Why can changing the length of the intake manifold increase the torque at different speeds?

Due to the intermittent and periodic intake process, a certain amplitude of pressure wave will be generated in the intake manifold. This pressure wave propagates and reflects back and forth in the intake system at the speed of sound. If the resonance system diagram consists of an intake manifold with a certain length and diameter and a resonance chamber with a certain volume, and its natural frequency is coordinated with the intake cycle of the valve, then at a certain speed, before the intake valve closes, a large pressure wave will be generated in the intake manifold, which will increase the pressure of the intake manifold and thus increase the intake air volume. This is the fluctuation effect of intake.

The purpose of variable intake manifold is to make full use of the intake fluctuation effect and minimize the intake speed difference between high and low engine speeds, thus improving the economy and power of the engine. Therefore, the engine is required to be equipped with a short intake manifold at high speed and heavy load; It is equipped with a long intake manifold at medium and low speed and medium and low load. Variable intake manifold is designed to meet this requirement.

When the engine is running at a low speed, the electronic control device of the engine instructs the control mechanism of the changeover valve to close the changeover valve, and at this time, air flows into the cylinder through the air filter and the throttle valve along the curved and slender intake manifold. The slender intake manifold improves the intake speed, enhances the inertia of airflow and increases the intake volume. When the engine is running at high speed, the on-off valve is opened, and air directly enters the short intake manifold through the air filter and throttle valve. The intake manifold is short and the intake resistance is small, which also increases the intake air volume.

Variable-length intake manifold can not only improve the dynamic performance of the engine, but also enhance the airflow intensity in the cylinder by increasing the intake speed of the engine at low and medium speeds, thus improving the combustion process and improving the fuel economy of the engine at low and medium speeds.

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