1. Field of the Invention
The present invention relates to an air-fuel mixture intake apparatus having a two-barrel or duplex carburetor for internal combustion engines.
2. Prior Art
There has been a strong need for internal combustion engines which will emit a reduced amount of pollutants such as carbon monoxide and unburned hydrocarbons and improve fuel economy without impairing engine performance and lowering the thermal efficiency of engines.
Various systems such as lean air-fuel mixture combustion systems or EGR systems have been practiced to reduce harmful components in the exhaust gas and achieve better mileage. These known systems have proven unsatisfactory in that in low-load operating conditions or especially in low-speed, low-load operating conditions, the volumetric efficiency of an air-fuel mixture introduced into a combustion chamber is low and an increased amount of exhaust gas tends to remain in the combustion chamber, and the air-fuel mixture in the chamber cannot easily be ignited. Furthermore, the speed of combustion and hence the speed of travel of flames are low, resulting in unstable fuel combustion in the combustion chamber. The foregoing systems thus have the disadvantages of low thermal efficiency and sluggish engine operation.
Improved engine performance, efficiency and fuel economy accompanied by better emission control can be best achieved by speeding up fuel combustion in combustion chambers. To increase the rate of combustion, there have been proposed many arrangements which are designed to burn an air-fuel mixture at a higher speed by developing disturbances in the air-fuel mixture, to promote fuel carburetion, and to uniformize fuel distribution among engine cylinders.
One of the proposed arrangements comprises an auxiliary intake passage for generating swirls in a combustion chamber during the intake stroke. Another proposal is composed of a combustion of primary and secondary intake passages, with primary and secondary throttle valves located closely to a combustion chamber in some applications. According to still another construction, a projection or a valve is disposed adjacent to an intake valve to produce a biased flow of airflow mixture.
The auxiliary intake passage is designed to introduce an air-fuel mixture into the combustion chamber at a high speed. With the cross section of a main intake passage being selected to suit high-speed, high-load engine operation, the speed of flow of the air-fuel mixture becomes reduced in lowload operating ranges in which the volumetric efficiency is small, with the results that sufficient swirls will not be generated in a combined flow of air-fuel mixtures from the main and auxiliary intake passages. The auxiliary intake passage is less effective to produce swirls than desired under medium and high load conditions in which the throttle valve is wide open and the boost pressure is relatively small. With the auxiliary intake passage, fuel tends to be less atomized during idling operation due to a bypassing flow of air-fuel mixture.
The speed of flow of air through the venturi of a carburetor is low and hence fuel is not fully atomized in low-load engine operation. Such insufficient fuel atomization causes fuel in a liquid form to flow down an intake passage into a combustion chamber, with the result that air and fuel will not be mixed uniformly and fuel will not be distributed uniformly among engine cylinders, resulting in poor fuel combustion in the engine cylinders.
To improve fuel combustion in low-load operating conditions, there has been devised a carburetor having a variable venturi which is variable in cross section in order to keep substantially constant the speed of flow of air through the venturi where a fuel discharge nozzle is located, irrespective of varying amounts of air flowing through the venturi. Although the variable venturi enables an engine to operate relatively stably and flexibly in a wide operating range from low load to full load conditions, it fails to effect stable air flow control when the throttle valve opens slightly because the venturi cross section does not change appreciably even if the opening of the throttle valve varies. Therefore, exhaust gas purification cannot be achieved by the variable venturi while the engine operates under small loads.
There have been known internal combustion engines equipped with a duplex or two-barrel carburetor or with primary and secondary intake passages for each engine cylinder, the secondary intake passage being put into service under certain load conditions. Such an intake system is more advantageous than single-carburetor intake systems in that it can effect better fuel atomization particularly in low to medium load ranges, cause more disturbances in the air-fuel mixture in a combustion chamber, and improve the rate of fuel combustion. An internal corbustion engine having a secondary throttle valve provided for each engine cylinder and actuatable when the engine is subjected to a higher load can prevent interference between the cylinders such as leakage of the air-fuel mixture therebetween on the secondary side, resulting in better fuel distribution among the engine cylinders. When secondary throttle valves are inadequate in their opening and closing motions or cannot be closed completely, the engine operation becomes as unstable as there are such defective secondary throttle valves since each engine cylinder is equipped with a secondary throttle valve. During deceleration, the secondary throttle valves subjected to bouncing or re-opening motion under a large negative pressure developed in the combustion chambers, with the consequences that stability and recovery of idling operation are poor, and the rpm of the engine during idling operation is relatively high, resulting in worse fuel economy. Furthermore, the secondary throttle valves open rapidly during acceleration, and hence the engine performance becomes impaired in the acceleration mode due to retarded fuel introduction into the engine cylinders.