Exhaust system of an internal combustion engine

An internal combustion engine having an intake valve and an exhaust valve, comprising:

• valve overlapping period control means for controlling a valve overlapping period during which both the intake valve and the exhaust valve are open;
• and
• equivalent length adjustment means for controlling an equivalent length of an exhaust passage necessary to cause an exhaust gas pulsation to return to an exhaust port in the form of a vacuum during said valve overlapping period, to reduce said equivalent length as an engine speed increases when said valve overlapping period is longer than a predetermined period, and for stopping a control of said equivalent length when said valve overlapping period is shorter than said predetermined period, said exhaust passage having a rapidly increased sectional area portion, said equivalent length being represented by a length from the exhaust valve to said rapidly increased sectional area portion, and said equivalent length adjustment means controlling the position of said rapidly increased sectional area portion to thereby reduce said equivalent length as the engine speed increases when said valve overlapping period is longer than said predetermined period, and stopping a control of the position of said rapidly increased sectional area portion when said valve overlapping period is shorter than said predetermined period.