SE1651227A1 - Fuel injection system - Google Patents

Abstract

The invention relates to a fuel injection system for supplying pressurised fuel, in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine. Said fuel injection system comprising a fuel tank (1), a feed line (2) for supply of the fuel into a fuel pump (3) having a pumping element (4) and an outlet valve (5), an injector (6) that receives pressurized fuel from said fuel pump (3) for delivering it for combustion in the internal combustion engine, a back-leak line (7) for removal of the fuel leaked from pressurized parts of the injector back to the fuel tank (1) via a return line (8), and a first return valve (9) installed between the back-leak line and the return line. Said first return valve is arranged to prevent fuel flow from the return line into the back-leak line and to allow fuel flow from the injector into the return line when fuel pressure in the back-leak line exceeds pressure in the return valve by a predetermined amount. Further, additional means is provided for connection of the back-leak line to the return line.(Fig.1)

Description

Fuel injection system TECHNICAL FIELDThis invention relates to a fuel injection system for supplying pressurised fuel,in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine.

BACKGROUND OF THE INVENTION High-volatility fuels, such as Dl\/IE, can present difficulties in the fast-actingsystems that operate in a cyclic manner and at relatively high pressures,which is typical of ICE fuel injection systems, in regards of hydraulic tightnessof system's elements. Such fuels evaporate at already quite lowtemperatures and lower pressures, making it difficult to prevent leakage sincesystems' valves would have to effectively be gas-tight; this is combined withthe requirement of reliable and durable operation of these same valves athigh hydraulic pressure and flow, the latter often meaning large acting forcesand contact pressures in the valve seats.

Failure to ensure a gas-tight seal in the valves of fuel injection systems canlead to both external and internal leakages. Although obviously best effortmust be executed to avoid leakages, it is not feasible to completely preventthem in all circumstances. External leakage usually appears at the injectornozzles and also in the dynamic seals of fuel pumps. Special means areknown to prevent these external leakages, whilst internal leakages areusually less critical in terms of safety and product quality. Tightness ofvarious valves in service is to a certain degree probabilistic and cannot beknown exactly once the vehicle/engine accumulated significant mileage.Some combinations of less tight vs. properly sealing valves could exacerbateconsequences of leakage at the more critical parts of the system.

An example of this is a high-pressure common rail fuel system of a dieselengine in which injectors are protected from being constantly subjected to fuel pressure from the return line. This protection can be arranged by anautomatic return isolating valve placed between the injector and the returnline such that when the return line pressure is higher than the injectorpressure at engine standstill, the valve closes to prevent the relatively largevolume of fuel from the return line to leak via non-tight nozzles into enginecylinders. This same valve is usually advantageously arranged to alsoperform the function of a pressure regulating valve that maintains a certainrelatively high injector backpressure during engine operation. When thisreturn isolating valve is tight, it would not allow injector pressure to comedown following an engine stop, and unless injector pressure is allowed toescape via the inlet path, injector would stay pressurized, increasing the riskof external leakage from the nozzle into the engine cylinder.

There can be reasons for maintaining relatively high pressure at injector inletuntil long after the engine is stopped, for instance in order to ensure goodengine startability. This then comes in conflict with the requirement to notsubject injector to fuel pressure in order to prevent leakage via nozzles intothe stopped engine.

There is thus a need for an improved fuel injection system that can solve theabove conflict of requirements and thereby reduce potential fuel leakagethrough injectors of a stopped engine.

SUMMARY OF THE INVENTION An object of the present invention is to provide an inventive fuel injectionsystem that reduces fuel leakage from injectors when the engine is notrunning. This object is achieved by the features of the characterising portionof claim 1.

The invention concerns a fuel injection system for supplying pressurised fuel,in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine, and comprises a fuel tank, a feed line for supply of the fuel into a fuel pump having a pumping element and an outlet valve, aninjector that receives pressurized fuel from said fuel pump for delivering it forcombustion in the internal combustion engine, a back-leak line for removal ofthe fuel leaked from pressurized parts of the injector back to the fuel tank viaa return line, and a first return valve installed between the back-leak line andthe return line, wherein said first return valve is arranged to prevent fuel flowfrom the return line into the back-leak line and to allow fuel flow from theinjector into the return line when fuel pressure in the back-leak line exceeds pressure in the return valve by a predetermined amount.

The invention is characterized in that said fuel injection system furthercomprises an additional means for connection of the back-leak line to thereturn line, which is arranged to connect the back-leak line to the return line at a lower level of the predermined amount when the engine is not running.

According to one embodiment of the invention, such additional means isarranged in the form of a second return valve for connection of the back-leakline to the return line. This valve is electrically controlled to open and reducepressure in the injector when the engine is not running, while being closedwhen the engine is running to ensure that necessary back-leak line pressure can be set by the first return valve. ln an alternative embodiment of the invention, the second return valve isarranged to be actuated by pressure in the engine's lubricating oil system,such that when the engine runs and the oil pressure is relatively high, thesecond return valve is closed, whilst when the engine is stopped and itslubricating oil pressure is relatively low then the second return valve is open.

According to another embodiment of the invention, the additional means isarranged in the form of the first return valve having a variable level of thepredetermined amount, such that when the engine is not running the predetermined amount is low and the first return valve allows connection of the back-leak line to the return line at a relatively low pressure, whereas whenthe engine is running the predetermined amount is relatively high and arelatively high pressure can be maintained by the return valve in the back- leak line, as necessary for normal operation of the fuel injection system.

According to one embodiment, the variable level of the predermined amountis achieved through an electric control. ln another embodiment, engine's lubricating oil pressure is utilized to vary the predetermined amount.

By means of arranging the additional means of connecting the back-leak lineto the return line, as well as enabling additional control of the predeterminedpressure level at which the back-leak line is connected to the return line, apossibility of de-pressurizing the injectors through the back-leak line isachieved when the engine is not running, which in turn reduces the amount offuel that can leak into the engine through closed nozzles if they lose part oftheir sealing ability due to wear.

Further advantages are achieved by implementing one or several of the features of the dependent claims.

BRIEF DESCRIPTION OF DRAWINGSln the detailed description of the invention given below reference is made to the following figures, in which: Figure 1 shows an embodiment of the fuel injection system according tothe present invention; Figure 2 shows another embodiment of the fuel injection systemaccording to the present invention; Figure 3 shows a further alternative embodiment of the injection system according to the present invention.

DESCRIPTION OF EXAIVIPLE E|\/|BODl|\/IENTS OF THE INVENTIONVarious aspects of the invention will hereinafter be described in conjunctionwith the appended drawings provided to illustrate and not to limit the invention, wherein like designations denote like elements.

Figure 1 shows a preferred embodiment of the fuel injection system 13according to the invention. The fuel injection system 13 is particularly suitablefor supplying pressurised high-volatility fuel, such as dimethyl ether (Dl\/IE) ora blend thereof, to an internal combustion engine, but the fuel injectionsystem 13 is equally suitable for conventional fuel, such as diesel oil. Thefuel injection system 13 comprises a fuel tank 1, a feed line 2 for supply ofthe fuel into a fuel pump 3 having a pumping element 4 and an outlet valve 5,an injector 6 that receives pressurized fuel from the fuel pump for delivering itfor combustion in the internal combustion engine (not shown), a back-leakline 7 for removal of the fuel leaked from pressurized parts of the injectorback to the fuel tank 1 via a return line 8, and a first return valve 9 installedbetween the back-leak line and the return line, wherein the first return valveis arranged to prevent fuel flow from the return line into the back-leak line andto allow fuel flow from the injector into the return line when fuel pressure inthe back-leak line exceeds pressure in the return valve by a predeterminedamount. The return valve can be realized in a simpler form as a spring-loaded relief/overflow valve, in which the predetermined amount of pressure(that can be also defined as valve opening pressure) is dependent chiefly onthe spring load and the pressure in the return line. Pressure in the return linegenerally follows the tank pressure and is relatively constant, being mostlydependent on fuel vapour pressure characteristic and the fuel temperature.Therefore, by selecting appropriate pre-load of the spring 15, the desiredlevel of pressure in the back-leak line can be set. This is usually needed toensure stable operation of the injector 6, especially when volatile fuel such asDl\/IE is used. ln the embodiment depicted by Fig.1, the first return valve 9 has anotherimportant function of an automatic isolating valve, preventing exposure of theinjector to tank pressure and back flow of fuel from tank to injector when theengine is turned off. The backflow could othen/vise occur in case injector'snozzle has lost its tightness, then fuel under vapour pressure would bedisplaced from the tank 1, return 8 and back-leak 7 lines into the injector andthe nozzle and escape into the engine, creating potentially dangeroussituations and a loss of fuel. To enable the first return valve 9 to perform thefunction of the isolating valve, valve 9 necessarily has a reliable and durablesealing ability, so that no leakage occurs when the valve is kept in the closedposition. Because of this, the return valve is also reliable and durable inpreventing leakage in the forward flow up until its opening pressure is reached.

As seen in Fig.1, there is also a significant volume contained between thefuel pump 3 and the injector 6. The function of the outlet valves 5 is to allowflow in the forward direction and stop the flow in the opposite direction, that isfrom injector 6 back through the pump 3. Therefore, when the engine isturned off, the outlet valves 5 assume the closed position, such that fuelvolume under a residual pressure would create similar risk of leakage via theinjector 6 as described above. ln the so-called common rail fuel injectionsystems, the volume contained between injectors and the fuel pump isparticularly large, because common rail 12 is placed between the pump andthe injector in order to maintain high pressure at the injector during theinjection process which usually takes place at a rate higher than themaximum flow rate the pump 3 can achieve. With the common rail 12 inplace, the volume of the fuel that could leak out of the improperly sealednozzle of the injector at engine standstill becomes quite significant, as do theundesirable effects of such a leakage. However, draining the common railwith the stopped engine back to a lower pressure may be undesirable,because of the need to subsequently re-fill it with fuel before engine can bestarted again. Comparatively large volume of the common rail may make the re-filling process inacceptably long. For that reason, there is a requirementfor the valves upstream of the common rail, such as valves 5, to be hydraulically tight.

The problem of potential fuel leakage at engine standstill from the commonrail via the injectors and out to the engine can be effectively tackled by theuse of automatic isolating valves 14 installed between the common rail 12and the nozzle of the injector 6. The opening pressure of the isolating valve isusually set higher than the residual pressure that settles in the common railafter the engine stop, so that the isolating valve 14 would close when theengine is stopped and prevent displacement of fuel from upstream of injector6 and valve 14 into the injector and its nozzle. However, even a smallleakage through the isolating valve 14, that can occur for a number ofreasons such as wear or dirt in fuel, in combination with the first return valve9 retaining its hydraulic tightness, would cause re-pressurization of injectorand thus maintain leakage via an unsealed nozzle in a stopped engine.

To avoid such a leakage, the invention according to Fig.1 makes use of anadditional means of relieving pressure in the back-leak line 7 in the form of asecond return valve 10. The fuel injection system is designed to keep thesecond return valve 10 closed during normal operation, such that anecessary relatively high backpressure in the backleak line is set andmaintained by the first return valve 9, and open the second return valve 10when the engine is stopped. The second return valve 10 mayadvantageously be equipped with a non-return valve function represented bycheck valve 16 in Fig.1, so as to automatically prevent tank emptying throughthe injector nozzle when the second return valve 10 is open. When thesecond return valve is open after an engine stop, fuel that may be enteringfrom the still pressurized common rail 12 into injector 6 and its nozzle via aleaking isolating valve 14, would be relieved back to the return line 8, and thus leakage via an unsealed nozzle of injector 6 would be reduced. ln Fig.1, the second return valve 10 is shown to be electrically actuated.However, its actuation may be arranged in other ways, depending on therequirements to a particular system and engine. For instance, the pressure inthe engine lubrication system may be used to actuate the valve, so that whenthe engine is started and oil pressure in the lubrication system rises, thatpressure acts on an appropriate actuating mechanism to close valve 10against the force of a return spring. When the engine stops, the oil pressure drops allowing the return spring to move the valve 10 back and open it. ln an alternative embodiment of the present invention, represented by Fig.2,the additional means 11 for relieving pressure from the back-leak line 7 isdesigned in the form of a mechanism for varying the opening pressure of thefirst return valve 9. This may achieve an advantage of lower cost and betterreliability, because no additional valve and hydraulic connections are thennecessary, and the number of fuel seal points is kept to a minimum. ln Fig.2,the additional means 11 is electrically actuated to reduce the openingpressure of the first return valve 9 when the engine is stopped, and increasethat pressure when the engine is started. Similarly, the means 11 may bedesigned to be a mechanism hydraulically actuated by the pressure in theengine lubrication system, as illustrated in Fig.3. When the engine is startedand oil pressure rises, the mechanism is actuated to compress the returnspring 15 and thereby rise the opening pressure of the first return valve 9.When the engine stops and the oil pressure falls, the mechanism 11 releasesthe return spring 15, the opening pressure of the valve 9 is reduced andexcessive pressure in the back-leak line 7 is relieved for a reduction of theleakage via unsealed nozzle of injector 6.

Variations of the fuel injection system according to the invention, asillustrated by the different embodiments, should not be interpreted as limitedexactly to a given embodiment, but said variations may be applied to otherembodiments as well when not inconsistent with each other.

Reference signs mentioned in the claims should not be seen as Iimiting theextent of the matter protected by the claims, and their sole function is to make claims easier to understand.

As will be realised, the invention is capable of modification in various obviousrespects, all without departing from the scope of the appended claims.Accordingly, the drawings and the description thereto are to be regarded asillustrative in nature, and not restrictive. For example, several injectors can beutilised to feed a multi-cylinder engine, they all may be connected to same common rail via their respective pressure-isolating valves.

Claims (1)

1. CLAIIVIS 1 _ A fuel injection system for supplying pressurised fuel, in particulardimethyl ether (Dl\/IE) or a blend thereof, to an internal combustionengine, said fuel injection system comprising a fuel tank (1), a feedline (2) for supply of the fuel into a fuel pump (3) having a pumpingelement (4) and an outlet valve (5), an injector (6) that receivespressurized fuel from said fuel pump (3) for delivering it for combustionin the internal combustion engine, a back-leak line (7) for removal ofthe fuel leaked from pressurized parts of the injector back to the fueltank (1) via a return line (8), and a first return valve (9) installedbetween the back-leak line and the return line, wherein said first returnvalve is arranged to prevent fuel flow from the return line into the back-leak line and to allow fuel flow from the injector into the return linewhen fuel pressure in the back-leak line exceeds pressure in thereturn valve by a predetermined amount, characterised in thatadditional means (10) is provided for connection of the back-leak line to the return line. A fuel injection system according to claim 1, characterised in that saidadditional means is arranged to be a second return valve (1 O). A fuel injection system according to claim 2, characterized in that said second return valve (1 O) is electrically controlled. _ A fuel injection system according to claim 2, characterised in that said internal combustion engine incorporates a lubricating oil system,wherein the oil pressure in that system is at a higher level when theengine is running and is at a lower level when the engine is notrunning, further wherein said second return valve (10) is arranged tobe actuated by pressure in said lubricating oil system such that the second return valve is closed when the pressure in the lubricating 11 system is at the higher level, and is open when the pressure in theIubricating oil system is at the lower level. _ A fuel injection system according to any of claims 2-4, characterized in that said second return valve incorporates a non-return valve function,allowing flow from the back-leak line and preventing flow in theopposite direction. _ A fuel injection system according to claim 1, characterized in that said additional means is arranged to vary said predetermined amount ofpressure difference, such that this amount is relatively high when the engine is running and is relatively low when the engine is not running. _ A fuel injection system according to claim 6, characterized in that said additional means is electrically controlled. _ A fuel injection system according to claim 7, characterised in that said internal combustion engine incorporates a Iubricating oil system,wherein the oil pressure in that system is at a higher level when theengine is running and is at a lower level when the engine is notrunning, further wherein said additional means (11) is actuated by thepressure in the Iubricating oil system such that said predeterminedamount is relatively high when the oil pressure is at a higher level and low when the oil pressure is at a lower level.