WO2011000043A1

WO2011000043A1 - Fuel injector gain compensation for sub-sonic flow

Info

Publication number: WO2011000043A1
Application number: PCT/AU2010/000828
Authority: WO
Inventors: Richard Albert George Woolford
Original assignee: Orbital Australia Pty Ltd
Priority date: 2009-06-30
Filing date: 2010-06-30
Publication date: 2011-01-06
Also published as: CN102483001A; BRPI1015944A2; CN102483001B

Abstract

A gaseous fuel direct injection fuel system (10) for a reciprocating piston internal combustion engine of a vehicle comprises a fuel tank (13) for receiving a gaseous fuel, a fuel injector (20) calibrated for sonic flow and controlled by.an electronic control unit (ECU) which controls the duration of the opening of the injector (20), as well as the points in the engine operating cycle at which the injector (20) is opened and closed. The ECU receives input signals from various sensors providing information relating to the operating conditions of the engine and, driver demands and determines the differential pressure across a fuel injector (20) to ascertain the occurrence of a condition corresponding to sub-sonic flow therethrough, and operates the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the, sub-sonic flow condition.

Description

Fuel Injector Gain Compensation for Sub-Sonic Flow Field of the Invention

The present invention relates to direct injection of gaseous fuels for internal combustion engines. The term "direct injection" refers to use of fuel injectors for injecting fuel directly into the combustion chambers of internal combustion engines.

The term "gaseous fuels" as used herein refers to compressed gas fuels such as compressed natural gas (CNG) and hydrogen (hfe), and liquefied gaseous fuels such as liquefied petroleum gas (LPG) and liquefied natural gas (LNG). Background Art

The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application. Gaseous fuels are known to have certain advantages over liquid fuels (such as petrol and diesel) for internal combustion engines, particularly in relation to cost and resultant exhaust emissions. Because of these advantages, there is an increasing trend towards the use of such fuels in internal combustion engines.

In certain applications, particularly in relation to engines for smaller vehicles such as auto-rickshaws, motorcycles and scooters, the fuel system utilises the vapour pressure of the gaseous fuel for the delivery of the gaseous fuel to and subsequently from the fuel injector, which is also typically the delivery injector for fuel to the engine. This arrangement is advantageous in terms of cost, as it avoids the need for a fuel pump or other delivery mechanism for delivering the gaseous fuel to the fuel injector.

The fuel injector then meters a quantity of gaseous fuel into the combustion chamber. The quantity of gaseous fuel delivered may be a function of the duration of opening of the injector. The gaseous fuel can, for example, be LPG which is stored in a fuel tank. Typically, a portion of the LPG is stored in the fuel tank in a liquid state while a gaseous portion of the LPG is stored above the liquefied portion. The fuel tank is connected to the fuel injector by a fuel supply line which incorporates a pressure regulator to regulate the supply of pressure of the gaseous fuel to the fuel injector.

The operation of the fuel injector is controlled by an electronic control unit which can control the duration of the opening of the fuel injector, the time at which the fuel injector begins to open relative to the engine cycle, and the time at which the fuel injector begins to close relative to the engine cycle. The electronic control unit obtains input signals from various sensors providing information on the operating conditions of the engine as well as the driver demand, and outputs control signals to certain engine components.

Operation of the fuel injector is effected by the control means in order to control the differential pressure across the fuel injector, the differential pressure being the difference between the supply pressure of the gaseous fuel being metered and injected by the fuel injector, and the prevailing pressure within the corresponding combustion chamber. The differential pressure is hence a determining factor in the actual quantity of fuel injected into the combustion chamber.

The fuel injector operates in a choked condition when open and is configured so as to promote sonic flow throughout a gaseous fuel delivery event. Sonic operation of the fuel injector ensures that the delivery of the gaseous fuel directly into the combustion chamber is able to occur irrespective of the pressures of the prevailing gases therein. If the fuel injector were not to operate under sonic flow conditions, the downsteam combustion chamber pressures may compromise the delivery event and affect the net amount of fuel being delivered into the combustion chamber.

For direct injection of gaseous fuels, there may often be a requirement for late fuel injection timing under light engine loads such as at idle. Late injection is generally advantageous in terms of achieving a desired fuel distribution about the spark plug at the time of ignition. However, under some such circumstances, for example possibly during cold operating conditions, particularly at engine start-up, ■ and/ or low gaseous fuel levels, the differential pressure across the fuel injector may reduce to a point at which sub-sonic flow will occur through the fuel injector.

Further, late injection timing is advantageous under normal operating conditions because an increase in the cylinder pressure can assist in extending the operating or dynamic range of the fuel injector. However, under some conditions, the delivery pressure will reduce, causing sub-sonic flow through the fuel injector.

Sub-sonic flow through the fuel injector will have the effect of changing the fuel delivery characteristics of the fuel injector which was calibrated according to sonic flow conditions. It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.

Disclosure of the Invention

The present invention stems from the realisation that a fuel injector controlling direct delivery of a gaseous fuel into a combustion chamber of an internal combustion engine can be operated to compensate for varying delivery and combustion chamber pressures upon the occurrence of a condition likely to cause sub-sonic flow through the fuel injector.

According to a first aspect of the invention there is provided a method of controlling a gaseous fuel direct injection system for an engine, the method comprising determining the differential pressure across a fuel injector to ascertain the occurrence of a condition corresponding to sub-sonic flow therethrough, and operating the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the sub-sonic flow condition. With this arrangement, the quantity of gaseous fuel delivered into a combustion chamber of the engine is a function of the duration of opening of the fuel injector.

The gaseous fuel injected may constitute the sole fuelling requirement for the engine operating cycle. Alternatively, the gaseous fuel injected may augment, or - A -

■ be augmented by, other fuel delivered to the combustion chamber during the engine operating cycle.

Preferably, the operation of the fuel injector is controlled by a control means such as an electronic control unit (ECU). Specifically, the ECU determines the parameters by which the fuel injector operates. The operating parameters may, for example, comprise the duration of the opening of the fuel injector, the time at which the fuel injector begins to open relative to the engine operating cycle, the time at which the fuel injector begins to close relative to the engine operating cycle, or any combination thereof. Preferably, the ECU monitors the delivery pressure of the gaseous fuel and also monitors a parameter indicative of the prevailing pressure in the combustion chamber relative to the engine cycle. In this way, the differential pressure can be evaluated or at least approximated.

The ECU may monitor the delivery pressure of the gaseous fuel through an input received from a pressure sensor associated with the fuel injector. The pressure sensor may measure the pressure of the gaseous fuel in a fuel rail to which the fuel injector is connected.

In the case of a reciprocating piston internal combustion engine, the combustion chamber pressure varies as a function of the position of the piston within the cylinder. With such an arrangement, the parameter being monitored to provide an indication of the prevailing pressure in the combustion chamber may comprise the crank angle of the engine.

The ECU may refer to a "look up" map or table to determine the typical prevailing pressure in the combustion chamber at a prescribed point or time in the engine cycle. Typically, the ECU would receive signals indicative of the crank angle before top dead centre (BTDC). The "look up" map or table would accordingly have an ordinate of crank angle BTDC with the output of the "look up" map or table being predicted combustion chamber pressure. In this way, for any particular crank angle BTDC the ECU could determine the expected prevailing pressure within the combustion chamber. From this the differential pressure can be determined and, if the differential pressure is at a level reflective of a sub-sonic flow condition through the fuel injector, the ECU would adjust the operating parameters of the fuel injector to compensate for the sub-sonic flow condition. In particular, the duration of opening of the fuel injector is modified according to the "look up" map or table, with the adjustment to opening duration being made as a function of the pressure differential across the fuel injector.

In a more refined system, the "look up" map or table could have ordinates of crank angle BTDC versus driver demand or throttle angle (degrees), or crank angle BTDC versus engine manifold pressure (kPa abs), with the output of the "look up" map or table being predicted combustion chamber pressure. Further accuracy in determining the combustion chamber pressure could be gained if the temperature of the inlet air within the engine intake manifold were able to be determined which could then be used to further modify the predicted combustion chamber pressure for a particular crank angle BTDC. In another arrangement, the occurrence of a sub-sonic flow condition can be determined by calculation using known gas flow formulae, rather than by monitoring a parameter indicative of the prevailing pressure in the combustion chamber relative to the engine operating cycle.

According to a second aspect of the invention there is provided a gaseous fuel direct injection system controlled by a method according to the first aspect of the invention.

Accorrding to a third aspect of the invention there is provided an engine having a gaseous fuel direct injection system controlled by a method according to the first aspect of the invention. According to a fourth aspect of the invention there is provided a gaseous fuel direct injection system for an engine, the injection system comprising means for determining the differential pressure across a fuel injector to ascertain the occurrence of a condition corresponding to sub-sonic flow therethrough and means for operating the fuel injector as a function of the differential pressure ■ during the occurrence of the sub-sonic flow condition to compensate for the subsonic flow condition.

According to a fifith aspect of the invention there is provided a fuel injection system for an engine, the fuel injection system comprising a fuel injector for direct injection of a gaseous fuel into a combustion chamber, means for determining the differential pressure across the fuel injector to ascertain the occurrence of a condition corresponding to sub-sonic flow therethrough, and means for operating the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the sub-sonic flow condition. According to a sixth aspect of the invention there is provided an engine having a gaseous fuel direct injection system, the injection system comprising means for determining the differential pressure across a fuel injector to ascertain the occurrence of a condition corresponding to sub-sonic flow therethrough, and means for operating the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the subsonic flow condition.

Brief Description of the Drawings

The invention will be better understood by reference to the following description of one specific embodiment thereof, as shown in the accompanying drawings in which:

Figure 1 is a schematic view of a gaseous fuel direct injection system according to the embodiment; and

Figure 2 is a graphical representation of the combustion chamber pressure in relation to the crank angle of a reciprocating piston engine fitted with the gaseous fuel direct injection system according to the embodiment.

Best Mode(s) for Carrying Out the Invention

Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The embodiment shown in the drawings is directed to a gaseous fuel direct injection fuel system 10 for a reciprocating piston internal combustion engine of a vehicle (not shown) which might typically be a smaller vehicle such as an auto- rickshaw, motorcycle or scooter.

The embodiment will be described in relation to use of a liquefied gas fuel; specifically LPG. However, it should be understood that any appropriate gaseous fuel may be used in relation to the invention. The fuel system 10 comprises a fuel tank 13 for receiving a gaseous fuel which in this embodiment comprises LPG. The LPG is stored in a liquefied state in the tank 13. Although the LPG is stored in a liquefied state in the fuel tank 13, a portion of the LPG exists in a gaseous state within the fuel tank above the liquefied LPG.

The LPG is delivered to a fuel rail 15 along a fuel supply line 17. A pressure regulator 19 is incorporated in the fuel line 17 to regulate the supply pressure of LPG to the fuel rail 15.

In this embodiment the engine is a multi-cylinder engine and the gaseous fuel is injected directly into the combustion chambers defined within the various cylinders of the engine by fuel injectors 20 connected to the fuel rail 15. As is conventional practice, the fuel injectors 20 are configured to operate in a choked condition when open; that is, the fuel injectors 20 are calibrated for sonic flow.

Operation of the fuel injectors 20 is controlled by an electronic control unit (not shown). The electronic control unit (ECU) can control the operating parameters of each fuel injector 20, particularly the duration of the opening of the injector 20, as well as the points in the engine operating cycle at which the injector 20 is opened and closed.

The ECU receives input signals from various sensors providing information relating to the operating conditions of the engine and driver demands. The ECU outputs various control signals, including in particular control signals relating to operation of the fuel injectors 20.

The sensors for providing information to the ECU include a pressure sensor 18 for sensing the pressure of the vaporised LPG within the fuel rail 15. In other words, the pressure sensor 18 monitors the delivery pressure of vaporised LPG to the fuel injectors 20.

The ECU is then able to vary the operating parameters of the fuel injectors 20 accordingly in order to meter the required quantity of injected gaseous fuel for controlling the air-to-fuel ratio of a combustible mixture in the corresponding engine cylinders to achieve acceptable performance, combustion stability and emissions control.

The ECU varies the operating parameters of each fuel injector 20 by controlling the duration of opening of the fuel injector 20, the time at which the fuel injector 20 begins to open relative to the engine operating cycle, the time at which the fuel injector 20 begins to close relative to the engine operating cycle, or any combination thereof.

In addition to monitoring the delivery pressure to each fuel injector 20, the ECU also monitors a parameter indicative of the prevailing pressure in the respective combustion chamber relative to the engine operating cycle. In this way, the differential pressure across each fuel injector 20 can be evaluated or at least approximated.

In this embodiment, the parameter being monitored comprises the crank angle within an engine operating cycle BTDC as it relates to a cylinder of the engine.

The ECU refers to a "look up" map or table to determine the typical prevailing pressure in the combustion chamber at a prescribed time in the engine operating cycle. Specifically, the ECU receives signals indicative of the crank angle BTDC.

The "look up" map or table has an ordinate of crank angle BTDC with the output of the "look up" map or table being predicted combustion chamber pressure. In this way, for any particular crank angle BTDC, the ECU can determine the expected prevailing pressure within the respective combustion chamber. From this information as well as information relating to the delivery pressure of the vaporised LPG as measured in the fuel rail 15, the differential pressure across the fuel injector 20 can be determined.

Predicted combustion chamber pressures in relation to different crank angles BTDC are determined by bench testing conducted in relation to the engine.

As well as crank angle BTDC, the "look up" map or table may also use driver demand or throttle angle (degrees) as a further ordinate, or engine manifold pressure (kPa abs), with the output of the "look up" map or table similarly being predicted combustion chamber pressure. Further accuracy in predicting the combustion chamber pressure can be gained by determining the temperature of the inlet air within the engine intake manifold which could then be used to further modify predicted combustion chamber pressures for particular crank angles BTDC.

Normally, each fuel injector 20 operates in a choked condition to promote sonic flow throughout the gaseous fuel delivery event. Sonic operation of the fuel injector 20 ensures that the delivery of the gaseous fuel directly into a corresponding combustion chamber is able to occur irrespective of the pressures of the prevailing gases therein. If the fuel injector 20 were not to operate under sonic flow conditions, the downsteam pressure in the combustion chamber may compromise the delivery event and affect the net amount of fuel being delivered to the combustion chamber.

During operation of the engine, the ECU determines the differential pressure across each fuel injector 20. Provided that the differential pressure across the fuel injector 20 is at a level corresponding to a sonic flow condition, no remedial action is required.

Under some operating conditions of the engine, the differential pressure may reduce to a level reflective of a sub-sonic flow condition. One circumstance where this condition might arise is where there is a reduction in the delivery pressure in the fuel rail 15, arising (for example) from cold operating conditions, and more particularly through a cold starting condition, where tank vapour pressure is low and/or a low gaseous fuel level condition exists leading to a reduction in the vapour pressure of the LPG, ultimately causing sub-sonic flow through the fuel injector 20. Another circumstance where this condition might arise is where there is a requirement to retard fuel injection timing for combustion stability; for example, at light engine loads such as idle. Late injection timing typically equates to their being a higher combustion chamber pressure at the point of injection, thereby reducing the differential pressure across the respective fuel injector 20, and potentially causing sub-sonic flow through the fuel injector 20.

Figure 2 is graphical representation of the combustion chamber pressure in relation to the crank angle of an engine cylinder. In Figure 2, the combustion chamber pressure, which is the cylinder pressure, is depicted by the line identified by reference numeral 21. Further, the delivery pressure in the fuel rail 15, which corresponds to the fuel injection pressure, is depicted by the line identified by reference numeral 23. In the arrangement shown, the differential pressure between the fuel injection pressure as depicted by line 23 and the combustion chamber pressure as depicted by line 21 promotes sub-sonic flow conditions in the band occurring between the two crank angles depicted by the broken lines identified by reference numerals 25 and 27.

It should be noted that the band within which sub-sonic flow conditions occurs can also be determinded for different fuel injection pressures by calculation. For example, in the representation shown in Figure 2, line 29 is determined by such calculations and its intersection with line 23 determines the start of the band within which sub-sonic flow occurs, specifically identified by broken line 25. It will hence be evident that, for example, for higher fuel injection pressures, the band within which sub-sonic flow occurs would move rightward in Figure 2 as line 23 (indicating fuel injection pressure) would consequently intersect line 29 and line 21 (indicating cylinder pressure) at higher points along each curve. That is, for higher fuel injection pressures, the sub-sonic flow band moves closer to TDC within an engine operating cycle, and vice versa for lower fuel injection pressures. Upon detecting that the differential pressure across the respective fuel injector 20 is at a level reflective of a sub-sonic flow condition, the ECU adjusts the operating parameters of the fuel injector 20 to compensate for the sub-sonic flow condition. Typically, the ECU would vary the duration of opening of the fuel injector 20 to enable delivery of an appropriate quantity of gaseous fuel to facilitate a desired level of combustion stability. More particularly, the duration of opening of the fuel injector 20 is modified according to the "look up" map or table, with the adjustment to opening duration being made as a function of the pressure differential across the fuel injector 20; that is, in the sub-sonic region the duration of opening of the fuel injector 20 is extended more the lower the pressure differential. The purpose of changing the duration of opening of the fuel injector 20 is to maintain the same 'target mass fuel' delivered to the engine cylinder (that is, if the pressure differential indicates sonic flow, then the duration of opening of the fuel injector may be Y milliseconds, but when the pressure differential indicates sub-sonic flow then the duration of opening of the fuel injector will need to be >Y milliseconds). The expectation is that the "look up" map or tables for the prevailing conditions extend the injector duration so as to maintain the same mass flow of fuel (and therefore a target air- to-fuel ratio in the combustion chamber).

Furthermore, a change between sonic flow and sub-sonic flow may possibly occur during an injection event. In such circumstances, the value referred to in the "look up" map or table may be determined by the value at the midpoint of the pulse width of the fuel injector 20 at that particular time.

From the foregoing, it is evident that the present embodiment provides a relatively simple yet highly effective arrangement for implementing compensation for the occurrence of sub-sonic flow conditions in a fuel injector arranged to deliver gaseous fuel directly into an engine cylinder. The embodiment models the combustion chamber pressure relative to the crank angle of the engine cylinder so that the ECU can make an assessment as to the prevailing pressure in the combustion chamber without requiring a specific calculation to be performed at each injection event (which would necessitate significant processing power). The calibration work can typically be performed through bench testing, thus reducing the ECU requirement during engine operation to no more than referencing a "look up" map or table.

It should be appreciated that the scope of the invention is not limited to the scope of the embodiment described. While the embodiment has been described in relation to LPG as the gaseous fuel, it should be appreciated that the invention may be applicable to any other liquefied gaseous fuel including liquefied natural gas (LNG) as well as compressed gas fuels such as compressed natural gas (CNG) and hydrogen (H₂).

Further, in the embodiment described, the LPG constituted the entire fuelling requirement for the engine; that is, only LPG was used to fuel the combustion process. In other embodiments, the LPG (or other gaseous fuel) may augment, or be augmented by, other fuel delivered to the combustion chamber during the engine operating cycle. The other fuel would typically be delivered by a separate delivery event. Where the other fuel is delivered by a direct injection process, it may be delivered by way of a separate fuel injector.

Further, while the embodiment has been described in relation to an assessment as to the prevailing pressure in the combustion chamber by modelling (without requiring a specific calculation to be performed at each injection event), it should be understood that an arrangement in which parameters are measured and a specific calculation is performed to determine the actual combustion chamber pressure at each injection event is within the scope of the invention.

Claims

■ The Claims Defining the Invention are as Follows:

1. A method of controlling a gaseous fuel direct injection system for an engine, the method comprising determining the differential pressure across a fuel injector to ascertain the occurrence of a condition corresponding to sub-sonic flow therethrough, and operating the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the sub-sonic flow condition.

2. The method according to claim 1 wherein the operation of the fuel injector is controlled by a control means operable to determine the operating parameters for the fuel injector.

3. The method according to claim 2 wherein the operating parameters comprise the duration of the opening of the fuel injector.

4. The method according to claim 2 or 3 wherein the operating parameters comprise, or further comprise, the time at which the fuel injector begins to open relative to the engine operating cycle,

5. The method according to claim 2, 3 or 4 wherein the operating parameters comprise, or further comprise, the time at which the fuel injector begins to close relative to the engine operating cycle.

6. The method according to claim 2 wherein the operating parameters are selected from a group consisting of: the duration of the opening of the fuel injector; the time at which the fuel injector begins to open relative to the engine operating cycle; and the time at which the fuel injector begins to close relative to the engine operating cycle.

7. The method according to any one of the preceding claims wherein the control means monitors both the delivery pressure of the gaseous fuel and a parameter indicative of the prevailing pressure in the combustion chamber relative to the engine cycle.

8. The method according to claim 7 wherein the control means monitors the delivery pressure of the gaseous fuel through an input received from a pressure sensor associated with the fuel injector.

9. The method according to claim 8 wherein the pressure sensor measures the pressure of the gaseous fuel in a fuel rail to which the fuel injector is connected.

10. The method according to claim 7, 8 or 9 wherein the control means refers to a "look up" map or table to determine the typical prevailing pressure in the combustion chamber at a prescribed time in the engine cycle.

11. The method according to any one of claims 7 to 10 wherein the engine comprises a reciprocating piston internal combustion engine during operation of which the combustion chamber pressure varies as a function of the position of the piston within the cylinder, and wherein the parameter being monitored to provide an indication of the prevailing pressure in the combustion chamber comprises the crank angle of the engine.

12. The method according to claim 11 wherein the control means receives signals indicative of the crank angle before top dead centre (BTDC) and wherein the "look up" map or table comprises an ordinate of crank angle BTDC to enable determination of a predicted combustion chamber pressure.

13. The method according to claim 12 wherein the control means can determine the expected prevailing pressure within the combustion chamber for any particular crank angle BTDC whereupon the differential pressure can be determined.

14. The method according to claim 13 wherein the control means adjusts the operating parameters of the fuel injector to compensate for the sub-sonic flow condition in the event of the differential pressure being at a level reflective of a sub-sonic flow condition through the fuel injector

15. The method according to claim 14 wherein the control means adjusts the operating parameters by modifying the duration of opening of the fuel injector according to the "look up" map or table, with the adjustment to opening duration being made as a function of the pressure differential across the fuel injector.

16. The method according to any one of claims 1 to 6 wherein the occurrence of a sub-sonic flow condition is determined by calculation using known gas flow formulae.

17. A gaseous fuel direct injection system controlled by a method according to any one of claims 1 to 16.

18.An engine having a gaseous fuel direct injection system controlled by a method according to any one of claims 1 to 16.

19. A gaseous fuel direct injection system for an engine, the injection system comprising means for determining the differential pressure across a fuel injector to ascertain the occurrence of a condition corresponding to subsonic flow therethrough and means for operating the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the sub-sonic flow condition.

20. A fuel injection system for an engine, the fuel injection system comprising a fuel injector for direct injection of a gaseous fuel into a combustion chamber, means for determining the differential pressure across the fuel injector to ascertain the occurrence of a condition corresponding to sub- sonic flow therethrough, and means for operating the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the sub-sonic flow condition.

21.An engine having a gaseous fuel direct injection system, the injection system comprising means for determining the differential pressure across a fuel injector to ascertain the occurrence of a condition corresponding to sub-sonic flow therethrough, and means for operating the fuel injector as a function of the differential pressure during the occurrence of the sub-sonic flow condition to compensate for the sub-sonic flow condition.

22. A method of controlling a gaseous fuel direct injection system for an engine, the method being substantially as herein described.

23.A fuel injection system for an engine, the fuel injection system being substantially as herein described with reference to the accompanying drawings.

24.An engine having a gaseous fuel direct injection system substantially as herein described with reference to the accompanying drawings.

Substitute Sheet

(Rule 26) RO/ AU