TECHNICAL FIELD

This disclosure relates to a fuel injection system for an internal combustion engine, wherein the fuel injection system comprises a fuel tank, a low pressure fuel feed pump, a high pressure fuel pump with a suction channel for supplying fuel to a high pressure pumping mechanism, and a first fuel return line, wherein the low pressure fuel feed pump is arranged for supplying fuel from the fuel tank to the suction channel, and wherein the first fuel return line is arranged for enabling excessively supplied fuel from the low pressure fuel feed pump to be re-circulated to the fuel tank.

The fuel injection system is particularly suitable for supplying DME fuel, or other similar high volatile fuels, including blends of DME fuel. The combustion engine is preferably of the compression-ignition type, such as a diesel engine. The engine is preferably provided in a heavy vehicle, such as a heavy truck, a construction vehicle or a bus.

BACKGROUND

Rising prices of crude oil-derived fuel and fears of its effect on our climate as well as of its imminent shortages have in recent years led to further developments in production processes of alternative fuels and internal combustion engines for their use. One potentially important renewable fuel that can be effectively produced from a variety of stocks including biomass is dimethyl ether (DME). With its soot-free combustion and high cetane number, DME is very well suited for diese!-type internal combustion processes. However, DME has a high volatility compared to normal diesel fuel and has to be pressurized to approximately 5 bar in order to be liquid at room temperature. There are a number of advantages of having fuel supplied in liquid form for injection into a diesel-type internal combustion engine, and thus fuel injection equipment (FIE) for DME or other similarly volatile fuel should be specially designed to prevent unwanted vaporization of the fuel and/or to effectively remove vapour cavities from such parts of the injection system where vapour can disrupt normal operation.

High-volatility fuels can be prevented from boiling by selecting a higher pressure and/or lower operating temperature. In a particular engine application, fuel must be supplied to the injection pressure-generating part of the FIE at an optimum combination of fuel feed pressure and temperature. In selecting that optimum combination, the cost and complexity of the fuel feed system are significant controlling factors. Imbalance towards a higher feed pressure would, whilst allowing operation at elevated temperatures, increase system cost, complexity, weight, and energy consumption, at the same time itself causing extra fuel heat-up. On the other side, lowering fuel feed temperature beyond a certain limit would also not be feasible. in order to shift the combination of fuel feed pressure and temperature towards a more energy-efficient and less expensive balance, a number of design solutions are used in prior art systems. On such solution is recirculation of fuel feed flow throughout the system at a rate exceeding the rate of fuel consumption by the engine. By this means, equalization of temperatures between the coldest parts (e.g. the fuel tank) and the hottest parts of the system is achieved and the risk of local vaporization is therefore reduced. Another approach is thermal insulation of parts of the system that are most exposed to heat sources. For a number of reasons, the most suitable type of FIE for high-volatility fuel, such as DME, is a common rail system well-known in the art. Such a system usually incorporates a multi-plunger high-pressure pump that via a common rail supplies fuel under high pressure to injectors. Over many years of development and use in the diesel engine industry, the typical high-pressure pump of a common rail system has evolved to incorporate the output control concept based on the inlet metering principle and fixed plunger displacement. This makes use of a single electronically controlled inlet metering valve (I V) to define the amount of fuel reaching the plungers during their respective iniet strokes. At a given feed pressure, varying the valve's restriction varies the output of the plungers which in turn - at a given engine consumption - varies the common rail pressure.

The single inlet metering valve, fixed displacement multi-plunger type of pump is widely accepted in common rail FIE on the grounds of its relative simplicity as compared to variable-displacement pumps. However, when high-volatility fuels are used, improvements to such a pump are necessary to ensure reliable performance across the full operating range of an engine. This is because in this type of pump there is a significantly large volume of fuel, situated in a suction channel leading fuel from the inlet metering valve to the plungers, the pressure in which most of the time needs to be controlled to a lower level than the available feed pressure for the purpose of pump output control. By definition, this volume of the suction channel is separate from the re-circulated fuel feed circuit, and is thus vulnerable to overheating and fuel vaporization. One way of alleviating this problem is to thermally insulate the suction channel from the surrounding parts. This works well down to a certain minimum fuel flow in the suction channel, but still vaporization can take place when the fuel flow is below that minimum or when there is no flow at all for a relatively long period of time. The operating conditions requiring extremely low or no output from the high-pressure pump and correspondingly low through-flow in the suction channel, can occur frequently and it can be necessary for the pump to change from that to full output almost immediately, such as when a hot engine needs to start or when high torque is demanded after a period of engine braking. Once fuel is allowed to vaporize in the suction channel, the hydraulic efficiency of the high-pressure pump is dramatically reduced and it can take long time before the pump can provide full output unless special action is taken.

It is known in the art to provide a separate bleed function for the suction channel, in order to be able to remove vapour from that channel instead of trying to liquefy it. The latter is usually a much longer process than vaporization, and to provide for liquefaction in a relatively hot pump, the feed pressure system would need to be capable of relatively high pressure and would therefore be unnecessarily complex, heavy and expensive. Bleeding the suction channel which is thermally insulated from the hot pump body, with a relatively cold and low-pressure fuel from the feed recirculation line, can simultaneously and effectively cool that channel and restore normal operation of the pump much easier. The bleed valve for the suction channel is normally electronically controlled, either by a separate electrical actuator or by means of an extended range actuator that simultaneously controls the bleed valve and the inlet metering valve. This adds cost because of the actuator(s) and the need of more sophisticated control algorithms for the engine management system (EMS). As disclosed in WO 2012/008892, electronic control of the bleed valve can be avoided by providing a pressure-controlled bleed function, which includes introducing a selector valve between the inlet and return ports of the high-pressure fuel injection pump. However, such a system is still quite complex.

There is thus a need for a simplified low cost fuel injection system for volatile fuels that provides a more reliable, durable and controllable performance across the full operating range of the engine.

SUMMARY

An object of the present disclosure is to provide a system for fuel injection where engine performance problems caused by fuel vapour in the fuel system is reduced. This object is achieved in two alternative ways, by the features of the characterising portions of independent claims 1 and 19 respectively. The disclosure concerns a fuel injection system for an internal combustion engine, where the fuel injection system comprises a fuel tank, a low pressure fuel feed pump, a high pressure fuel pump with a suction channel for supplying fuel to a high pressure pumping mechanism, and a first fuel return line. The low pressure fuel feed pump is arranged for supplying fuel from the fuel tank to the suction channel, and the first fuel return line is arranged for enabling excessively supplied fuel from the low pressure fuel feed pump to be re-circulated to the fuel tank.

This first alternative solution of the disclosure is characterized in that at least one non-return valve is placed in the fuel feed flow from the low pressure fuel feed pump to the suction channel and/or in the fuel return flow in the first fuel return line, wherein the at least one non-return valve is arranged to prevent fuel from flowing in a direction opposite the feed flow and/or re-circulation flow.

The disclosure also concerns a fuel injection system comprising a fuel tank, a low pressure fuel feed pump, and a high pressure fuel pump, wherein the low pressure fuel feed pump is arranged for supplying fuel from the fuel tank to the high pressure fuel pump. The high pressure fuel pump comprises a suction channel for supplying fuel to a high pressure pumping mechanism.

Unlike the first alternative of the disclosure, the fuel injection system of the second alternative comprises a bleed valve in the form of a spring-loaded non-return valve. The bleed valve is connected by its inlet to the suction channel and by its outlet to the fuel tank.

This second alternative solution of this disclosure is characterised in that the spring opening pressure of the bleed valve is set lower than an average relative feed pressure of the low pressure fuel feed pump.

The advantage of both alternatives of the disclosure is that they enable a relatively low cost fuel injection system that has a more reliable, durable and controllable performance across the full operating range of the engine. In particular, the startability of a hot engine is improved and the risk for loss of common rail pressure is reduced by means of the solutions of the disclosure. This is realised by limiting fuel vapour displacement in the fuel injection system and/or enabling simplified and low cost purging of the suction channel. The two disclosed alternative solutions can either be used and implemented separately or combined.

Certain operating conditions of the engine are more vulnerable to vaporisation of fuel in the fuel injection system. For example, during prolonged high engine idle operation, during prolonged engine braking and start-up of a warm engine there has been a very low fuel flow or no fuel flow at all in the suction channel for a relatively long period of time. Such low or non-existent fuel flow provides time for a high-volatile fuel present in the suction channel to be sufficiently heated by the warm environment in order to vaporise. If vapour in the suction channel is drawn into the high pressure pumping mechanism, the high compressibility of the fuel vapour strongly reduces the pumping ability, i.e. the volumetric efficiency, of the high pressure pumping mechanism. As a result of the reduced volumetric efficiency of the high pressure pumping mechanism, an insufficient amount of fuel is passed on to the common rail and subsequently to the fuel injector for injection into the internal combustion engine. Consequently, the internal combustion engine may experience lack of power and lack of responsiveness, the engine may come to a complete stop and a comparatively long cranking time may required for the engine to begin firing and start upon engine start-up.

If fuel vapour formed in the suction channel and the low pressure fuel feed pump is not running there is a risk that it displaces and flows backwards past the inlet metering valve and into the feed line and the first fuel return line. As the vapour migrates into these lines, it may eventually interfere with components of a fuel low pressure part of the fuel injection system, such as fuel filters or the low pressure fuel feed pump. Fuel vapour within the low pressure fuel feed pump may reduce the capacity of the low pressure fuel feed pump. Moreover, when restarting the engine again, the vapour is drawn into the suction channel where it disrupts the operation of the high pressure pumping mechanism, thereby prolonging the time required to start the engine as described above. With new, liquid fuel being fed into the system from the fuel tank, the system eventually purges itself from vapour. However, purging the feed line and/or fuel filters, etc. from vapour requires a substantial amount of time. As a consequence, the start-up process of the engine is prolonged, which of course disturbs the operation of the vehicle.

Similar to above, fuel vapour may also during stillstand of the low pressure fuel feed pump migrate from the suction channel past the inlet metering valve and into the first return line. At subsequent cranking of the engine the plungers may suck fuel vapour from the first return line back into the suction channel, thereby further prolonging the start-up procedure. Fuel vapour purged by a bleed valve of the suction channel and conveyed to the first return line via a second return line may equally be sucked into the suction channel during certain engine start-up operating conditions.

Thus, fuel vapour present in the suction channel and fuel vapour migrating into the feed line and first return line may ultimately cause the same problems - an internal combustion engine that cannot start or has a prolonged start-up process.

The first alternative solution of this disclosure reduces the engine performance problems caused by fuel vapour in the fuel system by providing at least one non-return valve in the fuel feed flow, from the low pressure fuel feed pump up to and including the suction channel and/or in the fuel return flow from the inlet metering valve to the tank. A non-return valve placed in the fuel feed flow from the low pressure fuel feed pump to the suction channel and arranged to prevent fuel and fuel vapour from flowing in a direction opposite the feed flow prevents vapour from migrating past the non-return valve and further into the fuel feed line against the feed direction. Hence, the amount of vapour that can migrate into the fuel feed line is reduced, largely depending on how far away from the suction channel the non-return valve is positioned. When the engine is started, there is no vapour or just a small amount of vapour that can be drawn from the fuel feed line into the suction chamber.

A non-return valve placed in the first fuel return line and arranged to prevent flow in the direction opposite to the re-circulation flow prevents any vapour present in the fuel return line between the valve and the fuel tank from being drawn into the suction channel by the negative pressure created during cranking of the engine. The vapour possibly present in the first fuel return line may either have seeped out from the suction channel at an earlier time, or may have been fed into the first fuel return line via another fuel line adapted for purging fuel vapour from e.g. the high pressure pumping mechanism or from the suction channel.

The second alternative solution of this disclosure reduces the engine performance problems caused by fuel vapour in the fuel system by providing a simplified and cost-efficient purging of the suction channel. This is realised by providing a bleed valve that is connected by its inlet to the suction channel and by its outlet to the fuel tank. The purpose of the bleed valve is to release or purge any fuel vapour from the suction channel and pass it on to the fuel tank. This is accomplished purely mechanically and automatically by using a bleed valve having a spring opening pressure set lower than an average relative feed pressure of the low pressure fuel feed pump. During normal operation of the engine in a medium to high load condition, the inlet metering valve is sufficiently opened to provide a certain level of flow of liquid fuel from the fuel tank to the fuel injectors, such that none or only low levels of fuel vapour may be generated in the suction channel. Liquid fuel is continuously pumped away from the suction chamber by the high pressure pumping mechanism and the volumetric efficiency of the high pressure pumping mechanism is relatively high. This continuous removal of fuel from the suction channel causes the pressure to drop in the suction channel to a level well below the average relative feed pressure of the low pressure pump and consequently also lower than the spring opening pressure of the bleed valve. Consequently, during normal operation of the high pressure pumping mechanism, the bleed valve remains closed.

However, during low load engine operation conditions, with none or only a small flow of fuel through the suction channel, liquid fuel vaporises due to the heat and accumulates in the suction channel. As a result, the volumetric efficiency of the high pressure pump will decrease as explained previously. Consequently, the rate at which fuel and fuel vapour is pumped away from the suction channel is reduced. Without the normal sucking effect of the high pressure pumping mechanism the pressure level within the suction channel approaches the pressure level of the low pressure fuel feed pump. With pressure within the suction channel increasing towards the average relative feed pressure of the low pressure pump, and thus to a level above the spring opening pressure of the bleed valve, the bleed vaive will eventually open enable purging of the fuel and fuel vapour of the suction channel. Therefore, the bleed valve automatically opens and releases the vapour from the suction channel, without need for any electronic or pilot operated control of the bleed valve.

For example, to come out of engine braking or some other driving condition with low fuel flow in an overheated high pressure pump, the system wants to see the high pressure pump output pressure rise. Failing to sense that, it keeps opening the inlet metering valve more and more until the pressure in the suction channel is raised above the valve opening pressure of the bleed valve. It can still be vapour there as fuel start to boil on coming into the suction channel, so then the bleed valve not only removes the vapour but allows the bleed flow to cool the pump until the plungers start filling up liquid instead of vapour. Until then, the volumetric efficiency of the pump can be so low that the bleed/cooling flow effect is achieved even on a running engine. In a state where the high pressure pump is at stil!stand but with normal operation of the low pressure fuel feed pump and an at least partly opened inlet metering valve, the pressure in the suction channel will be substantially equal to the pressure of the low pressure fuel feed pump. This scenario may be used during start-up of the engine by controlling the low pressure fuel feed pump to start operation a certain time period before starting operation of the high pressure pumping mechanism. Such a control method will automatically purge the suction channel and the feed line, depending on the length of the purging time.

According to the second alternative solution of the disclosure, purging of the suction channel during moments of low volumetric efficiency of the pumping mechanism and during pumping mechanism standstill is consequently automatically realised by setting the spring opening pressure of the bleed valve in relation to an operating pressure of the low pressure fuel feed pump. Volumetric efficiency of the pumping mechanism is reduced for example upon existence of fuel vapour in the suction channel, and pumping mechanism standstill occurs for example during start-up of the internal combustion engine, when the feed pump is operating but before the engine is cranked. The high pressure pumping mechanism is normally mechanically connected to the crankshaft of the engine.

The feed pressure of the low pressure pump is defined in relative terms since already the fuel tank is pressurised due to the volatility of the fuel. The pressure in the fuel tank varies and depends e.g. on the specific fuel used and operating conditions, such as fuel temperature and the ambient temperature. Hence, the term "relative feed pressure" relates to the difference between the pressure before and the pressure after the low pressure pump, i.e. between the inlet and outlet of the low pressure pump. The term "average" as used in connection to "relative feed pressure" relates to the relative feed pressure. The average relative feed pressure may be determined by sampling a measured relative feed pressure with a frequency of 1 Hz during any 10 minutes long period of time when the engine is running in a normal operation mode. This will produce 600 individual relative feed pressure measurements. Subsequently an average relative feed pressure is calculated for that period of time by summing up all 600 measured relative feed pressure values and divide the sum by 600. The fuel injection system normally targets a constant relative feed pressure, either by means of a controller, by a look-up table and knowledge of the current engine speed and inlet metering valve actuating position, or similar means. However, certain fluctuations in the relative feed pressure always occur, for example in response to large rapid changes in the inlet metering valve actuating position.

The advantage of the disclosed bleed valve compared to prior art is that it neither requires electronic control nor a selector valve provided between the inlet and return ports of the high-pressure fuel injection pump. This renders the fuel injection system less complex, cheaper and more robust. As the spring opening pressure of the bleed valve is set below the average relative feed pressure of the low pressure pump, no excess purging capacity is required from the low pressure pump in order to remove vapour from the suction chamber, contrary to a layout where the bleed va!ve has a opening pressure above the average relative feed pressure of the low pressure fuel feed pump, such that the low pressure fuel feed pump must be able to temporarily increase the feed pressure above the opening pressure of the bleed valve. The solution according to the disclosure thus allows for a low- capacity low pressure pump, which is cheaper and requires less space than a high-capacity pump.

Further advantages are achieved by implementing one or several of the features of the dependent claims. The fuel injection system may further comprise a single inlet metering valve that is arranged to control the flow of fuel entering the pumping mechanism via the suction channel. Such an arrangement provides a low cost and precise control of the output of the high pressure pump.

At least one pump non-return valve may be provided between the inlet metering valve and the suction channel or within the suction channel. The inlet of the at least one pump non-return valve is connected to the inlet metering valve, and the at least one pump non-return valve is arranged to prevent flow in the direction opposite the normal feed flow. The advantage provided by such an arrangement is that the at least one pump non-return valve prevents any fuel vapour present in the suction channel from escaping into either the fuel feed line or the first fuel return line. Hence, the time needed to restore normal operation after an occasion of fuel vaporisation in the suction channel is reduced. The at least one pump non-return valve also prevents fuel vapour from filling fuel filters and the low pressure pump.

A supply non-return valve may be provided in a fuel feed line connecting the low pressure fuel feed pump with the inlet metering valve. The outlet of the supply non-return valve is connected to the inlet metering valve, and the supply non-return valve is arranged to prevent flow in the direction opposite the feed flow. The advantage provided by such an arrangement is that the supply non-return valve prevents any fuel vapour present in the suction channel and/or between the inlet metering valve and the supply non-return valve from migrating further upstream into the fuel feed line. Hence, the time needed to restore normal operation after an occasion of fuel vaporisation in the suction channel is reduced. The at least one pump non-return valve also prevents fuel vapour from filling fuel filters and the low pressure pump. In comparison to the pump non-return valve, the supply non-return valve allows vapour formed in the suction channel to escape a somewhat further distance which may result in a slightly longer recovery time after an occasion of fuel vaporisation in the suction channel. On the other hand, if the supply valve is placed a distance away from the inlet metering valve it will also prevent vapour formed in the portion of the fuel feed line close to the in!et metering valve. Due to the vicinity to the high pressure pumping mechanism, this portion of the fuel feed line may be exposed to heating and there may be a risk of fuel vaporisation at low fuel feed flow. Of course, it is possible to provide the fuel injection system with both a pump non-return valve and a supply non-return valve.

A re-circulation non-return valve may be provided in the first fuel return line. The re-circulation non-return valve is arranged to prevent fuel and fuel vapour from flowing in a direction opposite the re-circulation flow. As described before, the advantage of such an arrangement is that the recirculation non-return valve prevents any vapour present in the fuel return line between the valve and the fuel tank from being drawn into the suction channel by the low pressure created at start-up of the engine, as well as preventing migration of fuel vapour from the suction channel downstream in the first return line.

Furthermore, the first fuel return line may be free from any adjustable flow control valves, and the feed pressure in the fuel feed line may instead be arranged to be controlled by varying the speed and/or displacement of the low pressure fuel feed pump. This means that the design is simplified compared to prior art solutions, since no adjustable flow control valves are needed for controlling the feed pressure. Simplified design increases the robustness and lowers the cost of a system.

The low pressure fuel feed pump may be of a fixed or variable displacement design and may be driven by an electric motor, and a variable fuel feed flow may be achieved by varying the rotational speed of the electric motor or the displacement of the pump. A fixed displacement pump enables reduced costs and reduced size compared with a variable displacement pump. The first fuel return line may be provided with a fixed hydraulic restriction. This fixed hydraulic restriction replaces the adjustable flow control valves commonly used to build up and control the fuel feed pressure in prior art solutions. The advantage of a fixed hydraulic restriction compared to adjustable flow control valves is that it does not require control. Thereby, the design is simplified. However, the use of a fixed hydraulic restriction requires an extended speed range control - especially towards slow speeds - of the low pressure pump compared to when using an adjustable flow control valve. The fixed hydraulic restriction may be connected in series with the recirculation non-return valve mentioned above. In such way, the low pressure fuel feed pressure may generate desired feed pressure in the supply line while simultaneously fuel vapour present in the first fuel return line is prevented from flowing in a direction opposite to the re-circulation flow.

The fuel injection system may comprise both at least one previously described non-return valve and a previously described bleed valve. The advantage of having both types of valves is that fuel vapour is both effectively removed from the suction channel as well as prevented from flowing towards the flow direction in the fuel feed line and/or first fuel return line. Consequently, the recovery after an occasion of fuel vaporisation is even quicker.

The first fuel return line may be arranged for enabling excessively supplied fuel from the low pressure fuel feed pump to be re-circulated from the inlet metering valve to the fuel tank. This enables a fuel cooling flow of the high pressure fuel pump.

The first fuel return line may be arranged for enabling excessively supplied fuel from the low pressure fuel feed pump not entering the suction channel to be re-circulated to the fuel tank. This enables a fuel cooling flow of the high pressure fuel pump.

An opening pressure of the re-circulation non-return valve and/or the supply non-return valve may be selected such that the re-circulation non-return valve and/or the supply non-return valve is always in an open state when the fuel feed pump is operating. Said non-return valves mainly serve to prevent fuel vapour from flowing past the inlet metering valve when the engine is not operating. The opening pressure may thus be relatively low, for example below 3 bar, preferably below 2 bar.

The re-circulation non-return valve may be connected upstream of the fixed hydraulic restriction. This avoids that fuel vapour can enter the hydraulic restriction and causing problems.

The spring opening pressure of the bleed valve may be within the range of 0.1 - 8.0 bar, specifically within the range of 1.0 - 8.0 bar, and more specifically within the range of 3.0 - 8.0 bar. These ranges are suitable ranges below the average relative feed pressure of a typical low pressure fuel feed pump. The bleed valve is a non-controlled valve, which simplifies the design and increases the robustness of the fuel injection system.

The low pressure fuel feed pump may be controlled to deliver a relative feed pressure that maximally varies within a range of plus/minus 30% around the average relative feed pressure at any time during normal operation of the engine, preferably within a range of plus/minus 20% around the average relative feed pressure at any time during normal operation of the engine, and more preferably within a range of plus/minus 10% around the average relative feed pressure at any time during normal operation of the engine. As mentioned before, no excess capacity is required from the low pressure fuel feed pump when using a bleed valve with a spring opening pressure set lower than the average relative feed pressure of the low pressure fuel feed pump. Ensuring that the low pressure fuel feed pump is not oversized limits the cost as well as the bestowed space of the pump. Now turning to the high pressure fuel pump again. Its high pressure pumping mechanism may comprise at least one plunger located in a pumping chamber, and an inlet valve and outlet valve associated with the pumping chamber. The inlet valve may be connected between the suction channel and the pumping chamber. The outlet valve may be connected between the pumping chamber and at least one fuel injector.

The previously mentioned bleed valve may be connected to the fuel tank via a second fuel return line. This second fuel return line may be connected to the first fuel return line downstream of the re-circulation non-return valve. The advantage of connecting the bleed valve to a fuel return line is that fuel removed as vapour from the suction channel is returned to the fuel tank where it returns to liquid state - due to the fuel tank temperature and pressure - and then can be reused. The advantage of connecting the second fuel return line to the first fuel return line instead of directly to the fuel tank is that line material and space is saved. The advantage of having the connection to the first fuel return line downstream of the re-circulation nonreturn valve is that the vapour carried in the second return line is prevented from migrating backwards, i.e. towards the flow direction, in the first fuel return line and into the suction channel again.

The first fuel return line may be provided with a fixed hydraulic restriction, and the second fuel return line may be connected to the first fuel return line downstream of the fixed hydraulic restriction. This is advantageous in terms of available pressure differential over the bleed valve. The second fuel return line may be connected to the first fuel return line at a return line connection point, and the fixed hydraulic restriction and the return line connection point may all be located within the high pressure fuel pump. This enables reduced installation of piping.

The first fuel return line may be free from any adjustable and/or controllable flow control valves. This enables a simplified layout and a more robust system, BRIEF DESCRIPTION OF DRAWINGS

In the detailed description below reference is made to the following figure, in which:

Figure 1 shows an example of a fuel injection system according to the present disclosure, comprising a supply non-return valve, Figure 2 shows a second example of a fuel injection system according to the present disclosure, comprising a pump non-return valve,

Figure 3 shows a third example of a fuel injection system according to the present disclosure, comprising a re-circulation non-return valve,

Figure 4 shows a fourth example of a fuel injection system according to the present disclosure, comprising a spring-loaded bleed valve for purging the suction channel,

Figure 5 shows a fifth example of a fuel injection system according to the present disclosure, comprising a bleed valve, non-return valves and a fixed hydraulic restriction, Figure 6 shows a symbolic illustration of the inlet metering valve, and

Figure 7 shows a sixth example of a fuel injection system according to the present disclosure, comprising an alternative routing of the second fuel return line. DESCRIPTION OF EXAMPLE EMBODIMENTS

Various aspects of the disclosure will hereinafter be described in conjunction with the appended drawings to illustrate and not to limit the disclosure, wherein like designations denote like elements, and variations of the described aspects are not restricted to the specifically shown embodiment, but are applicable on other variations of the disclosure.

Figure 1 shows an example of a fuel injection system 1 according to the present disclosure. Broadly, the system 1 comprises a low pressure fuel feed pump 3 feeding fuel from a fuel tank 2 via a fuel filter 20 to a high pressure fuel pump 5, which in turn delivers highly pressurised fuel to a common rail 31, which subsequently feeds high pressure fuel to fuel injectors 30 for injection into a combustion chamber of a cylinder of an internal combustion engine.

The fuel tank 2 is pressurised in order to maintain volatile fuels such as DME in a liquid state. The pressure held in the fuel tank 2 depends on the properties of the specific fuel and on ambient conditions such as temperature. The low pressure fuel feed pump 3 feeds liquid fuel in a fuel feed line 4 from the fuel tank 2 to the high pressure pump 5. A first fuel return line 7 is provided for re-circulation of excessive fuel to the fuel tank 2. The low pressure fuel feed pump 3 is controlled to provide a variable flow in such a way as to compensate for varying fuel consumption through the high pressure fuel pump 5 and the engine and by this means keep the pressure in the fuel feed line 4 within a necessary target range. The pressure in the fuel feed line is preferably kept constant but will generally vary to a certain extent due to fluctuations in demand and the like.

The high pressure pump 5 comprises an inlet metering valve 12, a suction channel 13 and at least one high pressure pumping mechanism 8, such as a plunger located in a pumping chamber 9 that is connected via an inlet valve 10 to the suction channel 13 and via an outlet valve 11 to a fuel injector 30 for injecting pressurized fuei into a combustion chamber of a cylinder of the internal combustion engine (not shown).

The inlet metering valve 2 is located upstream the suction channel 3 and is arranged for controlling the flow of fuel entering the suction channel 13 from the low pressure fuei feed pump 3. The inlet metering valve 12 is thus arranged to control the fuel output flow from the high pressure pump 5. The inlet metering valve is preferably located adjacent the suction channel for reducing the available volume between the inlet metering valve 12 and a pumping chamber inlet valve 10. The risk for vaporization of the fuel within this volume is relatively high due to the relatively high temperature generated by the high pressure pumping mechanism 8, as well as relatively low pressure during certain engine conditions, such as low engine power output conditions and engine stiilstand. Examples of low engine power output conditions are engine braking and engine idling. A relatively small volume between the inlet metering valve 12 and a pumping chamber inlet valve 10 facilitates improved evacuation of any fuel vapour within said volume upon request for increased engine output due to small amount of fuel vapour that can be developed within said small volume. Moreover, the relatively small amount of vaporized fuel is also beneficial in case vaporized fuel leaks out from the volume, either upstream past the inlet metering valve 12 or downstream via a suction channel bleed valve and fuel return line (not shown). The inlet metering valve 12 is preferably located in direct connection with the suction channel 13 for keeping said volume small.

An inlet metering valve outlet volume 39 is defined as the volume between the valve member of the inlet metering vaive 12 and the suction channel 13. This volume is preferably minimised for preventing the available volume for vaporised fuel. The suction channel is thus preferably located in the direct vicinity of the outlet of the inlet metering valve 12.

Fig. 6 is a symbolic illustration of the inlet metering valve 12 shown in fig. 1. The inlet metering valve 2 is located adjacent a branch point 40 of the fuel feed line 4, and fuel not entering the suction channel 13 is returned to the fuel tank 2 via the first fuel return line 7. The fuel feed line 4 thus connects an outlet of the low pressure fuel feed pump 3 with the branch point 40, and the first fuel return line 7 connects the branch point 40 with the fuel tank 2. The design enables excessively supplied fuel, i.e. fuel not entering the suction channel 13 via the iniet metering valve 12, to form continuous cooling flow of fuel through the high pressure pump 5 and back to a low pressure part 24 of the fuel injection system 1. This cooling flow is schematically depicted in fig. 6 by arrow 42 denoting fuel flow from the fuel feed pump 3 and arrow 41 denoting fuel flow not having entered the suction channel 13 and instead flowing back to the tank 2.

The inlet metering valve 12 preferably comprises an electrically controlled valve member that controls the level of flow through the inlet metering valve 12 and in to the suction channel 13. The inlet metering valve is preferably an electrically controlled proportional valve, for example of the solenoid type.

A branch volume 37 is formed at the branch point 40. This branch volume 37, which is defined essentially by the volume between the branch point 40 and a valve element of the inlet metering valve 12, is preferably keep small ensure that that as little fuel as possible is stiilstanding during a closed state of the inlet metering valve 12. Stiilstanding liquid fuel within the hot high pressure pump can quickly vaporise and cause problems upon subsequent opening of the inlet metering valve 12, Preferably, the branch volume is designed to be substantially part of the fuel recirculation path defined by the fuel feed line 4 and first fuel return line 7, such that substantially no fuel is left still standing at a closed state of the inlet metering valve 12 and with the low pressure fuel feed pump 3 running. The inlet metering valve 12 is consequently preferably located in the immediate vicinity of the branch point 40, thereby minimising the branch volume 37.

The pumping device 8 is illustrated in fig. 1 as three individual plungers 8. The plungers 8 are driven by a camshaft 25 via eccentric cams 27, The camshaft 25, which is rotationally supported by bearings 23 in a low-pressure volume 21 of a housing 22 of the high pressure pump 5, is generally mechanically connected to the crankshaft of the internal combustion engine. The inlet metering valve 12, the fuel injector 30, and the feed pump 3 are preferably controlled by an engine management system (EMS) {not shown). In figure 1 , a high pressure fuel injection pump 5 with three plungers 8 is shown, which plungers 8 are phase-shifted in their operating cycles. However, it is understood that the selection of just three plungers 8 is only an example. In actual fact the number of plungers in such a pump may vary depending on the application and the special conditions. Pumps with one, two, three, four, five, six or even more than six plungers can be used in connection with the invention. The invention is also not limited to the use of a plunger-type high pressure pump, even if this type of pump is preferred.

The system further comprises a supply non-return valve 18 located in the fuel feed line 4 and arranged to prevent fuel flow in a direction opposite to the feed flow, i.e. arranged to prevent flow of liquid and/or vaporised fuel in a direction from the high pressure pump 5 towards the low pressure fuel feed pump 3.

The fuel injection system in Fig. 1 works in the following way: the feed pump 3 draws fuel from the fuel tank 2 and pressurizes it to a certain relative feed pressure. The fuel feed passes through the supple non-return valve and the feed pressure is supplied to the inlet metering valve 12 of the high-pressure pump 5. Preferably, the EMS controls the feed pump 3 to achieve the required fuel feed pressure and at the same time establish fuel flow in excess of the amount required for power generation by the internal combustion engine. That excess amount of fuel flow constitutes a recirculation fuel flow, which helps keeping the fuel temperature relatively uniform throughout the feed pressure circuit so that local hot spots and vaporisation of fuel are with a high probability avoided, thereby ensuring stable fuel properties at the inlet of the inlet metering valve 12. The recirculation fuel flow is returned to the fuel tank 2 from the inlet metering valve 12 via a first fuel return line 7.

The fuel at feed pressure is then admitted through the inlet metering valve 12 to the suction channel 13 and further to the inlet ports of the pumping chambers 9 of the three pumping plungers 8. On the downward stroke, the plungers 8 fill in the mass of fuel that depends on the EMS- controlled opening of the inlet metering valve 12, and then pump it out of the pumping chamber 9 via the outlet vaive 11 and towards the injector 30 for injecting it into the internal combustion engine. The excess flow of fuel from the feed pump 3 is directed directly into the first fuel return line 7. At low power demand the inlet metering valve 12 remains in a nearly closed position, allowing only a relatively small amount of fuel into the suction channel 13, and at high power demand the inlet metering valve 12 is fully open, allowing a relatively large amount of fuel into the suction channel 13. The actuating position of the inlet metering valve 2 thus effectively controls the output flow from the high pressure pump 5, and the inlet metering valve is generally controlled to keep the fuel pressure in the common rail 31 on a stable level. The inlet metering valve 12 may be electrically controlled by the EMS.

In the event of overheating of fuel in the suction channel 13, which for instance can take place when the hot engine is stopped for a while and should be started again due to the lack of through-flow of fuel in the suction channel and the correspondingly poor cooling of that channel, the fuel in the suction channel 13 will begin to uncontrollably evaporate. The fuel vapour within the suction channel 13 may then start to leak from the suction channel driven by the higher fuel vapour pressure in the suction channel and other parts of the fuel injection system installed close to engine, compared to the vapour pressure in the low-pressure part 24 of the fuel injection system 1. Such vapour pressure difference is due to the difference in fuel temperatures, occurring naturally as the warm engine keeps heating up the fuel close to it while the fuel tanks remain relatively cold. In such circumstances, even a minor leakage rate would be sufficient to displace much of the fuel out of the suction channel 3, because a stopped hot engine will keep heating up its fuel for a long time. The supply non-return valve 18 prevents the fuel vapour from migrating into the fuel feed line 4 where it can cause problems by filling up the fuel filter 20 or the feed pump 3 or by being re-fed into the suction chamber 13 and disturbing the operation of the high pressure pump 5. The closer to the suction channel 13 the supply non-return valve 18 is placed, the smaller distance for the vapour to migrate into the fuel feed line 4. There may however be a risk of vapour forming in the portion of the fuel feed line 4 close to or within the high pressure pump 5 due to the heat generation in the high pressure pump 5. In order to be able to prevent that vapour from migrating further into the first feed line 4, the supply non-return valve 18 may be placed at a reasonable distance from the inlet metering valve 12. The supply nonreturn valve 18 is preferably incorporated into the high pressure pump 5. Alternatively, the supply non-return valve 18 is provided in the feed line 4 between the low pressure fuel feed pump 3 and the high pressure pump 5.

Figure 2 shows a second example of a fuel injection system 1 according to the present disclosure. It is identical to the fuel injection system 1 in Figure 1 , except in that it comprises a pump non-return valve 17 located within the suction channel 13 instead of a supply non-return valve 8 in the fuel feed line 4. The pump non-return valve 17 is arranged to prevent flow in the direction opposite the feed flow, and therefore it prevents any fuel vapour present in the suction channel 13 from escaping into either the fuel feed line 4 or the first fuel return line 7. Hence, the time needed to restore normal operation after an occasion of fuel vaporisation in the suction channel is reduced, and fuel vapour is prevented from filling the fuel filter 20 and the feed pump 3. The pump non-return valve 17 is preferably positioned as close to the inlet of the suction channel as possible to avoid that any fuel vaporises upstream of the pump non-return valve 17. Figure 3 shows a third example of a fuel injection system 1 according to the present disclosure. It is identical to the fuel injection system 1 in Figure 1 , except in that it comprises a re-circulation non-return valve 16 instead of a supply non-return valve 8. The re-circulation non-return valve 16 is located in the first fuel return line 7 and arranged to prevent liquid and/or vaporised fuel flow in the direction opposite to the re-circulation flow. Consequently, the re-circulation non-return valve 16 functions as to prevent any fuel vapour present in the fuel return line 7 between the re-circulation non-return valve 16 and the fuel tank 2 from being drawn into the suction channel 13 by the plungers 8 at start-up of the engine, mainly in situations where the normal recirculation fuel flow has not yet been properly established. This may occur when the low pressure fuel feed pump 3 starts operating relatively late in relation to operating start of the high pressure pump 5, and is further enhanced by the fact that the inlet metering valve 12 often is open to a large degree upon start of the combustion engine for building up sufficient pressure in the common rail 31. The fuel vapour possibly present in the first fuel return line 7 may either have seeped out from the suction channel 13 during stillstand of the engine, or may have been fed into the first fuel return line 7 via another fuel line (not shown) adapted for removing fuel vapour from e.g. the low-pressure volume 21 of the housing 22 or from a bleed valve of the suction channel 13, as will described more in detail below.

During normal operation, i.e. during operation of the low pressure fuel feed pump, all the above mentioned non-return valves 16, 17, 18 remain open due to the fuel feed flow. The opening pressure of each of the above mentioned non-return valves 16, 17, 18 is preferably relatively low to reduce the pressure drop over these valves during operation of the low pressure fuel feed pump 3. The opening pressure must only be large enough to prevent any fuel vapour from flowing backwards during engine shut-down. The opening pressure may typically be set in the range of 0.1 - 3 bar, preferably 0.1 - 2 bar and more preferably 0.1 - 1 bar. As the valves 16, 7, 18 are not repeatedly opened and closed during normal operation, there is hardly any wear of the abutting va!ve elements. Consequently, the non-return valves 16, 17, 18 retain their high sealing ability in the closed state even after long use, such that fuel vapour cannot flow past these valves during engine sti!lstand. Each of the disclosed examples of the fuel injection system 1 above can additionally comprise a bleed valve (not shown) at an end of the suction channel 13 opposite the inlet metering valve 12, which bleed valve is arranged for evacuating fuel vapour from the suction channel 13 back to the fuel via a second fuel return line (not shown). The bleed valve may be electronically controlled, servo controlled, an automatically controlled valve or the like.

Figure 4 shows a fourth example of a fuel injection system 1 according to the present disclosure, it is identical to the fuel injection system 1 in Figure 1 , except in that it comprises a bleed valve 14 and a second fuel return line 19 instead of a supply non-return valve 18. The bleed valve 14 is spring-loaded non-return valve. The bleed valve 14 is also a non-controlled valve. This means that the bleed valve 14 is neither electronically controlled nor controlled by an external control unit. The position of the valve member is instead determined solely by the differential pressure acting on the valve member and the spring force acting on the valve member. The bleed valve 14 is a non-return valve, also referred to a check-valve in literature, where a displaceable valve member controls the flow through the bleed valve 1 , and where a spring-force generated by a spring member urges the valve member to a closed position. As soon as the combined force from the hydraulic pressure acting on the valve member towards the open position is larger than the force exerted by the spring member towards the closed position, the valve member will open to enable flow through the bleed valve, from an inlet to an outlet of the bleed valve 14. The spring opening pressure of the bleed valve 14 corresponds to the force exerted by the spring member in the closed position of the bleed valve 14, i.e. the pressure that is required to open the bleed valve when no hydraulic pressure is urging the valve member towards the closed position.

The bleed valve 14 is connected by its inlet to the suction channel 13 and by its outlet to the fuel tank 2 via a second fuel return line 19. During normal operation of the high pressure pump 5, the bleed valve 14 automatically remains closed due to its required spring opening pressure being larger than the prevailing pressure in the suction channel 13. In the event of overheating of fuel in the suction channel 13, which for instance can take place when the hot engine is stopped or during engine braking due to relatively low through- flow of fuel in the suction channel13 and the correspondingly poor cooling of that channel 13, the fuel in the suction channel 13 will begin to uncontrollably evaporate. The evaporation can occur quickly and, even when the engine and the feed pump 3 are running, fresh liquid fuel entering the suction channel 13 through the open inlet metering valve 12 may immediately evaporate in low through-flow operating conditions, whilst on the other hand the high compressibility of vapour will drastically reduce the hydraulic efficiency of the pumping action of the plungers 8. Due to these phenomena, the pressure at the outlet of the high pressure pump 5 will begin to go down due to the reduced volumetric efficiency, and the pressure in the suction channel 13 increases towards the pressure level of the fuel feed line 4. Upon reaching a level greater than the spring opening pressure of the spring- loaded bleed valve 14, the bleed valve 14 is forced open. The opening of the bleed valve 14 is an automatic response to the increased pressure in the suction channel 13, and does not require any external control. The open bleed valve 14 and the suction channel 13 will become connected in series in a recirculation path via the second fuel return line 19, fuel tank 2 an the feed pump 3, allowing quick removal of the vapour from the suction channel 13 to the fuel tank 2 and also the cooling of the suction channel 13 by fresh liquid fuel incoming through the inlet metering valve 12. This will restore hydraulic efficiency of the plungers 8 such that the pressure in the suction channel is reduced again. When the pressure eventually reaches below the spring opening pressure of the bleed valve 14, the bleed valve 14 automatically closes again. By this means, norma! operation of the high-pressure pump 5 is restored. The opening of the bleed valve 14 is controlled by the pressure difference over the bleed valve 14, and it is thus advantageous to provide a relatively low pressure level in the second fuel return line 19, for enabling enough pressure difference to be developed over the bleed va!ve for opening of the bleed valve. For this purpose, the second fuel return line 19 is being connected to the first fuel return line 7 at a return line connection point 36 that is located downstream of a fixed hydraulic restriction 15 of the first fuel return line 7. The fixed hydraulic restriction 15 located in the first fuel return line enables build-up of fuel feed pressure in the circuit constituted by the fuel feed line 4 and first fuel return line 7. By providing the return line connection point 36 downstream the fixed hydraulic restriction 15 a relatively low pressure is ensured in the second fuel return line 19, such that a relatively high pressure difference can be established over the bleed valve 14. The second fuel return line 19 can alternatively be connected directly to the fuel tank 2 but it is preferred to connect the first and second fuel return lines 7, 19 at a connection point 36 for the purpose of avoiding lengthy piping installations.

Figure 5 shows a fifth example of a fuel injection system 1 according to the present disclosure. It is identical to the fuel injection system 1 in Figure 4, except in that it further comprises a supply-non return valve 18 located in the fuel feed line 4 and arranged to prevent fuel flow in a direction opposite to the feed flow, a re-circulation non-return valve 16 located in the first fuel return line 7 and arranged to prevent flow in the direction opposite to the recirculation flow, and a fixed hydraulic restriction 15 located in the first fuel return line 7 downstream of the re-circulation non-return valve 16. Note that first fuel return line 7 is free from any adjustable and/or controllable flow control valves. This applies equally to the examples shown in fig. 1-3. Adjustable and/or controllable flow control valves are herein defined as valves where a flow through the valve can be controlled externally of the valve, for example by means of an electronic control, pilot control, servo control, or the like. Non-adjustable and non-controllable flow control valves are automatic control valves where a flow passage is automatically controlled. Check valves and non-return valves are automatic valves that are automatically controlled based merely on the pressure difference over the vaive.

The fixed hydraulic restriction 15 enables build up of fuel feed pressure in the circuit constituted by the fuel feed line 4 and first fuel return line 7. The speed of the low pressure feed pump 3 is controllable within a wide range. The fixed hydraulic restriction 15 in combination with the wide range controllability of the speed of the feed pump 3 results in equally good controllability of the fuel feed pressure. The fixed hydraulic restriction 15 may be located anywhere in the first fuel return line upstream of the return line connection point 36, for example within the high pressure fuel pump 5 or within the low pressure part 24 of the fuel injection system. This applies equally to the examples shown in fig. 1-3.

During normal operation of the low pressure feed pump 3 and the high pressure pump 5, the bleed valve 14 automatically remains closed due to the prevailing pressure in the suction channel 13 being lower than its required spring opening pressure, while the supply non-return valve 18 and the recirculation non-return vaive 16 are kept open by the feed flow pressure. The fuel injection system according to fig.5 enables a relatively low cost fuel injection system that has a reliable, durable and controllable performance across the full operating range of the engine. Fuel vapour migration at engine stillstand due to internal leakage is effectively prevented by means of the supply non-return valve 18 and re-circulation non-return valve 16, and automatic fuel vapour purging of the suction channel is effectively realised by the low opening pressure of the bleed valve 14. Figure 7 shows a sixth example of a fuel injection system 1 according to the present disclosure. It is identical to the fuel injection system 1 in Figure 5 except in the installation of the second fuel return line 9. In figure 7, the return line connection point 36, the fixed hydraulic restriction 15 and the recirculation non-return valve 16 are all located within the high pressure fuel pump 5. This has the advantage that only two fuel lines must be installed between the low pressure part 24 of the fuel injection system 1 and the high pressure fuel pump 5.

Only a few examples of a fuel injection system 1 are shown in fig. 1 - 5, and it must be realised that many more combinations of the supply non-return valve 18, pump non-return valve 17, re-circulation non-return valve 16 and bleed valve 14 are possibly within the scope of the disclosure. For example, the fuel injection system 1 may comprise a re-circulation non-return valve 16 and a supply non-return valve 18, with or without a bleed valve 14.

Alternatively, the fuel injection system 1 may comprise a pump non-return valve 17, with or without a bleed valve 1 . The disclosure may thus be modified in various obvious respects, all without departing from the scope of the appended claims. Accordingly, the drawings and the description thereto are to be regarded as illustrative in nature, and not restrictive.

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