GB2312923A

GB2312923A - Heating i.c.engine fuel before injection

Info

Publication number: GB2312923A
Application number: GB9608775A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1996-04-27
Filing date: 1996-04-27
Publication date: 1997-11-12
Also published as: GB9608775D0

Abstract

A heat exchanger 10 is arranged upstream of fuel injectors, 20 for transferring heat from a heating fluid l4 to the fuel that is to be injected, the temperature of the fluid 14 is constantly maintained at a value not exceeding the boiling point of the fuel at the prevailing pressure of the fuel in the heat exchanger 10. The fluid 14 may be liquid heated electrically and the fluid circuit may include an insulated reservoir 66. The fluid 14 may be exhaust gas or engine coolant from two sources at temperatures above and below the temperature to be maintained. A valve ( 38, 39, Fig. 2,3 ) may control the flows from the two sources in response to a thermostat ( 36, 37 ) in the mixed flows. The heating temperature may be up to 50{C for port injection, 50 to 100{C for, direct petrol injection and 200 to 250{C for diesel injection.

Description

Fuel Heating System Field of the invention The present invention relates to an internal combustion engine burning a liquid fuel and having a pressurised fuel injection system within which the fuel is heated to improve fuel preparation.

Background of the invention It has already been proposed to heat the fuel in a fuel injection system to such an extent that flash boiling or flash atomisation of the fuel occurs when the fuel is subjected to a sudden pressure drop upon being discharged into the combustion chamber or intake port. The suddenly vaporised or flashed fuel is effective in causing the remaining liquid spray to break up, producing very fine atomisation and very short penetration distance of the spray.

The spray characteristics by flash atomisation is superior to many other known fuel injector sprays including high pressure liquid fuel spray and air-assisted or air-forced combined fuel and air sprays. Fine atomisation and short penetration distance of the spray are of course important requirements in spark ignition engines with port fuel injection (PFI) in order to minimise fuel impingement on the walls of the intake port. In direct injection spark ignition (DISI) engines, fine atomisation is desirable in order to avoid fuel impingement on the surfaces of the combustion chamber and improve ignitability of the stratified mixture charge. Lastly, in compression ignition (diesel) engines, flash atomisation improves fuel and air mixing surrounding the fuel spray.

In a spark ignition engine with port fuel injection, systems are known using an electric heating element operative only during cold starts to preheat the fuel during engine cold start. This heats the fuel rapidly to increase the evaporation of the lighter fraction of the fuel to enable the engine to start under sub-zero ambient conditions. Such heating can only be effective in producing flash atomisation if the heat stored in the heated fuel on account of its elevated temperature over and above the temperature of the fuel boiling point at the discharge pressure (this being termed excess sensible heat hereinafter) is comparable with the latent heat of vaporisation of the fuel. In this context, it should be explained that the latent heat of fuel is normally significantly greater than its specific heat and heating the fuel only to its boiling point at the discharge pressure would not be sufficient to achieve flash vaporisation because as soon as evaporation commences the temperature of the remaining fuel will drop significantly below the boiling point. Hence, it is necessary to heat the fuel significantly above the boiling point so that the excess sensible heat may counteract the cooling effect of the evaporation on the remaining fuel.

To give specific examples, for gasoline fuel in which the boiling point of the lightest fraction is typically 250C at ambient pressure, any heat added above this temperature would rank as excess sensible heat. On the other hand, for diesel fuel in which the boiling point of the lightest fraction is typically 1800C at ambient pressure, a significant higher temperature must be reached before any further additional heat could count as excess sensible heat.

There is a limit placed on the extent to which the fuel can be preheated by the boiling point of the fuel within the pressurised fuel injection system or the boiling point of the lightest fraction in the case of a multi-component fuel.

If any part of the fuel should commence to boil while still within the fuel injection system, then vapour lock will occur preventing distribution and metering of the fuel to the engine.

In summary therefore, it is desirable to preheat the fuel to achieve flash atomisation but heating the fuel to increase the excess sensible heat can cause serious problems, because the hotter the fuel the more difficult it is to control the heating and the higher the risk of boiling and vapour lock forming within the fuel injection system.

Known fuel heating systems heat the fuel rapidly with a hot heating element and rely on closed loop control to limit the power of the heating element after the fuel has reached a desired temperature below the boiling point within the fuel injection system. This however is not fail-safe from vapour lock because of possible overshoot in the fuel temperature due to hysteresis in the control system, and because of hot soak conditions after the engine is switched off when the fuel temperature could rise above the boiling point as the temperature between the different parts of the heating system is equalised.

It is believed that this is a reason why systems have not previously been used in conventional automobile engine to heat the fuel except for a short time during cold starts.

Object of the invention The present invention aims to provide a fuel heating system that is operative at all times that the engine is running to improve the combustion quality of the engine and which is safeguarded against overheating and the risk of vapour lock.

Summary of the invention According to the present invention, there is provided a liquid fuel heating system in an internal combustion engine comprising a heat exchanger arranged upstream of at least one fuel injector for transferring heat from a heating fluid only to the fuel that is to be injected by the injector into the engine, and means for continuously maintaining the temperature of the heating fluid at a value not exceeding the boiling temperature of the fuel at the prevailing pressure of the fuel in the heat exchanger.

The heating fluid flowing through the heat exchanger may be liquid or gas. For example, liquid drawn from the engine coolant circuit or exhaust gases drawn from the engine exhaust system may be used for heating the fuel. Such systems have the advantage of using waste heat from the engine and not therefore affecting the efficiency of the engine. However, such systems alone would not be effective during cold starts. As a further alternative, one may use a fluid circulating in a closed circuit and heating the fluid by an electrical heating element. In this case, the fuel can be heated during or before cold starts but the engine efficiency is reduced. Ideally, a combination of the two may be used to achieve rapid heating during cold starts and inexpensive heating during steady state operation.

In the present invention, the area and duration of thermal contact provided in the fuel heat exchanger are sufficiently large to bring the temperature of the fuel to substantially the same temperature as the heating fluid in the heat exchanger. In the limit, the fuel can only reach a maximum soak temperature equal to the temperature of the heating fluid. The system is therefore stable and remains fail-safe from vapour lock while the fuel pressure remains constant.

A non-return valve may be provided in the fuel supply upstream of the fuel heat exchanger to permit fuel flow only in the direction of the fuel heat exchanger and to maintain substantially the same fuel pressure in the fuel heat exchanger for a prolonged period after the fuel supply is switched off. This ensures that the fuel in the fuel heat exchanger can cool steadily after the engine is switched off, ahead of the fuel pressure being leaked away eventually, so that no vapour lock can occur within the fuel system.

If engine coolant or exhaust gases are used as heating fluids, they can be heated by the engine to a temperature greater than the boiling point of the lightest fraction of the fuel and it is necessary to regulate their temperature by controlled cooling before they are introduced into the heat exchanger. This may conveniently be effected by dividing the fluid flow into two streams, cooling only one of the two streams and remixing the two streams in the correct proportions to achieve the desired inlet temperature in the heating fluid to the heat exchanger.

In spark ignition engines, systems have been proposed that use the hot coolant in the engine to preheat the fuel, but the coolant temperature is too hot, being significantly higher than the boiling temperature of the lightest fraction of the fuel. This makes it difficult to stabilise the fuel temperature because of the unremitting temperature gradient which continues to push the fuel temperature above the boiling threshold unless it is checked by some control means to limit the heat transfer under all engine operating conditions. Moreover, there is an unavoidable occurrence of vapour lock in the fuel system every time after the engine is switched off when the fuel system undergoes a hot soak period during which the fuel temperature will continue to rise above its boiling point until it eventually equals the final temperature of the hot coolant. These are further reasons why fuel preheating using hot coolant has not been used in automobile applications. On the contrary, because of such considerations, heat transfer from the coolant to the fuel is deliberately suppressed in automobile engines by carefully isolating the fuel system from the coolant in order to heat the fuel as little as possible to avoid vapour lock.

In spark ignition engines, systems have also been proposed that use the hot exhaust gases from the engine to preheat the fuel. Here, the exhaust gas temperature is even hotter than the coolant temperature and much higher than the boiling temperature of the fuel, making it even more critical to stabilise the fuel temperature to avoid vapour lock. The problem is compounded by the fact the exhaust gas temperature is itself variable and changes rapidly under normal driving conditions. Once again, vapour lock during hot soak periods after the engine is switched off is unavoidable and severe. These considerations have prevented fuel preheating using hot exhaust gases from being used in automotive applications.

To achieve flash atomisation, preheating the fuel using exhaust gases as the heating fluid regulated according to the present invention to a stable inlet temperature in the range of 2000C to 2500C is fail-safe for heating diesel fuel pressurised at 20,000 to 30,000 KPa (200-300 bar). For gasoline fuel in spark ignition engines with port fuel injection (PFI) pressurised at 300 to 500 KPa (3-5 bar), using engine coolant as the heating fluid regulated to a stable inlet temperature of up to 500C to heat the fuel will be fail-safe from vapour lock. For direct injection spark ignition (DISI) engines where the fuel pressures is typically 5,000 to 7,000 KPa (50-70 bar), using engine coolant as the heating fluid regulated to a stable inlet temperature in the range of 500C to 1000C to heat the fuel will be feasible for introducing a large amount of excess sensible heat into the fuel for abrupt evaporation on passing through the fuel injector.

To improve cold start with fuel preheat, a quantity of hot heating fluid may be stored in an insulated reservoir which serves to retain the heat in the heating fluid after the engine is switched off and supply hot heating fluid to the fuel heat exchanger during subsequently cold start. If desired an electrical heating element may be provided in the insulated reservoir to raise the temperature of the heating fluid during cold starts.

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 is a schematic diagram showing the heating of the fuel supply to an engine using the heat of a heating fluid circulating in a closed circuit, Figure 2 is a schematic diagram showing the heating of the fuel supply to an engine using the heat of gases drawn from the engine exhaust system, and Figure 3 is a schematic diagram showing the heating of the fuel supply to an engine using the heat of the liquid coolant circulating in the engine coolant circuit.

Detailed description of the preferred embodiment Figure 1 shows fuel injectors 20 connected to a common fuel rail 26 by means of individual supply lines 21 that pass through a heat exchanger 10 and are in thermal contact within the heat exchanger 10 with a heating fluid 14 which is this case is a liquid. The fuel is supplied to the fuel rail 26 by a supply line 22 that leads from a fuel pump and contains a non-return valve 24. The other end of the fuel rail 26 is connected through a fuel pressure regulator 27 to a return line 28 leading back to the fuel tank. The heating fluid 14 flows along a closed circuit under the action of a pump 60. The outlet of the pump 60 is connected by a pipe 12 to the inlet end of the heat exchanger 10 the outlet of which is connected by a pipe 18 to a heating chamber 62 and a reservoir chamber 66 before being returned to the pump 60.

The heating chamber 62 contains a heating element 62 which may be an electric heating element or a tube through which engine coolant or exhaust gases flow. The chamber 66 has an insulating wall 68 and its purpose is to store a supply of hot heating fluid for use during cold starts.

Heat is transferred from the heating element 64 to the heating fluid 14 and by thermal contact in the heat exchanger 10 to the fuel flowing towards the fuel injectors 20. The fuel within the supply lines 21 is under high pressure and can be heated without risk of vapour lock to a temperature significantly higher than its boiling point at ambient pressure. In the present invention it is desired to heat the fuel in this manner so that its excess sensible heat should be comparable with the latent heat of vaporisation of the fuel without at the same time risking vapour lock.

For this purpose the inlet temperature of the heating fluid 14 is raised to the maximum permissible temperature of the fuel by regulating the heat output of the heating element 64. This being the highest temperature that the fuel can reach even if left for prolonged period in thermal contact with the heating fluid 14 ensures that there is never vapour lock or cavitation within the fuel supply lines 21 or the fuel injectors 20.

After the engine has been switched off, a significant volume of hot heating fluid will remain within the reservoir 66 and will retain its temperature because of the insulation 68.

On starting the engine the heating of the fuel can therefore commence almost immediately. If desired the heating fluid 14 can be circulated and heated electrically for a short time before engine ignition.

When the engine is switched off, the fuel rail 26 remains pressurised because of the non-return valve 24 and the risk of vapour lock through rapid depressurising of the fuel supply is avoided. Such heat soak as takes place during this time does not raise the temperature of the fuel significantly and only affects the temperature of the heating fluid outside the heat exchanger 10 because the circulation pump 60 is not activated.

The embodiment of Figure 2 does not use fluid circulating in a closed circuit but instead draws exhaust gases directly from the exhaust system of the engine for use as the heating fluid 14. In this embodiment as with the embodiment of Figure 3 described below, like reference numerals have been used to designate like components to avoid the need for repetition of the description. In particular the fuel side of the heat exchanger 10 is similar in all three embodiments and the major differences lie in the medium of the heating fluid.

Referring again to Figure 2, two inlets 42 and 44 draw exhaust gases from the exhaust pipe 40 at two points. The two inlets lead to a temperature regulating valve 30. The inlet 42 leads directly to the regulating valve 30 through a pipe 32 whereas the inlet 44 is connected to the regulating valve 30 by a pipe 34 that contains a radiator 46 radiating heat into the ambient atmosphere. The effect of this is that the gases in the pipe 32 are at a temperature higher than the desired regulated temperature of the heating fluid and the gases in the pipe 34 are below this temperature.

The two streams mix within the regulating valve 30 and affect the temperature of a bi-metallic element 36 that moves a proportioning valve 38 to maintain the mixed temperature constant. The outlet of the heat exchanger 10 in this case discharges through the pipe 18 and a shut-off valve 16 into the exhaust pipe at a point 48 in the exhaust pipe. Because of the shaping of the inlets 42, 44 and the outlet 48, the exhaust gases are circulated through the heat exchanger 10 without the need for a pump. The purpose of the shut-off valve 16 is to prevent such circulation in the event that the exhaust gases are so hot that the streams in both the pipes 32 and 34 are hotter than the desired fuel temperature.

Though the pipe 18 has been described and illustrated as returning the heating fluid to the exhaust pipe, it could be alternatively possible to return at least part of the gases to the intake system of the engine to serve as EGR gases.

Turning now to Figure 3, the heating fluid in this case is the engine coolant. The engine, as is conventional, has a block 50 and a cylinder head 40, a circulation pump 52. a thermostatic valve 54 and an external radiator 56. One stream of the heating fluid is drawn by an inlet 42 from a point immediately preceding the thermostat 54 where the engine coolant is at its hottest. A second stream of the heating fluid is drawn by an inlet 44 from a point immediately following the circulation pump 52 where the engine coolant is at its coldest. The second stream as with the case of the exhaust gases is cooled in a radiator 46 before it is introduced to the temperature regulating valve 30. Though the temperature regulating valve 30 is operating on different fluids in the embodiments of Figures 2 and 3, it has been allocated the same reference numerals because it serves an identical function. In this case a thermostatic insert 37 connected to a conical closure member 39 (similar to a conventional thermostat) is used to regulate the temperature of the heating fluid 14.

The return from the heat exchanger 10 in this case passes along the pipe 18 and the shut-off valve 16 to the inlet of the engine radiator 56 flowing along the pipe 48.

The heat exchanger 10 in this embodiment is slightly modified in that a single fuel supply line 23 in the heat exchanger 10 serves to heat the fuel to all the injectors 20.

Claims

1. A liquid fuel heating system in an internal combustion engine comprising a heat exchanger arranged upstream of at least one fuel injector for transferring heat from a heating fluid only to the fuel that is to be injected by the injector into the engine, and means for continuously maintaining the temperature of the heating fluid at a value not exceeding the boiling temperature of the fuel at the prevailing pressure of the fuel in the heat exchanger.

2. A liquid fuel heating system in an internal combustion engine as claimed in claim 1, wherein the thermal efficiency and the fluid flow rates through the heat exchanger are such that the exit temperature of the fuel is substantially equal to the temperature of the heating fluid.

3. A liquid fuel heating system in an internal combustion engine as claimed in claim 1 or 2, wherein a non-return valve is provided in the fuel supply upstream of the fuel heat exchanger to permit fuel flow only in the direction of the fuel heat exchanger and to maintain substantially the same fuel pressure in the fuel heat exchanger for a prolonged period after the fuel supply is switched off.

4. A liquid fuel heating system in an internal combustion engine as claimed in any one of claims 1 to 3, wherein the inlet temperature of the heating fluid to the fuel heat exchanger is regulated.

5. A liquid fuel heating system in an internal combustion engine as claimed in claim 4, wherein the inlet temperature of the heating fluid to the fuel heat exchanger is regulated upstream of the fuel heat exchanger by controlled mixing of two supply streams of heating fluid having temperatures above and below the desired inlet temperature.

6. A liquid fuel heating system as claimed in claim 5, wherein the two streams of heating fluid are derived from a common supply, one stream being taken directly from the common supply and the other through a second heat exchanger in which the heating fluid is cooled.

7. A liquid fuel heating system as claimed in claim 5 or 6, wherein the controlled mixing of the two streams of heating fluid is effected by means of a thermostat element maintained at the same temperature as the mixed stream and serving to operate a proportioning valve to set the relative proportion of the two mixed streams of the heating fluid arriving from the cooled and uncooled streams respectively.

8. A liquid fuel heating system in an internal combustion engine as claimed in any preceding claim, wherein the heating fluid comprises exhaust gases drawn from the exhaust system of the engine.

9. A liquid fuel heating system as claimed in claim 8, wherein the heating fluid is returned to the exhaust system after having passed through the fuel heating heat exchanger.

10. A liquid fuel heating system as claimed in claim 8, wherein the heating fluid serves as a supply of recirculated exhaust gases to the intake system of the engine after having passed through the fuel heating heat exchanger.

11. A liquid fuel heating system in an internal combustion engine as claimed in any of claim 1 to 7, wherein the heating fluid is liquid coolant drawn from the cooling system of the engine.

12. A liquid fuel heating system in an internal combustion engine as claimed in claim 11, wherein a quantity of the heating fluid is stored in an insulated reservoir serving to retain the heat in the heating fluid after the engine is switched off and supply the fuel heat exchanger with hot heating fluid during subsequent cold starts.

13. A liquid fuel heating system in an internal combustion engine, constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in Figure 1, Figure 2 or Figure 3 of the accompanying drawings.