Gas and liquid contact apparatus – Fluid distribution – Valved
Patent
1989-11-01
1992-04-28
Miles, Tim
Gas and liquid contact apparatus
Fluid distribution
Valved
261 502, 261DIG38, F02M 714
Patent
active
051086646
DESCRIPTION:
BRIEF SUMMARY
This invention relates to carburetor metering systems for supplying a fuel/air mixture.
It is known that substantial advantages are to be obtained, in terms of part load fuel economy and decrease of exhaust pollution, by operating a spark ignition engine with a fuel/air mixture having excess air over that required for just complete combustion of the fuel, that is with a lean mixture of fuel in air. Conventionally engines can readily tolerate excess fuel in the fuel/air mixture to a considerable degree. However, lean mixture operation requires precise control of mixture strength to ensure reliable operation without misfiring. Thus conventional carburetor system are generally unsuitable for supplying engines operating at lean mixture strengths.
A carburetor system suitable for supplying lean mixtures of fuel in air is disclosed in applicant's British Patent Specification No. 1,595,315 and corresponding U.S. Pat. No. 4,290,401. This carburetor comprises an evaporator for evaporating the fuel into a stream of air and a closed-loop control arrangement for maintaining the mixture strength at a required value in dependence on the temperature drop measured across the evaporator. Whilst such a carburetor is capable of operating adequately in a lean burn system, it has a fairly slow response time, set by the response time of the thermometric sensors, which can render the engine sluggish in operation. Robust, inexpensive thermometric sensors have response times of several seconds. More rapidly responding sensors are frail and expensive.
It is an object of the invention to provide a carburetor metering system capable of accurately controlling fuel flow over a wide range of air flows and mixture strengths.
According to the present invention, there is provided a carburetor metering system comprising a fuel valve for injecting liquid fuel into a stream of air, the valve comprising a valve member movable along a displacement axis with respect to a valve seat to vary the fuel flow through the valve, the valve member and the valve seat having substantially parallel surfaces which are inclined with respect to the displacement axis and which define therebetween a passage for the flow of fuel having a width corresponding to the distance apart of the surfaces and a length corresponding to the degree of overlap of the surfaces, the length of the passage varying substantially in proportion to the width as the valve member is moved along the displacement axis.
The system of the invention has been developed after detailed study of the mechanisms which affect fuel flow rates in conventional carburetors. As is well known, a pressure difference induced by the air flow is generally used to drive fuel through the metering orifice, and the fuel flow rate is caused to vary in dependence on the air flow rate. However, the fuel flow rate tends to vary unpredictably with air flow rate due to the fact that there are two different mechanisms which determine the relevant pressure differentials, one of which arises from the viscosity of the fluid and the other of which is dependent on momentum changes of the fluid. The relative magnitude of the two mechanisms varies with temperature and pressure, as well as with fluid flow and the proportions of the flow passage. Furthermore the relative magnitude of the two mechanisms is different for fuel and air in view of the different volume flow rates of fuel and air. The system of the invention is chosen so as to minimise the effects of momentum changes in the fuel orifice.
In a development of the invention the system further comprises an air control valve for adjusting the valve member to vary the fuel flow through the fuel valve on dependence on the air flow. Preferably the geometry of the air control valve is chosen such that the effect of the viscosity of the air on the pressure difference across the valve is negligible. Furthermore the relative geometries of the throughflow orifice of the fuel valve and the air control valve are preferably such that, with a constant pressure difference across the air
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Miles Tim
White John P.
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