1. Field of the Invention
The present invention is concerned with the structure of cylinder heads of internal combustion engines and, more particularly, with a cylinder head structure which improves flowing of a fuel mixture into an exhaust port.
2. Description of Related Art
In conventional engines, improvements in air intake and gas discharge or scavenging efficiencies have been accomplished by maximizing the area of exhaust ports. In addition, improvements in fuel combustion efficiency have been accomplished through the adaptation of compact combustion chambers, measurably improving engine output. In order to assure a sufficient valve opening area for such an engine, each cylinder is provided with multiple valves. The valve intake ports and exhaust ports of the multiple valve engine are located in close proximately. Because of this, in a valve overlap period, in which both an intake valve and an exhaust valve open, a fresh air fuel mixture introduced into the combustion chamber through the intake port is apt to be immediately expelled or blown through the exhaust port, resulting in a deterioration of emission performance, a decrease in engine output, and/or a reduction in fuel efficiency. Hence, consideration has been given to the establishment of walls around the intake port or the exhaust port. For example, Japanese Unexamined Patent Publication No. 55-104,519 discloses an engine in which a wall is formed so as to extend within the combustion chamber and surround the intake port, thereby directing the flow of fuel mixture introduced into the combustion chamber in an intended direction.
In an engine such as that described in the publication mentioned above, in which a wall surrounds an intake port, if a gap or space between the margin of an intake valve head, which is the portion of the valve having the largest diameter, and the wall is too large, then the wall will not effectively constrict a flow of fresh air which travels towards the exhaust port. A flow-through of fresh air, therefore, will not be effectively prevented. However, if this gap or space is small, resistance to intake air is increased, reducing the charging efficiency of fresh air. This leads to a reduction in engine output. In addition, if the protrusion of the wall is too large, a period of time in which air flow is constricted will be lengthened, increasing intake air resistance. This leads to a reduction in charging efficiency of fresh air and, consequently, to a decrease in engine output. It also becomes difficult to produce a tumble or turbulent flow of the fuel mixture within the cylinder, resulting in a deterioration in fuel combustion characteristics.
During valve overlap, exhaust gases within a combustion chamber are forced toward an exhaust port by incoming fresh air and are discharged or scavenged through the exhaust port. In order to improve scavenging efficiency, the cylinder head should be structured so as to make this scavenging action efficiently. Particularly, in order for engines equipped with superchargers to meet the demand to achieve improved knock resistance, it is desirable to promote scavenging action with the pressurized fresh intake air while the engine is supercharged, so as to realize a measurable reduction in the temperature of the fuel mixture within the cylinders caused by a reduction in the amount of gases remaining in the cylinders.