1. Field of the Invention
This invention relates to engine knocking control devices, and more particularly to a knocking control device which is improved so that it can control a knocking strength automatically to a predetermined value.
2. Description of the Prior Art
A so-called "knocking phenomenon" in which abnormal hammering sounds are caused in the engine of a vehicle while it is running is well known. The knocking phenomenon occurs in the case where the load is over a predetermined value and the ignition timing is excessively advanced. This knocking is undesirable because it cause unpleasant noise, and strong knocking provides strong gaseous vibration in the cylinder and causes abnormally high temperature locally in the cylinder, which may damage the engine.
FIG. 1 indicates variations in cylinder internal pressure with respect to engine crank angles. As is clear from FIG. 1, the pressure vibration is caused by the knocking when the crank angle is in the range of from 10.degree. after the top dead center (TDC) to 30.degree. after the top dead center. FIG. 2 is an enlarged view indicating the pressure vibration due to the knocking shown in FIG. 1. As is apparent from FIG. 2, this vibration component due to the knocking includes a damping oscillation signal having frequencies higher than several kilo-Hertz.
As is obvious from FIGS. 1 and 2, during the generation of knocking, the knocking vibration is superposed on the ordinary cylinder internal pressure, and therefore gaseous vibration and standing waves are generated in the cylinder as a result of which unpleasant knocking sounds are caused, and at worst, high temperature is created locally in the cylinder which may damage the engine. However, the knocking phenomenon itself produces no bad influence on the engine. Even in the case where knocking is caused by advancing the ignition timing, the fuel consumption percentage of the vehicle can be improved by increasing the combustion efficiency of the engine. In view of the improvement of fuel consumption percentage, the acceptance of appropriate knocking is suitable to provide the most effective running condition of the engine.
Accordingly, in order to obtain the most suitable engine operating efficiency and to maintain the knocking noise level lower than a predetermined value, it is necessary to control the knocking strength according to various conditions.
Furthermore, the provision of a device capable of freely controlling the strength of knocking has been desired by the manufacturers in order to obtain data concerning the design of anti-knocking engines by strictly investigating the knocking phenomenon and to approach the best octane number of fuel.
Heretofore, the knocking phenomenon has been detected by the hearing sense of a vehicle operator or mechanic and accordingly the detection is very low in accuracy and unstable. As is clear from FIGS. 1 and 2, the cylinder internal pressure should be utilized for detecting the knocking strength. Nevertheless, a method of detecting the knocking strength based on the hearing sense has been employed. It should be noted that this method detects the noise which is caused as a result of the cylinder internal pressure variation. The noise reaches the ear through the cylinder block, the cylinder head or the like. Accordingly, the noise is damped or changed in quality, and is therefore considerably different from the actual cylinder internal pressure variation. Thus, the conventional knocking strength measuring method cannot detect the knocking strength with high accuracy. Accordingly, it has been impossible to perform ignition control with high accuracy to maintain the knocking strength in a desired allowable range which is determined from engine efficiency and noise.
At the manufacturer, a full acceleration test of the vehicle is carried out to evaluate the vehicle or fuel anti-knocking characteristic. In general, data is obtained for engine design by measuring the vehicle running octane numbers or fuel octane numbers according to a corrective border line method.
The test is carried out by the sensory judgement of the knocking phenomenon utilizing the hearing sense of a person and by the manual ignition timing control, and therefore the test is disadvantageous in that the experimental data lack reproducibility. In order to eliminate this difficulty, or to maintain the knocking strength precisely in the desired allowable range, the provision of a device capable of freely controlling the knocking strength with high accuracy has been desired.