var engine_state = 0;

....

function start_stop_eng();

function start_stop_eng();
{
	if(engine_state == 0)
	{
          engine_state = 1;
	  ent_playloop(panzer,tankidle,77);
	}
	
	if(engine_state == 1)
	{
          engine_state = 0;
	  snd_stop(tankidle);
	}
}

....

action tank_controls()
{
 ....

    if(key_e)
     {
       start_stop_eng();
     }

}

...

///////////////////////////////////////////////////////////////////////////////////////////////////
// Icluding...
#include <acknex.h>
#include <default.c>
//
///////////////////////////////////////////////////////////////////////////////////////////////////
// Vehicle movement template... testing a tank... by Keinmann...
///////////////////////////////////////////////////////////////////////////////////////////////////

// Using the desert skirmish map, "Das Sandbox"...

// Declaring variables

var engine_state = 0;

//Declaring entity pointers

ENTITY* panzer;

// Defining movement vectors

VECTOR vSpeed, vAngularSpeed, vForce, vMove, vAhead;

// Defining sounds

SOUND* tankidle = "tankidle.wav";

// Declaring functions

// Tank movement code...

// Defining actions

action tank_controls()
{
	wait(1);
   set(my,POLYGON); // Make the engine interpret collisions by the polygon and not bounding box
	
	// Set the tank's physical properties.
      phent_settype(my,PH_RIGID,PH_BOX);
      phent_setmass(my,2,PH_SPHERE);
      phent_setfriction(my,5);
      phent_setelasticity(my,7,100);
      phent_setdamping(my,27,5);
	

	while(1)
	{
		 // Update camera positions
      camera.x = my.x - 55;      // Position relative to tank's x position
      camera.y = my.y + 0;      // Position relative to tank's y position
      camera.z = my.z + 25;      // Position relative to tank's z position
      // Set camera tilt and pan in order to look at the tank.
      camera.tilt = -15;
      camera.pan = 0;
      wait(1);
      
    if(key_e)
     {
     	 // ..............
     }
     
      //   Check to see if the player has fallen off the level i.e. the z value is less than minus 200
      if(my.z < -200)
      {
         phent_enable( me, 0 );      // To move physics enabled entities to an xyz position you must first disable physics for that entity
         my.x = 0;                  // Place at 0,0,80 (The 80 is so it spawns high enough above the level. You may need to change this with custom levels)
         my.y = 0;                  //
         my.z = 1000;
         phent_clearvelocity(my);   // Remove any speed and direction from the entity
         phent_enable( me, 1);      // Re-enable physics
      }
	}
}

// Collision detection/properties for objects
action terrain_prop()
{
   wait(1);
   c_updatehull(my,1);   // Update the "hull" of the object (helps engine with collisions)
   set(my,POLYGON);      // Make the engine interpret collisions by the polygon and not bounding box
}

///////////////////////////////////////////////////////////////////////////////////////////////////
// MAIN ///////////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////////////////

function main()
{
	video_mode = 8;
	shadow_stencil= ON;
	video_screen = 1;
	mouse_mode = 0;
	wait(1);
	
	//load level "Das Sandbox"...
	level_load("tryintankin.wmb");
	wait(2);
	
	// Gravity
   ph_setgravity(vector(0,0,-500));
   
   // Create sky
   ent_createlayer("skycube+6.tga", SKY | CUBE | VISIBLE, 0); 
   wait(2);
   
   // create player/tank entity
   panzer = ent_create("panzerIV_skin_wip.mdl", vector(-488,-848,1500), tank_controls);
}

action tank_controls()
{
 var e_pressed;
 ....
     if(!key_e)e_pressed = 0;
     if(key_e & e_pressed == 1)
     {
       e_pressed = 1;
       start_stop_eng();
     }

}

action tank_controls()
{
 ....

    if(key_e)
     {
       start_stop_eng();
       while(key_e) wait(1);//this will work but action will hang until user stops holding e.
     }

}

action tank_controls()
{
    var e_on_last_frame;
 ....
     while(.)....
         if(key_e && e_on_last_frame==0)//this will only work once if player holds e
          {
            start_stop_eng();
          }
          //just before the wait
          e_on_last_frame = key_e;//this will let you know if e was being pressed on prev frame.
          wait(1);
     }
}