The system for controlling the air/fuel mixture introduced into a lean burn engines includes a fuel/air mixing carburetor that is connected to the intake conduit to the engine and an air by-pass conduit that is also connected to the intake conduit. The main throttle valve is located downstream of the connection between the carburation conduit and the by-pass conduit so that mixing of the carbureted fuel/air mixture with the by-pass air occurs upstream of the main throttle valve. Accordingly, the main throttle valve does not control the air/fuel ratio but rather controls the quantity of fuel being introduced into the engine. The by-pass throttle valve may be controlled by an oxygen sensor in the exhaust system or by a fuel sensor in the intake system, when operating on natural gas, gasoline, hydrogen or alcohol fuel, as desired.
Robert W. Burrahm;David P. Meyers