GB2036175A

GB2036175A - Air/fuel mixture vaporizer using exhaust gas heat by direct measuring

Info

Publication number: GB2036175A
Application number: GB7938740A
Authority: GB
Original assignee: UK Secretary of State for Industry
Current assignee: UK Secretary of State for Industry
Priority date: 1978-11-23
Filing date: 1979-11-08
Publication date: 1980-06-25
Also published as: EP0012533A1; JPS55101757A; ES486234A1

Abstract

Air or air/fuel mixture is heated by the controlled addition of hot exhaust gas via apertures 1 8 in which the exhaust gas reaches sonic velocity at small throttle openings. A heat exchanger 23 including mixing devices can be provided through which the exhaust gas gives up some of its heat to the air/fuel mixture prior to the ingestion therein. Control means can include pressure and temperature sensors for effecting closure of the passage feeding the apertures 18 where the intake manifold vacuum is less than a predetermined value and/or when the intake manifold temperature reaches a predetermined value or alternatively be responsive to throttle position. <IMAGE>

Description

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SPECIFICATION

Improvements in or relating to internal combustion engines

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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 10 engine to assist in vapourising the fuel component.

It has been known for several years now that, for example if the liquid gasoline fed to a multi-cylinder spark ignition engine is vapour-15 ised 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 20 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 25 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 con-30 sumption. 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.

35 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 40 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 45 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 50 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 55 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 60 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, 65 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 70 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 capa-75 ble of transferring enough exhaust heat for 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 80 the air/fuel mixture, and hence loss of power.

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 85 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 90 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.

Accordingly, the present invention provides 95 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 100 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 105 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 110 downstream of the throttle.

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 115 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.

120 It is necessary to ensure that the exhaust gas does not mix with the ait/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 125 in which the air/fuel mixture is present should not exceed about 600°C at the most, while 200-400°C would be more acceptable for a variety of reasons.

In many instances it is desirable to provide 130 means for cooling exhaust gas flowing

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through the connecting passage.

in a preferred arrangement, the means for cooling is a heat exchanger through which heat in the exhaust gas flowing through the 5 connecting passage can be transferred to air or air/fuel mixture in the intake passage.

Conveniently the heat exchanger can be in the form of at least one heat transfer tube forming part of the connecting passage and 10 passing through the intake passage. The heat transfer tube can be externally finned to increase the efficiency of heat transfer.

In an advantageous arrangement a proportion of the exhaust gas flowing through the 15 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.

20 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 25 tube and the return passage communicate therewith.

Another convenient arrangement for cooling the exhaust gas and transferring heat to the air/fuel mixture prior to entry of the exhaust 30 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.

The heat exchanger will normally be located 35 in the intake passage downstream of the fuel 1 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 40 the fuel metering device, especially where the 1 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 45 limit the flow rate of exhaust gas into the 1

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 50 gas through the connecting passage will be 1 limited by the gas reaching sonic velocity in such a restricted portion.

Preferably the restricted portion takes the form of one of more orifices through which 55 the connecting passage opens directly into the 1 intake passage so that the exhaust gas is injected with a high velocity to promote mixing with the air/fuel mixture.

Preferably the connecting passage opens 60 into the intake passage just downstream of 1 the throttle, to promote better mixing between the exhaust gas and the mixture.

Although normally the throttle will be downstream of the point of fuel entry, as when the 65 fuel metering device is a carburettor of con- 1

ventional 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.

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

Fig. 2 is a schematic sectional view showing a modification of the system shown in Fig. 1.

Referring to Fig. 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.

The carburettor is of known form, in this instance a constant-depression type, 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 i 3 and its own weight to increase the fuel flow rate through the jet 8 when the air mass flow rate increases with engine speed.

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.

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.

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

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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 5 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 10 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 15 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 pro-20 vided with a plurality of fins 23. The upper end of the tube 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 25 heat to the air/fuel mixture, and also promotes better mixing in the air/fuel mixture passing thereover.

In an alternative arrangement, where such additional mixing is not essential, cooling of 30 the exhaust gas can be arranged merely by causing the exhaust 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 35 annular chamber such as 20, or through other configurations of passage in the wall, prior to entering the intake manifold.

In operation when the engine is running, air is drawn in through the downstream portion 2 40 and fuel is metered in the conventional manner by carburettor 3. The throttle plates 14 will be set to restrict the flow of air/fuel mixture to produce the desired engine speed, and the pressure in the intake manifold down-45 stream of the throttle plate 14 will therefore be lower than that in the exhaust manifold 1.

As a consequence exhaust gas from the exhaust manifold 1 is drawn up through the heat transfer tube 22 and injected into the 50 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 55 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 60 transfer of heat thereto and promote increased physical mixing of the fuel vapour and the air.

When the engine is idling with the throttle plate 14 almost closed, the pressure in the intake manifold is low (typically less than 65 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 70 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 75 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 80 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.

85 As an alternative, the rotatable ring 1 7 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 90 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.

95 A modification to the system shown in Fig.

1, using such a control valve, is shown in Fig.

2, wherein like reference numbers are used for like parts.

In the system shown in Fig. 2, the single 100 heat transfer tube 21 is replaced by a pair of finned heat transfer tubes 30, 31, each of which extends diametrically across the intake manifold 4. At their upper ends (as viewed in Fig. 2) the heat transfer tubes 30, 31 commu-105 nicate 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 Fig. 2) the tubes 30, 31 communicate with 110 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 115 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.

120 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 125 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 130 manifold 4. The control means 35 also has a

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temperature input 40 supplied via a line 41 from a temperature sensor 42 in the intake manifold.

The control means is such that the valve 33 5 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 10 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.

Exhaust gas can thus be ingested into the 15 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 20 gas is prevented by automatic closing of the valve 33 when the predetermined temperature is sensed by the sensor 42.

Heat is also supplied to the air/fuel mixture via the heat exchanger comprising the finned 25 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. 30 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. 35 There can be a plurality of pairs of tubes 1 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 40 through some or all of the tubes 31 or none 1 of them.

If a plurality of tubes 30, 31 are provided they are preferably arranged in a staggered array to promote greater homogeneity of the 45 air/fuel mixture. 1

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 50 inclined to the tubes 30, 31. Such mixing 1 elements can also be heated, to discourage the formation of pools of fuel on their surfaces.

As an alternative to the diaphragm-type 55 pressure sensor, the control valve 33 might 1 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 60 the throttle is opened towards full power, the 1 valve closes to reduce or shut off the supply of exhaust gas to the air/fuel inlet.

By way of a specific example, in one typical design substantially as described with refer-65 ence to Fig. 1, the internal diameter of the 1

heat transfer tube 22 was approximately '

3/8" and the external diameter approximately 7/16". The fins 23 attached to the outer surface of the tube has 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.

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, 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.

2. A system according to claim 1 including means for cooling exhaust gas flowing through the connecting passage.

3. A system according to claim 2 wherein 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.

4. A system according to claim 3 wherein the heat exchanger is in the form of at least one heat transfer tube forming part of the connecting passage and passing through the intake passage.

5. A system according to claim 4 wherein the heat transfer tube is externally finned.

6. A system according to claim 4 or claim 5 wherein 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.

7. A system according to claim 6 wherein the return passage communicates with an exhaust duct downstream of the said heat

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transfer tube, means 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 5 therewith.

8. A system according to claim 7 wherein the said means for inducing a pressure drop comprises a partial baffle in the exhaust duct between the points where the said heat trans-

10 fer tube and the return passage communicate therewith.

9. A system according to claim 2 wherein the means for cooling comprises an extended duct forming part of the connecting passage

15 and located within a wall of the intake passage.

10. A system according to any one preceding claim wherein the connecting passage has at least a portion which is of restricted

20 cross-sectional area so as to limit the flow rate of exhaust gas into the intake passage.

11. A system according to claim 10 wherein the cross-sectional area of the restricted portion is such that the exhaust gas

25 reaches sonic velocity therein under conditions of light engine load.

12. A system according to claim 10 or claim 11 wherein the restricted portion takes the form of one or more orifices through

30 which the connecting passage opens directly into the intake passage.

13. A system according to any one of claims 10 to 12 wherein the connecting passage opens into the intake passage just down-

35 stream of the throttle.

14. A system according to any one preceding claim comprising closing means for closing the connecting passage.

1 5. A system according to claim 14

40 wherein the closing means comprises a rotata-ble ring having at least one aperture therein corresponding to the opening of the connecting passage into the intake passage, rotation of the ring being effective to move the ring

45 between a position in which an aperture is aligned with an opening and a position in which there is no such alignment.

1 6. A system according to claim 14 or claim 1 5 comprising a valve for controlling

50 the flow of exhaust gas through the connecting passage.

17. A system according to any one of claims 14 to 16 comprising control means for controlling the closing means.

55 18. A system according to claim 17 wherein the control means comprises a pressure sensor for sensing the pressure in the intake passage downstream of the throttle.

19. A system according to claim 1 7 or

60 claim 18 wherein the control means comprises a temperature sensor for sensing the temperature in the intake passage downstream of the throttle.

20. A system substantially as hereinbefore

65 described with reference to Fig. 1 of the accompanying drawings.

21. A system substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.—1980.

Published at The Patent Office, 25 Southampton Buildings,

London, WC2A 1AY, from which copies may be obtained.