1. Field of the Invention
This invention relates to fuel injection systems for internal combustion engines and more particularly to a fuel injection system equipped with temperature compensating means.
2. Description of the Prior Art
With the fuel injection valve of a fuel injection system in an internal combustion engine, if the fuel temperature rises particularly under the high intake pipe vacuum conditions, there is the danger of causing a vacuum boiling phenomenon at the orifice of the fuel injection valve and thus failing to supply the desired amount of fuel. As a result, when the external temperature is high, the idling operation becomes unstable and there is the danger of the engine stalling in extreme cases.
With the recent fuel injection systems for internal combustion engines, a method is used in which the concentration of oxygen in the exhaust gases from an engine is detected and fed back to the fuel supply system thereby controlling the duration of opening of the fuel injection valve to provide the correct air-fuel ratio and thus the occurrence of the above-mentioned troubles is on the decrease. However, under conditions where the O.sub.2 sensor which is generally used for detecting the oxygen content of the exhaust gases is not operated, e.g., where the exhaust gas temperature is low, these troubles still tend to occur.
The phenomenon in which the quantity of fuel injected from the fuel injection valve decreases with rise in the fuel temperature will now be explained with reference to the experimental results shown in FIG. 1 of the accompanying drawings. In the Figure, the abscissa represents the fuel temperatures (.degree. C.) and the ordinate represents the rates of decrease of the fuel injection quantity at the different fuel temperatures, expressed as percentages, with the fuel injection quantity at a fuel temperature of 20.degree. C. being taken as 1.
Where gasoline is injected for a given time into the intake pipe of an engine through an electromagnetic fuel injection valve, if the fuel temperature rises from 20.degree. C. to 60.degree. C. when the vacuum in the intake pipe is 500 mmHg, the fuel quantity decreases by as much as 10%. When the intake pipe vacuum decreases so that it comes near to the atmospheric pressure, the rate of fuel decrease with rise in the fuel temperature decreases gradually as the vacuum changes to 420, 360 and 120 mmHg, respectively. Under high load operating conditions where the intake pipe vacuum is below 120 mmHg, there is practically no danger of troubles. The danger of troubles increases when the intake vacuum is near to 500 mmHg and the air-fuel ratio set to about 14:1 at 20.degree. C. leans out to about 16:1 at 60.degree. C. Presently, it is difficult to operate the engine at air-fuel ratios greater than 15:1 under idling conditions and the engine inevitably operates unstably.
On the other hand, when an engine equipped with a carburetor or electronically controlled fuel injection system (EFI) is started again under high temperature conditions, in the fuel supply pipe, the carburetor float chamber or in the injectors of the EFI system the density of the fuel is sometimes decreased by the high temperature and also the fuel is partially vaporized with the resulting further decrease in the density, thereby causing a phenomenon where the air-fuel mixture supplied to the engine is made leaner than is desired. This gives rise to a problem that the restarting performance is deteriorated and the engine idles unstably thus increasing the danger of the engine stalling.
Another problem is that with the exhaust emission regulations becoming severer in recent years, the idling air-fuel ratio is set as lean as possible and thus the danger of the engine stalling is increased further.