The butterfly valve carburetor regulates airflow through valve rotation, combining this with the carburetor's internal fuel injection mechanism to achieve precise air-fuel ratio control at different engine speeds. Its core principle lies in dynamically adjusting the intake air volume using changes in the butterfly valve's opening, while simultaneously regulating fuel supply through the carburetor's Venturi effect or pressure sensing device, thus maintaining the air-fuel mixture within an ideal range. This process involves the combined effects of mechanical structure, fluid dynamics, and combustion control; the following analysis focuses on the adjustment mechanism at different engine speeds.
At low engine speeds, the engine intake air volume is smaller, and the butterfly valve opening decreases accordingly. At this time, the carburetor controls fuel supply through the idle metering orifice and transition metering orifice. The idle metering orifice is designed as a long, narrow channel, utilizing the low-pressure zone created by the Venturi effect to draw fuel from the float chamber, forming a stable idle fuel mist. The butterfly valve's slight opening ensures a moderate airflow velocity, preventing poor fuel atomization due to excessively low velocity or an overly lean mixture due to excessively high velocity. Simultaneously, the transition injector within the carburetor intervenes when the throttle is partially open, supplementing fuel to prevent a temporary excessively lean mixture during the transition from low to medium speeds. This design ensures the air-fuel ratio remains stable near the stoichiometric ratio at low speeds, meeting the combustion requirements for cold starts and idling.
At medium speeds, the engine load increases, and the intake air volume significantly increases. The butterfly valve opening widens accordingly, increasing airflow velocity and strengthening the low-pressure zone generated by the Venturi effect. The carburetor's main jet then becomes the primary fuel supply channel; its orifice design is precisely calculated to provide a fuel quantity matched to the intake air volume at specific airflow velocities. Furthermore, the accelerator pump within the carburetor briefly operates when the throttle is rapidly opened, mechanically injecting additional fuel into the intake manifold to prevent a temporary excessively lean mixture due to the sudden increase in air. This dynamic compensation mechanism ensures air-fuel ratio stability during acceleration or load changes at medium speeds.
At high speeds, the engine intake air volume reaches its peak, and the butterfly valve is nearly fully open. At this point, the air velocity is extremely high, and the low-pressure zone of the Venturi effect may weaken due to the excessively high velocity, leading to insufficient fuel supply. To solve this problem, butterfly valve carburetors are typically equipped with enrichment devices, such as mechanical or vacuum enrichment valves. When the intake pressure or throttle opening exceeds a set value, the enrichment valve opens, and additional fuel enters the intake manifold through the enrichment orifice to compensate for fuel loss at high velocities. Simultaneously, the carburetor's float chamber is designed with a vent to ensure that the pressure in the float chamber balances with the intake manifold pressure at high speeds, preventing fluctuations in fuel supply. These designs work together to maintain the air-fuel ratio within a high combustion efficiency range at high speeds.
The precise control of the butterfly valve carburetor also depends on its matching and tuning with the engine. Parameters such as the carburetor's orifice size, injector angle, and float chamber fuel level must all be optimized according to the engine's displacement, compression ratio, and combustion characteristics. For example, high-compression engines require a leaner air-fuel mixture to prevent knocking, thus the main jet and idle jet of the carburetor are correspondingly smaller; while low-compression engines may require a richer mixture to ensure combustion stability. Furthermore, the carburetor's mounting angle and intake manifold length also affect mixture formation and distribution, requiring experimental determination of the optimal configuration.
Modern butterfly valve carburetors have also incorporated electronic control technology to further enhance precision. For instance, some carburetors are equipped with an oxygen sensor and an electronic control unit (ECU), which adjusts the air-fuel ratio by monitoring the oxygen content in the exhaust in real time. When the oxygen sensor detects an overly rich mixture, the ECU signals to reduce fuel supply; conversely, it increases fuel supply. This closed-loop control mechanism significantly improves the precision of air-fuel ratio adjustment, especially under transient conditions. However, electronically controlled carburetors are expensive and are still mainly used in high-end motorcycles or specific industrial applications.
Butterfly valve carburetors achieve precise air-fuel ratio control at different engine speeds through a clever combination of mechanical structure and fluid dynamics. Its low-speed idle jet, mid-speed main jet and accelerator pump, high-speed enrichment device, and matching tuning with the engine together constitute a complete adjustment system. With the application of electronic control technology, the adjustment precision and response speed of carburetors have been further improved, meeting the higher requirements of modern engines for combustion efficiency and economy. Although carburetor technology has been gradually replaced by electronic fuel injection systems, in certain fields, the butterfly valve carburetor still occupies a place due to its advantages of simple structure, low cost, and high reliability.
