1. Field of the Invention
This invention relates to a four-cycle spark-ignition internal combustion engine characterized by combining both (1) a measure in which the burnt gas remaining in the cylinder at the end of the exhaust stroke is replaced by a gas, such as exhaust gas, which is at a temperature approximating normal temperature and, (2) a measure in which, when a "knocking limit ignition point" exists, the ignition timing of the engine is controlled so as not to precede the "knocking limit ignition point," thereby to prevent the occurrence of knocking and to make it possible to use a high compression ratio. The expression "knocking limit ignition point" shall signify the ignition point up to which the ignition timing can be advanced without incurring knocking.
2. Description of Prior Art
Regarding a four-cycle spark-ignition internal combustion engine which operates on the Otto cycle (hereinbelow, termed the "engine"), it is well known that the thermal efficiency of the operation is higher as the compression ratio is higher. An engine of high compression ratio, however, is liable to exhibit knocking at a high engine output. Knocking is destructive to the engine and is dangerous, and hence, unless the occurrence of knocking is preventable, the compression ratio cannot be raised more.
In the case of operating an engine of a certain compression ratio under certain fixed circumstances, when the ignition timing is delayed, the tendency toward knocking is reduced but the thermal efficiency also is reduced. In the engines according to the conventional techniques, when the compression ratio is made high and the increased tendency toward knocking thus incurred is intended to be prevented by delaying the ignition timing, the lowering of the thermal efficiency attributed to the delay of the ignition timing is great and is more than the increased thermal efficiency based on the increase of the compression ratio, so that the overall thermal efficiency of the engine is reduced. For this reason and because the control of the ignition timing becomes delicate, it has scarcely been put into practical use to adopt a higher compression ratio by retarding the ignition timing.