A conventional valve timing control device comprises: a rotational shaft for opening and closing a valve; a rotational transmitting member rotatably mounted on the rotational shaft; a vane provided on the rotational shaft; a pressure chamber formed between the rotational shaft and the rotational transmitting member and divided by the vane into an advance chamber and a delay chamber; a first fluid passage communicated with the advance chamber for supplying and discharging a fluid; a second fluid passage communicated with the delay chamber for supplying and discharging the fluid; and a locking member for maintaining a relative position between the rotational shaft and the rotational transmitting member. Such a conventional variable timing device is disclosed, for example, in Japanese Patent Laid-Open Publication No. H01-92504 and in Japanese Patent Laid-Open Publication No. H09-250310.
In the conventional valve timing control device, the valve timing is advanced due to relative displacement between the rotational shaft and the rotational transmitting member when the fluid is supplied to the advance chamber and is discharged from the delay chamber. On the contrary, the valve timing is delayed due to the counter displacement between the rotational shaft and the rotational transmitting member when the fluid is discharged from the advance chamber and is supplied to the delay chamber.
Further, in the conventional valve timing control device disclosed in the publications, the vane transmits the rotation from the rotational transmitting member to the rotational shaft. Therefore, the rotational shaft always receives a force which expands the delay chamber while the internal combustion engine is running. When the internal combustion engine is stalled, the rotational shaft rotates so as to expand the delay chamber due to lack of enough fluid supply to hold the vane at the current position. Thus, the rotational shaft reaches the most delayed position where the delay chamber is the largest. In case the internal combustion engine is cranked at the most delayed position of the rotational shaft, the vane vibrates due to unstable transitional pressure so as to generate undesirable noise. Conventionally, the locking member maintains the predetermined relative position between the rotational shaft and the rotational transmitting member so that generation of such vibration of the vane is effectively prevented.
Moreover, air intake tries to flow into a cylinder by inertia even after the piston begins to go to the top dead center while the internal combustion engine is running at high speed. Therefore, volumetric efficiency may be improved by delaying closure of an air-intake valve so that the output of the internal combustion engine may be improved.
However, in the conventional valve timing control device, the most delayed position has to be set so that the air intake is sufficient to crank the internal combustion engine. This means that the closing timing of the air-intake valve is not optimized for the high-speed operation of the internal combustion engine. Thus, the volumetric efficiency cannot be improved by the inertia of the air intake. If the closing timing of the air intake valve is unreasonably optimized for the high-speed operation of the internal combustion engine, the air intake which is at first inhaled into the cylinder flows backward upon start of the internal combustion engine since the air intake does not have enough inertia and the air-intake valve continues to be opened even after the piston passes the bottom dead center and begins to go to the top dead center. Therefore, the internal combustion engine becomes hard to crank due to insufficient compression ratio and imperfect combustion. Further, in the conventional valve timing control device, due to low atmospheric pressure, a similar disadvantage may be expected at high altitudes if the air intake valve is set to be closed at around the bottom dead center of the piston.
Further, in the conventional valve timing control device, if the exhaust valve timing is similarly delayed, the amount of exhaust gas recirculating is increased by an extended overlapping time of the air-intake valve and the exhaust valve so that the internal combustion engine becomes hard to start.