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Carburetor engines with an inertia governor sometimes exhibit a self-excited oscillation in rotational speed, called hunting. The mechanism of hunting has been extensively studied for many years. It is our general understanding that the dynamic characteristic of the inertia governor is the primary factor to explain the phenomenon. However, the existence of another kind of hunting, called lean hunting, where lag in air-fuel ratio plays a more important role than the dynamic characteristic of the inertia governor, was reported by Tanaka(1,2). His previous papers, showing various experimental measurements of oscillating engine speed, air-fuel ratio and peak cylinder pressure, explain the nature of the phenomenon. To provide the foregoing work with a theoretical ground, we developed a mathematical formulation for the mechanism of lean hunting.

A significant wavewise change of air fuel ratio in line with the engine speed having a long intake pipe was studied experimentally and theoretically. The results show the pulsewise change in fuel flow plays the dominant role in the wavewise change in the air fuel ratio. It is found that this pulsewise fluctuation of the fuel flow forms an oscillation wave with both the amplitude and frequency becoming larger according to the engine speed resulted by the phase change of the pulsation wave in the intake pipe according to the engine speed. A modified frequency ratio of gas vibration in the intake pipe to that of engine intake stroke is proposed to explain this pulsation effect on the fuel flow and an effective simulator for this phenomena is established.

A quite interesting self-excited oscillation phenomenon in engine speed, which may not be explained with the classical theory of mechanical hunting, is studied experimentally. The effects of the various engine operating variables on the phenomenon are examined using a four cycle single cylinder gasoline engine with an inertia governor. It was found that the phenomenon occurs when engines are operated at a lean air fuel ratio under light load conditions, and that the hunting phenomenon is ascrlbable to the temporary shift in air fuel ratio from the steady state value. This shift in air fuel ratio occurs due to the fuel flow delay into the cylinder caused by the fact that the fuel flow into the cylinder cannot follow the movement of the throttle valve.

The hunting phenomenon which occurs at the lean air-fuel ratio and under light load conditions was studied in the previous paper and it was disclosed that this was not “mechanical hunting” and the fuel flow delay into the cylinder had a dominant effect on this “lean hunting”. This paper describes first, the experiments which demonstrate that such a fuel flow delay can occur by changing the throttle position sinusoidally in various periods, and that Pmax (the maximum pressure in the cylinder) responds quite differently according to the magnitude of the air-fuel ratio; secondly, the effects of ignition energy and ignition timing on the lean hunting were also studied.