PURPOSE: To improve exhausting efficiency by providing a primary exhaust port to perform natural exhaust and a secondary exhaust port which is sucked up by an air pump and is late in its closing time in each cylinder, and altering the closing time of the secondary exhaust port according to the opening condition of an engine.
CONSTITUTION: The cylinder bore of each cylinder is provided with a primary exhaust port 3 and a secondary one 4. The primary exhaust port 3 performs natural exhaust, and the secondary exhaust port 4 performs forced exhaust through an air pump 9. A rotary valve 8 is provided between the secondary exhaust port 4 and the air pump 9, and a timing control means 10 voluntarily controls the closing time of an exhaust valve at the secondary exhaust port 4 according to the operating condition of an engine during the opening period of said exhaust valve. The exhaust valve at the secondary exhaust port 4 is set in its valve closing time to be later than the exhaust valve at the primary exhaust port 3. Sucking exhaust can be therefore performed at the end of an exhaust stroke being less in an exhaust quantity to improve exhausting efficiency.
NOBEMOTO HIDETOSHI
OSUMI TOSHIHIKO
KURISU TORU
