In the conventional two-stroke internal combustion engine, the inner surface of the cylinder is furnished with an exhaust port and a set of scavenging ports, which are controlled with the reciprocating piston to open and close. The upper edge of the exhaust port is above the upper edge of the scavenging ports so as to have the exhaust port opened first. After the power stroke of the aforesaid two-stroke internal combustion engine being completed, the waste gas in the cylinder is to be exhausted by means of gas mixture entered into the cylinder via the scavenging ports. In the two-stroke internal combustion engine that uses a carburetor, the air and fuel are pre-mixed into a homogeneous gas mixture, which is then conveyed into the cylinder, and which is also used for clearing the waste gas in the cylinder simultaneously.
For instance, a Japan patent application No. 51-39112 teaches a two-stroke internal combustion engine, in which a valve is furnished for opening or closing the exhaust port so as to have the height of the upper edge of the exhaust port become variable; the upper edge of the exhaust port is almost positioned above the upper edge of the scavenging ports.
In the aforesaid conventional two-stroke internal combustion engine, the exhaust port is opened earlier than the scavenging ports, but is closed later than the scavenging ports; therefore, a given portion of the gas mixture is to be blown off from the exhaust port upon the mixture entering into the cylinder via the scavenging ports; in other words, a considerable quantity of fuel is to be blown into the exhausting passage without being burned, that blow-off condition not only increases the fuel consumption, but also increases the HC quantity contained in the exhausted gas; that blow-off condition is particularly serious during the engine running in idle or low speed condition. That is the reason why the small vehicle with a two-stroke engine exhausts more HC in the waste gas during being stopped and the idle running period.
It is well known that the content of HC in the waste gas exhausted can be reduced by lowering the upper edge of the exhaust port to about the same level of the upper edge of the scavenging ports so as to reduce the blow-off volume; in that case, the scavenging efficiency of the engine will be reduced, and, as a result, the power output under normal using condition will be reduced; therefore, it is deemed impractical by lowering the upper edge of the exhaust port to prevent from leaking. Moreover, the height of the upper edge of the exhaust port may be changed by furnishing a valve, which can be opened at high speed running condition so as to make the upper edge of the exhaust port higher for increasing the power output; however, when the valve is under closed condition, the upper edge of the exhaust port is still at an ordinary height, being unable to prevent the blow-off condition from taking place.
There is an internal combustion engine that has no carburetor; the fuel is to be sprayed directly into the cylinder. During the scavenging phase, the waste gas can be exhausted through the exhaust port, and a portion of fresh air can enter into the cylinder via the scavenging port without losing fuel; however, the atomizing efficiency of the aforesaid engine is much worse than the engine with carburetor, such as the fuel not being sprayed evenly.
The atomized fuel drops have to be gasified before being burned. But in high speed running condition, the atomized fuel drops can be burned completely within a very short combustion period. As a result of the combustion speed being limited, the power can not increased; during the engine running at a high speed, a lot of atomized fuel drops would be exhausted out of the exhaust port before being burned thoroughly; therefore, a heavy fuel consumption and air pollution will be resulted.