This invention relates to an ignition timing regulating device for an internal combustion engine, which senses the pressure of ambient atmosphere at, for example, the engine starting time and regulates the ignition timing of the engine on the basis of the value of sensed atmospheric pressure.
It is necessary to determine the ignition timing of an internal combustion engine depending on the state of the engine so that the engine can operate in its optimum operation mode. As is commonly known, it is the best mode of engine operation to ignite the engine under the condition of minimum advance at maximum torque or so-called MBT (minimum advance for best torque) when the factors such as the efficiency and fuel consumption of the engine are taken into account. To this end, it is necessary to change or regulate the ignition timing depending on the state of the engine so that the ignition can always occur under the condition which satisfies the MBT. This MBT is a function of the conditions of the engine and is primarily determined by main and auxiliary factors. The rotation speed of the engine and the pressure in the intake manifold are representative of the main factors, and the temperature of engine cooling water, EGR, etc. are representative of the auxiliary factors. The value of MBT is also variable depending on the value of atmospheric pressure. The value of MBT varies depending on the value of atmospheric pressure for the reason that, when the engine operates on, for example, a highland, the value of atmospheric pressure is lower than that on a lowland, that is, the volume density of air is reduced, with the result that the air-fuel ratio (A/F) in the carburetor shifts toward a richer side of the setting. According to the results of a test on a four-cylinder four-cycle internal combustion engine, the value of MBT varies relative to the value of A/F at the engine rotational frequency of 2000 rpm and the intake manifold pressure of -200 mm Hg in a manner as shown in FIG. 1. It is therefore necessary to regulate the advance so that it meets the variation in the value of MBT.
An engine ignition timing control device of electronic type has been proposed hitherto to regulate the advance so that it meets the value of MBT which varies in a manner as shown in FIG. 1. In the proposed ignition timing control device, the length of time required for the engine to make one complete revolution or to rotate through a predetermined angle is measured to detect the rotation speed of the engine, and a semiconductor type pressure sensor or a pressure responsive diaphragm is used to electrically sense the intake manifold pressure. In such an ignition timing control device, the value of advance .alpha..sub.N based on the detected rotation speed of the engine N and the value of advance .alpha..sub.P based on the sensed intake manifold pressure P are added to provide the sum (.alpha..sub.N +.alpha..sub.P), and on the basis of this sum, the ignition timing is primarily determined. Then, various connection factors are added to finally determine the ignition timing. An atmospheric pressure sensor of diaphragm type, semiconductor type or any other suitable type is additionally employed to sense the pressure of ambient atmosphere so as to retard the advance angle of the ignition timing depending on the value of sensed atmospheric pressure. This atmospheric pressure sensor is basically the same in function as the intake manifold pressure sensor in that both of them are pressure sensors. Hitherto, both of these two pressure sensors have been provided from the standpoint that they serve individually the separate functions.
The prior art ignition timing control device of the type above described is however expensive in that it includes the atmospheric pressure sensor and the intake manifold pressure sensor in spite of the fact that these two sensors are basically the same in their functions. Further, because of the recent demands for purification of engine exhaust gases, low fuel consumption, etc., it is required to sense the atmospheric pressure with high accuracy and to suitably compensate the value of sensed intake manifold pressure for the control of the ignition timing. However, a relatively inexpensive atmospheric pressure sensor comprising the combination of a diaphragm and a switch is capable of only sensing the pressure at a single point from the structural aspect, and a plurality of such sensors corresponding to the number of pressure sensing points are required to sense the pressures at such points. Further, employment of a pressure sensor of semiconductor or like type in the device for the purpose of improvement in the accuracy of pressure sensing leads inevitably to a further higher cost. Thus, an inexpensive device which can sense the atmospheric pressure with high accuracy is now demanded.