Engine

An engine comprising:

• first and second exhaust valves;
• first and second exhaust passages opened and closed by the first and second exhaust valves
• a first catalyst disposed in the first exhaust passage;
• second catalysts installed in the second exhaust passages so as to be located upstream of the first catalyst;
• a variable valve operating mechanism configured to adjust lifting amounts of the first and the second exhaust valves between approximately zero and a maximum lifting amount;
• and
• a lifting amount control unit configured to control lifting amounts of the first and the second exhaust valves with the variable valve operating mechanism such that an exhaust gas amount passing through the second exhaust passage is larger than that passing through the first exhaust passage, at least during start-up, and such that the exhaust gas amount passing through the first exhaust passage is larger than that passing through the second exhaust passage at least in a range of high-speed rotation and large-load operation of the engine after completion of warm-up operation.

An engine comprising:

• a cylinder head having at least one combustion chamber surface;
• first, second, and third exhaust valve seats opening in the combustion chamber surface, the third exhaust valve seat being smaller than the first and second exhaust valve seats;
• first, second, and third exhaust valves engaged with the first, second, and third exhaust valve seats;
• first, second, and third exhaust passages extending from the first, second, and third exhaust valve seats, respectively, toward an outer surface of the cylinder head, the third exhaust passage defining an exhaust port opening at the outer surface of the cylinder head, separate from the first and second exhaust passages;
• a first catalyst device having an upstream end communicating with the third exhaust passage but not the first or second exhaust passages;
• a second catalyst device having an upstream end communicating with the first and second exhaust passages;
• a variable valve operating mechanism configured to adjust lifting amounts of the first, second, and third exhaust valves between approximately zero and a maximum lifting amount;
• and
• a lifting amount control unit configured to control lifting amounts of the first, second, and third exhaust valves with the variable valve operating mechanism such that an exhaust gas amount passing through the third exhaust passage is larger than that passing through the first and second exhaust passages, at least during start-up, and such that the exhaust gas amount passing through the first and second exhaust passages is larger than that passing through the third exhaust passage at least in a range of high-speed rotation and large-load operation of the engine after completion of warm-up operation.

An engine comprising:

• first and second exhaust valves;
• first and second exhaust passages opened and closed by the first and second exhaust valves
• a first catalyst disposed in the first exhaust passage;
• second catalysts installed in the second exhaust passages so as to be located upstream of the first catalyst;
• and
• means for controlling lifting amounts of the first and the second exhaust valves such that an exhaust gas amount passing through the second exhaust passage is larger than that passing through the first exhaust passage, at least during start-up, and such that the exhaust gas amount passing through the first exhaust passage is larger than that passing through the second exhaust passage at least in a range of high-speed rotation and large-load operation of the engine after completion of warm-up operation.