Method for Operating an Internal Combustion Engine

CROSS-REFERENCE TO RELATED APPLICATIONS

0001This application is a continuation, under 35 U.S.C. §120, of copending International Application No. PCT/EP2010/007777, filed Dec. 18, 2010, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application No. DE 10 2010 007 071.8, filed Feb. 6, 2010; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

0002The invention relates to a method for operating an internal combustion engine having at least one working cylinder, in which a piston is moved up and down, so that, within an operating cycle of 720 degrees of the crankshaft, successively an intake stroke, a compression stroke, a power stroke and an exhaust stroke are performed, and the piston having at least three gas-exchange valves assigned thereto, the internal combustion engine further having an exhaust-gas line, and having at least one inlet port which leads into a gas exchange valve in the form of an inlet valve and which supplies, via this inlet valve, at least combustion air to the working cylinder, the internal combustion engine further having at least one outlet port which leads into a gas exchange valve in the form of an outlet valve and which, via this outlet valve, discharges at least exhaust-gas from the working cylinder into the exhaust-gas line, the internal combustion engine further having at least one exhaust-gas recirculation line which is connected to the exhaust-gas line of the internal combustion engine, wherein at least one of the gas exchange valves is used by at least one of the working cylinders as an EGR (exhaust-gas recirculation) valve and is supplied with exhaust-gas from an exhaust-gas recirculation line.

0003In case of a diesel engine, the exhaust-gas recirculation is one of the essential elements of a reduction of NO_ximplemented internally in the engine. A distinction is made between the “external” exhaust-gas recirculation (EGR), as is for example known from German Patent No. DE 199 61 292 C2, with cooled and uncooled alternatives and the “internal” EGR, as is for example known from German Patent Application Publication No. DE 10 2005 053 940 A1 which corresponds to U.S. Patent Application Publication No. U.S. 2006/0102121 A1. In case of the internal exhaust-gas recirculation, the remaining behind or recirculation of residual gas is usually implemented by influencing the gas exchange valve control times by means of an additional opening of inlet valves or outlet valves or a negative valve overlap via a variable valve train. The advantages of the internal EGR over the external EGR are in this case the short paths and fast response times as well as the ability for direct metering. A disadvantage is however the lack of the possibility of an effective cooling of the residual gas as in the external EGR.

0004In the known Miller/Atkinson cycle, the point in time for “inlet valve closes” (IC) is shifted to late. In this way, fresh gas or combustion air, herein also referred to as a cylinder charge, which is already present in the cylinder, is partially pushed out again into an inlet port for combustion air. This means a charging disadvantage which is compensated by a charging with a suitable boost pressure. Here, the boost pressure in the volume between the charger output and the engine inlet is to be controlled such that the boost pressure in each Miller operating point, i.e. each operating point in which the Miller/Atkinson cycle is being used, corresponds to the theoretical final compression pressure in the working cylinders of the internal combustion engine at the point in time “inlet valve closes.”

0005The positive effect of the Miller/Atkinson cycle on the NO_xemission and the ability for homogenizing the diesel mixture, by using an early or late point in time for closing the inlet valve, is known. Furthermore, the tendency to knock in high-pressure charging can be significantly reduced for spark ignition engine applications. Unlike in the spark ignition engine (Otto cycle engine), which can operate with a positive valve overlap due to the absence of geometric constraints, in a diesel engine it is generally not possible to easily implement the method by means of a phase adjuster. This is due to a mechanical collision of the valve with the piston in an advance adjustment and increased pumping work in a retard adjustment.

0006Furthermore, there is normally only one inlet valve available for the “Miller phase” prior to the start of compression, which can remain opened prior to the start of compression up to crank angles far beyond the charge exchange-BDC (bottom dead center of the reciprocating piston after the charge exchange, i.e. between an exhaust stroke and an intake stroke) for the purpose of implementing the “Miller process.” For the outflow of fresh gas from the working cylinder during the “Miller phase” prior to the start of compression, there is therefore only the volume of one inlet port available, even if two or more inlet ports are provided for the respective working cylinder.

SUMMARY OF THE INVENTION

0007It is accordingly an object of the invention to provide a method for operating an internal combustion engine which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which is improved with respect to the metering of the exhaust-gas recirculation and with respect to the implementation of a Miller/Atkinson cycle.

0008With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating an internal combustion engine, that includes the steps of:

0009providing an internal combustion engine having a working cylinder in which a piston is moved up and down so that, within a working cycle of 720 degrees of a crankshaft, successively an intake stroke, a compression stroke, a power stroke and an exhaust stroke are performed and which has at least three gas exchange valves assigned thereto, the internal combustion engine having an exhaust-gas line and having at least one inlet port which leads into a gas exchange valve configured as an inlet valve and which, via the inlet valve, supplies at least combustion air to the working cylinder, the internal combustion engine having at least one outlet port which leads into a gas exchange valve configured as an outlet valve and which, via the outlet valve, discharges at least exhaust-gas from the working cylinder into the exhaust-gas line, and the internal combustion engine having at least one exhaust-gas recirculation line which is connected to the exhaust-gas line of the internal combustion engine, wherein at least one of the gas exchange valves is used by the working cylinder as an EGR valve and is supplied with exhaust-gas from an exhaust-gas recirculation line; and

0010opening the EGR valve at least twice in a predetermined operating state of the internal combustion engine within a working cycle of 720 degrees of the crankshaft, wherein an opening duration of a first opening of the EGR valve at least overlaps in time with an opening of the outlet valve during the exhaust stroke and wherein an opening duration of a second opening of the EGR valve at least overlaps in time with an opening duration of the inlet valve during the intake stroke, wherein the EGR valve is closed between the first and the second opening.

0011In other words, according to the invention, there is provided a method for operating an internal combustion engine having at least one working cylinder in which a piston is moved up and down so that, within a working cycle of 720 degrees of the crankshaft, successively an intake stroke, a compression stroke, a power stroke and an exhaust stroke are performed and which has at least three gas exchange valves assigned thereto, having an exhaust-gas line, having at least one inlet port which leads into a gas exchange valve in the form of an inlet valve and which, via this inlet valve, supplies at least combustion air to the working cylinder, having at least one outlet port which leads into a gas exchange valve in the form of an outlet valve and which, via this outlet valve, discharges at least exhaust-gas from the working cylinder into the exhaust-gas line, and having at least one exhaust-gas recirculation line which is connected to the exhaust-gas line of the internal combustion engine, wherein at least one of the gas exchange valves is used by at least one of the working cylinders as an EGR valve and is supplied with exhaust-gas from the exhaust-gas recirculation line, the method being characterized in that in a predetermined operating state of the internal combustion engine, within a working cycle of 720 degrees of the crankshaft, the EGR valve is opened at least twice, wherein an opening duration of the first opening of the EGR valve at least overlaps in time with an opening of the outlet valve during the exhaust stroke and wherein an opening duration of the second opening of the EGR valve at least overlaps in time with an opening duration of the inlet valve during the intake stroke, wherein the EGR valve is closed between the first and the second opening.

0012This has the advantage that, without additional valves or actuators, at the same time an internal and external exhaust-gas recirculation is implemented within a working cycle of the internal combustion engine with a high metering accuracy for the recirculated exhaust-gas.

0013A simultaneous implementation of an exhaust-gas recirculation and a Miller/Atkinson cycle by means of an EGR valve without additional valves or actuators, wherein an additional volume for temporary storage of a cylinder charge, which has been expelled, is available in the form of an exhaust-gas recirculation line, is provided in that in a predetermined operating state of the internal combustion engine within a working cycle of 720 degrees of the crankshaft, the EGR valve is closed temporally after a bottom dead center between an intake stroke and a compression stroke so late after the beginning of the compression stroke and temporally after a closing of the inlet valve that a portion of the cylinder charge is pushed out into the exhaust-gas recirculation line before a compression stroke begins, so that the internal combustion engine is operated with a Miller/Atkinson cycle. In other words, a mode of the invention includes, in a predetermined operating state of the internal combustion engine within the working cycle of 720 degrees of the crankshaft, closing the EGR valve temporally after a bottom dead center between an intake stroke and a compression stroke so late after a start of the compression stroke and temporally after closing the inlet valve so that a portion of a cylinder charge is expelled into the exhaust-gas recirculation line before a compression stroke begins, so that the internal combustion engine is operated with a Miller/Atkinson cycle.

0014An internal exhaust-gas recirculation is implemented by selecting the opening duration of the first opening of the EGR valve such that an opening time (point in time of opening) of the EGR valve for the first opening occurs in time at or after an opening time of the outlet valve for the exhaust stroke and a closing time (point in time of closing) of the EGR valve for the first opening time occurs in time at or before a closing time of the outlet valve for the exhaust stroke. In other words, a mode of the invention includes selecting the opening duration of the first opening of the EGR valve such that an opening time of the EGR valve for the first opening is temporally at or after an opening time of the outlet valve for the exhaust stroke and such that a closing time of the EGR valve for the first opening is temporally at or before a closing time of the outlet valve for the exhaust stroke.

0015An external exhaust-gas recirculation is implemented by selecting the opening duration of the second opening of the EGR valve such that an opening time of the EGR valve for the second opening occurs in time at or after an opening time of the inlet valve for the intake stroke and a closing time of the EGR valve for the second opening occurs in time at or before a closing time of the inlet valve for the intake stroke. In other words, a mode of the invention includes selecting the opening duration of the second opening of the EGR valve such that an opening time of the EGR valve for the second opening is temporally at or after an opening time of the inlet valve for the intake stroke and such that a closing time of the EGR valve for the second opening is temporally at or before a closing time of the inlet valve for the intake stroke.

0016A further mode of the invention includes supplying the inlet valve only with fresh air or a basic amount of EGR.

0017Another mode of the invention includes supplying the EGR valve with pure exhaust-gas or fresh air mixed with exhaust-gas from a cooled or an uncooled low-pressure EGR circuit.

0018A further mode of the invention includes supplying the EGR valve with pure exhaust-gas or fresh air mixed with exhaust-gas from a cooled or an uncooled high-pressure EGR circuit.

0019An optimal adjustment of an operating point of the internal combustion engine with minimized pollutant emissions is achieved by supplying the inlet valve only with fresh air or a basic amount of EGR, by supplying the EGR valve with pure exhaust-gas or exhaust-gas which is mixed with fresh air, from a cooled or uncooled low-pressure EGR circuit and/or by supplying the EGR valve with pure exhaust-gas or fresh air mixed with exhaust-gas from a cooled or uncooled high-pressure EGR circuit.

0020Other features which are considered as characteristic for the invention are set forth in the appended claims.

0021Although the invention is illustrated and described herein as embodied in a method for operating an internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

0022The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0023<figref idref="DRAWINGS">FIG. 1</figref> is a schematic block diagram of a preferred embodiment of an internal combustion engine for performing the method according to the invention;

0024<figref idref="DRAWINGS">FIG. 2</figref> is a graph illustrating the piston movement and the valve movement of inlet and outlet valves and of a gas exchange valve that is used as an EGR valve, wherein a Miller/Atkinson cycle is implemented with little EGR;

0025<figref idref="DRAWINGS">FIG. 3</figref> is a graph illustrating the piston movement and the valve movement of inlet and outlet valves and of a gas exchange valve that is used as an EGR valve, wherein a Miller/Atkinson cycle is implemented with a lot of EGR; and

0026<figref idref="DRAWINGS">FIG. 4</figref> is a graph illustrating the piston movement and the valve movement of inlet and outlet valves and of a gas exchange valve that is used as an EGR valve, wherein an internal and an external EGR is implemented at the same time with the EGR valve opening twice in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

0027Referring now to the figures of the drawings in detail and first, particularly, to <figref idref="DRAWINGS">FIG. 1</figref> thereof, there is shown an exemplary embodiment of an internal combustion engine <b>10</b> for performing the method according to the invention. The internal combustion engine <b>10</b> includes working cylinders <b>12</b>, wherein each working cylinder <b>12</b> has inlet valves (intake valves) <b>14</b> and an outlet valve (exhaust valve) <b>16</b> assigned thereto. Further, each working cylinder <b>12</b> has a further gas exchange valve <b>18</b> assigned thereto. By way of a fresh air supply <b>20</b>, the working cylinders <b>12</b> are supplied, via the inlet valves <b>14</b> and corresponding inlet ports <b>22</b> opening into the inlet valves <b>14</b>, with fresh air <b>24</b> as combustion air and, by way of an exhaust-gas line <b>26</b>, exhaust-gas <b>30</b> is discharged from the working cylinders <b>12</b> via the outlet valves <b>16</b> and corresponding outlet ports <b>28</b> opening into the outlet valves <b>16</b>. A piston is disposed movably up and down in each working cylinder <b>12</b>, so that in each working cylinder, within a working cycle of 720 degrees of the crankshaft (crankshaft angle), successively an intake stroke, a compression stroke, a power stroke and an exhaust stroke is performed.

0028A compressor <b>32</b> of an exhaust-gas turbocharger <b>34</b> and a charge air cooler <b>36</b> are disposed in the fresh air supply <b>20</b>. A turbine <b>38</b> of the exhaust-gas turbocharger <b>34</b> and a particulate filter <b>40</b>, such as a diesel particulate filter, are disposed in the exhaust-gas line <b>26</b>. Furthermore, an exhaust-gas recirculation line <b>42</b> is provided for recirculating exhaust-gas <b>30</b> in the combustion process in the working cylinder <b>12</b>. This exhaust-gas recirculation line <b>42</b> is connected, downstream of the turbine <b>38</b>, to the exhaust-gas line <b>26</b> and has an EGR cooler <b>44</b> and a charge air cooler <b>46</b> for EGR, the latter being optionally embodied as an integral structural unit with the charge air cooler <b>36</b>. Furthermore, a bypass line <b>48</b> is provided in the exhaust-gas recirculation line <b>42</b>, which bypass line bridges the EGR cooler <b>44</b> and the charge air cooler <b>46</b> for EGR and which has a bypass valve <b>50</b>.

0029The respective fourth gas exchange valve <b>18</b> of each working cylinder <b>12</b> is neither connected to an inlet port <b>22</b> nor connected to an outlet port <b>28</b>, but is connected to the exhaust-gas recirculation line <b>42</b>. In this manner, one of the gas exchange valves of each working cylinder that is actuated by a respective camshaft serves directly as an EGR valve.

0030In <figref idref="DRAWINGS">FIGS. 2 to 4</figref>, a crank angle is plotted on a horizontal axis <b>52</b>, and a stroke movement is plotted on a vertical axis <b>54</b>. On the horizontal axis <b>52</b>, a bottom dead center (BDC) of the reciprocating piston before the charge exchange (between power stroke and exhaust stroke) is plotted at <b>56</b>, a top dead center (TDC) of the reciprocating piston during the charge exchange (between exhaust stroke and intake stroke) is plotted at <b>58</b> and a bottom dead center (BDC) of the reciprocating piston after the charge exchange (between intake stroke and compression stroke) is plotted at <b>60</b>. A first graph <b>62</b> illustrates the stroke movement <b>54</b> plotted over the crank angle <b>52</b> for the piston, a second graph <b>64</b> illustrates the stroke movement <b>54</b> over the crank angle <b>52</b> for the outlet valves <b>16</b>, a third graph <b>66</b> illustrates the stroke movement <b>54</b> over the crank angle <b>52</b> for the inlet valves <b>14</b> and a fourth graph <b>68</b> (dashed line) illustrates the stroke movement <b>54</b> over the crank angle <b>52</b> for those gas exchange valves <b>18</b> that are neither connected to an inlet port <b>22</b> nor connected to an outlet port <b>28</b>, but are connected to the exhaust-gas recirculation line <b>42</b> (EGR valves).

0031In the first alternative according to <figref idref="DRAWINGS">FIG. 2</figref> an external exhaust-gas recirculation takes place while the inlet valves <b>14</b> are open, and even afterwards. Here, the inlet valve <b>14</b> indeed closes in a normal manner in accordance with the third graph <b>66</b>, but the EGR valve <b>18</b> remains open longer in accordance with the fourth graph <b>68</b> and closes only after the closing of the inlet valve <b>14</b> according to the third graph <b>66</b> and after the BDC <b>60</b>. In this manner, a Miller/Atkinson cycle is implemented, because the EGR valve <b>18</b> remains open during the compression stroke after the BDC <b>60</b> according to the fourth graph <b>68</b>, so that a portion of the cylinder charge, which has previously been supplied via the inlet valve <b>14</b>, is expelled into the exhaust-gas recirculation line <b>42</b>, before the actual compression begins after closing also the EGR valve <b>18</b> according to the fourth graph <b>68</b>. By shifting the opening of the EGR valve <b>18</b> along the direction of the arrow <b>76</b>, the Miller/Atkinson cycle is more pronounced or less pronounced. At the same time, the height of the fourth graph <b>68</b> as well as the opening duration controls the amount of the external exhaust-gas recirculation. In the example according to <figref idref="DRAWINGS">FIG. 2</figref>, a small amount of exhaust-gas is recirculated. Altogether, the Miller/Atkinson cycle and the EGR are thus implemented at the same time by means of the EGR valve <b>18</b>, so that no additional valve control time adjustment (VVT, variable valve timing) is required for the inlet valves <b>14</b> and outlet valves <b>16</b>. The control of the inlet valves <b>14</b> and outlet valves <b>16</b> may be performed with fixed cams and without a camshaft adjustment or the like.

0032In the second alternative according to <figref idref="DRAWINGS">FIG. 3</figref>, the external exhaust-gas recirculation takes place during and after the inlet valves <b>14</b> are opened. In contrast to the first alternative according to <figref idref="DRAWINGS">FIG. 2</figref>, the fourth graph <b>68</b> is significantly higher and the opening time of the EGR valves <b>18</b> much longer, so that a large amount of exhaust-gas is recirculated. As is the case in the first alternative according to <figref idref="DRAWINGS">FIG. 2</figref>, the Miller/Atkinson cycle and at the same time the EGR is implemented by a late closing of the EGR valves <b>18</b>. The inlet valves <b>14</b> close in a normal manner, i.e. not for a Miller/Atkinson cycle. In the embodiment according to <figref idref="DRAWINGS">FIG. 2</figref> as well as in the embodiment according to <figref idref="DRAWINGS">FIG. 3</figref>, despite the implementation of a Miller/Atkinson cycle there is no expulsion of a portion of the cylinder charge into the inlet port <b>22</b>, but in the EGR line <b>42</b>.

0033In the third alternative according to <figref idref="DRAWINGS">FIG. 4</figref>, the EGR valves <b>18</b> are in each case opened twice within a working cycle of 720 degrees of the crankshaft, wherein each working cycle for each working cylinder includes in each case an intake stroke, a compression stroke, a power stroke and an exhaust stroke. The first opening of the EGR valves <b>18</b> according to the fourth graph <b>68</b> occurs between the BDC (bottom dead center) <b>56</b> and the TDC (top dead center) <b>58</b> while the outlet valve <b>16</b> is open according to the second graph <b>64</b>. This results in an internal EGR via the EGR valve <b>18</b>, because exhaust-gas is to some extent pushed out from the working cylinder not into the outlet port <b>28</b>, but rather into the EGR line <b>42</b> and is later again sucked into the working cylinder. The second opening of the EGR valves <b>18</b> according to the fourth graph <b>68</b> takes place between the TDC <b>58</b> and the BDC <b>60</b> while the inlet valve <b>14</b> is open in accordance with the third graph <b>66</b>. Here, on the one hand, the exhaust-gas of the previous working cycle that has previously been pushed out into the EGR line <b>42</b>, is again sucked into the working cylinder and, at the same time, exhaust-gas is externally recirculated. By shaping the fourth graph <b>68</b> to be higher for the second opening in comparison with the first opening of the EGR valve <b>18</b>, the previously expelled exhaust-gas is completely fed into the working cylinder and additionally exhaust-gas is externally supplied. This is indicated by arrow <b>78</b> for the first opening of the EGR valve <b>18</b> and arrow <b>80</b> for the second opening of the EGR valve <b>18</b>.

0034In the illustrated internal combustion engine it is provided that, in an engine configuration with more than two valves per working cylinder <b>12</b>, a gas exchange valve <b>18</b> is employed solely for the metering of exhaust-gas recirculation/residual gas. The feeding of this EGR valve <b>18</b> with a separate inlet port is carried out for example via the following variants:

0035Pure exhaust-gas from a cooled low-pressure EGR loop (optionally with its own compressor stage);

0036Pure exhaust-gas from a cooled high-pressure EGR loop;

0037Pure exhaust-gas from an uncooled high-pressure EGR loop;

0038Air/exhaust-gas mixture of the aforementioned variants.

0039In all variants, the actual inlet valves are supplied only with fresh air or a basic amount of EGR.

0040The control of the gas exchange valve <b>18</b> and optionally also of the further gas exchange valves <b>14</b>, <b>16</b> is implemented by means of a variable valve train (mechanical, electrical or hydraulic) and can, with respect to the point in time, take place in parallel to the intake as well as shifted in time. The metering of recirculated exhaust-gas is thus adjusted at each individual working cylinders <b>12</b> by positioning, lift height and control width (spread angle) of the control of the gas exchange valves <b>18</b> and, respectively, <b>14</b>, <b>16</b>.

0041The advantage of this configuration, in addition to the ability for a cycle-accurate metering of the EGR, is mainly the utilization of the entire exhaust-gas energy on the turbine side, whereas when compared to a conventional low pressure configuration only the fresh air mass flow has to be compressed. The EGR mass flow is drawn in via the EGR valve <b>18</b> directly by the engine. The optional coolers <b>44</b>, <b>46</b> and the bypasses <b>50</b> serve in this case for an air-side and exhaust-gas side temperature management. An optional additional switching valve <b>70</b> in a connecting line <b>72</b> between the exhaust-gas line <b>26</b> upstream of the turbine <b>38</b> and the exhaust-gas recirculation line <b>42</b> allows the use of high-pressure (HP) EGR and/or a low-pressure (LP) EGR. Optionally, a cooler <b>74</b> is additionally disposed in the connecting line <b>72</b>.

0042The fully variable EGR valve <b>18</b> is used, by a skillful arrangement of the control times, for implementing a Miller/Atkinson cycle with a late intake closing of the EGR valve <b>18</b>. By means of a multiple actuation of the EGR valve <b>18</b> within a working cycle, a combination of internal and external EGR is achieved via a single valve. The control times of all other inlet and outlet valves <b>14</b>, <b>16</b> can in this case remain fixed and can be actuated by a simple cam-driven control assembly. Unlike in the case of the conventional Miller/Atkinson cycle, the cylinder charge is not pushed back into the intake tract, but in the EGR tract or the EGR line <b>42</b>. The same applies for the internal EGR by means of the EGR valve <b>18</b> opening twice, as shown in <figref idref="DRAWINGS">FIG. 4</figref>.