WO2001086136A1

WO2001086136A1 - Induction and fuel delivery system for piston engine

Info

Publication number: WO2001086136A1
Application number: PCT/AU2000/001400
Authority: WO
Inventors: David Littlewood Johnson
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-05-11
Filing date: 2000-11-17
Publication date: 2001-11-15
Anticipated expiration: 2002-11-11
Also published as: ZA200209124B; AUPQ748100A0

Abstract

Apparatus forming part of the induction tract of a piston internal combustion engine in which a minor part of the flow of induction air passes through heat-exchange means (47, 48) in which it is heated by exhaust gases above the boiling temperature of the engine fuel; said fuel being heated in heat exchange means and an atomised flow being discharged into and completely evaporated in said minor part of the flow of induction air, the resultant fuel-air mixture being combined with and thoroughly mixed with the major part of the flow of induction air prior to its entry into the engine inlet ports; said major part of the flow of induction air being optionally heated to regulate the temperature of the combined flow of induction air.

Description

INDUCTION AND FUEL DELIVERY SYSTEM FOR PISTON ENGINE This invention relates to methods and apparatus for use in piston internal combustion engines for conditioning the induction airstream and for conditioning fuel discharged into the induction airstream.

The object of the present invention is to improve the fuel economy of piston internal combustion engines by providing means to control the temperature of the induction airstream and to control the temperature of fuel discharged into the induction airstream. A further object of the invention is to provide thorough mixing of the fuel with the induction airstream, thereby delivering, as nearly as possible, an homogenous mixture to each cylinder in an engine and allowing a more efficient burning of the fuel .

In the operation of gasoline-fuelled piston internal combustion engines, it is normal practice for fuel to be discharged into the induction airstream in the form of finely divided liquid droplets. Droplet delivery has the advantage that fuel in this form displaces a minimum of air, thereby making available a maximum of oxygen for combustion of the fuel in the engine cylinder. While some evaporation of the droplet fuel does occur during passage of the mixture through the induction tract and further evaporation occurs as a result of the adiabatic temperature rise associated with compression of the charge in the cylinder, part of the fuel remains in liquid droplet form at the onset of ignition. As ignition is initiated in the vapour content of the charge, it is therefore necessary to provide a greater quantity of fuel in the charge in order to ensure a sufficiently rich vapour mixture for ignition to reliably occur under all operating conditions. In conventionally carburetted engines, lean mixtures are provided for low power operation and additional fuel is released through power enrichment provisions during high BMEPs.

It has been demonstrated that, once a self- propagating flame front has been established in the cylinder, it will reliably propagate throughout the generality of a charge having a much leaner mixture strength than that required for reliable ignition to occur. Many systems have been proposed for exploiting this effect by providing a locally enriched mixture in the ignition region. Such charge stratification methods are well known in the art, but have not found favour because of the extra cost and complexity they entail. In some engines, reliability of ignition in lean mixtures has been improved by providing larger, more energetic or multiple ignition sources in each cylinder.

It is also well known in the art that, in engines in which the cylinders are supplied with mixture through a multi-branched inlet manifold, it is difficult to ensure an even distribution. As a result, some cylinders tend to receive a leaner mixture than others, making a compensatory increase in overall mixture strength necessary. Excepting where individual carburettors feed individual cylinders through straight inlet tracts, this effect is common in conventionally carburetted engines. The problem of unequal mixture distribution is not mitigated by throttle box fuel injection arrangements, which retain conventional inlet manifolding provisions.

Inequalities in mixture distribution as a result of induction tract inadequacies have been commonly overcome by injecting fuel in droplet form directly into the opening of the inlet port, from whence it is entrained in the induction airflow and carried into the cylinder. While port injection of a quantity of fuel accurately appropriate to the instantaneous operating parameters of an engine does mitigate the unequal mixture distribution problem, there is less opportunity for evaporation of the droplet fuel in the short travel distance involved and some will still remain in droplet form at the onset of ignition. Thus, absent charge stratification or more elaborate ignition provisions, a richer than necessary mixture strength is still required in order to ensure reliable ignition.

It is well known that diesel engines commonly operate at lambda figures exceeding 1.5. This means that in excess of 150% of the air required for a stoichiometric mixture is supplied to a cylinder. Such high lambda figures are a consequence of the timed injection of droplet fuel employed in the diesel engine and are largely responsible for its excellent fuel economy. In contrast, gasoline- fuelled engines of conventional arrangement seldom exceed lambda figures of 1.1, with lower figures occurring at higher powers. However, it has been shown that, in the same engines, a charge comprising dry gaseous fuels, for instance, propane, thoroughly mixed with air to provide an homogenous mixture will provide reliable ignition at lambda figures of 1.3 at relatively high BMEPs and there is evidence that operation up to lambda figures of around 1.5 can readily be achieved. While separate complete evaporation of gasoline fuel in the manner employed with liquefied petroleum gas fuel is not practical due to difficulties in vaporising certain essential additives contained in gasoline, there is evidence that, if gasoline can be fully evaporated in the induction airstream and thoroughly mixed with the charge air, operation at relatively high BMEPs is achievable at lambda figures similar to the best achievable with dry gaseous fuels. To achieve a high degree of evaporation of droplet fuel prior to entry of the mixture to the cylinder, heating of all or part of the charge air and, in some cases, the fuel, is necessary.

It is conventional belief that, in air breathing engines, charge temperature should be maintained as low as possible to provide increased charge density, thereby improving volumetric efficiency. It is also believed that the onset of detonation is delayed by maintaining a low charge temperature. It can be demonstrated that high charge temperatures may, in fact, be provided or tolerated with beneficial effect, with volumetric efficiency being restored in various ways and premature detonation not proving to be a problem.

According to the present invention, liquid gasoline fuel is preferably heated by engine waste heat in a suitable heat exchanger to a temperature close to its normal boiling point and is maintained in a pressurised state to prevent boiling off. Induction air is heated by exhaust gases in a suitable heat exchanger to a temperature well above the boiling point of gasoline. The heated fuel is supplied to suitable carburetion or fuel injection means, where it may undergo further heating, from which it is discharged into the heated induction air stream where it undergoes flash evaporation. The heated air stream constitutes only part of the induction airstream and the resultant mixture is diluted as required by further air to provide an induction airstream of controlled temperature. The induction airstream is mixed as thoroughly as possible before passing to the cylinders of an engine, effectively as an homogenous dry gas.

The benefits of the present invention, where fully realised, are achievement of reliable ignition at all phases of engine operation using leaner than normal mixtures, no reduction in power output, improved fuel economy and reduction of pollutants in the exhaust stream.

The various aspects of the present invention will be more readily understood by reference to the following description of preferred embodiments given in relation to the accompanying drawings in which:

Figure 1 is a longitudinal cross-sectional view through a horizontal plane of an exhaust manifold of an engine incorporating a simple form of the present invention;

Figure 2 is a longitudinal cross-sectional view through a vertical plane of the arrangement depicted in Figure 1; Figure 3 is a longitudinal cross-sectional view of a simple form of the present invention incorporated into the lower exhaust tract of an engine; Figure 4 is a transverse cross-sectional view of part of the arrangement depicted at Figure 3;

Figure 5 is a transverse cross-sectional view through a vertical plane of a what is proposed as a production form of the present invention incorporated into an engine;

Figure 6 is a longitudinal cross-sectional view through a horizontal plane of the arrange- ment depicted at Figure 5, with some components shown cut away;

Figure 7 is a longitudinal cross-sectional view through a vertical plane of the arrangement depicted at Figure 5 with certain components deleted and others shown in ghosted form;

Figure 8 is a longitudinal cross-sectional view of a mixer unit for use with the arrangement depicted at Figures 1 and 2. With reference to Figures 1 and 2, liquid gasoline fuel is preferably heated to around its unpressurised boiling point in a small shell and tube-type heat exchanger (not shown) of the type well known in the art using waste heat from the cooling system of an engine and is maintained in a pressurised state to prevent its boiling off. Said heated fuel is supplied through fuel supply line 1 to circular manifold 2 and thence to a plurality of modulatable fuel injectors 3 which discharge a flow of atomised fuel into plenum 4. Excess fuel is returned to the fuel system through return line 5. Air is supplied through suitable filtration means (not shown) to inlet duct 6 and thence to plenum 4, the inlet to said plenum being arranged such that said flow of air enters it more or less tangential ly, causing a rapid rotation of the air within said plenum, the discharge of atomised fuel from said fuel injectors being entrained in said rotating airflow within said plenum and undergoing flash evaporation without wetting out on the walls of said plenum. From said plenum, said fuel-air mixture passes via mixture inlet line 7 into heat exchange tubes 8. Total evaporation of all evaporable components of said gasoline occurs within said heat exchange tubes and the velocity of airflow through said heat exchange tubes is such as to retain all non-evaporable components of said gasoline in full entrainment. Exhaust manifold 10 is of the so-called log type, made in the form of a single length of suitable cross-sectional shape, hot exhaust gases passing from the exhaust ports of an engine cylinder head (not shown) , through short ducts 9 and into the interior of said exhaust manifold where it passes over said heat exchange tubes before being exhausted through outlet 11. Heated fuel-air mixture from said heat exchange tubes passes via mixture outlet line 12 to be discharged into throat 13 of venturi 14 situated in inlet manifold supply duct 15, the depression generated by said venturi acting to promote the flow of fuel-air mixture through said heat exchange tubes . Air entering said inlet manifold supply duct 15 first passes through suitable filtration means and is preferably heated by suitable heat exchange means (not shown) using engine waste heat to a temperature such that the temperature of the mixture flow downstream of said venturi may be as high as 200°C. Preferably, the heating of said airflow is controlled by thermostatic control means (not shown) sensing the temperature of the flow of mixture downstram of said venturi. Auxiliary fuel injector 41 is provided in the region of venturi 14 to provide fuel for starting and, if required, for acceleration and power enrichment purposes. A butterfly valve (not shown) is provided in manifold supply duct 15 to control the flow of fuel-air mixture through venturi 14.

With reference to Figure 8, a mixer unit 23 is provided upstream of the inlet manifold of an engine, said mixer unit comprising tubular body part 20 provided with attachment flanges 16, 19 at each end. Situated along the interior of said tubular body part are a plurality of short tubular mixing elements 17 made somewhat smaller in diameter than said tubular body part and orientated such that their flanged upstream ends are attached to the inner surface of said tubular body part. A plurality of holes 18 are provided in each said mixing element, the arrangement of said mixing elements and holes being such as to promote mixing of the air passing through said mixer unit with minimal impediment to flow. Mixture outlet line 12 passes more or less radially into the interior of said tubular body part and terminates in delivery tube 21 arranged coaxial ly with said tubular body part, the opening 22 of said delivery tube being situated just above the point of attachment of the first said mixing element such that a venturi effect is generated adjacent said opening, the resultant depression acting to promote a discharge flow from opening 22 and, thereby, the flow of fuel-air mixture through said heat exchange tubes. In an alternative embodiment (not shown), opening 22 is situated in the throat of a suitable venturi provided at the upstream end of said mixer unit, the depression generated within said venturi acting to promote the flow of fuel-air mixture through said heat exchange tubes.

With reference to Figures 3 and 4, a heat exchange unit is incorporated into the lower exhaust tract of an engine. In this arrangement, exhaust pipe 24 is connected to the main exhaust system of an engine (not shown) by flange 25, exhaust gases from said exhaust system passing along the annular space 28 between coaxially arranged exhaust gas duct 29 and inner mixture duct 30 and discharging through connection duct 31 connected to the lower part of said main exhaust system by flange 32. Positioned coaxially around said exhaust gas duct is outer mixture duct 33, said outer mixture duct being surrounded by coaxially arranged outer sheath 34, the annular space 35 between said outer mixture duct and said outer sheath preferably being filled with a suitable insulating material . Dump duct 26 is connected to exhaust pipe 24 and passes to the lower part of said main exhaust system. Dump valve 27, preferably in the form of a butterfly valve, is provided in said dump duct and its position is modulated to regulate the temperature of fuel-air mixture arriving at throttle butterfly valve 36 situated at the proximal end of said outer mixture duct to a temperature of approximately 200°C. A flow of air from suitable filtration means (not shown) enters the distal end 37 of said outer mixture duct and passes along the annular space 38 between said outer mixture duct and said exhaust gas duct and through the bore 39 of said inner mixture duct, one or more modulatable fuel injectors 42 positioned just inside the distal end of said outer mixture duct discharging a flow of atomised gasoline into said flow of air. Optionally, to promote its evaporation, said gasoline is first heated using engine waste heat in a suitable heat exchange unit (not shown) to a temperature around its unpressurised boiling point. In passing through annular space 38 and bore 39, said fuel-air mixture takes up heat from said exhaust gases passing through annular space 28, the degree of heating being regulated by the volume of exhaust gases dumped through dump duct 26. The final temperature of said fuel-air mixture may be as high as 200°C. Total evaporation of all evaporable components of said gasoline occurs within said annular space and said bore and the velocity of airflow is such as to retain all non-evaporable components of said gasoline in full entrainment. Preferably, the position of dump valve 27 is regulated by thermostatic means sensing the temperature of fuel-air mixture arriving at the distal end of said outer mixture duct. Said flow of fuel-air mixture debouches from the distal end of said outer mixture duct and into the inlet manifold (not shown) of an engine through opening 40. Auxiliary fuel injector 41 is provided just upstream of opening 40 and provides a flow of atomised gasoline for starting and, if required, for acceleration and power enrichment purposes. With reference to Figure 5, 6 and 7, the present invention is incorporated into the cylinder head of an engine (not shown) in what is proposed as a production form. The position of the cylinder head 43 and its rocker cover 44 are depicted in the figures ghosted in in fine line. In the arrangement depicted, the original exhaust manifold is replaced by one or more heat exchange units comprising casing 45, perforated end plates 46, 47 and heat transfer tubes 48. Inlet box 49 is provided between the end plates 46 of adjacent said heat exchange units and cold air inlet duct 50 conducts a flow of air to said inlet box. Preferably, the flow of air through inlet duct 50 first passes through suitable filtration means (not shown) and is regulated by control valve 52 situated in short duct 51, said short duct being connected to said inlet duct by flanges 53, 54 on their respective ends being sealingly fixed together. Formed on the ends of casings 45 between perforated end plates 47 and closure plates 55 are collector boxes 56, said collector boxes being connected to hot air transfer ducts 57 by collector ducts 58. Unions 59 are provided in said hot air transfer ducts close to said collector ducts to permit detachment of said hot air transfer ducts. The ends of heat transfer tubes 48 are sealingly fixed to apertures in perforated end plates 46, 47 of each said heat exchange unit and extend throughout the length of said heat exchange units. Said heat transfer tubes are preferably made convoluted, to provide increased heat transfer surface area and better accommodate thermal expansion, and from a suitable thin, heat resisting material. Short ducts 60 sealingly fixed to said cylinder head by flanges 83 conduct the flow of exhaust gases from the exhaust ports of said engine and into said heat exchange units, baffles and deflectors (not shown) being optionally provided within casings 45 to prevent excessive localised heating of said heat transfer tubes. From said heat exchange units, said exhaust gases pass out through apertures 61 provided in the outer surfaces of casings 45, into exhaust collector domes 62 sealingly fixed to the outer surface of said casings, through apertures 64 provided in the lower surfaces of said exhaust collector domes and into exhaust system branches 63, 65 sealingly connected to said exhaust collector domes, from whence they pass into exhaust pipe 66. Cold air entering through inlet duct 50 passes to inlet box 49, through heat transfer tubes 48 where it takes up heat from said exhaust gases, into collector boxes 56 and thence via collector ducts 58 to hot air transfer ducts 57. Said hot air transfer ducts pass above said engine to the intake manifold side and turn to become distributor ducts 68 running generally parallel to the axis of said engine. Preferably, said hot air transfer tubes and said distributor ducts are enclosed throughout their lengths by shrouds 69, the annular space between them preferably being filled with a suitable insulating material . Large feeder ducts 67 sealingly connected to said distributor ducts descend to sealingly join small feeder ducts 70 which are sealingly connected to plenums 71 into which modulatable fuel injectors 72 discharge a flow of atomised fuel. Preferably, said gasoline fuel is preheated using waste engine heat in a suitable heat exchanger unit and is supplied to said fuel injectors through fuel rail 74. Said fuel injectors are preferably being made from a suitable heat conductive material and take up heat from said plenums, further heating said f el. Said small feeder ducts preferably join said plenums tangential ly such that a rapid rotation is generated in the airflow passing through said plenums. Said fuel undergoes flash evaporation in the hot airstream discharged from said plenums and said air and vaporised fuel are mixed thoroughly as they passes though short mixture ducts 73 in the form of a high speed vortex and into the inlet ports of said engine cylinder head. Total evaporation of all evaporable components of said gasoline occurs within said short mixture ducts and the velocity of airflow is such as to retain all non-evaporable components of said gasoline in full entrainment.

Muff 75 encloses the lengths of casings 45 and exhaust collector domes 62 except for aperture 76, the outer surface of said muff being developed into warm air collector channel 78. Preferably, said muff and said warm air collector channel are insulated by a layer 77 of suitable insulating material on their external surfaces. Apertures 82 are provided in the upper surface of said warm air collector channel and short ducts 84 conduct a flow of warm air from said apertures into warm air duct 80. Preferably, said warm air duct is insulated throughout its length by a layer of insulation 79 on its outer surface. Said warm air duct passes across said engine to the inlet manifold side where it connects at its distal end 81 to the induction system air filtration means of said engine. Thermostatic control means sensing the temperature of induction air are provided to mix as required warm air from said warm air duct with cold ambient air. With particular reference to Figure 5, a throttle plate (not shown) operated by shaft 85 is provided in throttle box 89. Inlet manifold 86, the position 88 of which is denoted in broken line, connects said throttle box to mounting flange 87. Preferably, said inlet manifold comprises a plurality of runners in the form of long convolutions to achieve acoustic efficiency. In alternative arrangements (not shown), said engine is turbo-charged or supercharged and said charge heating provisions are arranged to provide less charge heating, thereby providing the same final charge temperatures as in the naturally aspirated arrangement .

Where volumetric efficiency is compromised by the said charge heating, the resultant power losses are preferably recovered through improved induction tract design, turbo-charging or supercharging.

To achieve a reduction of oxides of nitrogen as a result of charge heating, ignition advance is reduced, means to ensure thorough mixing of the fuel- air mixture are provided and maximum charge turbulence is generated through inlet port shaping, squish surfaces and combustion chamber shaping.

Where the fuel-air mixture is injected directly into the engine inlet ports effectively in the form of a dry gas gas, the maximum degree of mixing is generated by the design of plenums 71 and short mixture ducts 73 and maximum charge turbulence is generated through inlet port shaping, squish surfaces and combustion chamber shaping. Where they are employed, fuel injectors are of the conventional arrangement well known in the art and controlled by electronic means which reference throttle position, engine RPM, manifold air pressure, cylinder head temperature, exhaust oxygen and inlet air temperature in the manner well known in the art.

Where they are employed, air and fuel temperature regulation devices are preferably controlled by stepper motor-operated valves controlled by electronic means which reference, as appropriate, throttle position, engine RPM, manifold air pressure, cylinder head temperature and inlet air temperature in the manner well known in the art.

Because the fuel-air mixture is effectively utilised as a dry gaseous fuel, acceleration and power enrichment not normally required. Where acceleration or power enrichment proves necessary, this is provided by an auxiliary injector situated close to the throttle butterfly, for example, feature number 41 in Figure 3.

With particular reference to Figures 5 and 6, in an alternative embodiment, provision is made to inject fuel directly into the upstream ends of the three hottest heat transfer tubes 48 adjacent short ducts 60 from injectors (not shown) accommodated within inlet box 49.

Where said fuel-air mixture is heated to relatively high temperatures, it is diluted with cool ambient air in said induction tract to provide an optimum final charge temperature. Preferably, the temperature of said fuel-air mixture as it enters the engine cylinder head should be in the range 80°C to 120°C.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. Apparatus forming part of the induction tract of a piston internal combustion engine in which a minor part of the flow of induction air passes through heat-exchange means in which it is heated by exhaust gases to a temperature above the boiling temperature of the liquid fuel on which said engine is operated; in which said liquid fuel is heated in heat exchange means; in which an atomised flow of said heated liquid fuel is discharged into said minor part of the flow of induction air in which it undergoes complete evaporation, said evaporated fuel being entrained in and thoroughly mixed with said heated minor part of the flow of induction air; and in which the resultant mixture of heated air and evaporated fuel is combined with the major part of the flow of induction air prior to its entry into the engine inlet ports, said major part of the flow of induction air being optionally heated to regulate the temperature of the combined flow of induction air.

2. Apparatus according to Claim 1 in which said minor part of the flow of induction air is heated to a temperature in the range ambient to 500°C.

3. Apparatus according to Claim 1 in which said minor part of the flow of induction air is heated by up to 100°C above the boiling temperature of the liquid fuel on which said engine is operated.

4. Apparatus according to Claim 1 in which the temperature of said major part of the flow of induction air is regulated such that, when it is combined and mixed with said minor part of the flow of induction air, the temperature of the total flow of induction air is maintained above the condensation temperature of said fuel.

5. Apparatus according to Claim 4 in which the temperature of said total flow of induction air is maintained at least 20°C above the condensation temperature of said fuel.

6. Apparatus according to Claim 1 in which the temperature of said minor part of the flow of induction air is regulated by thermostatically- controlled valve means which control the rate of flow through and thereby the residence time in said heat exchange means .

7. Apparatus according to Claim 1 in which said greater part of the induction airflow is heated when required in a muff formed around said induction air heat exchange means.

8. Apparatus according to Claim 7 in which the temperature of said greater part of the induction airflow is regulated by thermostatically-controlled valve means which control the rate of flow through and thereby the residence time in said muff.

9. Apparatus according to Claim 1 in which said liquid fuel is heated in heat exchange means to a minimum of 15°C above its boiling point.

10. Apparatus according to Claim 1 in which said liquid fuel is heated by engine coolant in a suitable heat exchanger.

11. Apparatus according to Claim 1 in which said liquid fuel is heated by exhaust gases in a suitable heat exchanger.

12. Apparatus according to Claim 1 in which said liquid fuel is heated in a suitable shell and tube type heat exchanger.

13. Apparatus according to Claim 1 in which said liquid fuel is heated in a suitable plate type heat exchanger.

14. Apparatus according to Claim 1 in which said heated liquid fuel is maintained in a pressurised state until its atomisation through a nozzle or injector, thereby preventing its boiling off.

15. Apparatus according to Claim 1 in which said induction air heat exchange means comprise an elongate casing through the length of which pass one or more tubes , said exhaust gases passing through said casing around the exterior of said tubes and said minor part of the flow of induction air passing through said tubes and taking up heat from said exhaust gases.

16. Apparatus according to Claim 15 in which a single, long, convoluted tube passes through said casing.

17. Apparatus according to Claim 15 in which a plurality of more or less straight tubes pass from end to end through said casing.

18. Apparatus according to Claim 17 in which said tubes are made corrugated to provide greater heat transfer surface area.

19. Apparatus according to Claim 15 in which said casing is made in the form of two shorter parts, cool air entering through an inlet box positioned between adjacent ends of said casing parts and passing outwardly through said tubes to the ends of said casing parts.

20. Apparatus according to Claim 15 in which said casing is made in the form of three or more shorter parts.

21. Apparatus according to Claim 1 in which heated air from said induction air heat exchange means is conducted across the engine from the exhaust manifold side to the induction manifold side through tubular air transfer ducts.

22. Apparatus according to Claim 21 in which said air transfer ducts are clad externally with a thermal insulation material .

23. Apparatus according to Claim 21 in which said air transfer ducts are provided with disconnection means to facilitate their removal.

24. Apparatus according to Claim 1 in which the temperature of said combined flow of induction air is regulated by thermostatically-controlled valve means which mix cold ambient air with heated air.

25. Apparatus according to Claim 1 in which said minor part of the flow of induction air constitutes between 10 and 25 per cent of said total flow of induction air.

26. Apparatus according to Claim 1 in which a mixer unit is provided in the induction tract of said engine to thoroughly mix said vapourised fuel and said heated or unheated component parts of said total flow of induction air.

27. Apparatus according to Claim 26 in which said mixer unit comprises a tubular body part within the bore of which are coaxially fixed a plurality of short tubular elements having the same cross- sectional shape as said bore, having an axial length approximately that of the width or diameter of said bore, and having a cross-sectional area approximately 75% of that of said bore, each said tubular element being perforate, provided with a plurality of narrow slots or small apertures.

28. Apparatus according to Claim 1 in which air transfer ducts convey said minor part of said flow of induction air from said induction air heat exchange means to a plurality of feeder ducts which, in turn, convey said flow to a plurality of plenum chambers into each of which a modulatable fuel injector discharges a flow of atomised fuel, the fuel-air mixture so generated discharging into and mixing with the main induction airflow.

29. Apparatus according to Claim 28 in which said feeder ducts enter said plenum chambers tangential ly, thereby generating a high speed vortex within each, said vortices promoting rapid mixing of said fuel-air mixture.

30. Apparatus according to Claim 28 in which said plenum chambers and said modulatable fuel injectors are made substantially from metal and said modulatable injectors are heated by conduction from said plenum chambers and thereby add heat to the fuel passing through them.

31. Apparatus according to Claim 17 in which fuel is discharged into the upstream ends of the hottest tubes in said induction air heat exchange means .