The present invention relates to a vapor lock preventing mechanism in a motor-driven fuel pump.
Conventionally, there is disclosed in Japanese Patent Laid-open Publication No. 62-214294 a motor-driven fuel pump provided with a vapor jet in a pump chamber for eliminating a fuel vapor generated in the pump chamber at high temperatures or drawn with a fuel upon sucking of the fuel into the pump chamber and thereby preventing vapor lock.
Such a vapor jet is normally provided at a high-pressure position in the pump chamber. In the case of a multi-stage motor-driven pump, the vapor jet is provided at a high-pressure position in a first-stage pump chamber on a suction side of the pump section, so as to efficiently eliminate the fuel vapor under the high fuel pressure. However, if a large amount of fuel vapor is generated in the case of using a light gasoline as the fuel, for example, the fuel vapor resides widely in the pump chamber to pass the position of the vapor jet or generate vapor lock in the worst case.
Such a problem is considered to be eliminated by enlarging a diameter of the vapor jet or forming the vapor jet at a higher-pressure position to thereby improve a vapor discharging ability. However, the fuel is largely leaked with the fuel vapor through the vapor jet to cause a reduction in fuel discharge quantity of the pump and a reduction in pump ability at an ordinary temperature.
In another type two-stage motor-driven fuel pump disclosed in Japanese Utility Model Publication No. 63-100686, a first impeller has a thickness and a vane depth greater than a second impeller to thereby increase a gradient of fuel pressure increase in a first pump chamber, thereby early diminishing the fuel vapor generated in the first pump chamber or efficiently eliminating the fuel vapor from the vapor jet.
However, in the latter case, since the first impeller and the second impeller have different shapes and sizes as mentioned above, a common member for each pump stage cannot be utilized, and an overall size of the pump section is increased.