
83.    
84.    
85. }

 

Try

Code (csharp):

1.  
2. showEngine = (GameObject)Instantiate(enginePrefab, engine.transform.position, engine.transform.rotation);
3.