As for a two-cycle internal combustion engine, it is generally known that in an exhaust stroke a part of the fuel mixture, which is fed into a cylinder chamber, flows out of an exhaust port to an exhaust flow passage along with combustion gas and is exhausted to the outside, thus causing air pollution.
The engine of Japanese Utility Model Publication No. 55-4518 is proposed as an example of a solution to the aforesaid problem. According to the above, a variable valve is provided in an air supply flow passage, which introduces air into a scavenging flow passage, connected to a scavenging port, owing to the negative pressure in the crank chamber before starting a scavenging stroke, the variable valve passing an extremely small quantity of air, including a zero flow, in an operation state of low rotation and low load operation of the engine, and increasing the flow rate of air in states other than the aforesaid operation state. Thus, in a scavenging stroke in which the scavenging port is opened, air is fed into a fuel flow passage from the crank chamber to form a layer of air between the combustion gas and a scavenging flow in the cylinder chamber, thereby preventing blow-by of the fuel mixture. In addition, the aforesaid air supply quantity is zero or very small at the time of low rotation and low load operation of the engine, thus preventing an excessive rarefaction of the fuel mixture, eliminating poor ignition, and stabilizing the combustion operation. Moreover, it is described that the aforesaid air supply quantity into the cylinder chamber increases at the time of low rotation and low load operation of the engine, thus effectively fulfilling the aforesaid operation of preventing blow-by of mixture.
The engine of Japanese Laid-open Patent No. 58-5423 is proposed as another example. According to that document, a crank chamber compression two-cycle internal combustion engine has an exhaust port and a scavenging port in the sidewall of a cylinder chamber, and the exhaust port and the scavenging port are opened and closed by a sidewall of the piston. Air is sucked into a scavenging flow passage, connected to the scavenging port through an air supply flow passage, due to negative pressure in the crank chamber, and sucked air is fed into the cylinder chamber prior to the fuel mixture which is sent from the crank chamber at beginning of a scavenging stroke in which the scavenging port is opened. At this time, it is intended that the scavenging port is not opened to the crank chamber due to the sidewall of the piston even at a lower dead center, and that the scavenging flow passage, connected to the scavenging port, is at least more than twice as long as that of the conventional crank chamber compression two-cycle internal combustion engine. Moreover, the total volume of the scavenging port and the scavenging flow passage is designed to be 20% or more of the stroke volume. Thus, an initial part of the scavenge, which is blown to exhaust, can be almost entirely an air component with an extremely low fuel content. Accordingly, the quantity of an initial scavenge, which is not mixed with the fuel mixture in the crankcase, can be selected so as to be an optimum value according to the volume of the scavenging flow passage. When a liquid fuel, such as gasoline or the like, is used, a large quantity of liquid fuel, adhering to the wall surface of the scavenging flow passage, evaporates, due to the high speed flow of sucked air accompanied by pulsation, and is mixed in the initial part of scavenge and blown to exhaust with the scavenge, thereby significantly reducing the stratified scavenging effect of this system. It is described, however, that the use of fuel gas almost prevents the mixing of the fuel into the sucked air in the scavenging flow passage.
In the aforesaid Japanese Utility Model Publication No. 55-4518, the quantity of air supplied is zero or very small at the time of low rotation and low load operation of the engine, thus preventing excessive rarefaction of the fuel mixture, eliminating poor ignition, and stabilizing the combustion operation. Moreover, the aforesaid quantity of air supplied into the cylinder chamber increases at the time of low rotation and low load operation of the engine, thus effectively fulfilling the aforesaid operation of preventing blow-by of mixture. However, in recent years, a demand for purification of the exhaust has increased more and more, the emission regulation has tightened up, and the purification of the exhaust gas at the time of the whole range of rotation of the engine, as well as at the time of low rotation and low load operation of the engine, is desired. For instance, in California 1999 Regulation as an example, it is demanded that the emission rate of total hydrocarbon (hereinafter referred to as "THC") be not more than 50 [g/HP*h]. Therefore, there is a disadvantage in that it is difficult for the above regulation to be only satisfied with the engine of Japanese Utility Model Publication No. 55-4518.
According to the aforesaid Japanese Laid-open Patent No. 58-5423, the scavenging flow passage is designed to be at least more than twice as long as that of the conventional crank chamber compression two-cycle internal combustion engine, and the total volume of the scavenging port and the scavenging flow passage is designed to be 20% or more of the stroke volume. However, this is an art applied only to fuel gas. With the use of fuel gas, blow-by is prevented. On the contrary, with the use of a liquid fuel, such as gasoline or the like, a large quantity of liquid fuel adhering to the wall surface of the scavenging flow passage evaporates, due to the high speed flow of sucked air accompanied by pulsation, and is mixed in the initial part of the scavenge and blown to exhaust with the scavenge. In addition, since the scavenging flow passage is provided outside of the crankcase, there arise disadvantages in that the crankcase is increased in size and the production becomes difficult.