The present invention relates to control systems for multiple cylinder internal combustion engines, and in particular to a control system which combines closed-loop mixture control and split engine operations.
It is known that fuel economy is achieved under light load conditions by operating a multiple cylinder engine on partial cylinders if the reduced engine power can adequately operate the vehicle. When the engine load is relatively heavy the engine is operated on full cylinders. The whole cylinders are divided into a first group which is operated only during heavy load conditions and a second group which is operated at all times. This method of engine control is known as split engine operation. On the other hand, closed-loop mixture control systems are also known and widely used as an effective means of eliminating noxious gaseous components. Such systems employ an exhaust gas sensor and a three-way catalytic converter disposed downstream of the gas sensor to effect simultaneous oxidation of hydrocarbon and monoxide and reduction of nitrogen oxides when the air-fuel ratio is precisely controlled to within a predetermined range, known as converter window which corresponds to the stoichiometric air-fuel ratio.
However, the above known methods cannot be combined together without giving rise to a problem in that the deactivated cylinders operate as a pump to introduce air into the exhaust system thereby increasing oxygen contents in the emissions, which results in a false gas sensor signal. To eliminate this problem, a prior method involved the use of a shutoff valve to direct the stream of pumped air through a passage that bypasses the catalytic converter during partial cylinder mode, and switch the direction of the gas flow to the catalytic converter during the full cylinder mode.
However, because of the inherent delay time it takes the exhaust gas to reach the location of the shutoff valve, the signal that controls the shutoff valve must be precisely timed in relation to the delay time and if improperly timed a false gas sensor signal would result. Copending U.S. patent applications U.S. Ser. No. 046,309, filed June 7, 1979; U.S. Ser. No. 046,350, filed June 7, 1979; and U.S. Ser. No. 048,156, filed June 13, 1979 disclose a system in which split engine control and feedback mixture control are properly combined by the use of a first and second exhaust conduits respectively connected to the first and second group cylinders and a common exhaust passage downstream of the first and second exhaust conduits for directing the gases exhausted through the upstream side first and second exhaust conduits. A first set of gas sensor and catalytic converter is disposed in the second exhaust conduit and a second set of gas sensor and catalytic converter is disposed in the common conduit. The feedback control circuit is selectively responsive to the gas concentration signal provided by the first or upstream side gas sensor when the engine is operated on the second group cylinders (partial cylinder mode) or responsive to the signal from the downstream side gas sensor when the engine is operated on full cylinders (full cylinder mode).
During the partial cylinder mode, however, the air pumped through the deactivated first group cylinders tend to reduce the temperature of the downstream side gas sensor as well as the downstream side catalytic converter to thereby reduce its conversion efficiency. Therefore, when the engine operational mode is switched from partial to full cylinder modes, the signal from the downstream side gas sensor is inappropriate for feedback control operation for a certain interval of time.