GB2328715A

GB2328715A - Apparatus for preconditioning hydrocarbon fuel vapour for i.c. engines

Info

Publication number: GB2328715A
Application number: GB9718205A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1997-08-29
Filing date: 1997-08-29
Publication date: 1999-03-03
Also published as: GB9718205D0

Abstract

The apparatus comprises a first heated vessel 112 integral with or attached to the exhaust manifold 122 and communicating with a second pressurised vessel 114 containing a heating element 115. A compressor 111 for introduces gases containing fuel vapour into the pressurised vessels 112, 114 and a pressure regulator 113 maintains a predetermined gas pressure of typically 5 to 10 atmospheres in the pressurised vessels 112, 114. The gases containing fuel vapour are heated in the pressurised vessels 112, 114 for a sufficient time and at a sufficient temperature and pressure for the chemical composition of a substantial proportion of the fuel vapour to be modified before they are dispensed by a gas metering valve 119 into the air intake system 116 of the engine 100.

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 Patent 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 the 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.

The Applicants have also proposed previously in copending GB Patent Application No. 9717023.7 to provide a housing heated by heat derived from the engine exhaust gases and a plurality of baffles disposed within the housing to divide the heated interior of the housing into a sinuous gas flow path such that the fuel remains in the housing for a sufficient time and at a sufficient temperature while traversing the sinuous gas flow path in the heated interior of the housing to be thermally preconditioned.

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 preconditioning the fuel efficiently. The slow thermal reactions taking place require constant application of heat over a prolonged length of time and providing a sufficiently long sinuous gas flow path, as earlier suggested, creates other problems in that its large size increases the thermal capacity thereby increasing the warm-up time during cold starts and delaying the light-off of the exhaust catalytic converter.

US-A-4,407,661 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 thermal reactions taking place within the mixture is still relatively slow.

Summary of the invention According to the present invention, there is provided an apparatus for preconditioning hydrocarbon fuel vapour to be supplied to an internal combustion engine, comprising a heated pressurised vessel, a compressor for introducing gases containing fuel vapour into the pressurised vessel, and means for dispensing gases from the pressurised vessel at a controlled rate to the engine, the gases containing fuel vapour being sufficiently heated and compressed within the pressurised vessel for the chemical composition of a substantial proportion of the fuel vapour to be modified before being introduced into the engine.

It is preferred to provide a pressure regulator to maintain the pressure within the pressurised vessel at a predetermined value to standardise the reaction conditions within the pressurised vessel and to simplify the metering of vapour from the vessel to the engine.

On account of the pressurising and heating of the vessel in the present invention, the mean free path of the gas molecules is reduced so that they collide with each other more frequently and more energetically. The thermal preconditioning of the fuel vapour which would normally take place slowly under near atmospheric pressure conditions is speeded up significantly when the pressure is increased. The greater the pressure, the greater will be the chemical reaction rate and it is preferred that the pressure should be increased by the compressor to several atmospheres, typically 5 to 10 atmospheres. Such pressure may be achieved by providing a positive displacement compressor for compressing the gases containing fuel vapour into the pressurised vessel, the compressor drawing the gases from a vapour supply at near atmospheric or sub-atmospheric pressure.

The high pressure allows a smaller pressurised vessel to be used and achieves the desired reaction rate without significantly increasing the thermal capacity of the overall system. Consequently, the warm-up time is not increased unduly nor is the light-off of the exhaust catalytic converter unduly delayed.

Heating of the pressurised vessel may be provided by a heating element or by heat derived from the engine exhaust gases by thermal contact with the exhaust manifold walls.

The gases containing fuel vapour, having been thermally preconditioned in the pressurised vessel, may be metered accurately to the engine by the use of a calibrated gas metering valve, when the pressure in the pressurised vessel is regulated to a predetermined value. Such a gas metering valve and its associated control system for dispensing to the engine a predetermined flow rate of fuel vapour according to the engine speed and load operating conditions may be similar to those used in engines supplied with a gaseous fuel such as compressed natural gas (CNG). The pressurised fuel vapours may be injected into the engine intake manifold or intake ports. Alternatively, they may be injected directly into the engine combustion chambers.

The volume of the pressurised vessel is sufficiently large to store a substantial mass of gases containing fuel vapour under pressure in relation to the through flow of the gases supplied to and discharged from the pressurised vessel under steady flow conditions. Under these conditions, the residence time of the gases in the pressurised vessel will be sufficiently long to allow the desired thermal reactions to take place. After the engine has stopped, the vessel also serves as a pressurised storage reservoir for the preconditioned fuel vapour ready to be used when the engine is next started.

The Applicants have proposed in copending GB Patent Application No. 97161S6.6 to provide a vapour extraction system that continuously separates gasoline fuel into a lighter vapour fraction and a heavier liquid fraction. It is particularly advantageous for the compressor to draw the vapour fraction from the proposed vapour extraction system for the preconditioned fuel in the present invention because the lighter vapour fraction of the gasoline fuel is the easier to decompose thermally into partial products.

Fuel vapour may, alternatively or additionally, be introduced into the preconditioning pressurised vessel by the compressor drawing vapour from a vapour canister which stores fuel vapour evaporated from the liquid fuel storage tank of the engine.

The pressurised vessel 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 sufficiently large thermal capacity allowing it to conserve isothermal reaction conditions within its interior for preconditioning the fuel vapour.

As an alternative, the pressurised vessel 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 vessel should run at substantially the same temperature as the exhaust manifold.

The fuel vapour may be modified chemically in different ways depending on the absence or presence of other gases supplied with the fuel vapour into the preconditioning pressurised vessel. In the absence of all other gases, some of the fuel vapour may be cracked under pressure to smaller molecules. If oxygen is supplied with the fuel vapour, some of the vapour may be thermally oxidised under pressure to form partial oxidation products. If carbon dioxide, water vapour and not oxygen are supplied with the fuel vapour (i.e., the fuel vapour is mixed with exhaust gases resulting from burning a stoichiometric or rich mixture), some of the vapour may be reformed under pressure to form hydrogen and carbon monoxide. All these processes, which normally take place slowly under near atmospheric pressure conditions are speeded up significantly in the present invention where the pressure in the preconditioning vessel is increased to several atmospheres, and a high temperature is maintained throughout the heated interior of the vessel.

To assist and speed up even further the above thermal reaction processes under high pressure and temperature, appropriate catalytic materials, such as copper, silver or gold, may be incorporated in the heated interior of the fuel preconditioning pressurised vessel.

The fuel vapour preconditioning system of the invention does not have 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, which could be in gaseous or liquid form, can be supplied to the engine in any conventional manner, such as by being injected at low pressure into the engine intake ports or injected at high pressure directly into the engine combustion chambers.

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 fuel vapour, and Figure 2 shows a vapour extraction system.

Detailed description of the drawings Though the invention can be implemented by drawing fuel vapour from any vapour supply, the preferred embodiment of the present invention relies on the availability of a continuous supply of the lighter vapour fraction of a gasoline fuel separated from a heavier liquid fraction in the manner that will now be described with reference to Figure 2, 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 liquid fuel injection system for the engine comprising a fuel circulation pump 32 that supplies fuel under pressure into the 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 returning 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 form of a hydrocarbon storage material such as activated carbon to increase the quantity of vapour that can be 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 fuel vapour preconditioning system of the present invention, 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 liquid fuel injectors 108 that inject fuel into the intake ports leading to the combustion chambers of the engine. The engine has an intake manifold 106 with a main throttle 104.

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 first pressurised vessel 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. Two openings are provided near the opposite ends of the vessel 112, one being a vapour supply opening connected by way of a regulating valve 113 to a positive displacement compressor 111 which draws fuel vapour from the pipe 110 of the vapour extraction system and delivers it under pressure into the pressurised vessel 112.

The pressure regulator 113 is set to maintain a predetermined pressure of several atmospheres in the pressurised vessel 112. The higher the pressure used, the smaller one can make the vessel 112 and thereby minimise the increase in thermal capacity of the exhaust manifold.

The other opening to the pressurised vessel 112 is a discharge opening connected to a second pressurised vessel 114 which in turn has a discharge opening connected to the intake manifold 106 downstream of the main throttle 104 by way of a pipe 117 and a gas metering valve 119. The second pressurised vessel 114, which is pressurised to the same pressure as the first pressurised vessel 112 and is surrounded by heat insulating materials, has a heating element 115 in its interior for heating the gases containing fuel vapour to a higher temperature, if necessary, than that reached in the first pressurised vessel 112 heated by the exhaust manifold 122.

The gas metering valve 119 and its associated control system for dispensing to the engine a predetermined flow rate of fuel vapour according to the engine speed and load operating conditions may be similar to those used in engines supplied with a gaseous fuel such as compressed natural gas (CNG), and it is therefore not deemed necessary to described them in greater detail.

The compressor 111 also draws recirculated exhaust gases from the exhaust manifold 122 by way of an EGR pipe 123 and a EGR regulating valve 125, and the EGR gases are mixed with the fuel vapour in the pipe 110. The chemical reactions between the fuel vapour and the recirculated exhaust gases taking place under pressure in the preconditioning vessels 112, 114 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.

The combined volume of the pressurised vessels 112, 114 is sufficiently large to store a substantial mass of gases containing fuel vapour under pressure in relation to the through flow of the gases supplied to and discharged from the pressurised vessels under steady flow conditions. This ensures that the residence time of the gases in the pressurised vessels will be sufficiently long to allow the desired thermal reactions to take place within the pressurised vessels.

Claims

1. An apparatus for preconditioning hydrocarbon fuel vapour to be supplied to an internal combustion engine, comprising a heated pressurised vessel, a compressor for introducing gases containing fuel vapour into the pressurised vessel, and means for dispensing gases from the pressurised vessel at a controlled rate to the engine, the gases containing fuel vapour being sufficiently heated and compressed within the pressurised vessel for the chemical composition of a substantial proportion of the fuel vapour to be modified before being introduced into the engine.

2. An apparatus as claimed in claim 1, wherein a positive displacement compressor is provided for compressing the gases containing fuel vapour into the pressurised vessel, the compressor drawing the gases from a vapour supply at near atmospheric or sub-atmospheric pressure.

3. An apparatus as claimed in claim 2, wherein the vapour supply 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 vapour supply comprises a vapour canister storing fuel vapour evaporated from a fuel storage tank of the engine.

5. An apparatus as claimed in any preceding claim, wherein the pressurised vessel is heated by a heating element.

6. An apparatus as claimed one of claims 1 to 4, wherein the pressurised vessel is heated by heat derived from the engine exhaust gases by thermal contact with the exhaust manifold walls.

7. An apparatus as claimed in claim 6, wherein the pressurised vessel is formed integrally with the exhaust manifold and has a wall thickness comparable with that of a cast exhaust manifold.

8. An apparatus as claimed in claim 6, wherein the pressurised vessel is formed separately from the exhaust manifold and is held in thermal contact with the exhaust manifold.

9. An apparatus as claimed in any preceding claim, wherein catalytic materials are provided in the heated interior of the pressurised vessel in order to assist and speed up the thermal reaction processes taking place within the vessel.

10. An apparatus as claimed in any preceding claim, wherein a pressure regulator is provided to maintain the pressure in the pressurised vessel to a predetermined value.

11. An apparatus as claimed in claim 10, wherein a calibrated gas metering valve is provided for dispensing the pressurised gases from the pressurised vessel to the engine.

12. An internal combustion engine having an apparatus as claimed in any preceding claim, wherein the pressurised gases containing fuel vapour is injected into the engine intake system.

13. An internal combustion engine having an apparatus as claimed in any one of claims 1 to 11, wherein the pressurised gases containing fuel vapour is injected directly into the engine combustion chambers.