Internal-combustion engines – Charge forming device – Liquid fuel evaporating by extended fuel film
Patent
1995-02-21
1996-10-15
Okonsky, David A.
Internal-combustion engines
Charge forming device
Liquid fuel evaporating by extended fuel film
123337, 481896, 261107, F02M 2504
Patent
active
055643990
DESCRIPTION:
BRIEF SUMMARY
This invention relates to carburettor metering systems and is concerned more particularly, but not exclusively, with carburettor metering systems for supplying air/fuel mixture to small gasoline engines, whose exhaust emissions are subject to legislative control.
Small engines are typically single cylinder four-stroke engines of low cost, and are used in applications such as lawnmowers and outboard motors. Such single cylinder engines draw in air/fuel mixture intermittently, and this causes problems in fuel metering which are not present in multi-cylinder engines such as are typically used in the automotive field. Furthermore, small engines, unlike automotive engines, operate either at a governor controlled speed or with a fixed relationship between speed and load, and usually have fixed ignition timing. Existing carburettor metering systems for such small engines have a tendency to produce inhomogeneous air/fuel mixtures containing unevaporated fuel droplets which tend to increase the quantity of hydrocarbons in the exhaust emissions. It is also known that a homogeneous mixture allows operation with a weak air/fuel mixture, at part load, which has the advantage of reducing emissions of oxides of nitrogen and of carbon monoxide.
Generally carburettor metering systems produce a pressure difference related to air flow, and use this pressure difference to propel fuel from a constant pressure container into the air flow, usually as a more or less atomised spray. In order to produce an air/fuel mixture of highly uniform consistency, with the above mentioned benefits in terms of exhaust emissions, it is known to supply the fuel to a fabric wick through which the warmed air is passed to produce a substantially dry fuel vapour. In such systems, however, it is preferable that the power control throttle is placed downwind of the wick so that there are no significant pressure changes at the wick when the power demand changes such as might lead to large transient increases or decreases in mixture strength as the throttle is closed or opened.
Because of the position of the throttle in such systems, it is not possible to use a conventional venturi carburettor metering system which relies on the throttle plate to play a part in metering at low load. Furthermore the alternative of a constant depression, variable geometry carburettor metering system (typified by the well known S.U. carburettor) is complex and precluded in small engine applications on the ground of cost.
There is an optimum mixture strength for each load, which minimises exhaust emissions. For governed engines, or engines with a fixed relationship between speed and load, the mixture quantity varies in a known way with load, and can thus be used to control mixture strength. The optimum relationship is effected by engine design and must be determined by experiment. Current legislation in California specifies emission of carbon monoxide, and of combined hydrocarbons and oxides of nitrogen. In lean operation, with air/fuel ratios greater than about 17:1, the nitrogen oxide emissions fall as the mixture is made leaner, while, at very lean mixtures, hydrocarbon emissions start to rise. Carbon monoxide emissions are low and practically constant for ratios greater than about 16:1. The result is that the total emissions are low over a range of mixture strengths, which allows reasonable tolerance in approximating the optimum.
Legislation specifies limits based on tests at idle, quarter load, half load, three quarter load, and full load. For these conditions a typical engine might require air/fuel ratios of about 17:1 at idle, 18:1 at quarter load, 19:1 at half and three quarter loads, and 12:1 at full load. This latter rich mixture is to obtain full load while keeping total emissions as low as possible. The richer mixture at low load is needed, in part, because ignition timing is fixed.
The invention seeks to provide a novel carburettor metering system which is particularly suitable for such an application.
According to the present invention there is provided a carbu
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Okonsky David A.
Phillips Peter J.
White John P.
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