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(11) | EP 2 009 754 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Pencil ignition coil and boot |
| (57) The invention relates to a pencil ignition coil and boot for ignition of an internal
combustion engine's spark plug and an assembly of the same with a spark plug. The
coil has an ignition terminal which is provided with an annular male protrusion (12)
and a boot (15) for housing the spark plug (18). The boot (15) has an annular side
wall which forms a female cavity which can receive the annular male protrusion (12)
of the pencil coil. The female cavity and the male protrusions are dimensioned to
allow the male protrusion of the terminal (12) to project past a spark plug engaging
portion (21) of the boot that receives an enlarged portion (18a) of the spark plug
(18) head. This provides an overlap which ensures that the boot is locally loaded
under compression rather than shear during the ignition cycle.
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TECHNICAL FIELD
[0001] The present invention relates to a pencil ignition coil and boot, particularly, but not exclusively, a pencil ignition coil and boot for use in an internal combustion engine spark ignition system of an automobile, and to an assembly of the same with a spark plug.
BACKGROUND OF THE INVENTION
[0002] Many spark ignition systems are provided with pencil type ignition coils which are mounted between a flexible boot and seal within the spark plug bore. One problem with these systems is that due to the harsh working environment, the pencil coils are normally subjected to a repeated number of cycles which, in time, can cause stress fractures to occur in the boot wall which in turn leads to premature failure of the coil.
SUMMARY OF THE INVENTION
[0003] According to the present invention, there is provided a pencil ignition coil and boot for a spark plug ignition system, the pencil ignition coil and boot comprising:
an ignition terminal having an annular male protrusion,
a boot for housing a spark plug, the boot having an annular side wall provided with a female cavity for receiving the annular male protrusion therein, and defining a spark plug receiving cavity having a profiled inner surface including an engaging portion so configured as to effect in use a friction fit with an enlarged diameter portion of a head of a spark plug receivable therein;
wherein the female cavity of the boot is of a depth, and the male protrusion of a length, which allows the male protrusion of the terminal to project past the engaging portion of the plug receiving cavity in which an enlarged diameter portion of the spark plug head is received in use.[0004] The advantages of this arrangement are at least two-fold: it increases the path length for electrical tension attempting to punch through from the ignition coil to the outside of the outer side wall and also changes the response of the pencil coil and boot to modes of movement in order to ensure that when a spark plug head is located in the boot the boot is locally loaded in compression rather than in shear between the male protrusion and an enlarged end portion of the spark plug head. These both contribute to an increase in the expected lifetime of the apparatus.
[0005] In particular, this loading condition arises through the relative geometry and juxtaposition of the female cavity in the boot, the male protrusion of the ignition terminal, and the plug receiving cavity in the boot. As will be familiar, the plug receiving cavity is profiled to enable a spark plug to be secured within the boot due to a friction fit between its enlarged head and a correspondingly profiled inner surface of the receiving cavity in the boot. For example, the correspondingly profiled inner surface of the boot defines a neck portion of minimised cross-sectional area through which an enlarged head of a spark plug is inserted and beyond which the enlarged head is retained.
[0006] In the prior art, the male protrusion of the terminal is spaced from the neck portion of the surface of the plug receiving cavity in which an enlarged diameter portion of the spark plug head is received in use. The male protrusion creates a shear load at or near the base of the female cavity, for example at an inner wall defining such cavity.
[0007] In accordance with the invention the boot is so configured that the male protrusion of the terminal is enabled to project past the engaging portion of the plug receiving cavity in which an enlarged diameter end portion of the spark plug head is received in use, in particular such that the male protrusion of the terminal bears upon the boot in use in the vicinity of this engaging portion, and in particular in the vicinity of the neck portion. As a result the material of the boot is compressed, at least in one point, between the male protrusion and the enlarged end portion of the spark plug head, for example at the neck portion.
[0008] Preferably the female cavity of the boot is provided between and defined by an annular outer wall and an annular inner wall. The male protrusion of a terminal thus bears upon paired engagement surfaces comprising an inner surface of the annular outer wall and an outer surface of the annular inner wall when inserted into the boot. Preferably a spark plug receiving cavity is provided and defined by the wall of a suitably profiled axial recess. A spark plug thus bears upon an engagement surface comprising an inner surface of the axial recess when inserted into the boot.
[0009] In accordance with the invention the boot is so configured that the male protrusion of the terminal is enabled to project past an engaging portion of the plug receiving cavity. Preferably, this is effected in that at least a portion of the axial recess of the plug receiving cavity is defined by a suitably profiled inner surface of the annular inner wall.
[0010] Particularly preferably the female cavity and for example the annular inner wall thereof is configured to ensure that, during operation, the boot is substantially loaded in compression at the area adjacent the end of the male protrusion, for example in that an inner surface of the annular inner wall defines an engaging portion of the spark plug receiving cavity, for example a neck portion of minimised cross-sectional area adapted to be maintained in compression between the male portion and a spark plug head in use. Thus the material of the annular inner wall is compressed, at least in one point, between the male protrusion and the enlarged end portion of the spark plug head.
[0011] According to the present invention in a further aspect, there is provided a pencil ignition coil, boot and spark plug assembly for an ignition system, the assembly comprising a pencil ignition coil and boot as above described with a spark plug engaged therewith.
[0012] Specifically, an assembly comprises:
an ignition terminal having an annular male protrusion,
a boot for housing a spark plug, the boot having an annular side wall provided with a female cavity for receiving the annular male protrusion therein, and defining a spark plug receiving cavity having a profiled inner surface;
a spark plug received within the spark plug receiving cavity such that an enlarged diameter portion of the head of the spark plug is in friction fit with an engaging portion of the profiled inner surface of the cavity;
wherein the female cavity of the boot is of a depth, and the male protrusion of a length, which allows the male protrusion of the terminal to project past the enlarged diameter portion of the spark plug head.BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-
Fig. 1 is a cross sectional side view of a prior art pencil coil not in accordance with the present invention, where the pencil coil is mounted between a pair of elastomeric members in a typical spark plug bore;
Fig. 2 is a cross sectional view in the direction A-A of the coil of Fig. 1 where a punch through crack is shown;
Fig. 3 is a further schematic cross sectional side view of the coil in Fig. 1;
Fig. 4 is a more detailed view of the coil and spark plug interface of Fig. 3;
Fig. 5 is a schematic cross sectional side view of the coil according to the present invention; and
Fig. 6 is a more detailed view of the coil and spark plug interface of Fig. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] A known type of pencil coil 1 is shown in Figs. 1 and 2 and has a male terminal 2, resistor 3 and shock absorbing spring 4. The pencil coil 1 is shown in Fig. 1 inserted into a boot 5 which has an annular female cavity provided by opposing outer 6 and inner 7 annularly projecting side walls. The male terminal 2 is received within and engages upon paired engagement surfaces comprising an inner surface of the annular outer wall 6 and an outer surface of the annular inner wall 7. This can generate shear forces, especially in the vicinity of the base of the walls. An elastomeric seal 9 is also provided at the top of the pencil coil 1.
[0016] Spark plug 8 is secured within the boot 5 due to a friction fit between its enlarged head and a correspondingly profiled inner surface of the boot 5. Specifically, the inner surface of the boot comprises an axial recess profiled in particular to define a neck 10a of reduced diameter and a head locating cavity 10 beyond it. In use a spark plug 8 having an enlarged diameter head portion 8a is inserted through the neck 10a to be received and retained in the cavity 10, a friction fit being maintained between at least a part of the inner surface of the boot and the spark plug.
[0017] It can be seen that, in the prior art arrangement shown in Fig. 4, there exists a small distance between the largest external diameter 8a of the head of the spark plug 8 and the lower end of the pencil coil terminal 2 which results in an effective "line of sight" between the enlarged head portion and the outer wall of the boot 5. This geometrical layout will be referred to as a negative overlap -01 (see Fig.4.).
[0018] During the lifetime of the engine in which the pencil coil and boot are installed, the interface between them is repeatedly subjected to modes of shear, compression, tension etc. caused by the ignition cycle. This has been found to result in cracks in the boot wall which are often localised in the crack area C, depicted in Figs. 1 to 4. Once a crack has fully formed from the spark plug to the outside of the boot wall, the electrical tension between the spark plug 8 and the outside of the boot wall 6 causes an electrical punch through which results in failure in ignition of the spark plug 8.
[0019] A pencil ignition coil and boot according to the present invention will now be described, with reference to Figs. 5 and 6.
[0020] Pencil coil 11 has a male terminal 12 and a resistor 13. The pencil coil 11 is shown inserted into a boot 15 which has an annular female cavity provided by opposing outer 16 and inner 17 side walls. The male terminal 12 is received within and engages upon paired engagement surfaces comprising an inner surface of the annular outer wall 16 and an outer surface of the annular inner wall 17.
[0021] Spark plug 18 is secured within the boot 15 due to a friction fit between its enlarged head and a correspondingly profiled inner surface of the boot 15. Again, this is effected in that the inner surface of the boot comprises an axial recess profiled to define a neck 21 of reduced diameter and a head locating cavity 20 beyond it into which a spark plug 18 having an enlarged diameter head portion 18a is inserted, a friction fit being maintained between an engaging surface on at least part of the inner surface of the boot, and especially in the vicinity of the neck, and the spark plug head.
[0022] At the other end of the pencil coil an elastomeric seal 9 is provided such that the pencil coil 11 is mounted between a pair of elastomeric components (seal 9 and boot 15).
[0023] The outer 16 and inner 17 side walls of the boot 15 are formed so as to provide a female cavity which has a greater depth (relative to the prior art arrangement). Furthermore, the male terminal protrusions 12 of the pencil ignition coil 11 are longer than those of the prior art arrangement such that they co-operate into engagement with the greater depth of the female cavity relative to the head locating cavity 20 when compared with the prior art.
[0024] Once the boot 15 is placed onto a spark plug 18, the spark plug head 18a penetrates deeper into the bore of the pencil coil boot arrangement due to the altered geometry of the male terminal and female cavity and spark plug head receiving cavity. This results in a positive overlap +02 (see Fig.6.) between the spark plug enlarged diameter head in the head receiving cavity and the lower part of the male terminal protrusion 12. In this way, the "line of sight" between the spark plug's enlarged head and the outer wall of the boot 15 (which is present in the prior art arrangement) has been removed. This contributes to preventing the electrical punch through problem previously defined.
[0025] The altered geometry also changes the reaction of the boot and pencil coil interface to the different movement modes since it ensures that the boot 15 is locally loaded (in the area adjacent the bottom of the terminal) in compression rather than in shear (as in the prior art arrangement). In particular it can be seen that the spark plug head receiving cavity in the region of the enlarged diameter head is defined by a suitably profiled inner surface of the annular inner wall 17 in particular including a neck 21. The head engages against and bears upon this region of the inner surface of the annular inner wall and by virtue of the relative juxtaposition with the annular male protrusion 12 in the female cavity reacts against the forces created by the inserted male protrusion. This region of the boot 15 is consequently loaded locally in compression between the inserted male terminal protrusion 12 and the inserted spark plug head 18a.
[0026] Ensuring that the boot 15 is loaded locally in compression greatly reduces the tendency for any cracking to occur in the boot 15 at this location which, in turn, greatly increases the lifetime of the pencil coil 11 and boot 15.
[0027] The male terminal and female cavity are also provided with indents / protrusions constituted of a retaining lip 22 and a corresponding retaining recess 23 which mate with one another once the pencil coil 11 has been inserted into the boot 15. The retaining lip 22 is provided on the outer wall 16 of the boot 15 and the retaining recess 23 is provided on the inner wall 17 of the boot 15. This assists in ensuring that these components remain in good communication with one another and provides confirmation that they are properly connected to one another during the installation procedure.
1. A pencil ignition coil and boot for a spark plug ignition system the pencil ignition
coil and boot comprising:
an ignition terminal having an annular male protrusion (12),
a boot (15) for housing a spark plug (18), the boot (15) having an annular side wall provided with a female cavity for receiving the annular male protrusion (12) therein, and defining a spark plug receiving cavity (20) having a profiled inner surface including an engaging portion so configured as to effect in use a friction fit with an enlarged diameter portion (18a) of a head of a spark plug receivable therein;
wherein the female cavity of the boot (15) is of a depth, and the male protrusion of a length, which allows the male protrusion of the terminal (12) to project past the engaging portion (21) of the plug receiving cavity (20).2. A pencil ignition coil and boot according to claim 1, wherein the female cavity of
the boot (15) is provided with an additional depth which provides a separation gap
between the end of the male protrusion (12) and the bottom of the female cavity when
the male protrusion (12) resides within the female cavity.
3. A pencil ignition coil and boot according to either of claims 1 or 2, wherein the
female cavity of the boot (15) is provided between an annular outer wall (16) and
an annular inner wall (17).
4. A pencil ignition coil and boot according to claim 3 wherein a spark plug receiving
cavity (20) is defined by the wall of a suitably profiled axial recess, at least a
portion thereof comprising a suitably profiled inner surface of the annular inner
wall (17).
5. A pencil ignition coil and boot according to claim 4 wherein an inner surface of the
annular inner wall defines a neck portion (21) of minimised cross-sectional area adapted
to be maintained in compression between the male protrusion (12) and a spark plug
head (18a) in use.
6. A pencil ignition coil and boot according to any one of claims 3 to 5, wherein a retaining
lip (22) is provided on one of the outer or inner walls (16, 17) of the boot (15)
and a corresponding retaining recess (23) is provided on the male protrusion (12).
7. A pencil ignition coil and boot according to any preceding claim, wherein the length
of the male protrusion (12) and the female cavity is configured to ensure that, during
operation, the boot (15) is substantially loaded in compression at the area adjacent
the end of the male protrusion (12).
8. An assembly for a spark plug ignition system, comprising:
an ignition terminal having an annular male protrusion (12),
a boot (15) for housing a spark plug (18), the boot (15) having an annular side wall provided with a female cavity for receiving the annular male protrusion (12) therein, and defining a spark plug receiving cavity having a profiled inner surface;
a spark plug (18) received within the spark plug receiving cavity such that an enlarged diameter portion (18a) of the head of the spark plug (18) is in friction fit with the engaging portion (21) of a profiled inner surface of the cavity;
wherein the female cavity of the boot (15) is of a depth, and the male protrusion (12) of a length, which allows the male protrusion of the terminal to project past the enlarged diameter portion (18a) of the spark plug head.