Abstract: A method for directing an up shift operation for a vehicle having an engine and a manual transmission generates a direction of an up-shift operation when the vehicle speed exceeds a reference speed determined for each gear position. The reference speed of a medium speed gear is varied according to the degree of acceleration at the time of the previous up-shift operation. If the up-shift operation from a low speed gear to the medium speed gear is effected while the vehicle is not accelerating rapidly, the reference speed for an up shift operation from the medium speed gear is lowered. Further, if a cruising state of the vehicle with the medium speed gear after the acceleration is continued for more than a predetermined time period, the reference speed for an up-shift operation from the medium speed gear is also lowered.

Abstract: An air/fuel ratio control system for an internal combustion engine provides a mixture leaner than a predetermined reference value when a detected engine load is in a light load range. In order to produce a required engine power during a transitional period, the system detects the speed of variation of an operating parameter used for the calculation of air/fuel ratio and controls the air/fuel ratio toward the reference level even though the magnitude of the detected engine load is in the light load range. Preferably, an EGR rate is also made smaller upon occurrence of a rapid change of the operating parameter so as to prevent the power down of the engine during the transitional period.

Abstract: A secondary air introducing apparatus for introducing secondary air into the exhaust manifold of an internal combustion engine. The apparatus includes a secondary air introducing passage connected between the air cleaner and the exhaust manifold of the engine, a check valve in the passage arranged to operate in response to pulsation pressure in the exhaust manifold to supply clean secondary air thereto, a first expansion chamber interposed in the passage on an upstream side of the check valve, and a second expansion chamber interposed in the passage on an upstream side of the first chamber. At least the check valve and the first chamber are fixedly mounted on the intake manifold.