METHOD FOR OPERATING AN INTERNAL COMBUSTION ENGINE USING A GASEOUS FUEL, AND INTERNAL COMBUSTION ENGINE

Abstract

The disclosure relates to a method for operating an internal combustion engine comprising at least two cylinders and a system for fuel injection, in which the fuel is withdrawn from a primary tank and supplied to at least one rail in a form significantly compressed compared with atmospheric pressure, and a plurality of cylinders draw the gaseous fuel from a rail used collectively, wherein, during operation of the internal combustion engine, the pressure target value of the gaseous fuel stored in the rail is controlled to or otherwise held at a constant value or a variable target value, which changes only in a small range B, irrespective of the engine operating point.

Claims

1. Method for operating an internal combustion engine comprising at least two cylinders and a system for fuel injection, in which fuel can be withdrawn from a primary tank and can be supplied to at least one rail in a form significantly compressed compared with atmospheric pressure, and a plurality of cylinders can draw gaseous fuel from a rail used collectively, wherein during operation of the internal combustion engine a rail pressure target value of the gaseous fuel stored in the rail is held at a constant value or a variable target value, irrespective of an engine operating point, which value changes only in a small range B.

2. Method according to claim 1, wherein at least one cylinder is actively shut off during operation of the internal combustion engine under partial load.

3. Method according to claim 2, wherein a dynamic cylinder shut-off having a constant or varying number of cylinders deactivated simultaneously is performed, wherein the number of cylinders to be deactivated is preferably determined depending on a current engine load.

4. Method according to claim 3, wherein the number of deactivated cylinders is changed in an alternating manner in successive operation cycles of the internal combustion engine.

5. Method according to claim 1, wherein the small range B changes, with respect to a rated value of the rail of the internal combustion engine.

6. Method according to claim 1, wherein a supply of fuel into the plurality of cylinders takes place via port injection, and a constant or lastingly approximately constant rail pressure target value is in an order of magnitude between 8 bar and 25 bar.

7. Method according to claim 6, wherein an effective rail volume, with respect to a total displacement of the internal combustion engine, is between 4% and 40%.

8. Method according to claim 7, wherein the supply of fuel into the combustion chambers takes place via direct injection.

9. Method according to claim 8, wherein a low-pressure direct injection takes place, and the constant or lastingly approximately constant rail pressure target value is in an order of magnitude between 10 bar and 20 bar.

10. Method according to claim 8, wherein a high-pressure direct injection takes place, and the constant or lastingly approximately constant rail pressure target value is in an order of magnitude between 150 bar and 500 bar.

11. Method according to claim 8, wherein the effective rail volume, with respect to the total displacement of the internal combustion engine, is in the range between 1% and 32%.

12. Method according to claim 1, wherein the gas exchange valves of a deactivated cylinder can be operated differently from a case of said cylinder being kept active, and in this case the gas exchange valves of a deactivated cylinder remain completely closed.

13. Method according to claim 3, wherein opening times of injectors, existing for respective main injections, are not or are only slightly changed depending on the engine load.

14. Method according to claim 1, wherein a rail pressure pump is operated in a clocked manner for charging the rail with fuel, wherein a clock rate of the rail pressure pump is in particular synchronized to such time periods within which an injection of fuel into an active cylinder of the internal combustion engine is due.

15. Method according to claim 1, wherein the gaseous fuel is hydrogen or natural gas, or contains hydrogen in molecular form or natural gas.

16. Gas engine comprising at least two cylinders and a system for fuel injection, in which the fuel can be withdrawn from a primary tank and can be supplied to at least one rail in a form significantly compressed compared with atmospheric pressure, and the gaseous fuel can be supplied to a plurality of cylinders from the rail used collectively, wherein the internal combustion engine comprises an engine controller which is configured to carry out the method according claim 2.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0039] Further advantages and properties of the disclosure will be set out in the following, with reference to the figures, in which:

[0040] FIG. 1: is a schematic block circuit diagram of a conventional internal combustion engine having common rail injection,

[0041] FIG. 2: is a time graph illustrating the relevant state variables in the case of fuel injection,

[0042] FIG. 3: shows different sequential views for a possible cylinder deactivation of an internal combustion engine,

[0043] FIG. 4: shows schematically illustrated speed-torque full load curves depending on the active cylinder number (the solid lines in each case), and the corresponding full load curves in alternating operation (the dotted lines) and a schematically indicated fuel consumption characteristic diagram of the 4-cylinder engine.

DETAILED DESCRIPTION

[0044] The disclosure relates to a method for operating an internal combustion engine which draws a gaseous fuel and comprises a system for fuel injection. In this respect, an internal combustion engine according to the disclosure of this kind can be based on the fundamental structure of a common rail system according to FIG. 1, which comprises a rail 2. The supply of fuel into the combustion chambers (not shown) takes place over opening time periods of the respective injectors 1, which draw the gaseous fuel, in particular hydrogen, from a rail 2. This is supplied with fuel, withdrawn from the primary tank (not shown), by means of a rail pressure pump 6. In the embodiment shown, the volume flow from the primary tank to the rail pressure pump 6 is controlled by the motor controller 3 by means of the volume flow control valve (VCV) 5.

[0045] A pressure sensor 7 detects the rail pressure, i.e. the pressure inside the rail 2, the value of which is taken into account in the motor control 3. The gaseous fuel can be discharged from the rail 2 via a pressure control valve (PCV) 4 that can be controlled by the motor controller 3. In a simple embodiment, presupposing that a sufficiently large fuel inflow can be covered, the rail pressure can be controlled to or otherwise held at a defined target value simply by actuation, determined by the engine controller 3, of the valves 4, 5. Holding in another manner can be achieved for example by a controller. In an embodiment, the internal combustion engine comprises a rail pressure pump which can be operated according to need. In addition to the starting value of the pressure sensor 7, the engine controller 3 obtains numerous further operating variables and parameters of the drive system 10, as well as quantitative information relating to operating interventions, of which in FIG. 1, merely by way of example, the measured value signal flow of a crankshaft sensor 11 and the measured value signal flow of a camshaft sensor 12 are shown. In addition to the valves 4, 5, the engine controller 3 can also specify the opening and closing of a relevant injector 1. Therefore, inter alia a temporary deactivation of individual cylinders of the engine controller 3 is expedient.

[0046] What is characteristic for the disclosure at this point is in particular the novel engine controller 3 for controlling the target pressure value within the rail 2, in particular in conjunction with a load-dependent dynamic cylinder shut-off.

[0047] The method according to the disclosure and/or the internal combustion engine according to the disclosure is characterized in that the rail pressure target value exhibits a very significantly reduced dependency on the speed/torque operating point of the internal combustion engine, and is optionally even constant. Clearly, in the case of the internal combustion engine according to the disclosure, the same power range must be covered, starting at an idle operation, up to the operating points along the full-load characteristic curve. Furthermore, the possible use of the internal combustion engine according to the disclosure should be able to be fulfilled in particular for applications which have high dynamics requirements.

[0048] The reproducibility of a fuel injection amount is significantly worse in the case of shorter opening time periods of an injector 2, while maintaining all other conditions influencing this, than in the case of fuel injections during a longer opening time period, which can be explained with reference to the graph shown in FIG. 2. The simplified time curves of the injector current 14, the injection rate 15 and the rail pressure 13 during an injection process are shown. Before the injection process considered, the rail pressure 13 was raised to a particular level. The energization of the injector 1 results in its opening, and thus initiates a fuel inflow from the rail 2 in the direction of the combustion chamber, whereupon the rail pressure 13 reduces. In reality, too, there is a certain time delay between the electrical current flow 11 to the injector 1 and the through-flow of fuel 12, which is shown on the graph in an excessive order of magnitude for clarity.

[0049] The actuation of an injector 1 of this kind, which is also suitable for delivering high injection rates, usually takes place via a solenoid valve, i.e. ultimately via an electromagnet. Thus, the electrical circuit, which is arranged inside the injector 1, exhibits a comparatively high inductance. It is known that the presence of an inductance in an electrical connection leads to a delayed current strength change along said connection. Since quick response times are essential for the opening and closing of an injector 1, the electrical circuit for energizing the electromagnets is correspondingly expanded, with the aim of compensating the delay caused by the inductance of the solenoid, which is possible to a certain extent. However, focusing on this measure is irrelevant in the consideration to be taken here. In addition to the delay for electrical reasons, a delay for mechanical reasons is added, caused by the movement of the valve needle into its opening position. An artefact of the electrical compensation measures for reducing the switch-on time duration of the electromagnet is the brief sharp excessive increase in the injector current 14.

[0050] While the injector 1 opens, a fuel inflow already begins, and while the injector already closes there is initially still a fuel inflow, the time duration of which is shown exaggerated in FIG. 2 for reasons of clarification. During these switching transitions, the pressure drop along the injector fuel path is far less well defined than in the case of a fully open injector 1, since in the case of a fully open injector 1 a constant or at least approximately constant flow resistance exist in the fuel path considered. On account of what is known as the injector bouncing, not even the movement processes of the closure element of an injector 1 are exactly reproducible. In the case of a comparatively large store of fuel in the rail 2 and a low injection rate, the latter remains approximately constant, provided that the rail pressure changes only on account of the injection process considered, i.e. reduces slowly.

[0051] Accordingly, in order to achieve a high injection amount accuracy longer injector opening times are advantageous, because the fuel amounts which are rather undefined during the switching transitions make up a smaller portion of the total amount of fuel that has been injected during a complete injection process (from the start to the end of an injection).

[0052] In order that the extremely short opening times of the injectors 1, which result in a common rail system according to the prior art if the internal combustion engine is in the lower partial load operation and, most markedly, in idle operation, are avoided, the disclosure provides for a deliberate cylinder deactivation in the event of a low capacity utilization of the internal combustion engine. The latter avoids, in the case of an internal combustion engine according to the disclosure, a significant reduction in the number of main injections in which there are comparatively short opening times of the injectors 2. Furthermore, with respect to the main injections, the entire collective time of the injector opening times which occur at a low to medium capacity utilization of the internal combustion engine is very significantly increased. Short or even extremely short injector opening times are therefore very disadvantageous, because in this case the majority of the entire fuel supply amount occurs during said switching transitions, and therefore there is a comparatively large deviation between the respectively corresponding actual values and target values of a fuel portion of an injection process. According to the disclosure, the variability of the rail pressure target value is limited to a very low quantity, and optionally entirely prevented, depending on the capacity utilization of the internal combustion engine. Similar applies for the rail pressure actual value. Clearly, the rail pressure actual value must be permanently in an order of magnitude by which, optionally, a fuel supply for the operating situation of the maximum fuel requirement can currently be covered, or at least be very close to such a pressure level, such that a corresponding increase in the rail pressure, and thus a corresponding fuel supply, is possible within a very short time. Holding the rail pressure largely constant has the additional benefit that the spray hole geometry of an injector nozzle can be formed such that it can be designed as optimally as possible for a very particular pressure level of the fuel to be injected. With regard to a dynamically operated internal combustion engine, as a result of implementing a measure of this kind, i.e. for an internal combustion engine according to the disclosure, the fuel spray patterns of the injections have a technically far more advantageous appearance, which is in turn far more favorable for the entire combustion process.

[0053] With respect to the structural design thereof, the system according to the disclosure can additionally differ with respect to conventional systems in that the capacity provided, in the rail 2, for temporary fuel storage, is dimensioned so as to be substantially larger than in the case of internal combustion engines not according to the disclosure. More precisely, in an internal combustion engine according to the disclosure the capacity K provided for the temporary storage of fuel is significantly larger with respect to the chemical energy content of the fuel in the rail 2, with respect to the corresponding total displacement volume V. For improved readability of the following text, the term or the magnitude of the characteristic numerical relationship c=K/V is introduced. The characteristic relationship c is influenced essentially by the volumetric energy density of the fuel, taking into account the rail pressure.

[0054] According to the prior art of corresponding and otherwise comparable internal combustion engines, the characteristic numerical relationship c is dependent to a significant extent on the current engine delivery power. In the case of an internal combustion engine according to the disclosure, the characteristic numerical relationship c=K/V is optionally constant in all speed/torque operating points and, when viewed in reality, exhibits a low to very low dependency on the current engine delivery power. For the following text, the designation effective rail volume is used. This is the entire inner volume in which the fuel compressed under a high pressure level, referred to as the rail pressure, is located. This includes the inner volume of the rail 2 plus the total volume of all the connection lines between the actual rail 2 and the respective interior portions of the injectors 1, which are still filled with the fuel under rail pressure, even in the case of a closed injector 1.

[0055] In the applications of a cylinder deactivation known from the prior art, a higher efficiency of the internal combustion engine should be achieved in lower and medium partial load operation, and/or an increase in the exhaust gas temperature should be achieved while preventing an increased fuel consumption, which temperature increase is required or at least advantageous for the subsequent treatment of the exhaust gas. Although use can also be made of these advantages in the case of the internal combustion engine according to the disclosure, primarily, however, the aim of a virtually constant target pressure in the rail 2 is pursued by the selective cylinder deactivation, in the case of the internal combustion engine according to the disclosure. The internal combustion engine according to the disclosure, operated using a gaseous fuel, offers significantly greater dynamics compared with conventional gas engines, which is a significant advantage for numerous applications, and is fundamentally decisive, for some applications, for a gas engine being suitable for the relevant application at all.

[0056] The system or method according to the disclosure can be applied in the case of a cylinder direct injection, but also in the case of a port injection of the fuel. In each case typical maximum rail pressures, in particular for a hydrogen engine, are approximately 20 bar, in the case of a port injection and similarly for a low-pressure direct injection. A typical maximum rail pressure for a high-pressure hydrogen direct injection is approximately 300 bar. It is known to a person skilled in the art that a high-pressure direct injection is the better concept from a purely technical perspective. However, the low pressure is very likely valid on account of its significantly lower complexity. The pressure range of the above-mentioned injection concepts can be summarized as follows:

[0057] Direct injection (low-pressure concept)->LPDI:

[0058] variable rail pressure, depending on the engine load, of between 8 bar and 20 (25) bar

[0059] Direct injection (high-pressure concept)->HPDI:

[0060] variable rail pressure, depending on the engine load, of between 150 bar and 300 (400) bar

[0061] Port Injection:

[0062] variable rail pressure, depending on the engine load, of between 8 bar and 20 (25) bar

[0063] For each of these three concepts, a specific, possible embodiment according to the disclosure of a hydrogen motor is cited in the following. The following table contains the dimensions of the rail 2 used in each case, and the effective rail volume, in each case, of these three embodiments. In that, in a manner deviating from those currently available, such rails are used of which the dimensioning and shaping corresponds to a technical improvement up to a technical optimization with respect to the concept according to the disclosure, the respective effective rail volumes can be increased again.

[0064] The displacement of the internal combustion engine specifically studied in this case is approximately 81 in each case. The maximum hydrogen supply amount for a main injection (in one cylinder) in each case reaches a total of approximately 80 mg. At 30° C. and a pressure of 300 bar, the specific weight of hydrogen is approximately 20 kg/m.sup.3. At 30° C. and a pressure of 50 bar, the specific weight of hydrogen is approximately 3.5 kg/m.sup.3. The following table shows some key data which are essential for tests of the present disclosure already carried out.

TABLE-US-00001 TABLE 1 Information relating to the rails used at present for the respective test engines of the three concepts Quotient [%] of the maximum Information relating to amount of an Information the lines between the H2 injection relating to the rail and the injectors with respect rail of the Flow effective H2 amount to the total relevant H2 diameter Lengths Total Rail Rail in the H2 content test engine [mm] [mm] volume [l] volume [l] volume [l] rail [mg] of the rail HPDI 3.5 4 × 420 0.01616 0.03982 0.05598 997 5.6 (250 bar) LPDI 7 4 × 420 0.06464 0.03982 0.1045 365.75 21.9 (50 bar) Port 7 4 × 420 0.06464 0.03982 0.1045 110 6.6 (15 bar)

[0065] The above table shows the values of a 4-cylinder hydrogen engine. The characteristic numerical value specified for this, in percent, which quantifies the ratio of the amount of hydrogen located in the rail, under operating pressure, and the maximum amount of a main injection, based in each case on the mass of hydrogen. Taking into account the energy density and the engine efficiency, the value for other fuels or different fuel mixtures can be easily calculated.

[0066] The numerical values for internal combustion engine having a different number of cylinders may deviate. In the case of a similar number of cylinders, a proportional scaling according to the number of cylinders may be reasonably correct.

[0067] The internal combustion engine according to the disclosure can be operated in such a way that, in the event of a constant delivery power in each case, always the same number of cylinders is actively operated, irrespective of the particular value of the delivery power, in two successive operating cycles of the full engine in each case, i.e. in the case of a 4-stroke engine after passing through a crankshaft angle of 720° in each case. In this case, however, it is ensured that it is not always the same cylinders which are deactivated during successive operating cycles, but rather the deactivation is divided over all available cylinders. Possible deactivation patterns are shown by way of example in FIG. 3, for a four-cylinder (left-hand side) and a six-cylinder engine (right-hand side). In the case of the four-cylinder engine shown, either the cylinders 2, 4, or alternatively 1, 3, are deactivated alternately. In the case of the six-cylinder engine, the cylinders 1, 3, 5 or 2, 4, 6 are deactivated together, in alternation. Alternatively, more than the two patterns for deactivation of the cylinders can also be performed; see FIG. 3 bottom right.

[0068] In the case of the internal combustion engine designed by way of example as a 4-cylinder combustion engine, it may be the case that the target output power required over a particular time period in fact cannot be provided by two-cylinder operation (2 of the 4 combustion chambers are operated actively, and the remaining two combustion chambers are deactivated, while the power that can be provided in three-cylinder operation is significantly above the target delivery power. In such a design, instead of a permanent three-cylinder operation, alternating operation is proposed. In this case, for example in a first operating cycle of the 4-cylinder combustion engine three cylinders are operated actively, while a fuel supply is stopped in the remaining cylinder, and the gas exchange valves thereof remain closed throughout. In the following operating cycle, in contrast, only two cylinders are operated actively, while no fuel supply takes place in the remaining two cylinders, and the gas exchange valves thereof remain closed throughout.

[0069] If the capacity utilization rate for a three-cylinder operation is correspondingly low, and the flutters are tolerable for this, when a corresponding target output power is present in a first and following second operating cycle of the full engine, in each case just two cylinders are operated actively, while in the following third operating cycle of the full engine three cylinders are operated actively. Clearly, the mode of operation illustrated in this case can be transferred to other capacity utilization rates of a 4-cylinder combustion engine, and likewise clearly to those internal combustion engines which comprise a different number of cylinders. In order to illustrate this aspect, FIG. 4 shows, on the basis of a 4-cylinder engine, schematically indicated full load characteristic curves (the maximum torque plotted against the speed) depending on the number of active cylinders (the solid lines), and the full load characteristic curves in the case of alternating operation (the dotted lines) of the 4-cylinder engine.

[0070] Ignoring the geometric deviations, which are necessary in order that the connection lines and a rail pressure sensor 7 can be fastened, the known rails 2 have the geometric shape of a prism, and in this case usually the shape of a circular cylinder. The same also applies for the actual storage volume and the external volume of a rail 2. An embodiment of this kind has the advantages that (i) the manufacture is comparatively simple on account of this shaping, (ii) the integration of the rail 2 on the internal combustion engine is comparatively simple and only a comparatively small installation space is required for a rail 2 of this kind, which space is not available for other attachment components, (iii) a high strength is promoted with respect to the lateral surface thereof, and furthermore that (iv) such a shaping of the rail 2 simplifies the respective connection lines between the respective injectors.

[0071] The system may use a rail pressure pump 6 that can be operated as needed. Optionally, a rail pressure pump 6 according to the disclosure is used, which can be operated in a clocked manner During operation under deactivation of one or more cylinders, optionally the clock rate of the rail pressure pump 6 for fuel supply into the rail 2 is synchronized to the time periods within which the fuel injection of the active combustion chambers is due, wherein this synchronization does not have to take place simultaneously with the fuel injection.

[0072] The operation according to the disclosure of an internal combustion engine according to the disclosure requires the capability for provision of the (approximately) identical rail pressure to already be possible in the low speed range or already in idle operation, as in the operating point of the maximum supply of fuel. Accordingly, driving the rail pressure pump 2 using a fixed speed ratio with respect to the crankshaft would be very disadvantageous.

[0073] As explained above, a gas engine according to the disclosure offers a significant potential for improvement of dynamics, as a result of which, when the rail 2 is designed having correspondingly large dimensions—i.e. the rail 2 has a significantly larger inner volume, ultimately even similarly to an internal combustion engine which is operated by a liquid fuel—the air supply can be decisive for the limiting of the dynamics. Such a limitation to a higher level can in turn be reduced in that the air supply comprises a suitable boost system.

[0074] The advantages of the combustion engine according to the disclosure can be briefly summarized again below:

[0075] A rail 2 having a very much larger inner volume can be used.

[0076] This is in turn very advantageous for a fuel of low density and/or high compressibility.

[0077] As a result, the control path already obtains a high I portion which promotes the control accuracy.

[0078] An embodiment according to the disclosure of the internal combustion engine as a gas engine exhibits significantly increased engine dynamics compared with a gas engine not according to the disclosure, because in the case of a load increase the rail pressure does not first have to be raised to an increased target value. In the case of a fuel which is supplied to the rail 2 in a gaseous aggregate state, such an increase requires a comparatively long period of time, and specifically also if no delivery of fuel from the rail 2 takes place at all in the meantime.

[0079] An internal combustion engine according to the disclosure simplifies the rail pressure control, because the rail pressure target value can be held at a constant value, irrespective of the speed/torque engine operating point, or at a target value which, although variable, changes only comparatively slowly and within a very small range, because this at least optionally also does not have any direct dependency on the current operating point of the internal combustion engine, even in the case of a dynamic load cycle.

[0080] Since in the case of operation according to the disclosure of an internal combustion engine, collectively those time periods between the respective sequences of a fuel supply interval, consisting in each case of a main injection and optionally pre- and post-injections, are significantly longer, the fuel portion supplied to the combustion chambers under flow conditions that can be reproduced significantly better is increased. Accordingly, with respect to the main injections collectively, the fuel portions actually supplied are better determined. This in turn allows for an improved quantity determination of the reducing agent supplies in each case.

[0081] The clear range reduction of the rail pressure allows for a design of an injector 2 that is optimized to an accordingly very particular pressure level, in particular the spray hole geometry thereof, which allows for a high degree of reproducibility of a respective fuel injection amount, and the spray image of the fuel entering the combustion chamber, which in turn favors such a design of the injector 2 which can be optimized, on account of a significantly more strongly determined spray image, in such a way that precisely the spray image within said small range is particularly advantageous for the combustion process.

[0082] Simplified raising of parameters for such injectors 2 which are intended to be operated in an internal combustion engine according to the disclosure.

[0083] With respect to the main injections, the minimum injection times occurring are significantly longer. Accordingly, there is a comparatively longer time period available for corrections of the injection amount with respect to the available calculating time for such correction calculations and the actuatory implementation of these corrections. Furthermore, more time quotas are available during a current main injection, in which quotas other “tasks” can be performed, e.g. safety functions.

[0084] Large amounts of fuel, which would have to be discharged from the rail 2 into a buffer tank, which may be provided, or into the primary tank—the latter presupposing that the fuel provided and the on-board equipment allow this—or would have to be correspondingly discharged in another way, are omitted.

[0085] Collectively, there is an increase in the efficiency, since compared with an internal combustion engine which is also operated using all cylinders in low and medium partial load operation, the active cylinders of an internal combustion engine according to the disclosure are collectively operated under a significantly increased medium pressure.

[0086] An increase in the exhaust gas temperature in low and medium partial load operation of the internal combustion engine results, which in turn increases the effectiveness of the exhaust gas post-treatment and/or offers a starting point for a potential increase in said effectiveness, with respect to the selective catalytic reaction.

LIST OF REFERENCE CHARACTERS

[0087] Fuel injector 1 [0088] Rail 2 [0089] Engine controller 3 [0090] Pressure control valve PCV 4 [0091] Volume flow control valve VCV 5 [0092] Rail pressure pump 6 [0093] Pressure sensor 7 [0094] Drive system 10 [0095] Measured value crankshaft sensor 11 [0096] Measured value camshaft sensor 12 [0097] Rail pressure time curve 13 [0098] Injector current time curve 14 [0099] Time curve of the fuel injection rate 15