GB2328248A - Apparatus for preconditioning hydrocarbon fuel for i.c. engines - Google Patents

Abstract

The apparatus comprises a housing 112 arranged to be heated by the exhaust manifold 122 and having supply and discharge openings. A plurality of baffles 114 is disposed within the housing 112 to divide the heated interior of the housing 112 into a sinuous gas flow path leading from the supply opening to the discharge opening. Fuel in liquid or vapour form is introduced into the housing 112 by way of the supply opening and the preconditioned fuel from the discharge opening is delivered to the engine intake system. A hole (123, fig.3) may be provided between the housing 112 and the exhaust manifold 122. The fuel is heated for a sufficient time and at a sufficient temperature while traversing the sinuous gas flow path for the chemical composition of a substantial proportion of the fuel to be modified before it is introduced into the air intake system of the engine. The nature of the chemical reaction will depend on whether the engine is running on a lean or rich mixture.

Description

Engine Fuel System Field of the invention The present invention relates to an apparatus for preconditioning hydrocarbon fuel to be supplied to an internal combustion engine.

Background of the invention It has previously been proposed in an engine having an external exhaust gas recirculation (EGR) system to meter fuel not only into the air drawn into the engine by way of the intake system but also into the EGR pipe so that part of the fuel may be vaporised within the EGR pipe. It is also known to pass fuel through a heat exchanger to precondition it before it is introduced into the air intake system of the engine.

The Applicants have previously proposed in copending PCT Application No. PCT/GB97/01187 to ensure that air is present in the EGR gases to mix with the fuel vapour in the EGR pipe and to make the section of the EGR pipe lying between the point at which the fuel vapour is mixed with the air and the point at which the resulting mixture enters the intake system sufficiently long, and its temperature sufficiently high, to enable a proportion of the fuel vapour to be oxidised thermally by the air in the mixture to produce a substantial quantity of partial oxidation products in the EGR pipe.

Such preconditioning of the fuel is known to improve ignition and reduce the duration of combustion in an internal combustion engine but hitherto no practical solution has been proposed for heating the fuel. The slow thermal reaction between the fuel and the air requires constant application of heat over a prolonged length of time and merely insulating the EGR pipe, as earlier suggested, does not suffice to precondition the fuel.

US-A-4,074,661 proposes passing the mixture through a heat exchanger that is heated by a separate burner, but this is not acceptable as it is wasteful of fuel. This is also not an effective solution because the mixture does not dwell for a sufficient length of time in the heat exchanger.

Summary of the invention With a view to mitigating at least some of the foregoing disadvantages, the present invention provides an apparatus for preconditioning hydrocarbon fuel to be supplied to an internal combustion engine, comprising a housing arranged to be heated by heat derived from the engine exhaust gases and having supply and discharge openings, and a plurality of baffles disposed within the housing to divide the heated interior of the housing into a sinuous gas flow path leading from the supply opening to the discharge opening, means for introducing fuel in liquid or vapour form into the housing by way of the supply opening and means for delivering preconditioned fuel from the discharge opening to the engine intake system, the fuel being heated for a sufficient time and at a sufficient temperature while traversing the sinuous gas flow path for the chemical composition of a substantial proportion of the fuel to be modified before it is introduced into the air intake system of the engine.

In the invention, the heat for preconditioning of the fuel is derived from the exhaust gases, that is to say from waste heat generated by the engine. Furthermore, the thermal conditions necessary for the chemical composition of the fuel to be modified are achieved by extending the path traversed by the fuel and thereby increasing the time during which the fuel is exposed to a high temperature.

The Applicants have proposed in copending GB Patent Application No. 9716156.6 to provide a vapour extraction system that continuously separates the fuel into a lighter vapour fraction and a heavier liquid fraction. It is particularly advantageous to use the vapour fraction for the preconditioned fuel both for reasons of safety and because the lighter fraction of the fuel is the easier to decompose into partial products.

Vapour may alternatively or additionally be introduced into the housing drawn from a vapour canister for storing vapours evaporated from the fuel storage tank of the engine.

The housing is preferably formed integrally with the exhaust manifold and has a wall thickness comparable with that of a cast exhaust manifold. This gives the housing a large thermal capacity allowing it to conserve isothermal reaction conditions within its interior for preconditioning of the fuel.

As an alternative, the housing may be formed separately from the exhaust manifold but held in ample thermal contact with it. The thermal contact should allow heat flow over a large area so that the housing should run at substantially the same temperature as the exhaust manifold.

In one embodiment of the invention, the fuel discharge opening of the housing may be connected to a venturi section in the intake passage leading to the intake system of the engine. This would provide a sub-atmospheric pressure for maintaining a gas flow through the housing that is proportional with the intake air flow to the engine. This would also act as a vacuum source for a vapour extraction system or for another vapour source such as a vapour canister.

The fuel may be modified chemically in different ways, depending on the absence or presence of other gases in the housing. In the absence of all other gases, some of the hydrocarbon fuel may be cracked to smaller molecules. If oxygen is present, some of the hydrocarbon fuel may be thermally oxidised to form partial oxidation products. If carbon dioxide and water vapour are present but not oxygen (i.e. the housing contains exhaust gases resulting from burning a stoichiometric or rich mixture), some of the hydrocarbon fuel may be reformed to hydrogen and carbon monoxide. All these are slow processes and require the fuel to be subjected to a high temperature for a prolonged period of time. These conditions are achieved in the present invention when the engine is running and the exhaust manifold is hot.

If the fuel is to be reacted with exhaust gases, the housing may be provided with an opening in the wall common to the exhaust manifold and the housing to allow a flow of exhaust gases to enter the housing and subsequently discharge into the intake system of the engine in a manner similar to a conventional exhaust gas recirculation (EGR) system. By operating the engine with an overall lean mixture, the exhaust gases entering the housing will contain surplus oxygen for thermal oxidation of the fuel.

Alternatively, by operating the engine with a stoichiometric or rich mixture, the exhaust gases entering the housing will contain little or no oxygen and the fuel will be reformed by the carbon dioxide and water vapour present in the exhaust gases.

If desired, ambient air may be mixed with the fuel before being introducing into the housing, this being the case when fuel vapour is purged through a vapour canister for the fuel storage tank.

To assist and speed up the above thermal reaction processes, appropriate catalytic materials, such as copper, silver and gold, may be incorporated on the surfaces of the walls and the baffles within the housing.

The fuel preconditioning system of the invention is not intended to treat all the fuel to be burnt in the engine but only enough to generate reactive products to assist in the burning of the remaining fuel. The remainder of the fuel can be supplied to the engine in any conventional manner, such as by being injected at low pressure into the intake ports or injected at high pressure directly into the combustion chambers of the engine.

The presence of reactive products can render the mixture to be burnt in the engine prone to auto-ignition.

This can be advantageous when the engine is operating under part load conditions but is to be avoided under high loads in order to avoid knock and engine damage. The fuel preconditioning system should only therefore supply preconditioned fuel to the engine under low and part load operating conditions.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an engine with an intake and exhaust system fitted with an apparatus of the invention for preconditioning the fuel, Figure la shows a detail of Figure 1, Figure 2 is a view similar to Figure 1 showing an alternative embodiment of the invention, and Figure 3 shows a vapour extraction system.

Detailed description of the drawings Though the invention can be implemented using only liquid fuel, the preferred embodiment of the present invention relies on the availability of a continuous supply of vapour fuel. This can be derived by separating gasoline fuel into a lighter vapour fraction and a heavier liquid fraction in the manner that will now be described with reference to Figure 3, this being the subject of the above mentioned copending GB Patent Application No. 9716156.6.

An engine 10 has an intake manifold 16, a main throttle 14 and an intake passage containing a venturi 12.

A fuel injection system for the engine comprising a fuel circulation pump 32 that supplies fuel under pressure into a fuel rail 34 from which fuel is dispensed to the individual cylinders of the engine by fuel injectors 18. The pressure in the fuel rail 34 is regulated by a relief valve 36 that derives a reference pressure from the intake manifold 16.

Surplus fuel is spilled by the relief valve 36 into a fuel return pipe 38.

While it is conventional for the pump 32 and the return pipe 38 to be directly connected to the main fuel storage tank, designated 20 in the drawing, they are connected instead to a volatising chamber 30 that contains a much smaller quantity of fuel. The volatising chamber 30 is connected to the main fuel tank 20 by a supply pipe 24 containing a fuel lifter pump 22 and the level of fuel within the chamber 30 is maintained constant by means of a float 28 and a valve 26.

An evaporator 40 is disposed in the vapour filled space of the chamber 30 above the liquid level and in the path of the fuel returned by way of the fuel return pipe 38. The return fuel is sprayed over the evaporator and the latter is designed to have a large surface area that is coated with a film of fuel. The large surface area may be achieved by using a matrix of capillaries or a porous or sintered block for the evaporator 40. Neither the evaporator 40 nor the fuel in the chamber 30 is heated and evaporation relies on the reduced pressure in the vapour space, the dispersion of the spray droplets, the large surface area of the evaporator 40 and such heat as the return fuel picks up during its circulation flow. The matrix of the evaporator 40 may be formed of a hydrocarbon storage material such as activated carbon to increase the quantity of vapour that can readily be extracted under dynamic conditions.

To maintain the vapour space in the volatising chamber 30 below atmospheric pressure, a pipe 42 leading from it is connected by way of a first pipe 46 and a regulating valve 56 to the venturi 12, by way of a second pipe 44 and a regulating valve 54 to the intake manifold 16 and by way of a third pipe 110 to the engine intake system, as will be described below by reference to Figure 1.

The pipe 42 is also connected by way of a pipe 48 and a regulating valve 58 to a vapour canister 50 that is itself connected to the ullage space of the main fuel tank 20 by a pipe 52. Instead of the pipe 48 being connected to the pipe 42 to allow fuel vapour stored in the vapour canister 50 to be purged directly into the venturi 12, it is alternatively possible as represented by the pipe 48' shown in dotted lines to route the purge flow from the vapour canister 50 through the volatising chamber 30.

In Figure 1, an engine designated 100 is fitted with fuel injectors 108 that inject fuel directly into the combustion chambers of the engine. The engine has an intake manifold 106 with a main throttle 104 arranged downstream of a venturi 102. The exhaust system of the engine includes an exhaust manifold 122, that is normally cast and has a large wall thickness, connected to a downpipe 124 that is normally of lighter gauge piping.

A housing 112 with thick walls is either cast integrally or mounted in intimate thermal contact with the exhaust manifold 122 so that its interior operates at substantially the same temperature as the exhaust manifold 122. The interior of the housing 112 is partitioned by baffles 114 to define a sinuous gas flow path. The baffles 114a and 114b, as shown in greater detail in Figure la, are thin plates with curved edges that seal against each other or against the interior of the housing 112 about the periphery, and have openings on opposite sides to force the gas flow to follow a sinuous path while traversing the housing 112 from bottom to top. Two openings are provided near the opposite ends of the housing 112, one being a vapour supply opening connected to the pipe 110 of the vapour extraction system shown in Figure 3 and the other being connected by a discharge pipe 116 and a flow regulating valve 118 to the venturi 102.

The venturi vacuum serves to draw fuel vapour from the volatising chamber 30 of the vapour extraction system through the housing 112 and into the air intake system of the engine. While traversing the housing 112, the fuel is heated over a prolonged time and at a high temperature such that its chemical composition is modified. Such preconditioning increases the concentration of reactive products and thereby increases the ignitability of the mixture in the combustion chamber.

The embodiment of Figure 2 differs from that of Figure 1 in the following respects. First, a hole 123 is provided in the wall between the housing 112 and the exhaust manifold 122 to admit exhaust gases into the housing 112. This will cause recirculation of exhaust gases and will allow the fuel vapour to mix and react with the EGR gases in the housing 112 before the gases enter the air intake system of the engine. In this case also the pipe 116 and the regulating valve 118 can be replaced by a conventional EGR pipe 117 and an EGR regulating valve 119. The EGR pipe 117, unlike the pipe 116 in Figure 1, will be at a pressure higher than atmospheric because of the exhaust back pressure, and for this reason a vapour pump 111 is required to draw vapour from the pipe 110 of the vapour extraction system and introduce it under pressure into the housing 112. In this embodiment the vapour and EGR mixture is introduced directly into the intake manifold 106 downstream of the main throttle 104. A further difference to be noted is that the engine of Figure 2 has port fuel injection through fuel injector 109 instead of direct injection.

The chemical reaction between the fuel vapour and the recirculated exhaust gases will depend on the composition of the exhaust gases. Where the engine is running with a lean mixture, the exhaust gases will contain surplus oxygen and the fuel vapour will be thermally reacted to form partial oxidation products. Where the engine is running with a rich mixture, on the other hand, the fuel vapour will be partially reformed without oxidation by combining with the carbon dioxide and steam in the exhaust gases to form carbon monoxide and hydrogen.

Claims (9)

1. An apparatus for preconditioning hydrocarbon fuel to be supplied to an internal combustion engine, comprising a housing arranged to be heated by heat derived from the engine exhaust gases and having supply and discharge openings, and a plurality of baffles disposed within the housing to divide the heated interior of the housing into a sinuous gas flow path leading from the supply opening to the discharge opening, means for introducing fuel in liquid or vapour form into the housing by way of the supply opening and means for delivering preconditioned fuel from the discharge opening to the engine intake system, the fuel being heated for a sufficient time and at a sufficient temperature while traversing the sinuous gas flow path for the chemical composition of a substantial proportion of the fuel to be modified before it is introduced into the air intake system of the engine.

2. An apparatus as claimed in claim 1, wherein means are provided for supplying fuel to the housing in vapour form.

3. An apparatus as claimed in claim 2, wherein the means for supplying fuel to the housing in vapour form comprises a vapour extraction system for continuously separating gasoline fuel into a lighter vapour fraction and a heavier liquid fraction.

4. An apparatus as claimed in claim 2, wherein the means for supplying fuel to the housing in vapour form comprises a vapour canister of a fuel storage tank of the engine.

5. An apparatus as claimed in any preceding claim, wherein the housing is formed integrally with the exhaust manifold and has a wall thickness comparable with that of a cast exhaust manifold.

6. An apparatus as claimed in any one of claims 1 to 4, wherein the housing is formed separately from the exhaust manifold and is held in thermal contact with the exhaust manifold.

7. An apparatus as claimed in any preceding claim, further comprising an opening for admitting exhaust gases into the housing to mix and react with the fuel to be preconditioned.

8. Apparatus as claimed in any preceding claim, wherein catalytic materials are incorporated on the surfaces of the walls and the baffles within the housing in order to assist and speed up the thermal reaction processes taking place within the housing.

9. Apparatus for preconditioning hydrocarbon fuel supplied to an internal combustion engine, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.