Patent Publication Number: US-10306926-B2

Title: Electronic smoking device

Description:
PRIORITY CLAIM 
     This application claims priority to EP Patent Application No. 15159775.4, filed Mar. 19, 2015, and now pending. 
     FIELD OF INVENTION 
     The present invention relates generally to electronic smoking devices and in particular electronic cigarettes. 
     BACKGROUND OF THE INVENTION 
     An electronic smoking device, such as an electronic cigarette (e-cigarette), typically has a housing accommodating an electric power source (e.g. a single use or rechargeable battery, electrical plug, or other power source), and an electrically operable atomizer. The atomizer vaporizes or atomizes liquid supplied from a reservoir and provides vaporized or atomized liquid as an aerosol. Control electronics control the activation of the atomizer. In some electronic cigarettes, an airflow sensor is provided within the electronic smoking device which detects a user puffing on the device (e.g., by sensing an under-pressure or an air flow pattern through the device). The airflow sensor indicates or signals the puff to the control electronics to power up the device and generate vapor. In other e-cigarettes, a switch is used to power up the e-cigarette to generate a puff of vapour. 
     SUMMARY OF THE INVENTION 
     In one aspect an atomizer for an electronic smoking device is provided which comprises at least a first heating wire and a second heating wire. The first and second heating wires are wound together to form a common heating coil. Further, the first and second heating wires differ in at least one physical parameter resulting in different thermal properties of the first and second heating wire. 
     Different physical parameters of the first and second heating wires may relate to at least one of the following non-exclusive list of physical parameters of the heating wires: material; structure; wire locations or winding axes location within the common heating coil; wire diameter, diameters of a turn of the first and second heating wires; sizes and structures of cross sectional areas, surface profiles, length, and others. 
     Further provided is an electronic smoking device with the inventive atomizer. 
     The characteristics, features and advantages of this invention and the manner in which they are obtained as described above, will become more apparent and be more clearly understood in connection with the following description of exemplary embodiments, which are explained with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, same element numbers indicate same elements in each of the views: 
         FIG. 1  is a schematic cross-sectional illustration of an exemplary e-cigarette; 
         FIG. 2  is a schematic view of an atomizer in a first embodiment; 
         FIG. 3  is a section view through the atomizer of  FIG. 2 ; 
         FIG. 4  is a section view through an atomizer in a second embodiment; 
         FIG. 5  is a view of a heating coil perpendicular to its winding axis in a third embodiment; 
         FIG. 6  is a view of a heating coil perpendicular to its winding axis in a fourth embodiment; 
         FIG. 7  is a schematic view of an atomizer in a fifth embodiment; 
         FIG. 8  is a vertical cut through an atomizer in a sixth embodiment; 
         FIG. 9  shows a vertical cut through an atomizer in a seventh embodiment; and 
         FIG. 10  shows a cartomizer with an atomizer in an eighth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Throughout the following, an electronic smoking device will be exemplarily described with reference to an e-cigarette. As is shown in  FIG. 1 , an e-cigarette  10  typically has a housing comprising a cylindrical hollow tube having an end cap  16 . The cylindrical hollow tube may be single piece or a multiple piece tube. In  FIG. 1 , the cylindrical hollow tube is shown as a two piece structure having a battery portion  12  and an atomizer/liquid reservoir portion  14 . Together the battery portion  12  and the atomizer/liquid reservoir portion  14  form a cylindrical tube which is approximately the same size and shape as a conventional cigarette, typically about 100 mm with a 7.5 mm diameter, although lengths may range from 70 to 150 or 180 mm, and diameters from 5 to 20 mm. 
     The battery portion  12  and atomizer/liquid reservoir portion  14  are typically made of steel or hardwearing plastic and act together with the end cap  16  to provide a housing to contain the components of the e-cigarette  10 . The battery portion  12  and an atomizer/liquid reservoir portion  14  may be configured to fit together by a friction push fit, a snap fit, or a bayonet attachment, magnetic fit, or screw threads. The end cap  16  is provided at the front end of the battery portion  12 . The end cap  16  may be made from translucent plastic or other translucent material to allow an LED  20  positioned near the end cap to emit light through the end cap. The end cap can be made of metal or other materials that do not allow light to pass. 
     An air inlet may be provided in the end cap, at the edge of the inlet next to the cylindrical hollow tube, anywhere along the length of the cylindrical hollow tube, or at the connection of the battery portion  12  and the atomizer/liquid reservoir portion  14 .  FIG. 1  shows a pair of air inlets  38  provided at the intersection between the battery portion  12  and the atomizer/liquid reservoir portion  14 . 
     A battery  18 , a light emitting diode (LED)  20 , control electronics  22  and optionally an airflow sensor  24  are provided within the cylindrical hollow tube battery portion  12 . The battery  18  is electrically connected to the control electronics  22 , which are electrically connected to the LED  20  and the airflow sensor  24 . In this example the LED  20  is at the front end of the battery portion  12 , adjacent to the end cap  16  and the control electronics  22  and airflow sensor  24  are provided in the central cavity at the other end of the battery  18  adjacent the atomizer/liquid reservoir portion  14 . 
     The airflow sensor  24  acts as a puff detector, detecting a user puffing or sucking on the atomizer/liquid reservoir portion  14  of the e-cigarette  10 . The airflow sensor  24  can be any suitable sensor for detecting changes in airflow or air pressure such a microphone switch including a deformable membrane which is caused to move by variations in air pressure. Alternatively the sensor may be a Hall element or an electro-mechanical sensor. 
     The control electronics  22  are also connected to an atomizer  26 . In the example shown, the atomizer  26  includes a heating coil  28  which is wrapped around a wick  30  extending across a central passage  32  of the atomizer/liquid reservoir portion  14 . The coil  28  may be positioned anywhere in the atomizer  26  and may be transverse or parallel to the liquid reservoir  34 . The wick  30  and heating coil  28  do not completely block the central passage  32 . Rather an air gap is provided on either side of the heating coil  28  enabling air to flow past the heating coil  28  and the wick  30 . The atomizer may alternatively use other forms of heating elements, such as ceramic heaters, or fiber or mesh material heaters. Nonresistance heating elements such as sonic, piezo and jet spray may also be used in the atomizer in place of the heating coil. 
     The central passage  32  is surrounded by a cylindrical liquid reservoir  34  with the ends of the wick  30  abutting or extending into the liquid reservoir  34 . The wick  30  may be a porous material such as a bundle of fiberglass fibers, with liquid in the liquid reservoir  34  drawn by capillary action from the ends of the wick  30  towards the central portion of the wick  30  encircled by the heating coil  28 . 
     The liquid reservoir  34  may alternatively include wadding soaked in liquid which encircles the central passage  32  with the ends of the wick  30  abutting the wadding. In other embodiments the liquid reservoir  34  may comprise a toroidal cavity arranged to be filled with liquid and with the ends of the wick  30  extending into the toroidal cavity. 
     An air inhalation port  36  is provided at the back end of the atomizer/liquid reservoir portion  14  remote from the end cap  16 . The inhalation port  36  may be formed from the cylindrical hollow tube atomizer/liquid reservoir portion  14  or maybe formed in an end cap. 
     In use, a user sucks on the e-cigarette  10 . This causes air to be drawn into the e-cigarette  10  via one or more air inlets, such as air inlets  38  and to be drawn through the central passage  32  towards the air inhalation port  36 . The change in air pressure which arises is detected by the airflow sensor  24  which generates an electrical signal that is passed to the control electronics  22 . In response to the signal, the control electronics  22  activate the heating coil  28  which causes liquid present in the wick  30  to be vaporized creating an aerosol (which may comprise gaseous and liquid components) within the central passage  32 . As the user continues to suck on the e-cigarette  10 , this aerosol is drawn through the central passage  32  and inhaled by the user. At the same time the control electronics  22  also activate the LED  20  causing the LED  20  to light up which is visible via the translucent end cap  16  mimicking the appearance of a glowing ember at the end of a conventional cigarette. As liquid present in the wick  30  is converted into an aerosol more liquid is drawn into the wick  30  from the liquid reservoir  34  by capillary action and thus is available to be converted into an aerosol through subsequent activation of the heating coil  28 . 
     Some e-cigarette are intended to be disposable and the electric power in the battery  18  is intended to be sufficient to vaporize the liquid contained within the liquid reservoir  34  after which the e-cigarette  10  is thrown away. In other embodiments the battery  18  is rechargeable and the liquid reservoir  34  is refillable. In the cases where the liquid reservoir  34  is a toroidal cavity, this may be achieved by refilling the liquid reservoir  34  via a refill port. In other embodiments the atomizer/liquid reservoir portion  14  of the e-cigarette  10  is detachable from the battery portion  12  and a new atomizer/liquid reservoir portion  14  can be fitted with a new liquid reservoir  34  thereby replenishing the supply of liquid. In some cases, replacing the liquid reservoir  34  may involve replacement of the heating coil  28  and the wick  30  along with the replacement of the liquid reservoir  34 . A replaceable unit comprising the atomizer  26  and the liquid reservoir  34  is called a cartomizer. 
     The new liquid reservoir  34  may be in the form of a cartridge having a central passage  32  through which a user inhales aerosol. In other embodiments, aerosol may flow around the exterior of the cartridge  32  to an air inhalation port  36 . 
     Of course, in addition to the above description of the structure and function of a typical e-cigarette  10 , variations also exist. For example, the LED  20  may be omitted. The airflow sensor  24  may be placed adjacent the end cap  16  rather than in the middle of the e-cigarette. The airflow sensor  24  may be replaced with a switch which enables a user to activate the e-cigarette manually rather than in response to the detection of a change in air flow or air pressure. 
     Different types of atomizers may be used. Thus for example, the atomizer may have a heating coil in a cavity in the interior of a porous body soaked in liquid. In this design aerosol is generated by evaporating the liquid within the porous body either by activation of the coil heating the porous body or alternatively by the heated air passing over or through the porous body. Alternatively the atomizer may use a piezoelectric atomizer to create an aerosol either in combination or in the absence of a heater. 
       FIG. 2  shows a schematic view of an atomizer  26  in a first embodiment.  FIG. 3  shows a cut through the inventive atomizer  26  of  FIG. 2  in a plane going through a center or winding axis  43 ,  53  of the first and second heating wires  40 ,  50 . The atomizer  26  as shown in  FIG. 2  can be incorporated into the e-cigarette  10  of  FIG. 1  or other e-cigarettes which employ an atomizer  26 . An atomizer  26  atomizes or vaporizes the liquid stored in the e-cigarette  10  of  FIG. 1 . The atomizer  26  in the embodiment shown in  FIG. 2  includes a heating coil  28 . The heating coil  28  comprises at least a first heating wire  40  and a second heating wire  50 . The first and second heating wires  40 ,  50  are wound together to form the common heating coil  28 . The first and second heating wires  40 ,  50  are parallel to each other. Each of the heating wires  40 ,  50  which constitute the common heating coil  28  are wound to form a first heating coil  41  and a second heating coil  51 , respectively. Each loop of a heating wire  40 ,  50  is in the following named a turn  42 ,  52 . In  FIG. 2 , the first and second heating coils  41 ,  51  extend from left to right with a horizontal center or winding axis  43 ,  53 . The first heating wire  40  has two end portions  47  (one on the left, shown in  FIG. 2 ; one on the right, not shown) which are used to electrically contact the first heating wire  40 . Similarly, the second heating wire  50  has two end portions  57  which are used to electrically contact the second heating wire  50 . The first and second heating wires  40 ,  50  are electrically contacted in parallel. 
     As can be seen from  FIG. 2 , the first and second heating coils  41 ,  51  extend in the same direction and are staggered along the winding or center axis  43 ,  53  with respect to each other such that one turn  42  of the first heating wire  40  is neighbored by a turn  52  of the second heating wire  50 . In the center region of the heating coil  28 , the one turn  42  of the first heating wire  40  is neighbored by two turns  52  of the second heating wire  50 . The two heating coils  41 ,  51  are wound together to form a single or common heating coil  28 . In other words, the first and second heating coils  41 ,  51  are displaced along the center axis  43 ,  53  with an offset such that at least one turn  42  of the first heating coil  41  is placed between two turns  52  of the second heating coil  51 , vice versa. The center axis  43  of the first heating coil  41  is identical to a center axis  53  of the second heating coil  51 . A turn  42  of the first heating wire  40  directly contacts two turns  52  of the second heating wire  50 . This allows the heating coil  28  to heat up quickly and to reduce a delay until an aerosol is produced from the liquid. 
     The first and second heating wires  40 ,  50 , or the first and second heating coils  41 ,  51  according to all embodiments of the invention have at least one different physical parameter resulting in different thermal properties of the heating wires  40 ,  50  and heating coils  41 ,  51 . In the embodiment shown in  FIG. 2  and  FIG. 3 , the diameter  44  of the first heating wire  40  is greater than the diameter  54  of the second heating wire  50 . Both heating wires  40 ,  50  are solid wires with constant cross section  46 ,  56  and planar outer surfaces. Due to the different physical parameters, here different wire thicknesses, the first and second heating wires  40 ,  50  heat differently. The first and second heating wires  40 ,  50  have different heating profiles or heat transfer characteristics and heat up and cool down at different speeds. Thin wires like the second heating wire  50  heat up and cool down very fast reducing the delay until an aerosol is produced by the atomizer  26  in contact with a liquid of an e-cigarette  10 . Also, thin wires generally reach a maximum temperature faster while thick wires take longer to heat up but also retain their heat for longer. 
     The combination of at least two heating wires  40 ,  50  having different physical parameters provides in comparison with a heating coil consisting of a single heating wire a more complex heat transfer characteristic of an atomizer  26  in an e-cigarette  10  and thus a more complex aerosol generation upon contact with the liquid stored in the e-cigarette  10 . The vaping experience may become more multidimensional using the different thermal properties of different heating wires  40 ,  50  combined into a common heating coil  28 . In the state of the art, this has been achieved with staggered power delivery to a single wire, while according to an embodiment such electronics could be omitted or structured less complex resulting in a cheaper and simpler way to achieve a similar goal with improved performance outcomes of the atomizer  26  and consequently the e-cigarette  10 . 
     By using two different heating wires  40 ,  50  with different thermal properties, e.g. by using two different heating wire thicknesses together in a single heating coil  28 , the surface area of the heating coil  28  for liquid contact is greater in comparison with a heating coil  28  formed from a single heating wire. This improves an aerosol generation in an electronic smoking device  10  the inventive atomizer  26  is supplied to. 
       FIG. 3  shows the different diameters  44 ,  54  of the first and second heating wires  40 ,  50  resulting in different sizes of their cross-sectional areas  46 ,  56 . The first heating wire  40  has a larger diameter  44  and thus a larger cross-sectional area  46  than the second heating wire  50 . The thicker first heating wire  40  has a lower resistance and provides more heat than the second heating wire  50 . The turns  42  of the first heating wire  40  directly contact the neighboring turns  52  of the second heating wire  50  resulting in a dense heat transfer characteristic of the common heating coil  28 . 
     The first and second heating wires  40 ,  50  of the first embodiment shown in  FIGS. 2 and 3  differ in a single physical parameter, its wire thickness. However, the invention is not limited thereto. The heating wires  40 ,  50  can differ in a physical parameter, like the structure or size of cross-sections, surface profiles, materials etc. Some examples will be described in the following embodiments. The different embodiments can also be combined together such that the heating wires  40 ,  50  differ in two, three or a larger number of physical parameters leading to a complex heating profile of the common heating coil  28 . 
     For instance, the heating wires  40 ,  50  of  FIG. 2  and  FIG. 3  may also be formed of a different material. One heating wire  40 ,  50  may be formed of a metal, one of a ceramic. Or both may be formed of different metals. One heating wire  40 ,  50  may be formed of a compound material or alloy, the other may be formed of a single material. The group of possible heating wire materials may comprise, for example, nickel, chromium, iron, aluminum, copper and alloys thereof as well as ceramics. Different heating wire materials will lead to different thermal properties and heat characteristic of the different heating coils  41 ,  51  resulting in a complex heat transfer pattern of the common heating coil  28  formed thereof. 
       FIG. 4  is a vertical cut through an atomizer  126  with a common heating coil  128  in a second embodiment. The second embodiment differs from the first embodiment of  FIGS. 2 and 3  in that the turns  142 ,  152  of first and second heating coils  141 ,  151  do not directly contact each other but are spaced apart from each other along the center axis  143 ,  153  of the heating coils  141 ,  151 . Again, in a region spaced apart from the outer turns  142  of the heating coil  128 , one turn  142  of the first heating wire  140  is neighbored by two turns  152  of the second heating wire  150 , vice versa. Yet, the turns  142  of the first heating wire  140  and the turns  152  of the second heating wire  150  do not directly contact each other but are spaced apart from each other. This can be achieved by forming the first and second heating coils  41 ,  51  rigid, e.g. by use of a ceramic or metal material. Spacing apart the two heating coils  141 ,  151  results in a larger wire surface usable for liquid contact, but also results in a less dense heating profile of the common heating coil  128 . 
       FIG. 5  is a cut through an atomizer  226  with a common heating coil  228  perpendicular to its winding axis in a third embodiment. Thus, the view is through the heating coil  228  along its length. Here, the first heating coil  241  and the second heating coil  251  have the same diameter  249 ,  259  of a turn  242 ,  252 , but their center axes  243 ,  253  are displaced from each other. The displacement of the heating coils  241 ,  251  increases the complexity of the heating profiles of the common heating coil  228 . However, the displacement of the heating coils  241 ,  251  with respect to each other alone does not result in a different thermal property of the first heating coil  241  with respect to the second heating coil  251 . In order to have different thermal properties, they have to differ in an additional physical parameter. In the embodiment shown, the first and second heating coils  241 ,  251  are formed of different materials. A relative displacement of the second heating coil  251  may be limited such that the cross-sections of the turns  242 ,  252  of the first and second heating coils  241 ,  251  overlap. Preferably, the displacement of the center axis  253  of the second heating coil  251  from the center axis of the first heating coil  241  may be lower than a radius of a turn  42  of the first heating coil  241 . 
       FIG. 6  is a view of an atomizer perpendicular to its winding axis in a fourth embodiment. In comparison to the third embodiment in  FIG. 5 , the heating coils  341 ,  351  have a common center axis  343 ,  353 , but the diameter of a turn  342  of the first heating coil  341  is larger than the diameter of a turn  352  of the second heating coil  351 . Thus, along the length of the common heating coil  328 , its surface profile consists of valleys and peaks resulting in a complexity of a heat profile of the common heating coil  328 . The two heating wires  340 ,  350  of this embodiment may have the same wire thicknesses but due to their different winding diameters  349 ,  359 , the resulting heating coil  328  has a complex shape. In the preferred embodiment shown, the size of the winding diameter  359  of the second heating coil  351  is such that the cross-sections of the heating wires  340 ,  350  would still overlap in the cut shown in  FIG. 6 . 
       FIG. 7  is a schematic view of an atomizer  426  in a fifth embodiment. The fifth embodiment differs from the first embodiment in  FIG. 2  in that the structure of the two heating wires  440 ,  450  differs. The first heating wire  440  is a twisted ribbon wire formed of a helix of spiral before it is wound to form the heating coil  428  together with second heating wire  450 . Thus, the first heating wire  440  has a through-hole in its middle extending along its length and has varying cross sections  446  along its length. The second heating wire  450  is a solid wire with a non-varying circular cross-section  456  and a planar surface. Due to their different structure, the first and second heating wires  440 ,  450  shown in  FIG. 2  have different surface profiles. The surface profile of the first heating wire  440  before being wound into a heating coil  441  consists of alternating valleys and peaks, whereas the surface profile of the second heating wire  450  is planar. The valleys of the first heating wire  440  increase the wire area usable for liquid contact and therefore improve the aerosol formation, while the solid second heating wire  450  provides more heat since it has a lower resistance than the first heating wire  440 . 
     The examples of a ribbon heating wire  440  and a round heating wire  450  are not limiting. Any physical shape or structure of the heating wires  440 ,  450  can be envisaged, e.g. a stranded wire, an oval wire, a wire with a surface structure etc. 
       FIG. 8  is a vertical cut through an atomizer  526  in a sixth embodiment. Here, the first and second heating wires  540 ,  550  have a different length and thus contribute with a different number of turns  542 ,  552  to the common heating coil  528 . As exemplarily shown, while the first heating wire  540  contributes with six turns  542  to the common heating coil  528 , the second heating wire  550  is shorter and contributes only with three turns  552  to the common heating coil  528 . 
       FIG. 9  shows a vertical cut through an atomizer  626  in a seventh embodiment. In this embodiment, a third heating wire  660  is additionally provided, wherein the first, second and third heating wires  640 ,  650 ,  660  are wound together to form the heating coil  628 . One turn  642  of the first heating wire  640  contacts on one side a turn  652  of a second heating wire  650  and on the other side one turn  662  of the third heating wire  660 . The first, second and third heating wires  640 ,  650 ,  660  in  FIG. 9  are formed of different materials. While the first heating wire  640  is formed of a nickel-chromium alloy, the second heating wire  650  is formed of iron-chromium-aluminum alloy and the third heating wire  660  is formed of a nickel-iron alloy. Due to their different materials, the heating wires  640 ,  650 ,  660  have a different resistivity and therefore heat differently. Apart from their materials, the heating wires  640 ,  650 ,  660  do not differ in the embodiment shown. However, the third heating wire  660  could differ from the first and second heating wires  640 ,  650 , respectively, also in any other physical parameter apart of its material. 
     Also more than three heating wires could be provided to form the common heating coil of the atomizer. Preferably, all least two of the group of heating wires differ in at least one physical parameter. In another embodiment, none of the provided heating wires would have an identical set of physical parameters as any other of the group of heating wires in the common heating coil. 
     Electronic smoking devices may be structured such that the liquid reservoir can be removed from an electronic cigarette together with the atomizer and can be replaced by a new, refilled atomizer/liquid reservoir portion  14  being called a cartomizer.  FIG. 10  shows a cartomizer  700  for an electronic smoking device  10  according to an embodiment. The cartomizer  700  comprises a liquid reservoir  34  as described in the context of  FIG. 1  with an atomizer  26  of  FIG. 2 . However, all other embodiments of the atomizer described in the context of this invention may be used as an atomizer in  FIG. 10 . The atomizer/liquid reservoir portion  14  can be separated from the battery portion  12  and the cartomizer  700  can be removed and replaced. The first and second heating wires  40 ,  50 , respectively the first and second heating coils  41 ,  51  of the atomizer  26  have electronic contacts  710 , which upon fixation of the atomizer/liquid reservoir portion  14  to the battery portion  12  provide electrical contact to the battery  18 . However, the electronic smoking device may be configured differently, for instance with an opening in its housing through which the cartomizer is removable or replaceable. 
     In summary, an atomizer for an electronic smoking device is provided comprising at least a first heating wire and a second heating wire. The first and second heating wires differ in at least one physical parameter and are wound together to form a common heating coil. Due to the at least one different physical parameter, the first and second heating wires have different thermal properties and heat differently. The first and second heating wires have different heating profiles or heat transfer characteristics. The first and second heating wires heat up and cool down at different speeds. In one aspect, a greater surface area for liquid contact may be provided. Different physical parameters of the first and second heating wires may relate to at least one of the following features: 
     the first and second heating wires are formed of a different material, 
     the first and second heating wires have a different structure, 
     the locations of the first and second heating wires within the common heating coil  28  differ, 
     the diameter of the first and second heating wires differ, 
     the diameters of a turn of the first and second heating wires differ, 
     the first and second heating wires have different sizes or structures of cross sectional areas in a common cut through the heating coil, 
     one of the first and second heating wires has a varying cross section while the other has a differently varying cross section or a constant cross section, 
     the first and second heating wires have different surface profiles, surface treatments, or surface coatings, and 
     the first and second heating wires have a different length. 
     Only one, a group of or all of the above features may be present in an embodiment of the invention. 
     Preferably, the first heating wire is wound into a first heating coil, the second heating wire is wound into a second heating coil, wherein the first and second coils extend in the same direction and are staggered along the winding axis with respect to each other such that one turn of the first heating wire is neighbored by at least one turn, preferably two turns of the second heating wire. The two heating coils are wound together to form a single heating coil. Preferably, the turns of the two heating coils are displaced along the winding axis with an offset such that one turn of the first heating coil is placed between two turns of the second heating coil, vice versa. Preferably, the center axis of the first heating coil is parallel to a center axis of the second heating coil. In one aspect, the center axis of the first heating coil is identical to a center axis of the second heating coil. This saves space and increases the heating performance of the heating coil. 
     In one aspect, a turn of the first heating wire directly contacts two turns of the second heating wire. This would allow the heating coil to heat up quickly and to reduce the delay until aerosol is produced. In one aspect, a diameter of the first heating wire is different from the diameter of the second heating wire. This may provide a simple way to achieve different heating profiles. In one aspect, the first heating wire and the second heating wire are made of different materials. In another aspect, a surface profile of the first heating wire is different from a surface profile of the second heating wire. Preferably, a cross section of a first heating wire varies differently along its length compared to a cross section of the second heating wire. A cross section of a first heating wire may vary along its length wherein the cross section of the second heating wire may be constant. In a vertical cut through the heating coil, a cross section of a first heating wire  40  may be different from a cross section of the second heating wire. For example, although wires having a round cross section are commonly available in various materials and diameters, one or more of the heating wires, or parts of it, may have a non-round cross section, such as a flatter ribbon-like wire. 
     The number of end portions of the heating coil is double the number of heating wires. Preferably, the first end portion of the first heating wire and the end portion of the second heating wire are coupled together into a first common contact portion and the second end portions of the first and second heating wires are coupled together into a second common contact portion, respectively. 
     In yet another embodiment, a third heating wire is additionally provided, wherein the first, second and third heating wires are wound together to form the heating coil. Preferably, one turn of the first heating wires contacts on one side one turn of a second heating wire and on the other side one turn of the third heating wire. The third heating wire differs from the first and/or second heating wire in at least one physical parameter, e.g. its material; its cross section; its variation of its cross section along its length; its length; and its surface profile such that the third heating wire has a different thermal property than the first and second heating wires. 
     In one embodiment, an electronic smoking device is provided comprising: a housing, a liquid reservoir provided inside the housing, and an atomizer as described above. The invention is not limited to a heating wire with a wick but may be used with any other element for providing the liquid to the heating wire. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims. 
     LIST OF REFERENCE SIGNS 
     
         
           10  e-cigarette 
           12  battery portion 
           14  atomizer/liquid reservoir portion 
           16  end cap 
           18  battery 
           20  light emitting diode (LED) 
           22  control electronics 
           24  airflow sensor 
           26  atomizer 
           28  heating coil 
           30  wick 
           32  central passage 
           34  liquid reservoir 
           36  air inhalation port 
           38  air inlets 
           40  first heating wire 
           41  first heating coil 
           42  turn of first heating wire 
           43  center axis of first heating wire 
           44  diameter of first heating wire 
           45  surface profile of first heating wire 
           46  cross section of first heating wire 
           47  first end portion of first heating wire 
           48  second end portion of first heating wire 
           49  diameter of turn of a first heating wire 
           50  second heating wire 
           51  second heating coil 
           52  turn of second heating wire 
           53  center axis of second heating wire 
           54  diameter of second heating wire 
           55  surface profile of second heating wire 
           56  cross section of second heating wire 
           57  first end portion of second heating wire 
           58  second end portion of second heating wire 
           59  diameter of turn of a second heating wire 
           660  third heating coil 
           662  turn of third heating wire 
           700  cartomizer 
           710  electrical contacts of heating wires