Ignition timing control apparatus for an internal combustion engine
BACKGROUND OF THE INVENTION 
     The present invention relates to an ignition timing control apparatus for an internal combustion engine, and more particularly to an ignition timing control apparatus for an internal combustion engine suitable for controlling an ignition timing of an engine for a motor vehicle having exhaust gas purification means. 
     In the internal combustion engine for the motor vehicle, a negative pressure advancing device is arranged in a distributor and a negative pressure slightly upstream of a full close position of a throttle valve in an intake air path is detected so that an ignition timing is advanced in accordance with an operation condition of the engine represented by the detected negative pressure. It is intended to economize fuel and improve engine performance by advancing the ignition timing. 
     In recent years, in order to reduce noxious components exhausted from the engine of the motor vehicle having the negative pressure advancing device, the advancement of the ignition timing is suppressed under a particular operation condition. Under the particular operation condition, the ignition timing is retarded relative to an output timing so that a maximum combustion temperature is lowered and nitrogen oxide in the exhaust gas is reduced and an exhaust gas temperature is raised to promote oxidization of carbon monoxide and hydrocarbon. In this manner, the noxious components are reduced. 
     It has been proposed to allow the advancement of the ignition timing by the negative pressure advancing device when an engine is operated at a high speed gear ratio and forcibly suppress the operation of the negative pressure advancing device when the engine is operated at other than the high speed gear ratio. 
     In such an engine, a shift switch for detecting a fourth speed or a fifth speed is arranged in a transmission, and when the shift switch is actuated to indicate the fourth or fifth speed condition, the intake negative pressure supplied to the negative pressure advancing device of the distributor is prevented from being blocked. A similar arrangement may be used in an motor vehicle hoving an automatic transmission engine. 
     It has also been proposed to permit the advancement of the ignition timing by the negative pressure advancing device when an engine coolant temperature is below a predetermined temperature or when the engine is operated under a cool condition and forcibly suppress the operation of the negative pressure advancing device when the engine coolant temperature is above the predetermined temperature or when the engine is operated under a warm condition and a transmission gear is at other than the high speed gear position. 
     In such an engine, however, a load to the engine is not included in the condition to forcibly suppress the operation of the negative pressure advancing device. Accordingly, under a light load condition of the engine, surging occurs, which causes the deterioration of the operation performance of the motor vehicle and the increase of fuel consumption. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an ignition timing control apparatus of an internal combustion engine which forcibly suppresses the operation of the negative pressure advancing device under a light load condition of the engine. 
     In accordance with the present invention, the ignition timing control apparatus comprises negative pressure advancing means for controlling a governor advancing device arranged at a distributor in accordance with an operation condition of the engine to advance an ignition timing; gear position sensing means for producing a high speed gear position signal when a transmission gear is at a high speed gear position; temperature sensing means for producing a temperature signal when the engine is at least in a warm-up condition; load sensing means for producing a light load signal when the engine is operated under a light load condition; and means for permitting the control to the governor advancing device by the negative pressure advancing means when one of the high speed gear position signal, the temperature signal and the light load signal is produced and inhibiting the control to the governor advancing device by the negative pressure advancing means when none of the high speed gear position signal, the temperature signal and the light load signal is produced. 
     According to the present invention, the advancement control to the governor advancing device by the negative pressure advancing device is inhibited only when the transmission gear is at other than the high speed gear position, the engine temperature is above a predetermined temperature and the engine is operated under a high load condition, and the ignition timing is advanced by the negative pressure advancing device in accordance with the operation condition of the engine whenever the engine is operated under the light load condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows one embodiment of an ignition timing control apparatus for an internal combustion engine of a motor vehicle in accordance with the present invention; 
     FIG. 2 shows a structure of a vacuum switch shown in FIG. 1; 
     FIG. 3 shows a structure of a negative pressure switching solenoid valve shown in FIG. 1; and 
     FIG. 4 shows a structure of a negative pressure control device shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a preferred embodiment of the present invention. A shift switch 4 for producing a high speed gear position signal when a transmission gear is at a high speed gear position is arranged at a transmission 2. In the case of a manual transmission, the shift switch 4 produces the high speed gear position signal when the gear is selected to a fourth or fifth speed, and in the case of an automatic transmission, the shift switch 4 produces the high speed gear position signal when the gear ratio is at a high gear ratio in a drive range or an over-drive range. 
     A coolant temperature switch 6 is provided as temperature sensing means for sensing a cool condition of the engine in which a temperature of the engine is below a predetermined temperature. The coolant temperature switch 6 senses the temperature of coolant 8 of the engine. It is deactuated when the engine is at a temperature suitable for a normal operation and actuated to produce the temperature signal when the engine is at a low temperature during a warm-up condition or at a high temperature which may cause overheat of the engine. 
     A vacuum switch 14 for sensing the light load condition of the engine is provided in an intake manifold 12 downstream of a throttle valve 26. As shown in FIG. 2, the vacuum switch 14 has a pressure chamber 52 sectioned by a diaphragm 50, a movable contact 54 fixed to the diaphragm 50, a stationary contact 58 fixed to a switch case 56 and a compression spring 62 inserted in the pressure chamber 52 for normally biasing the diaphragm 50 downwardly as viewed in the drawing to keep a contact 60 in a released position. The pressure chamber 52 is connected to the intake manifold 12 through a negative pressure pipe 16 and an adaptor 18. Under the light load condition in which the negative pressure in the intake manifold 12 is high, the hihg negative pressure of the intake manifold 12 is introduced into the pressure chamber 52 of the vacuum switch 14. As a result, the movable contact 54 of the vacuum switch 12 is pulled upward against the force of the compression spring 62 and the contact 60 is made so that the light load condition is detected and the light load signal is produced. 
     The shift switch 4, the coolant temperature switch 6 and the vacuum switch 14 are connected to a control circuit 20. When the signal from one of those switches is supplied to the control circuit 20, it connects a power supply 22 to a negative pressure switching solenoid value 24. As shown in FIG. 3, the negative pressure switching solenoid value 24 has an exciting coil 64 and a negative pressure switching value 66. When the power is supplied through the control circuit 20 to the coil 64 to excite the coil 64 and switch the negative pressure switching valve 66, a negative pressure pipe 30 connected to an advance port 28 formed slightly upstream of a fuel close position of the throttle valve 26 is communicated with a negative pressure pipe 36 connected to a negative pressure advancing device 34 which is arranged on a side of a distributor 32 to control the ignition timing in accordance with the operation condition of the engine. When the power is not supplied to the coil 64 through the control circuit 20, an air suction port 38 is communicated with the negative pressure pipe 36 so that air is supplied to the negative pressure advancing device 34. 
     Referring to FIG. 4, the negative pressure advancing device 34 comprises a diaphragm 72a to which one end of a rod 70 connected to an ignition timing control movable base of a governor advancing device of the distributor 32 is fixed, an advancing pressure chamber 72b sectioned by the diaphragm 72a for pulling the rod 70 leftward as viewed in the drawing, and a compression spring 72c for biasing the diaphragm 72 rightward as viewed in FIG. 4. The negative pressure pipe 36 is connected to the pressure chamber 72b so that the negative pressure advancement is attained. When the exciting coil 64 of the negative pressure switching solenoid valve 24 is not energized, the pressure chamber 72b of the negative pressure advancing device 34 is connected to the atmosphere, and when the exciting coil 64 is energized, the pressure chamber 72b of the negative pressure advancing device 34 is connected to the advance port 28. 
     The operation of the ignition timing control apparatus thus constructed is now explained. 
     In an idling condition after the start of the engine, the atmospheric pressure upstream of the throttle valve 26 is introduced into the advance port 28 due to a closed position of the throttle valve 26. Under this condition, the atmosphere pressure is introduced into the negative pressure advancing device 34 of the distributor 32 without regard to the position of the negative pressure switching solenoid valve 24 and the ignition timing is not advanced. 
     In the open position of the throttle valve 26, the intake negative pressure in the vicinity of the throttle valve 26 is introduced into the advance port 28. Under this condition, the ignition timing advancement is controlled in accordance with the position of the negative pressure switching solenoid valve 24. For example, when the temperature signal indicating that the engine coolant temperature is below the predetermined temperature is produced by the coolant temperature switch 6, or when the high speed gear position signal indicating that the gear is at the high speed position is produced by the shift switch 4, or when the light load signal indicating that the negative pressure in the intake manifold 12 is high, that is, the engine is operated in the light load condition is produced by the vacuum switch 14, the exciting coil 64 of the negative pressure switching solenoid valve 24 is energized by the control circuit 20 so that the intake negative pressure taken from the advance port 28 is supplied to the negative pressure advancing device 34 of the distributor 32. As a result, the ignition timing determined by the distributor 32 in accordance with the intake negative pressure leads to the governor advance angle. 
     In the present embodiment, when at least one of the signals from the shift switch 4, the coolant temperature switch 6 and the vacuum switch 14 is supplied to the control circuit 20, the ignition timing is advanced. Accordingly, when the engine is operated under the light load condition, the vacuum switch 14 produces the light load signal so that the pressure chamber 72b of the negative pressure advancing device 34 is connected to the advance port 28 and the ignition timing is advanced in accordance with the magnitude of the intake negative pressure in the vicinity of the throttle valve 26 even if the transmission gear is at other than the high speed gear positon or the engine coolant temperature is above a predetermined temperature. Through this control, when the transmission gear is shifted to other than the high speed gear position after the warm-up of the engine and the engine is operated under the light load condition, the engine output is increased, the drivability of the car is improved and the fuel consumption is reduced to compare with a prior art system in which the ignition timing is not advanced in accordance with the intake negative pressure. 
     When the throttle valve 26 is fully opened under the high load of the engine, the intake negative pressure is lowered and the negative pressure introduced into the advance port 28 is also reduced. Under this condition, the pressure introduced into the negative pressure advancing device 34 of the distributor 32 is equal to or close to the atmospheric pressure without regard to the position of the negative pressure solenoid valve 24 and the ignition timing is not advanced. Accordingly, only the governor advance control is effective so that the generation of the nitrogen oxide in the exhaust gas is suppressed.