System for increasing the temperature of an air/fuel mixture delivered to an internal combustion engine

Abstract

 A system for utilising heat in the exhaust gas of an internal combustion engine to assist in vapourising the fuel in the air/fuel mixture supplied to the engine. To avoid the problems of overheating and cracking the fuel at large engine loads, the heating is effected by means of a connecting passage (30, 21) through which exhaust gas can be ingested into the intake passage (4). To control the exhaust gas ingestion rate at small loads, a restricted portion (19) can be provided in the connecting passage, in which the exhaust gas reaches sonic velocity at small throttle openings. A heat exchanger (23) can be provided through which the exhaust gas gives up some of its heat to the air/ fuel mixture prior to the ingestion therein. The homogeneity of the air/fuel mixture can be enhanced by the provision of obstructions in various configurations extending across the intake passage. Such obstructions are preferably heated to inhibit formation of pools of liquid fuel thereon. Control means (33) can be provided for controlling the flow of the exhaust gas through the connecting passage at higher engine loads or when the engine is fully warme up. Such control means can include pressure and temperature sensors (39 and 42) for effecting closure of the connecting passage where the intake manifold vacuum is less than a predetermined value and/or when the intake manifold temperature reaches a predetermines value.

Description

[0001] This invention relates to internal combustion engines, and in particular to a system for increasing the temperature of the supply of air/fuel mixture for combustion in such an engine to assist in vapourising the fuel component.

[0002] It has been known for several years now that, for example if the liquid gasoline fed to a multi-cylinder spark ignition engine is vapourised before entering the intake manifold then significant improvements in engine performance may be obtained. These improvements can be the result of two factors. Firstly vapourising the liquid fuel improves mixture homogeneity thereby allowing an engine to operate on leaner mixtures with improved thermal efficiency. Secondly if the heat that is used to vapourise the liquid fuel is available immediately on starting the engine then the warm-up time during which the engine is on choke is reduced. Reducing the time an engine is on choke is of particular assistance in reducing the exhaust emission from the engine but is also helpful in reducing fuel consumption. For optimum performance a fuel vapourising system should ensure thorough mixing of fuel and air even at light load conditions to provide a dry homogeneous mixture.

[0003] The hot exhaust gases produced by an engine are an obvious heat source to use for vapourising liquid fuel, but the difficulty for many years has been to use exhaust gas heat without cracking the fuel as a result of local overheating, or overheating the . charge as a whole, especially under full throttle conditions. A heat exchanger capable of achieving fast warm-up under engine idling conditions must have a large heat transfer service. For this reason it is found that with known exhaust gas heated systems, as a result of the large heat transfer surface, an increasing amount of heat is transferred to the mixture so that its temperature rises to a greater extent as engines speed and load are increased. This results from the considerable increase in exhaust gas temperature which occurs with increasing throttle opening. For example, the exhaust gas temperature of a petrol engine can typically vary from about 400 to 500 degrees C under idling conditions to about 800 degrees C at full throttle. As a result, at full power conditions, the mixture is hotter than desired. A number of undesirable consequences follow from this. Firstly, the density of the air/fuel mixture is reduced, resulting in a lower mass flow rate of mixture at any given engine speed, and hence a reduced maximum power output. Secondly, the hot mixture is more susceptible to detonation ("knock") especially when the engine is running under a large load, or even to ignition of the mixture whilst in the intake. Thirdly, cracking of the fuel may occur, leading to loss of power and deposits on surfaces within the intake. Another factor is the power loss which can be caused, especially towards full power operation, by the pipe friction losses in the heat exchanger. A large heat exchanger capable of transferring enough exhaust heatfor rapid warm-up will normally also cause a considerable pressure drop in the intake manifold. This results in reduced density of the air/fuel mixture, reduced mass flow rate.of the air/fuel mixture, and hence loss of power.

[0004] An ideal mixture heating system is thus one which rapidly transfers large amounts of heat to the air/fuel intake when an engine is idling following a cold start and that progressively varies the rate of transfer of heat to the mixture as power is increased in such a way that the increase in air/fuel temperature is not substantially more towards full power operation than at idling. At the same time the use of a large heat exchanger with consequent pressure loss in the intake system should be avoided. The invention seeks to provide a system that has the desired characteristic.

[0005] Accordingly, the present invention provides a system for increasing the temperature of an air/fuel mixture to assist in vapourising liquid fuel droplets contained therein prior to combustion in an internal combustion engine, said engine having an intake passage through which air can be drawn into the engine, a fuel metering device for introducing liquid fuel into air within the intake passage to produce an air/fuel mixture therein, and a throttle in the intake passage for controlling the rate of flow of air/fuel mixture through the intake passage, wherein a connecting passage is provided through which in use a proportion of the engine exhaust gas can flow from the engine into the intake passage at a location downstream of the throttle.

[0006] In an engine which has an exhaust duct on the same side as the intake passage, the connecting passage may link the intake passage and exhaust duct. In the event that the exhaust duct is on the opposite side of the engine to the intake passage, it may be preferable that the connecting passage links more or less directly from an engine exhaust port to the intake passage.

[0007] It is necessary to ensure that the exhaust gas does not mix with the air/fuel mixture at a temperature sufficiently high to cause the mixture to ignite, and desirably the exhaust gas temperature at entry to the intake passage in which the air/fuel mixture is present should not exceed about 600⁰ C at the most, while 200-400°C would be more acceptable for a variety of reasons.

[0008] In many instances it is desirable to provide means for cooling exhaust gas flowing through the connecting passage.

[0009] In a preferred arrangement, the means for cooling is a heat exchanger through which heat in the exhaust gas flowing through the connecting passage can be transferred to air or air/fuel mixture in the intake passage.

[0010] Conveniently the heat exchanger can be in the form of at least one heat transfer tube forming part of the connecting passage and passing through the intake passage. The heat transfer tube can be externally finned to increase the efficiency of heat transfer.

[0011] In an advantageous arrangement a proportion of the exhaust gas flowing through the said at least one heat transfer tube is returned to the exhaust duct through a return passage, only the remaining proportion flowing through the connecting passage into the intake passage.

[0012] Preferably the return passage communicates with the exhaust duct downstream of the said heat transfer tube, means being provided for inducing a pressure drop in the exhaust duct between the points where the heat transfer tube and the return passage communicate therewith.

[0013] Another convenient arrangement for cooling the exhaust gas and transferring heat to the air/fuel mixture prior to entry of the exhaust gas into the intake passage is to provide an extended duct forming part of the connecting passage and located within a wall of the intake passage.

[0014] The heat exchanger will normally be located in the intake passage downstream of the fuel metering device, but in some instances, eg to avoid the possibility of overheating fuel particles in the air/fuel mixture it may be preferable to locate the heat exchanger upstream of the fuel metering device, especially where the fuel is of a kind particularly sensitive to overheating. In a preferred embodiment the connecting passage has at least a portion which is of restricted cross-sectional area, so as to limit the flow rate of exhaust gas into the intake passage. It is envisaged that when the pressure in the intake passage downstream of the throttle is low, ie under conditions of light or no engine load, the flow rate of exhaust gas through the connecting passage will be limited by the gas reaching sonic velocity in such a restricted portion.

[0015] Preferably the restricted portion takes the form of one of more orifices through which the connecting passage opens directly into the intake passage so that the exhaust gas is injected with a high velocity to promote mixing with the air/fuel mixture.

[0016] Preferably the connecting passage opens into the intake passage just downstream of the throttle, to promote better mixing between the exhaust gas and the mixture.

[0017] Although normally the throttle will be downstream of the point of fuel entry, as when the fuel metering device is a carburettor of conventional form depending on the venturi effect to draw in fuel, it is also envisaged that the throttle might be placed upstream thereof, as may be the case with single point fuel injection. In the latter event the connecting passage can communicate with the intake passage upstream of the fuel metering device. This could be advantageous in some circumstances in that the hot exhaust gas can be well mixed with the cold air intake before introduction of the fuel, so as to avoid local overheating and cracking of some fuel droplets.

[0018] The invention will now be further described by way of example only with reference to the accompanying drawings, of which Figure 1 is a schematic sectional view showing a system in accordance with the invention, and

[0019] Figure 2 is a schematic sectional view showing a modification of the system shown in Figure 1.

[0020] Referring to Figure 1 there is shown a portion of an exhaust manifold 1 and of an intake passage for a multi-cylinder spark ignition internal combustion engine (engine not shown). The intake passage is defined by an upstream portion 2 in which there is provided a fuel metering device in the form of a conventional carburettor generally indicated as 3, and downstream thereof an intake manifold 4. The upstream section 2 and the intake manifold 4 are sealed together by means of a gasket 5.

[0021] The carburettor is of known form, in this instance a constant- depressiontype, in which a piston 6 carries a tapered needle 7 which can move vertically in a fuel jet 8 to meter fuel which is supplied by a fuel pipe 9. The piston 6 has a shoulder 10, whose lower side is exposed to atmospheric pressure while its upper side is exposed via a bore 11 through the piston 6 to the pressure obtaining within the venturi section 12 of the carburettor. The piston 6 thus moves upwards under the influence of the pressure forces against the action of a spring 13 and its own weight to increase the fuel flow rate through the jet 8 when the air mass flow rate increases with engine speed.

[0022] The carburettor 3 thus has the normal function of metering liquid fuel into the air stream at such a rate as to produce the desired air/fuel mixture, and any form of carburettor or other fuel metering device may be used which will achieve this effect.

[0023] In the upstream section 2, downstream of the carburettor 3 there is provided a variable throttle comprising a throttle plate 14 or butterfly valve pivotable (by means not shown) about a pivot point 15, for controlling the flow rate of the air/fuel mixture in the intake passage.

[0024] At the upstream end of the intake-manifold 4, ie just downstream of the throttle plate 14, there is formed an annular recess 16 in the inner wall of the intake manifold in which there is accommodated a ring 17. The ring 17 has one or more small radial holes 18 therein which can be aligned with a similar number of small radial holes 19 in the wall of the recess 16. The ring can be rotated by means (not shown) to bring the sets of holes 18, 19 into alignment or to blank off some or all of them from one another, for a purpose explained hereinafter. The holes 19 open into an annular chamber 20 formed in the wall of intake manifold 4, which chamber communicates with an axial passage 21 also formed in the wall of the intake manifold 4. The passage 21 communicates with a heat transfer tube 22 which passes diametrically through the intake manifold 4 and opens into the exhaust manifold 1. The exterior of the tube where it passes through the intake manifold is provided with a plurality of fins 23. The upper end of the tupe 22 is blanked off by a plug 24. The finned tube 22 acts as a heat exchanger to cool the exhaust gas prior to entering the air/fuel intake and to provide heat to the air/fuel mixture, and also promotes better mixing in the air/fuel mixture passing thereover.

[0025] In an alternative arrangement, where such additional mixing is not essential, cooling of the exhaust gas can be arranged merely by causing the e.xh' st gas to flow through a passage formed within the wall of the manifold 4, eg through an axial passage such as 21, by causing the gas to flow around an annular chamber such as 20, or through other configurations of passage in the wall, prior to entering the intake manifold.

[0026] In operation when the engine is running, air is drawn in through the downstream portion 2 and fuel is metered in the conventional manner by carburettor 3. The throttle plates 14 will be set to restrict the flow df air/fuel mixture to produce the desired engine speed, and the pressure in the intake manifold downstream of the throttle plate 14 will therefore be lower than that in the exhaust manifold 1.

[0027] As a consequence exhaust gas fro the exhaust manifold 1 is drawn up through the heat transfer tube 22 and injected into the intake manifold 4 through the passage 21, the chamber 20, and the holes 18, 19 located immediately downstream of the throttle plate. The air and liquid fuel droplets emerging from the throttle plate mix with the hot injected exhaust gas and then pass across the outer surface of the heat transfer tube 22, and the fins 23 which serve to increase the heat transfer rate.. Both.of these processes simulate increased evaporation of the fuel by transfer of heat thereto and promote increased physical mixing of the fuel vapour and the air.

[0028] When the engine is idling with the throttle plate 14 almost closed, the pressure in the intake manifold is low(typically less than about 15" Hg). Consequently it is possible to draw large amounts of exhaust gas into the intake system. The flow of exhaust gas is therefore limited by making the total cross-sectional area of each set of holes 18, 19 suitably small, so that flow through these holes will reach sonic velocity at light load conditions when the intake manifold 4 is at low pressure. As engine load increases so the intake manifold pressure rises and the flow of the exhaust gas into the intake manifold is reduced. In this manner, the problem of overheating the charge at high engine speeds and/or loads can be overcome. In the event that flow of the exhaust gas into the intake manifold is too high when full power conditions are reached then the ring 17 may be rotated to blank off some or all of the holes 18 in the ring from their corresponding holes 19 in the wall of the recess 16.

[0029] As an alternative, the rotatable ring 17 and the recess 16 can be dispensed with, so that the holes 19 open directly into the intake manifold 4. Further control of the flow of exhaust gas into the intake manifold can be provided, if desired, by means of a control valve which could conveniently be provided in the passage 21, or elsewhere in the flowpath by which exhaust gas is conveyed to the air/ fuel inlet passage.

[0030] A modification to the system shown in Figure 1, using such a control valve, is shown in Figure 2, wherein like reference numbers are used far like parts.

[0031] In the system shown in Figure 2, the single heat transfer tube 21 is replaced by a pair of finned heat transfer tubes 30, 31p each of which extends diametrically across the intake manifold 4. At their upper ends (as viewed in Figure 2) the heat transfer tubes 30, 31 communicate with each other through a.chamber 32 in the wall of the intake manifold . The passage 21 communicates directly with the chamber 32. At their lower ends (as viewed in Figure 2) the tubes 30, 31 communicate with the exhaust duct, the tube 30 at a point upstream from the tube 31. In the exhaust duct between the openings for the tubes 30, 31 there is provided an annular baffle for inducing a pressure drop between these two openings when exhaust gas is flowing through the duct. Exhaust gas is thus induced to flow up the tube 30 and down the return tube 31, hence transferring heat to the air/fuel mixture.

[0032] A control valve 33 actuated through a mechanism shown schematically at 34 is provided in the connecting passage 21, for the purpose of controlling the flow of exhaust gas therethrough. The mechanism 34 is actuable by a control means 35. The control means 35 includes a diaphragm-type pressure sensor which has a pressure input 36 at atmospheric pressure, and a pressure input 37 via a line 38 from a pressure tapping 39 in the inlet manifold 4. The control means 35 also has a temperature input 40 supplied via a line 41 from a temperature sensor 42 in the intake manifold.

[0033] The control means is such that the valve 33 is opened when the inlet manifold vacuum (corresponding to the difference in pressure input at 36 and 37) exceeds a certain value, and the temperature of the mixture as sensed by the sensor 42 exceeds a predetermined certain value (say about 60°C). If either of these two conditions is not satisfied, the valve 33 is closed by the control means 35 acting through the mechanism 34.

[0034] Exhaust gas can thus be ingested into the air/fuel mixture through the passage 21 and holes 18, 19 only under conditions when the intake manifold pressure is low, eg at idling during warm-up. Furthermore, overheating of the air/fuel mixture by ingestion of exhaust gas is prevented by automatic dosing of the valve 33 when the predetermined temperature is sensed by the sensor 42.

[0035] Heat is also supplied to the air/fuel mixture via the heat exchanger comprising the finned heat transfer tubes 30, 31. This arrangement thus provides for a continuous supply of heat to the air/ fuel mixture, which is supplemented at appropriate times by ingestion of a proportion of exhaust gas via passage 21.

[0036] In an alternative arrangement, the valve 33 can be situated in the tube 30 or the chamber 32 so as to control the flow of exhaust gas up tube 30 and down the return tube 31, as well as through the connecting passage 21.

[0037] There can be a plurality of pairs of tubes such as 30, 31 for upward and return flow of exhaust gas across the air/fuel intake and back to the exhaust duct. In that event the valve 33 may be arranged to control the flow through some or all of the tubes 31 or none of them.

[0038] If a plurality of tubes 30, 31 are provided they are preferably arranged in a staggered array to promote greater homogeneity of the air/fuel mixture.

[0039] To further promote mixture homogeneity there can be further provided one or more mixing elements, extending across the intake passage, preferably in staggered array and inclined to the tubes 30, 31. Such mixing elements can also be heated, to discourage the formation of pools of fuel on their surfaces.

[0040] As an alternative to the diaphragm-type pressure sensor, the control valve 33 might be controlled by a variety of means depending upon the object desired. For example, a mechanical linkage might be provided between the valve and the throttle plate 14, so that as the throttle is opened towards full power, the valve closes to reduce or shut off the supply of exhaust gas to the air/fuel inlet.

[0041] By way of a specific example, in one typical design substantially as described with reference to Figure 1, the internal diameter of the heat transfer tube 22 was approximately

" and the external diameter approximately 7/1611 The fins 23 attached to the outer surface of the tube had a total surface area approximately 10 times the surface area of the tube 22. With such a design, the mass flow rate of exhaust gas passing into the intake manifold at engine idling conditions would be approximately 12 per cent of the total exhaust gas flow from the engine and this would reduce progressively to 1 or 2 per cent at full power conditions.

[0042] An early trial embodiment of the invention when fitted to a multicylinder spark-ignition petrol engine provided a mixture temperature in the air/fuel intake at engine entry of 80°C under idling conditions, and 50°C at full load. It is envisaged that further development will achieve mixture temperatures of about 60°C at idle and 30-40°C at full load.

Claims

1. A system for increasing the temperature of an air/fuel mixture to assist in vaporising liquid fuel droplets contained therein prior to combustion in an internal combustion engine, said engine having an intake passage through which air can be drawn into the engine, a fuel metering device for introducing liquid fuel into the air within the intake passage to produce an air/fuel mixture therein, and a throttle in the intake passage for controlling the rate of flow of air/fuel mixture through the intake passage, characterised by a connecting passage (21, 22) through which in use a proportion of the engine exhaust gas can flow from the engine into the intake passage (2,4) at a location downstream of the throttle (14).

2. A system according to claim 1 characterised by means (22,23) for cooling exhaust gas flowing through the connecting passage.

3. A system according to claim 2 characterised in that the means for cooling comprises a heat exchanger (22,23) through which heat in the exhaust gas flowing through the connecting passage can be transferred to air or air/fuel mixture in the intake passage.

4. A system according toclaim 3 characterised in that the heat exchanger is in the form of at least one heat transfer tube (22) forming part of the connecting passage and passing through the intake passage.

5. A system according to claim 4 characterised in that the heat transfer tube is externally finned (23).

6. A system according to claim 4 or claim 5 characterised in that a proportion of the exhaust gas flowing through the said at least one heat transfer tube is returned to the exhaust duct through a return passage (31), only the remaining proportion flowing through the connecting passage (21) into the intake passage.

7. A system according to claim 6 characterised in that the return passage communicates with an exhaust duct (1) downstream of the said heat transfer tube, means (32) being provided for inducing a pressure drop in the exhaust duct between the points where the said heat transfer tube and the return passage communicate therewith.

8. A system according to claim 7 characterised in that the said means for inducing a pressure drop comprises a partial baffle (32) in the exhaust duct (1) between the points where the said heat transfer tube (30) and the return passage (31) communicate therewith.

9. A system according to claim 2 characterised in that the means for cooling comprises an extended duct (21) forming part of the connecting passage and located within a wall of the intake passage,

10. A system according to any one preceding claim characterised in that the connecting passage has at least a portion (19) which is of restricted cross-sectional area so as to limit the flow rate of exhaust gas into the intake passage,

11. A system according to claim 10 characterised in that the cross-sectional area of the restricted portion (19) is such that the exhaust gas reaches sonic velocity therein under conditions of light engine load.

12. A system according to claim 10 or claim 11 characterised in that the restricted portion takes the form of one or more orifices (19) through which the connecting passage opens directly into the intake passage.

13. A system according to any one of claims 10 to 12 characterised in that the connecting passage opens into the intake passage just downstream of the throttle (14).

14. A system according to any one preceding claim characterised by closing means (17,33) for closing the connecting passage.

15. A system according to claim 14 characterised in that the closing means comprises a rotatable ring (17) having at least one aperture (18) therein corresponding to the opening (19) of the connecting passage into the intake passage, rotation of the ring being effective to move the ring between a position in which an aperture is aligned with an opening and a position in which there is no such alignment.

16. A system according to claim 14 or claim 15 characterised in that the closing means comprise a valve (33) for controlling the flow of exhaust gas through the connecting passage.

17. A system according to any one of claims 14 to 16 characterised by control means (35) for controlling the closing means.

18. A system according to claim 17 characterised in that the control means comprises a pressure sensor (39) for sensing the pressure in the intake passage downstream of the throttle.

19. A system according to claim 17 or claim 18 characterised in that the control means comprises a temperature sensor (42) for sensing the temperature in the intake passage downstream of the throttle.

Drawing