GB2392957A

GB2392957A - Method of recharging the fuel pressure accumulator of an i.c. engine fuel injection system

Info

Publication number: GB2392957A
Application number: GB0303968A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-09-13
Filing date: 2003-02-21
Publication date: 2004-03-17
Also published as: GB0303968D0

Abstract

The accumulator 10 contains a quantity of liquid fuel, eg LPG, DME, gasoline or alcohol, in a lower chamber 14 pressurised by inert compressed gas in an upper chamber 16. The fuel is delivered to a fuel rail 44 via a pressure regulator 40. The system is used eg in research laboratories for running engines for short periods without the need for a mechanical fuel pump. For continuous operation of the engine, a forcing chamber 20 is provided for periodically forcing fuel into the lower accumulator chamber 14 via a non-return valve 18. The forcing chamber 20 is divided by a piston 22, or a diaphragm, into an upper chamber 24 containing a quantity of liquid fuel and a lower chamber 26,28 containing a quantity of a fluid (eg propane, chlorofluorohydrocarbon, DME, chloromethane or ammonia) of low boiling-point, low critical-point temperature and high critical-point pressure. Heat from source 50 raises the temperature of the fluid above its critical-point to generate sufficient pressure to force fuel from the upper chamber 24. The forcing chamber 20 can be refilled with fuel by cooling the chamber 24 and using a transfer pump 32 to transfer fuel from a tank 34.

Description

- 1 2392957

METHOD OF RECHARGING A FUEL PRESSURE ACCUMULATOR

_eld of the invention 5 The present invention relates to a method for charging and recharging a fuel pressure accumulator.

Background of the invention

10 It is known in engine research laboratories to use a fuel pressure accumulator for forcing liquid fuel at high pressure to the fuel injection system of a test engine without relying on a mechanical fuel pump to provide the high pressure fuel. The pressure energy in the fuel pressure accumulator is provided by a pocket of inert compressed gas such as nitrogen supplied from a high pressure nitrogen gas bottle. However, such an arrangement is only suitable for running the engine for a short period of time because the pressurized fuel accumulator has a 20 limited fuel storage capacity and has to be de-pressurised and re-pressurised again each time it is refilled with fresh fuel. Also such an arrangement would be impractical for mobile application on a vehicle where a source of inert compressed gas is not readily available.

For continuous operation of the engine without disrupting the fuel supply to the fuel injection system, a mechanical fuel pump is typically used for maintaining a steady high fuel pressure to the fuel injection system. To so ensure durability of the fuel pump, it is essential that the fuel being pumped should have sufficient viscosity for good lubrication of the moving parts within the pump. This has been satisfactory for most gasoline and diesel fuels, but problematic for other fuels such as Liquefied Petrolum Gas as (LPG) and DiMethyl Ether (DME) which have very low viscosity and could cause pump wear.

- 2 LPG has been used as an alternative to gasoline fuel in spark ignition engines by virtue of its higher octane number, better combustion properties, lower exhaust emissions and cleaner fuel constituents containing no lead 5 or sulphur. It is a mixture of propane and butane such as occurring in nature or produced synthetically. LPG is normally stored in liquid form at room temperature at relatively low pressures of A to 12 bar depending upon ambient temperature, but is generally used in gaseous form lo by first evaporating the LPG in a separate vaporizer before introducing the gas into the air intake system of the engine. There are however further benefits to use the LPG in liquid form through a high pressure fuel injection system by injecting the liquid LPG fuel into the intake port or directly into the combustion chamber of the engine, but the fuel in the LPG liquid injection system must be pressurized significantly above the LPG storage pressure in order to prevent vapour lock from forming anywhere within the liquid fuel injection system that may be caused by local heating.

DME has been considered as an alternative to diesel fuel in compression ignition engines by virtue of its higher cetane number, lower autoignition temperature, higher thermal efficiency, lower NOx emissions and smoke-free 25 combustion. It has a boiling point of -24.9 C at ambient pressure and is conveniently stored in liquid form at room temperature at approximately 5 bar storage pressure.

However, in order to make DME a practical fuel for use in an engine fuel injection system, an alterative method of 30 pressurizing the fuel to that of using a mechanical fuel pump is desirable to avoid pump wear.

Summary of the invention

35 In order mitigate at least some of the above-mentioned problems, there is provided a method of recharging a fuel pressure accumulator supplying an engine fuel injection

- 3 system with liquid fuel at high pressure, wherein the fuel pressure accumulator is recharged from time to time by forcing a quantity of fresh liquid fuel through a delivery non-return valve into the pressure accumulator, the fuel 5 being contained in a forcing chamber having a piston or diaphragm dividing the said chamber into a first chamber containing a quantity of the fuel and a second chamber containing a sealed quantity of suitable fluid of low boiling point, low critical- point temperature and high lo critical-point pressure, and the fuel being forced out of the forcing chamber against the pressure in the fuel pressure accumulator when a higher pressure is generated in the second chamber by heating the sealed quantity of suitable fluid to or above its critical-point temperature.

The sealed fluid contained in the forcing chamber may be propane having a boiling point at atmospheric pressure of -42 C and heated to its criticalpoint temperature of 96.8 C generating a critical boiling pressure of 42. 1 bar. It may 20 be a chlorofluorohydrocarbon, e.g. CHC1F2 (which is inert but does not contribute significantly to ozone depletion) having a boiling point at atmospheric pressure of -40.8 C and heated to its critical-point temperature of 96 C generating a critical boiling pressure of 49.2 bar. It may be DME 25 having a boiling point at atmospheric pressure of -24.9 C and heated to its critical-point temperature of 127 C generating a critical boiling pressure of 53.7 bar. It may be chloromethane CHCl3 having a boiling point at atmospheric pressure of -23. 9 C and heated to a critical point So temperature of 143 C generating a critical boiling pressure of 66.1 bar. It may be ammonia having a boiling point at atmospheric pressure of -33.3 C and heated to a critical point temperature of 132.4 C generating a critical boiling pressure of 111.9 bar.

Thus by choosing the sealed fluid, or a mixture of fluids, with boiling properties in the range of those of the

- 4 above examples, a high recharging pressure may be generated by heating the fluid to a relatively low critical-point temperature. Such temperature is readily available using a fuel burner, an electric heater, or waste heat from the 5 engine coolant or exhaust gas.

It is known in hydraulics to store energy in a pressure accumulator. The pressure accumulator is a high pressure vessel containing a column of hydraulic fluid with pocket of lo inert compressed gas trapped on top of the fluid. Pressure energy is stored in the accumulator by pumping more hydraulic fluid into the accumulator by means of a hydraulic pump so that the pocket of inert gas is further compressed by the increased volume of hydraulic fluid forced into the accumulator. When energy is to be extracted from the pressure accumulator, some of the hydraulic fluid is released at high pressure from the accumulator driven by the pocket of inert compressed gas, and this hydraulic fluid is used to power a hydraulic device using the stored energy in 20 the inert compressed gas.

In the present invention, the fuel pressure accumulator is operated in a similar principle and has a similar construction to that of the hydraulic pressure accumulator, 25 but differs from the latter in that pressure energy stored in the inert compressed gas in the accumulator is recharged not by using a hydraulic pump forcing liquid into the accumulator, but instead by using the boiling pressure of the sealed fluid to force the liquid into the accumulator 30 using the principle of a thermodynamic pump.

In the invention, while liquid fuel at high pressure is continuously extracted from the fuel pressure accumulator to supply an engine fuel injection system, the accumulator is recharged from time to time from the forcing chamber without interrupting the fuel supply to the fuel injection system thus ensuring continuous operation of the engine for an

- 5 indefinite period of time, and this is achieved without relying on a mechanical liquid fuel pump.

In the invention, the forcing chamber itself must be 5 refilled with fresh fuel before recharging the fuel pressure accumulator. This is achieved by removing the heating or isolating the sealed fluid from the heating, and allowing the fluid to condense thus lowering the pressure in the forcing chamber, and providing a fuel transfer pump for lo transferring liquid fuel from a fuel storage tank at substantially similar pressure through a supply non-return valve into the forcing chamber.

Preferably the chosen sealed fluid contained in the 5 forcing chamber has a vapour pressure at ambient temperature lower than that of the liquid fuel in the fuel storage tank at the same temperature. In this case, there is no need of the fuel transfer pump and the liquid fuel will flow automatically from the fuel storage tank into the forcing 20 chamber.

The whole process of refilling the forcing chamber and recharging the fuel pressure accumulator according to the method of the present invention may be fully automated by 25 using suitable sensors and actuators controlled by a central processing unit.

The invention and the associated rechargeable fuel pressure accumulator have many advantages over a mechanical JO fuel pump in that the liquid fuel stored in the fuel pressure accumulator is at substantially ambient temperature thus maintaining the fuel in a subcooled liquid state when delivered to the fuel injection system. Also because a batch of liquid fuel is always stored under high pressure in 5 the fuel pressure accumulator, it is available at any time for immediate starting of the engine without having to wait for the fuel pressure to build up as would be the case with

- 6 a mechanical fuel pump. It also avoids cavitation problems that sometimes occur at the suction side of the pump.

Finally the rechargeable fuel pressure accumulator has lower cost and better durability, especially for fuels of low 5 viscosity such as LPG and DME.

The invention is also suitable for use with other fuels including gasoline and alcohol fuels. It may also be used in an engine designed to run with two different fuels at lo least one of which is pressurised by a rechargeable fuel pressure accumulator.

In one type of dual-fuel engine selectable to operate with one or another fuel, such as LPG or gasoline, the same fuel rail and fuel injectors may be used common to both fuels individually, where the two fuels are supplied in liquid form and are pressurized separately at least one by a rechargeable fuel pressure accumulator. A two-way valve may be used to direct one or the other high pressure fuel supply 20 to the common fuel rail thus enabling liquid LPG to be injected through the same fuel injectors as gasoline in a port-injected or direct-injected spark ignition engine.

This gives excellent fuel preparation through flash evaporation of the injected liquid LPG and mitigates the 25 loss in engine performance normally experienced when the engine is switched to run with evaporated gaseous LPG.

In another type of dual-fuel engine where two fuels, such as DME and diesel fuel, are supplied simultaneously in 30 different proportions to the engine, the DME is supplied in liquid form and pressurized by a rechargeable fuel pressure accumulator for injection through one set of fuel injectors into the intake ports of the engine to form a lean homogeneous DME fuel charge, while the diesel fuel is 35 pressurized separately by a mechanical fuel pump and injected through another set of fuel injectors directly into the combustion chambers of the engine to form within each

- 7 combustion chamber a localized diesel fuel charge surrounded by a premixed lean homogeneous DME fuel charge. This type of dual-fuel combustion system is particularly suitable for a compression ignition engine running in homogeneous charge 5 compression ignition (HCCI) mode with DME, combined with diesel type combustion in varying proportions covering the full range of engine loads.

Brief description of the drawing

The invention will now be described further, by way of example, with reference to a single drawing showing a schematic view of a fuel pressure accumulator with a system for recharging it with fuel using the method of the present invention. Detailed description of the preferred embodiment

In the drawing of Figure 1, a fuel pressure accumulator go 10 is shown with a piston 12 with ring seals separating the accumulator chamber into an upper chamber 16 containing an inert compressed gas and a lower chamber 14 closed off by a non-return valve 18 and containing a quantity of liquid fuel pressurized by the inert compressed gas in the upper chamber 5 16. The high pressure fuel is delivered through a pressure regulator 40 to a fuel rail 44 of an engine fuel injection system having fuel injectors 46.

In so far described, the fuel pressure accumulator 10 30 is known and commonly used in engine research laboratories for forcing liquid fuel at high pressure to the fuel injection system of a test engine without relying on a mechanical fuel pump to provide the high pressure fuel.

However, such an arrangement is only suitable for running 35 the engine for a short period of time because the pressurised fuel accumulator has a limited fuel storage capacity and has to be de-pressurised each time it is

- 8 refilled with fresh fuel. Also such an arrangement would be impractical for mobile application on a vehicle where a source of inert compressed gas is not readily available.

5 For continuous operation of the engine, the fuel pressure accumulator 10 must be recharged from time to time with fresh fuel without interrupting the fuel supply to the fuel injection system. This is achieved in the present invention by providing a forcing chamber 20 for forcing from lo time to time a quantity of fresh liquid fuel through the delivery non-return valve 18 into the pressure accumulator 10. The forcing chamber 20 has a piston 22 with ring seals separating the forcing chamber into an upper chamber 24 containing a quantity of fresh liquid fuel and a lower chamber 26, 28 containing a sealed quantity (shaded volume) of suitable fluid of low boiling point, low critical-point temperature and high critical-point pressure collected by gravity at the bottom of the chamber 26, 28. During the recharging operation, the fuel in the upper chamber 24 is 20 forced out of the forcing chamber 20 through the delivery non-return valve 18 to the lower chamber 14 of the accumulator 10 against the pressure of the inert compressed gas in the upper chamber 16 of the accumulator 10, and to generate the necessary higher forcing pressure, a heat 5 source 50 is applied to boil the sealed fluid in the lower chamber 28 of the forcing chamber 20 and raise the temperature of the fluid to or above its critical-point temperature. So The sealed fluid contained in the chamber 26, 28 may be propane having a boiling point at atmospheric pressure of -42 C and heated to its critical-point temperature of 96.8 C generating a critical boiling pressure of 42.1 bar. It may be a chlorofluorohydrocarbon, e.g. CHClF2 (which is inert but 35 does not contribute significantly to ozone depletion) having a boiling point at atmospheric pressure of -40.8 C and heated to its critical-point temperature of 96 C generating

- 9 a critical boiling pressure of 49.2 bar. It may be DME having a boiling point at atmospheric pressure of -24.9 C and heated to its critical-point temperature of 127 C generating a critical boiling pressure of 53.7 bar. It may 5 be chloromethane CHCl3 having a boiling point at atmospheric pressure of -23.9 C and heated to a critical point temperature of 143 C generating a critical boiling pressure of 66.1 bar. It may be ammonia having a boiling point at atmospheric pressure of -33. 3 C and heated to a critical o point temperature of 132.4 C generating a critical boiling pressure of 111.9 bar.

Thus by choosing the sealed fluid, or a mixture of fluids, with boiling properties in the range of those of the above examples, a high recharging pressure may be generated by heating the fluid to a relatively low critical-point temperature. Such temperature is readily available using a fuel burner, an electric heater, or waste heat from the engine coolant or exhaust gas (represented generally by the 20 heat source 50).

In Figure 1, the forcing chamber 20 itself must be refilled with fresh fuel before recharging the fuel pressure accumulator 10. This is achieved by removing the heat 25 source 50 or isolating the sealed fluid from the heat source 50, and cooling the chamber 26, 28 to condense the fluid thus lowering the pressure in the forcing chamber 20. A fuel transfer pump 32 then transfers liquid fuel from a fuel storage tank 34 at substantially similar pressure through a 30 supply non-return valve 30 into the forcing chamber 24.

In a special case where the fuel in the storage tank 34 is LPG and the sealed fluid in the forcing chamber 26, 28 is DME or chloromethane, because the vapour pressure of LPG is 35 greater than that of DME or chloromethane at the same temperature, there is no need of the fuel transfer pump 32 and the LPG will flow automatically into the chamber 24.

- 10 The whole process of refilling the forcing chamber 20 and recharging the fuel pressure accumulator 10 may be fully automated by using suitable sensors and actuators controlled by a central processing unit (not shown).

The invention of Figure 1 and the associated fuel pressure accumulator 10 have many advantages over a mechanical fuel pump in that the liquid fuel stored in the fuel pressure accumulator 10 is at substantially ambient lo temperature thus maintaining the fuel in a subcooled liquid state when delivered to the fuel injection system. Also because a batch of liquid fuel is always stored under high pressure in the fuel pressure accumulator 10, it is available at any time for immediate starting of the engine 5 without having to wait for the fuel pressure to build up as would be the case with a mechanical fuel pump. It also avoids cavitation problems that sometimes occur at the suction side of the pump. Finally the rechargeable fuel pressure accumulator 20, 10 has lower cost and better so durability, especially for fuels of low viscosity such as LPG and DME.

Claims

- 11 CLAIMS

1. A method of recharging a fuel pressure accumulator supplying an engine fuel injection system with liquid fuel 5 at high pressure, wherein the fuel pressure accumulator is recharged from time to time by forcing a quantity of fresh liquid fuel through a delivery non-return valve into the pressure accumulator, the fuel being contained in a forcing chamber having a piston or diaphragm dividing the said 0 chamber into a first chamber containing a quantity of the fuel and a second chamber containing a sealed quantity of suitable fluid of low boiling point, low critical- point temperature and high critical-point pressure, and the fuel being forced out of the forcing chamber against the pressure 5 in the fuel pressure accumulator when a higher pressure is generated in'the second chamber by heating the sealed quantity of suitable fluid to or above its critical-point temperature. 20 2. A method of recharging a fuel pressure accumulator as claimed in claim 1, wherein the sealed fluid contained in the forcing chamber is propane.

3. A method of recharging a fuel pressure accumulator 25 as claimed in claim 1, wherein the sealed fluid contained in the forcing chamber is a chlorofluorohydrocarbon, e.g. CHC1F2.

4. A method of recharging a fuel pressure accumulator 30 as claimed in claim 1, wherein the sealed fluid contained in the forcing chamber is DME.

5. A method of recharging a fuel pressure accumulator as claimed in claim 1, wherein the sealed fluid contained in the forcing chamber is chloromethane CHC13.

- 12 6. A method of recharging a fuel pressure accumulator as claimed in claim 1, wherein the sealed fluid contained in the forcing chamber is ammonia.

5 7. A method of recharging a fuel pressure accumulator as claimed in claim 1, wherein the sealed fluid contained in the forcing chamber has boiling properties in the range of those of the fluids claimed in claims 2 to 6.

lo 8. A method of recharging a fuel pressure accumulator as claimed in claim 1, wherein the sealed fluid contained in the forcing chamber is a mixture of two or more fluids having boiling properties in the range of those of the fluids claimed in claims 2 to 6.

9. A method of recharging a fuel pressure accumulator as claimed in any preceding claim, wherein while fuel at high pressure is continuously extracted from the fuel pressure accumulator to supply an engine fuel injection 20 system, the accumulator is recharged from time to time from the forcing chamber without interrupting the fuel supply to the fuel injection system.

10. A method of recharging a fuel pressure accumulator 25 as claimed in any preceding claim, wherein the forcing chamber is refilled with fresh fuel before recharging the fuel pressure accumulator by removing the heating or isolating the sealed fluid from the heating and allowing the fluid to condense thus lowering the pressure in the forcing 30 chamber, and providing a fuel transfer pump for transferring liquid fuel from a fuel storage tank at substantially similar pressure through a supply nonreturn valve into the forcing chamber.

35 11. A method of recharging a fuel pressure accumulator as claimed in any preceding claim, wherein the sealed fluid contained in the forcing chamber has a vapour pressure at

- 13 ambient temperature lower than that of the liquid fuel in the fuel storage tank at the same temperature.

12. A method of recharging a fuel pressure accumulator 5 as claimed in claim 11, wherein the sealed fluid contained in the forcing chamber is DME or chloromethane, and the liquid fuel in the fuel storage tank is LPG.

13. A method of recharging a fuel pressure accumulator lo as claimed in any preceding claim, wherein the liquid fuel in the fuel storage tank recharged into the fuel pressure accumulator include LPG, DME, gasoline and alcohol fuels.

14. A rechargeable fuel pressure accumulator recharged according to the method as claimed in any preceding claim.