The present invention relates to a fuel supply system and more particularly to a system for optimising the supply of air/fuel mixture to the individual cylinders of an internal combustion engine.

in Otto-cycle i.e. engines, the mixture distribution to the cylinders via the inlet manifold depends on a number of factors including engine speed and location of cylinder (i.e. whether it is an inner cylinder or an outer cylinder); in addition it depends strongly on the temperature of the inlet manifold, which can be heated by heat-exchange with the exhaust gases or the cooling medium of the vehicle engine. However, the mixture distribution required for optimal operation of the engine depends on different factors in different ways so that the operation of the system and the design of the inlet manifold involve compromises.

The present invention seeks to provide a fuel supply system which operates as well as possible over a wide range of operating conditions.

According to a first aspect of the present invention there is provided a fuel supply system for an internal combustion engine comprising air/fuel mixture inlet means to the cylinders and means for heating at least part of the inlet means, characterised in that means are provided for controlling the rate at which heat is supplied by the heating means to the inlet means in dependence on one or more engine operating parameters.

An advantage of this system is that control of the temperature of the inlet means enables an optimum

mixture distribution to the cylinders to be achieved under a wide range of operating conditions.

Preferably, values representing the optimal temperatures of the inlet means for different engine operating points are stored and appropriate values are selected for the controlling means. Thus, after adapting the fuel supply system to the i.e.engine, the inlet means is readily maintained at an optimal temperature so as to minimise A - deviations.

In a preferred embodiment the heating means comprises a heat exchanger and the controlling means controls the rate of flow of the. heating fluid through the heat exchanger. The heating fluid may be the coolant of the cooling system of the vehicle.

The heat exchanger may be made integral with the inlet means. Thus the inlet means may comprise an air/fuel mixture inlet passage surrounded by a first generally cylindrical wall which is surrounded by one or more passages for the heating fluid and then a second generally cylindrical wall. Preferably the first wall is thin relative to the second wall. Preferably the first wall has high thermal conductivity and a low thermal capacity, whilst the second wall is of thermally insulating material.

An advantage of this heat exchanger is that the temperature of the inlet means can be quickly changed to match the existing speed-load conditions of the motor.

The inlet means is preferably an inlet manifold.

According to a second aspect of the present invention there is provided air/fuel mixture inlet means for an internal combustion engine comprising an inlet passage for the air/fuel mixture surrounded by a wall, characterised in that the wall is of relatively thin material with high thermal conductivity and low thermal capacity and is surrounded at a spacing by a further wall of relatively thick material and defining with the first wall one or more passages for the flow of a heating medium. The further wall is preferably of a thermally insulating material.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

Fig.l is a block diagram of a fuel supply system with heating of the inlet manifold;

Fig.2 is a cross-section of a preferred inlet manifold pipe;

Figs.3a_, b_ and c_ are graphs showing the air-fuel ratios supplied to the individual cylinders of an i.e. -engine under different conditions of inlet anifiold heating;

Fig.4a_ is a schematic diagram of an inlet manifold; and

Fig.4b is a series of graphs showing the air-fuel ratios supplied to the individual cylinders of an i.e.engine at different engine speeds but without the heating control of the present invention.

Fig.l shows a motor fuel supply system 10 in which an air/fuel mixture is supplied from a carburettor or

central injection unit 11 to an inlet manifold 20. The manifold is heated by engine coolant and its temperature is detected by a sensor 16. The manifold 20 distributes the mixture to the individual cylinders of a motor 12 and the exhaust gases leave at point 13.

The exhaust gases are analysed at 14, and any deviations from the stoichiometric ratio (i.e. deviations of the air/fuel ratio - from its ideal value of unity) are detected by an electrical/mechanical controller 15 so that appropriate correcting action can be taken. The controller also receives inputs from the manifold temperature sensor 16 and from the motor 12. Among the operating parameters detected at the motor 12 and fed to the controller 15 are the ^-.-values of the individual cylinders.

When adapting the induction system to the engine of a car, the optimal temperatures of the engine coolant are determined as a function of the engine operating parameters. These values are stored as a set of parameters. In use, these optimal temperature values for each operating point can be obtained by controlling the flow rate of engine coolant.

Thus, in dependence on its various inputs, the controller 15 controls the rate of flow of the heating fluid to the inlet manifold 20. The adjustment is effected by a control member, e.g. a valve 21, in the passage from the heating fluid supply 22. By controlling the rate of flow f , the temperature of the inlet manifold wall and hence of the mixture therein is also controlled. With optimal inlet manifold temperature the ^ -deviations of the individual cylinders become minimal, to ensure optimal running of the engine over wide ranges of operating conditions.

To enable a quick response to be provided to a change in control requirements, there is preferably used an inlet manifold having a portion with a cross-section as shown in Fig.2. The air/fuel induction passageway 31 is surrounded by a thin wall 32 of good thermal conductivity and low thermal capacity so that the contents of passageway 31 are in good thermal communication with the heating fluid in surrounding passageways 33. The outer wall 34 of the passageways is relatively thick and is preferably of thermally insulating material. An advantage of this manifold is that quick temperature changes thereof may be effected to quickly match the temperature to the current speed- load conditions of motor 12.

Figs 3a,3b and 3c respectively show the Λ-values of the four cylinders of an i.e. engine with the inlet manifold too strongly heated, unheated and optimally heated.

Before describing Fig.3 in further detail, reference is made to Fig.4a_, which shows the transport of the air/fuel mixture in a siamese-type inlet manifold 20. The mixture is supplied from a first distribution point 41 via secondary distribution points 42,43 to the four cylinders which fire in the order 1-3-4-2.

Because of various effects resulting from the firing sequence, and thus the order of suction, an air-fuel ratio deviation arises between the inner and the outer cylinders. The manifold comprises two symmetrical halves and during the suction phases of one half, the other half of the manifold is inactive. There is always a thin film of fuel stored on the wall of the manifold, and in the inactive half of the manifold much of this film evaporates. During the following suction

phase, the inner cylinder, which comes first in the suction sequence, receives most of the vaporised fuel and therefore a much richer mixture than the outer cylinder. The lower the r.p. . of the engine the longer is the period of suction-inactivity and the more of the wall film mass can evaporate. Thus the resulting air/fuel deviation between the inner and the outer cylinder increases. This low speed behaviour is illustrated at the left of Fig.4, with Fig.4b showing the corresponding air/fuel ratio deviations.

At higher speeds, illustrated at the right of Fig. , a centrifugal effect dominates. The liquid fuel is atomized into droplets, when passing through the injection valve. The smaller droplets are quickly vaporized and transported with the airstream even around sharp edges. The bigger droplets are thrown off course by centrifugal forces and enter the outer cylinders. This is called the centrifugal effect and causes the mixture of the outer cylinders to become richer and that of the inner ones to become leaner. The higher the r.p. . the greater is this effect.

Thus, returning to Fig.3, at low engine speeds, e.g. 2000 Rev/min, with too high a temperature. Fig 3a_, the two inner cylinders become richer than the outer cylinders. With an unheated manifold, Fig.3^, and under the influence of internal guide barriers or ribs 44 which force part of the flow of the wall film in a predetermined direction, the fuel tends to flow to the outer cylinders so that they become too rich. Ribs 44 are typically a few millimetres high. With an optimum inlet manifold temperature with a controlled operating point, ^" a very good mixture distribution can be achieved, see Fig 3c_.

An advantage of the above-described system is that it can compensate for both suction effects at low engine speeds and centrifugal effects at high engine speeds. It can be applied to any model or make of car without major redesign work being necessary.

Various modifications may be made to the above-described arrangement. For example instead of being heated by the engine coolant, the inlet manifold may be heated by part of the exhaust gases from the engine, with a suitable control element in the exhaust system. Alternatively a separate controllable heating means may be specifically provided for the inlet manifold.

The engine may have any number of cylinders. The cross-section shown in Fig.2 may be of a common part of the inlet manifold for supplying air/fuel mixture to all the cylinders in turn, or one of a plurality of similar respective parts for the individual cylinders. Alternatively it may represent a section of any other part of the pipe system between the carburettor and the cylinders.