Method and Apparatus for Operating a Spark-Ignited Reciprocating-Piston Internal Combustion Engine

This invention relates to methods and apparatus for operating spark-ignited reciprocating-piston internal combustion engines in which the flow of intake and exhaust gases is controlled by periodically actuated intake and exhaust valves and exhaust gases are recirculated.

German Offenlegungsschrift No. 2 125 368 discloses an arrangement for internal exhaust gas recirculation in an internal combustion engine in which periodically actuated intake and exhaust valves are controlled in such a way that, during the opening of the intake valve, the exhaust valve is opened briefly by an additional lobe on an exhaust valve control cam and, as a result, exhaust gas is recirculated from an exhaust duct into the combustion chamber by the downward motion of the reciprocating piston during the overlap phase between the intake valve and the exhaust valve. At idling speeds, this actuation of the exhaust valve during the intake stroke ceases, this being accomplished by operation of hydraulic regulating tappets. This appears to provide a possible improvement in the smoothness of operation of the internal combustion engine resulting from increased turbulence in the fresh-gas charge, i.e., by the homogenization of the fresh gas supplied as a result of the exhaust gas introduction. There is no indication in this disclosure as to how the gases are guided in the internal combustion engine.

European published application No. 0 063 305 A1 discloses a spark-ignited reciprocating-piston internal combustion engine having an adjustable-stroke intake valve which is recessed relative to the combustion chamber in order to burn leaner mixtures. To obtain reliable ignition of the lean mixture, a chamfer is arranged in the form of a guide duct provided on one side in the region of the transition from the inlet cross-section of the intake duct to the combustion chamber, and this guide duct guides the induced mixture toward the spark plug.

Accordingly, it an object of the present invention to provide a method and apparatus for operating a spark-ignited reciprocating-piston internal combustion engine which overcomes disadvantages of the prior art.

Another object of the invention is to provide a method and an apparatus for operating a spark-ignited reciprocating-piston internal combustion engine which achieves significant reductions in fuel consumption, especially in the partial-load operating range, and wherein an undesired increase in HC and CO emissions is avoided without causing any disadvantages in the full-load operating range.

These and other objects of the invention are attained by providing exhaust gas recirculation arranged to produce charge stratification in the combustion chamber so that the fresh gas concentration is increased in the vicinity of the spark plug. This results in a significant reduction in fuel consumption, especially in the partial-load operating range, and the internal recirculation of comparatively high quantities of exhaust gas simultaneously avoids increases in HC and CO emissions. The high tolerance of the combustion method used herein for recirculated exhaust is achieved by the use of a specific charge stratification of exhaust gas and fresh gas, largely avoiding homogenization.

To recirculate a comparatively large quantity of exhaust gas from the exhaust duct, the exhaust gas valve lift curve, and optionally the intake valve lift curve, is adjusted in the direction of a retarded crank angle, i.e., it is shifted to a large extent into the intake stroke of the cylinder, so that the downward movement of the reciprocating piston produces a sufficient vacuum to draw in significant quantities of exhaust gas. To assure that this comparatively large quantity of exhaust gas does not result in a mixture which is difficult to ignite in the combustion chamber, the invention furthermore provides a swirling flow of the recirculated exhaust gas which rotates about an axis extending parallel to the direction of motion of the reciprocating piston. This swirling flow, together with the quantity of fresh gas flowing in through the intake valves, leads to a specific charge stratification in the combustion chamber so that an increased fresh gas concentration is available in the vicinity of the spark plug for the combustion chamber.

It is thus important to the invention that a comparatively large quantity of exhaust gas is recirculated from the exhaust duct. This is achieved by a specific shift in the exhaust valve lift curve, and optionally the intake valve lift curve, relative to the crank angle, and the recirculated exhaust gas has a specific swirling flow imposed on it as it flows out of the exhaust duct which, together with the inflowing stream of the fresh gas, brings about a charge stratification in such a way that there is a readily ignited mixture in the vicinity of the spark plug.

The invention can be employed both with internal combustion engines having two valves per combustion chamber, i.e. having one intake valve and one exhaust valve, and with multi-valve internal combustion engines. Since, in the case of two-valve internal combustion engines, only one camshaft is generally used to control both the intake and exhaust valves, that camshaft can be adjusted overall relative to the crank angle to provide a comparatively simple embodiment, so that both the exhaust valve lift curve and the intake valve lift curve are adjusted synchronously. Such synchronous adjustment is advantageous since an improved and increased stratification/concentration gradient is established in this case whereas in the case of adjustment exclusively of the exhaust-valve lift curve, a tendency towards homogenization, which is undesirable per se, is observed in certain regions of the combustion chamber.

The swirling flow imposed upon the recirculated exhaust gas is preferably produced by appropriate configuration of the exhaust duct, and this involves an arrangement similar to that previously provided solely to the configuration of the intake duct to promote swirling of the fresh gas mixture.

In a particularly preferred embodiment, at least one of the exhaust valves, together with the corresponding exhaust duct flow cross-section, is recessed relative to the combustion chamber and a chamfer is provided along one side of the duct between this flow cross-section and the combustion-chamber land. The chamfer is arranged so that, because of the presence of the chamfer on one side and the absence of a chamfer on the opposite side, the exhaust gas which emerges and is recirculated flows into the combustion chamber in a defined direction tangential to the cylinder axis, particularly in the case of small valve strokes of the exhaust valves. This sets up a pronounced rotation of the recirculated exhaust gases in the combustion chamber.

In an advantageous embodiment, where there are a plurality of exhaust valves per combustion chamber, e.g. two per combustion chamber, it is furthermore possible for the flow directions provided by the chamfers to be a mirror image, giving rise to a corresponding number of swirling flows rotating in opposite directions to one another about parallel axes.

In the case of multi-valve internal combustion engines, provision can preferably be made for at least one of the exhaust valves to be capable of being decoupled from the periodic drive by the camshaft, making it possible to increase the swirl of the exhaust gas in a specifically intended manner by virtue of an asymmetric inflow. In a further advantageous arrangement utilizing a deactivatable exhaust valve, provision can be made for only the exhaust valve which is not deactivated to have the abovementioned chamfer.

In the simplest case, the flow motion imposed upon the fresh gas flowing in through the intake valve or valves is that set up by virtue of a conventional duct configuration. In this context, the position of the intake cross-section is important for the charge stratification.

Preferably the fresh gas intake is arranged to impose a swirling flow with a comparatively low level of swirl in comparison with the swirling flow of the exhaust gas.

For a multi-valve internal combustion engine, the fresh gas preferably has a tumbling flow imposed upon it, i.e., the fresh gas rotates about an axis running transversely to the cylinder axis. In a further advantageous arrangement, the intensity of the tumbling flow can be adjusted by flaps arranged, for example, in the intake ducts.

In a further advantageous embodiment for multi-valve internal combustion engines with at least two exhaust valves, provision can be made for at least one of the exhaust valves to close prematurely. For this purpose, a corresponding cam is provided with a flank which closes one valve prematurely relative to the other exhaust valve cam.

The adjustment of the exhaust and/or intake valve lift curves can be effected using any conventional arrangement for shifting such curves relative to the crank drive angle. This shift can be performed either with a so-called two-point positioning device or with a device movable to any point in a range.

Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings in which:

FIG. 1a is a perspective and schematic view illustrating a representative embodiment of a multi-valve internal combustion engine in accordance with the invention;

FIG. 1b is a perspective and schematic view illustrating a representative embodiment of another multi-valve internal combustion engine according to the invention;

FIG. 2 is a perspective and schematic view illustrating a representative embodiment of a two-valve internal combustion engine according to the invention;

FIG. 3 is a schematic plan view showing a combustion-chamber land on a multivalve internal combustion engine according to the invention;

FIG. 4 is a sectional view taken along the line IV--IV of FIG. 3 and looking in the direction of the arrows;

FIG. 5 is a graphical representation showing valve lift curves with a shifted exhaust-valve lift curve; and

FIG. 6 is a graphical representation showing valve lift curves with two shifted curves.

In the typical embodiment of the invention shown in FIG. 1a, a spark-ignited reciprocating-piston internal combustion engine includes a cylinder 1 in which a piston 2 is slideably mounted. A combustion chamber 4 defined between the piston 2 and a combustion-chamber land 3 is supplied with fresh fuel gas through two intake valves 5 mounted in an intake duct 14.

The combustion exhaust gases are discharged from the combustion chamber 4 through two exhaust valves 6 and 7 mounted in an exhaust duct 8. The intake valves 5 and the exhaust valves 6 and 7 are driven periodically in a conventional manner by cams on one or more camshafts (not shown) which are driven in conjunction with the crankshaft of the internal combustion engine.

The variation with time of the valve lift against the crank angle of the crankshaft is illustrated in FIG. 5 in the form of an exhaust valve lift curve A and an intake valve lift curve E. Between the curves A and E therein is a comparatively small valve overlap phase 9.

A conventional arrangement for changing the position of at least the exhaust valve lift curve A relative to the crank angle is provided between the crankshaft and the camshaft in such a way that the lift curve can be shifted to the curve position indicated by A', thereby establishing an enlarged valve overlap phase 10. The arrangement for changing the position of the valve lift curve relative to the crank angle can also be employed in such a way that both camshafts of a multi-valve engine are adjusted synchronously as shown by the shifted curves A' and E' in FIG. 6.

When a valve lift curve shifting arrangement is used with cylinders 1 having only two gas exchange valves as shown in FIG. 2 and having one camshaft with cams arranged rigidly thereon, the change in position relative to the crank angle produces the curve shifts shown in FIG. 6, in which both the exhaust-valve lift curve A and the intake valve lift curve E are shifted by the same amount to the positions A' and E', with the result that there is no change in the valve overlap phase 9.

The intake and exhaust valves 5, and 6 and 7, respectively, each control the flow through corresponding flow cross-sections 13 of the intake duct 14, and 15 and 16 of the exhaust duct 8, the flow cross-sections being provided with corresponding valve seat rings 11 and 12.

As shown in FIGS. 3 and 4, at least one flow cross-section 15 of the exhaust duct 8 is recessed in the combustion-chamber land 3 relative to the combustion chamber 4 and a duct-like chamfer 17 is provided on one side of this flow cross-section 15, in a region of transition to the combustion-chamber land 3. The maximum extent of this chamfer 17 in the radial direction relative to the flow cross-section 15 is indicated by a vector R which points in a direction tangential to an axis Z extending parallel to the direction of motion of the reciprocating piston 2. The unchamfered region of the combustion chamber land 3 which is opposite the chamfer 17 provides a circular arc-shaped mask 18 for the exhaust valve 6 in a first stroke region.

When a plurality of exhaust valves 6 and 7 are provided, the directions R of the chamfers 17 can differ from each other to establish a specific swirling flow D.

The combustion chamber 4 has a spark plug 19 mounted in an approximately central position.

The operation of the reciprocating-piston internal combustion engine described above is as follows. In one or more specific load ranges, a partial load range for instance, the exhaust valve lift curve A is shifted to its position A' during a downward motion of the piston 2 by the arrangement for changing the position of the valve lift curves relating to the crank angle. As a result, the exhaust valve 6 or valves 6 and 7 are open during the fresh gas intake stroke. Because of the relative orientations of the vector R of the chamfer 17 and the mask 18 with respect to the cylinder axis 2, a specific swirling flow D is established in a first stroke region of the exhaust valves 6 and 7 since the exhaust gas can leave the exhaust duct 8 only through the gaps formed between the exhaust valves 6 and 7 and the chamfers 17. The mask 18 is effective in this manner unless the valve stroke exceeds the length of the mask 18 which would allow the exhaust gas to propagate in the mask direction as well. With the embodiment shown in FIGS. 1a and 3, the positions of the vectors R of the chamfers 17 is chosen in such a way that the recirculated quantity of exhaust gas in the swirling flow D rotates about the axis Z. The valve lift curve A or of both the curves A and E may be shifted relative to the crank angle by amounts up to 60.degree. of crank angle, preferably between 20.degree. and 45.degree.. This assures that a comparatively large amount of exhaust gas is internally recirculated. For a two-valve engine such as shown in FIG. 2, the intake of fresh gas through the intake duct 14 can take place either without swirl, i.e., in a manner defined exclusively by a conventional configuration of the intake duct 14, or with a swirl, the level of swirl of this flow of fresh gas generally being significantly lower than that of the recirculated flow of exhaust gas.

For multi-valve engines such as shown in FIG. 1a and 1b, the flow of fresh gas is preferably supplied in such a way that a comparatively slight tumble flow is established, i.e., the stream of fresh gas also rotates about an axis Q extending transversely to the axis Z. To influence the level of tumble, adjustable flaps, for example, can be provided in conventional manner in the outlet duct 14.

With a plurality of exhaust valves 6 and 7 as shown in FIG. 1a, the formation of a chamfer 17 for only one of the valves can be sufficient, in which case the other of the exhaust valves 6 and 7 is closed during the internal exhaust-gas recirculation, resulting in increased swirl intensity. Maintaining one of the exhaust valves 6 and 7 closed in this manner can be accomplished either by deactivating the valve by switchable hydraulic bucket tappets, for example, or by appropriate configuration of the cam shape for that valve.

Because of the swirling flow of recirculated exhaust gas along the periphery of the cylinder wall and of the fresh gas flowing in through the flow cross-section or cross-sections 13, a clear charge stratification is established in the combustion chamber 4, with an increased concentration of fresh gas occurring in the region of the spark plug 19, so that ignition is performed reliably despite the relatively large quantities of exhaust gas in the chamber. A significant reduction in fuel consumption can be achieved by recirculating a comparatively large quantity of exhaust gas, particularly in partial load ranges of the internal combustion engine in which the load demand is not high, and no rise in HC or CO emissions is observed.

In the arrangement shown in FIG. 1b, the vectors R of two chamfers 17 can be arranged in mirror-image fashion relative to one another so that two swirling flows D and D', each rotating about a corresponding axis Z and Z' parallel to the direction of motion in the cylinder, are established in opposite directions. The sum of the swirl levels here is zero. As in all the other embodiments of the invention, the density of the inflowing exhaust gas is relatively low in comparison with the fresh gas density because of the exhaust-gas temperature, i.e., the exhaust gas streams are relatively light in weight.

Consequently, they arc pushed aside by the inflowing fresh gas and are taken along in diluted form in the tumbling motion of the fresh gas.

Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.