Abstract: An ignition coil has primary and secondary coils, a central core and a pair of split cores. One of the split cores has a first opposing face orthogonal to a coil axial direction X facing a core front end face of the central core in the coil axial direction X. The respective split cores have second opposing surfaces mutually facing each other in a lateral direction Y at a rear end of the central core. At least one of the second opposing surfaces and one of rear end surfaces of the central core contacting the least one of the second opposing surfaces collectively constitute a pair of oblique surfaces and come to close to the other one of the second opposing surfaces in the lateral direction Y as a portion of each of the oblique surfaces recedes further from the first opposing face in the coil axial direction X.

Abstract: An ignition coil for an internal combustion engine includes a plug boot with a hole having a length extending in an axial direction of the ignition coil. The length of the hole has a portion which includes a plurality of small distance-to-center sections and a plurality of large distance-to-center sections which are arranged alternately in a circumferential direction of the hole. Each of the small distance-to-center sections is located at a first distance away from the center of the hole, while each of the large distance-to-center sections is located at a second distance away from the center of the hole. The second distance is larger than the first distance. The small distance-to-center sections are placed to be contactable with an outer periphery of the coil spring. This structure of the ignition coil has enhanced resistance to mechanical vibration of the coil spring and also has enhanced withstand voltage.

Abstract: An ignition coil for an internal combustion engine includes a plug boot with a hole having a length extending in an axial direction of the ignition coil. The length of the hole has a portion which includes a plurality of small distance-to-center sections and a plurality of large distance-to-center sections which are arranged alternately in a circumferential direction of the hole. Each of the small distance-to-center sections is located at a first distance away from the center of the hole, while each of the large distance-to-center sections is located at a second distance away from the center of the hole. The second distance is larger than the first distance. The small distance-to-center sections are placed to be contactable with an outer periphery of the coil spring. This structure of the ignition coil has enhanced resistance to mechanical vibration of the coil spring and also has enhanced withstand voltage.

Abstract: An ignition coil has primary and secondary coils, a central core and a pair of split cores. One of the split cores has a first opposing face orthogonal to a coil axial direction X facing a core front end face of the central core in the coil axial direction X. The respective split cores have second opposing surfaces mutually facing each other in a lateral direction Y at a rear end of the central core. At least one of the second opposing surfaces and one of rear end surfaces of the central core contacting the least one of the second opposing surfaces collectively constitute a pair of oblique surfaces and come to close to the other one of the second opposing surfaces in the lateral direction Y as a portion of each of the oblique surfaces recedes further from the first opposing face in the coil axial direction X.