The concept of providing lean air/fuel mixtures to engines is not new. For example, U.S. Pat. No. 1,916,938 which issued Jul. 4, 1933 describes the use of a by-pass air conduit in a carbureted engine to reduce the amount of fuel in the air/fuel mixture to increase the efficiency of the engine. In the '938 patent, the system described utilizes a standard carburation system with a by-pass conduit which provides additional air to the engine under certain operating conditions.
U.S. Pat. Nos. 4,084,563; 4,182,295; and 4,751,907 illustrate various systems that have been designed for providing additional air to the engine aside from that normally provided through the carburation system. The supplementary air provides a leaner fuel mixture which not only makes the engine operate more efficiently, but also reduces some of the pollutants produced when the engine is operating. In each of the above patents, and all of the systems are somewhat different, the by-pass air conduit with its attendant control valve connects with the air inlet to the engine downstream of the main throttle valve. Introducing the extra air below the main throttle valve allows a wider range of air/fuel ratios. However, any change in the main throttle valve position drastically disrupts the air/fuel ratio by shifting the balance of flow between the two air paths. The foregoing systems may be described as those systems wherein the mixing of the carbureted air/fuel and of the by-pass air occurs downstream of the main throttle valve control for the engine.
An object of this invention is to provide a system wherein the extra or by-pass air is introduced upstream of the main throttle valve and thus any change in the main throttle valve position affects the flow in both the flow paths maintaining a nearly constant air/fuel ratio over varying engine speeds.
In connection with the prior art air/fuel control systems, there have also been utilized sensors located in the intake or the exhaust systems for determing the fuel in the intake or the content of oxygen or O.sub.2 in the exhaust system of the engine. Both of these sensors have been utilized to control the by-pass throttle valve and in some instances the main throttle valve in an effort to control the air/fuel mixture over the various operating conditions of the engine.
With the mixing of air and fuel upstream of the main throttle valve, it is only necessary to control the by-pass throttle valve to provide the appropriate air/fuel mixture for the engine over its operating range.