In addition to the measures explained in the foregoing, an inlet-and-exhaust system of favorable design with regard to oscillation condition of the gas does much to improve the intake and exhaust efficiency of the engine. The pulsating intake of the fuel-and-air mixture and discharge of the exhaust gases initiates oscillation in the system. At the end of the suction stroke the fuel-and-air mixture in the inlet duct flows at high velocity to the inlet valve, which is in the process of closing.

No engine design should ignore the potentially destructive and energy wasting effects of resonance, but very little anti-resonance development has been applied since the IC engine was first introduced. The closure slows down the rush of the mixture, which impinges on the valve and causes a buildup of pressure in front of it.

As a result, the fuel mixture continues to flow into the cylinder even after the piston has passed through bottom dead center and is rising again to start the compression stroke. But the inlet valve now closes completely and deflects the mixture back along the inlet duct. This is what initiates the oscillation in the duct.

At the open end of the duct the fuel mixture is again deflected, and the cycle is repeated. If in case when the inlet valve opens again, the pressure wave in the inlet duct is moving toward the valve, the mixture will immediately enter the cylinder. The system is now in a state of resonance. As the inlet valve opens wider and the piston moves downwards, the pressure in the inlet duct drops, while the velocity rises to its maximum. Toward the end of the suction stroke the inlet valve begins to close again, so that the flow is again retarded, the pressure builds up, and the oscillation phenomena are repeated.

Optimum charging of the cylinder with the fuel mixture is achieved when the frequency of oscillation coincides with the opening and closing frequency of the valve so as to produce resonance, as envisaged above when the inrush of fuel mixture finds the valve just opening to let it into the cylinder. Evidently this will occur only at one particular engine speed. At other speeds the volumetric efficiency will be lower. Long inlet ducts provide good charging at high speeds, whereas short ones are better at relatively low speeds.

The development of oscillation and resonance is counteracted by the flow resistance in the inlet duct, the constriction and turbulence at the throttle valve (in the carburetor), and the damping effect occurring at the open intake end of the duct. Figs 1 and 2 show the oscillation system in a four cylinder engine: the inlet pipes all emerge from a single connection at the carburetor, so that damping occurs there. The charging is therefore poorer than in the case of the inlet system of a fuel-injection spark-ignition engine as illustrated in Figs 3 and 4.