Four-stroke gasoline engine with direct injection and method for valve control

Abstract

The invention relates to a method for actuating intake valves and exhaust valves of a four-stroke direct-injection gasoline engine. At low torque, a parallel retardation of the intake valves and exhaust valves is carried out, so that, at the commencement of the intake stroke, the exhaust valve is still open and the intake valve is still closed after top dead center (TDC) of the piston. High internal exhaust gas recirculation (EGR) occurs, irrespective of the pressure in the intake manifold. The resulting internal EGR is sufficient to displace the 50% MFB timing to its optimal position after TDC and to reduce nitrogen oxide emissions.

Description

FIELD OF THE INVENTION

[0001] The invention relates to a method for the control of at least one intake valve and of at least one exhaust valve of a four-stroke gasoline engine with direct injection, under part load the exhaust valve being opened, at the commencement of the intake stroke, after the top dead center of the piston. The invention relates, furthermore, to a four-stroke gasoline engine with direct injection, containing at least one intake valve and at least one exhaust valve which are capable of being opened and closed via at least one phase-adjustable camshaft, and also to an engine control for setting the phase displacement of the camshaft.

BACKGROUND OF THE INVENTION

[0002] Four-stroke internal combustion engines with spark ignition (gasoline engines) are subdivided into those with external mixture formation and those with internal mixture formation. Where external mixture formation is concerned, the air/fuel mixture is generated outside the cylinders, for example by means of a carburetor or by port fuel injection into the intake pipe. In these engines, the power output is controlled via throttling of the air supply.

[0003] In gasoline engines with internal mixture formation, fuel is directly injected into the combustion chambers (cylinders). Direct injection gasoline (DI-G) engines can be operated with a lean air/fuel mixture. The air/fuel mixture is either distributed homogeneously or in a stratified manner in the cylinder space. Furthermore, at relatively low engine torque, DI-G engines can be operated without throttling the air supply. Consequently, pumping losses in the low torque mode are avoided and fuel efficiency is improved.

[0004] The operation of a DI-G engine affects combustion, particularly in the case of stratified charge. Fuel injection takes place within a relatively narrow time window before top dead center (TDC) of the piston, i.e., during the compression stroke when the spark plug is capable of igniting the air/fuel mixture. Moreover, in comparison with an engine with homogeneous mixture formation, the flame propagation speed is high. Due to the effects mentioned above, the timing of when 50% of the mass of fuel is burned (50% MFB), does not occur at an optimal time in terms of efficiency. To optimize fuel efficiency, 50% MFB timing occurs about 8-10° after TDC of the piston in the expansion or working stroke. In the prior art, the 50% MFB timing for DI-G engine occurs considerably earlier.

[0005] To solve this problem, exhaust gas recirculation (EGR) may be used. Recirculation of burned exhaust gases into the combustion chamber causes the combustion of the air/fuel mixture to be slowed during the next working stroke. Consequently, the timing 50% MFB timing is retarded and becomes closer to the optimal timing mentioned above. EGR may be external, in which exhaust gases are conducted from the exhaust manifold to the intake manifold through pipes and valves. On account of the additional components, however, this method is relatively costly. Internal EGR can be provided, in which burned exhaust gases flow back into the cylinder via an open exhaust valve (cf. DE 199 48 298 A1). Internal EGR can be provided when the intake valves or exhaust valves are variably activatable, for example via one or two camshafts with adjustable phasing. In the known systems, at the end of the exhaust stroke, valve overlap, i.e., when both intake and exhaust valves are open at the same time, is set around TDC, with the result that the desired return or dwell of exhaust gases in the cylinder occurs.

[0006] It was found, however, that the last-mentioned method of internal EGR does not, in DI-G engines, lead to satisfactory results or to a sufficient shift of the 50% MFB timing.

SUMMARY OF THE INVENTION

[0007] The method according to the invention for the control of the at least one intake valve and of the at least one exhaust valve of a four-stroke gasoline engine with direct injection is based, on the one hand, on the fact that, under part-load conditions of the engine, the at least one exhaust valve is still held open after the top dead center of the piston at the commencement of each intake stroke. During the downward movement of the piston which commences after the top dead center, therefore, burnt exhaust gas can be sucked in again from the exhaust manifold through the open exhaust valve. The method is defined, furthermore, in that, at the commencement of the intake stroke, the at least one intake valve is held closed after the top dead center of the piston.

[0008] Thus, in contrast to known methods of valve control, at the commencement of the intake stroke, when the piston moves down again and thereby generates a vacuum in the cylinder, no or only a slight overlapping opening of intake valves and exhaust valves takes place. Instead, at this time, only the exhaust valve is held open, so that exhaust gases can flow back. Since, because of the closed intake valve, no inflowing air from the intake manifold counteracts the backflow of the exhaust gases, a sufficiently large exhaust gas quantity can be collected in the cylinder. By contrast, with the intake valve open, it would not be possible to collect such an exhaust gas quantity, since, in a gasoline engine with direct injection, there is no throttling of the air supply and consequently, also, no corresponding vacuum prevails in the intake manifold. By contrast, with the method according to the invention, even in an engine of this type, sufficiently high internal exhaust gas recirculation is possible, which can be utilized for a displacement of the combustion center of gravity in the direction of the optimal position. Contrary to external exhaust gas recirculation, no additional structural measures on the engine are necessary in order to achieve this result.

[0009] According to a preferred version of the method, the closing of the at least one exhaust valve takes place in the intake stroke and the opening of the at least one intake valve takes place in the intake stroke, in such a way that the combustion center of gravity at which 50% of the fuel is burnt is displaced into the optimal time point. This optimal time point is typically at a crankshaft angle of between 8 and 10° after the top dead center of the piston. Since the combustion center of gravity can be delayed correspondingly, maximum combustion efficiency can be achieved.

[0010] According to a preferred development of the method, the at least one intake valve and the at least one exhaust valve of the gasoline engine are operated in the intake stroke, under part load, in such a way that the respective opening times do not overlap one another. Preferably, the exhaust valve is closed approximately at the same time point at which the intake valve begins to open. By a valve overlap being ruled out, this ensures that the inflow of air through the intake valve at no time counteracts the return flow of the exhaust gases through the open exhaust valve. A very accurately determinable exhaust gas quantity can thereby be recirculated into the cylinder.

[0011] Under part load, at the commencement of the intake stroke, the at least one exhaust valve of the gasoline engine is still held open preferably up to a crankshaft angle of at least 10°, particularly preferably of at least 45°, after the top dead center of the piston. In a similar way, at the commencement of the intake stroke, under part load, the at least one intake valve of the gasoline engine is still held closed preferably up to at least 10°, particularly preferably up to at least 45°, after the top dead center of the piston. It was shown that, in the case of such actual settings of the valve control, on the one hand, sufficiently high internal exhaust gas recirculation occurs, which is sufficient for displacing the combustion center of gravity into the optimal position, and, on the other hand, a sufficient quantity of fresh air can also be sucked in.

[0012] According to a preferred embodiment of the method, the at least one intake valve and the at least one exhaust valve of the gasoline engine are actuated by a common camshaft of adjustable phase. The basic setting of a camshaft of this type is, in general, that the exhaust valve closes at the end of the emission stroke at the top dead center of the piston, while the intake valve begins to open at the same time point with the commencement of the intake stroke. This setting can be advanced or retarded in parallel by means of a common phase-displaceable camshaft for both valves. The method explained above is in this case carried out via a late setting of the camshaft under part-load conditions of the engine.

[0013] According to another embodiment of the method, the at least one intake valve and the at least one exhaust valve of the gasoline engine are actuated in each case by a specific camshaft of adjustable phase. Although the control of the intake and exhaust valves via specific separately adjustable camshafts is more complicated in structural terms, it nevertheless allows greater flexibility in engine control.

[0014] The invention relates, furthermore, to a four-stroke gasoline engine with direct injection, which contains at least one intake valve and at least one exhaust valve which are capable of being opened and closed via at least one phase-adjustable camshaft. Furthermore, the gasoline engine contains an engine control which can perform the setting of the phase displacement of the camshaft and which is designed to the effect of carrying out a method of the type explained above. That is to say, the engine control can activate the at least one camshaft, in particular, in such a way that, at the commencement of the intake stroke, with the engine being under part load, after the top dead center of the piston the exhaust valve is still held open and at the same time the intake valve is still held closed. The engine control may be implemented, in particular, in the form of a microcomputer. By means of a gasoline engine of this type, the advantages of the method described can be achieved, that is to say sufficient internal exhaust gas recirculation for displacing the combustion center of gravity into its optimal position.

[0015] According to a development of the four-stroke gasoline engine described above, the at least one intake valve and the at least one exhaust valve have in each case a specific camshaft separately adjustable by the engine control. By virtue of the separate adjustability of the phases of the intake valve and exhaust valve, high flexibility can be achieved in engine control, by means of which optimal settings can be carried out under all operating conditions.

[0016] The at least one camshaft of the gasoline engine is preferably arranged overhead, that is to say so as to run on the cylinder head.

BRIEF DESCRIPTION OF THE FIGURES

[0017] The invention is explained in more detail below by way of example, with reference to the figures in which:

[0018] FIG. 1 shows a valve control diagram for a DI-G engine, according to the prior art; and

[0019] FIG. 2 shows a valve control diagram according to the present invention.

DETAILED DESCRIPTION

[0020] In FIGS. 1 and 2, valve lift height is plotted against crank angle degrees in the x-axis. Curves 1, 3, and 13 are for an exhaust valve and curves 2, 4, and 14, are for an intake valve. The intake and exhaust valves are arranged in a known way oin the head of the cylinders of a four-stroke gasoline engine. The valves control the entry of fresh air and the exit of the burned exhaust gases.

[0021] In FIG. 1, curves 1 and 2 show valve control during normal operation, according to the prior art. In this case, during the exhaust stroke, the exhaust valve is opened between bottom dead center (BDC) and TDC of the piston and is otherwise closed. The intake valve (curve 2) opens between TDC and the next BDC. No, or only a minimal overlap, of the valve openings occurs at TDC.

[0022] Furthermore, FIG. 1 illustrates actuation, conventional in the prior art, of the valves under low torque. The exhaust valve timing is retarded, according to curve 3 and the intake valve is advanced, according to curve 4. Such separate phase adjustment is possible with two separate, and separately adjustable, camshafts for the intake and exhaust valves. Valve overlap near TDC causes exhaust gases to enter the intake manifold during the end of the exhaust stroke. Burned exhaust gases are inducted out of the intake manifold at the beginning of the intake stroke. Internal EGR is control in this way. It was found, however, that, in four-stroke DI-G engine, the internal EGR possible by valve overlap, as shown in FIG. 1, is insufficient to cause the 50% MFB timing to adjust to the optimal timing (approximately 8-10° after TDC). The reason that more internal EGR cannot be obtained by valve overlap, as shown in FIG. 1, is that there is insufficient vacuum for drawing exhaust gases does not prevail in the intake manifold because of the lack of throttling.

[0023] To achieve sufficient internal EGR under low torque conditions of the engine, and, furthermore, to reduce the formation of nitrogen oxides, which is possible by EGR, the valve timing, according to the present invention, is shown in FIG. 2.

[0024] Referring now to FIG. 2, curve 1 for the exhaust valve and curve 2 for the intake valve illustrate normal operation according to the prior art; these correspond to curves 1 and 2 in FIG. 1. According to the invention, both the intake valves and the exhaust valves are operated, equally retarded, shown as curves 13 and 14, in FIG. 2. Such a parallel retardation of the intake valves and exhaust valves can be achieved both by a common phase-displaceable camshaft for the intake valves and exhaust valves and by separate camshafts for the intake valves and exhaust valves. For structural reasons, it is preferable, for the camshafts to be arranged overhead as a SOHC (single overhead camshaft) configuration or a DOHC (double overhead camshaft) configuration.

[0025] According to the invention, according to curve 13, the exhaust valve is open at the commencement of the intake stroke up to about 45° after TDC. The intake valve is still closed in the same period of time and begins to open only when the exhaust valve is closed. What is achieved, thereby is that, at the commencement of the intake stroke, exhaust gases are drawn back out of the exhaust manifold through the open exhaust valve regardless of the conditions in the intake manifold. Consequently, in spite of the unthrottled operation of a DI-G engine, sufficiently high internal EGR can be achieved, which allows optimization of time of 50% MFB into the optimal timing and at the same time reduces nitrogen oxide formation.

Claims

1. A method for controlling at least one intake valve and at least one exhaust valve of a four-stroke direct-injection gasoline engine under low torque operation, comprising: retarding an opening of the intake valve so that the intake valve opens after top dead center (TDC) of the piston.

2. The method of claim 1 wherein the intake valve retardation is adjusted so that the 50% fuel mass fraction burned time occurs at about 9 crank angle degrees after TDC of expansion.

3. The method of claim 1 wherein the intake valve opening occurs after a closing time of the exhaust valve.

4. The method of claim 1 further comprising: adjusting a closing time of the exhaust valve to occur at least 45 crank angle degrees after TDC of intake.

5. The method of claim 1, further comprising: adjusting said opening time of the intake valve to occur at least 45 crank angle degrees after TDC.

6. The method of claim 1 wherein the intake valve and the exhaust valve are actuated by a common camshaft of adjustable phase.

7. The method of claim 1, wherein the intake valve is actuated by an intake camshaft of adjustable phase and the exhaust valve is actuated by an exhaust camshaft of adjustable phase.

8. A four-stroke, direct-injection gasoline engine having at least one intake valve, at least one exhaust valve, and a camshaft, comprising:

an engine controller electrically coupled to the engine and the camshaft, said engine controller causes a phase displacement of the camshaft to cause an opening of the intake valve so that the intake valve opens after top dead center (TDC) of the piston.

9. The engine of claim 8 wherein the engine controller causes the exhaust valve closing time to occur at least 45 degrees after TDC.

10. The engine of claim 8 wherein the engine controller causes the intake valve opening time to occur at least 45 degrees after TDC.

11. The engine of claim 8 wherein the camshaft actuates the intake valve, comprising: an exhaust camshaft actuating the exhaust valve.

12. The engine of claim 8, wherein the camshaft is arranged overhead.