The invention relates to a method for operating an internal combustion engine, especially of a motor vehicle. In the method, the fuel, which is to be injected for a combustion, is injected directly into a combustion chamber during an induction phase and during a compression phase. Furthermore, the invention relates to an internal combustion engine, especially for a motor vehicle. The engine has an injection valve with which the fuel, which is to be injected for a combustion, can be injected directly into a combustion chamber during an induction phase and during a compression phase. The engine has a control apparatus for controlling (open loop and/or closed loop) the fuel mass injected into the combustion chamber.
Systems for the direct injection of fuel into the combustion chamber of an internal combustion engine are generally known. In these systems, a so-called stratified operation and a so-called homogeneous operation are distinguished. The stratified operation is used especially for smaller loads; whereas, the homogeneous operation is applied for larger loads applied to the engine.
In stratified operation, the fuel is injected into the combustion chamber during the compression phase of the engine in such a manner that, at the time point of the ignition, a fuel cloud is located in the direct vicinity of the spark plug. This injection can take place in different ways. Thus, it is possible that the injected fuel cloud is already located at the spark plug during or directly after the injection and is ignited by the spark plug. Likewise, it is possible that the injected fuel cloud is conducted to the spark plug via a charge movement and is only then ignited. In both combustion methods, no uniform fuel distribution is present, instead, a stratified charge is present.
The advantage of the stratified operation is that there, with a very small fuel quantity, the applied smaller loads can be taken care of by the engine. Larger loads can, however, not be satisfied with the stratified operation.
In homogeneous operation, which is provided for such larger loads, the fuel is injected during the induction phase of the engine so that a swirling and therefore a distribution of the fuel can still easily take place in the combustion chamber. To this extent, the homogeneous operation corresponds approximately to the operation of internal combustion engines wherein fuel is injected into the intake manifold in the conventional manner. As required, the homogeneous operation can be used also for smaller loads.
So that an injection of fuel in the compression phase of the engine can take place, that is, in stratified operation, it is generally necessary that a high pressure is present for this injection. If this is not the case, then the possibility is present of a so-called backblowing of a combusted mixture of the previous combustion as well as of the inducted combustion air from the combustion chamber back into the pressure store.
After the engine has been at standstill, the high pressure, which is necessary for the stratified operation, is usually not present. For this reason, a switchover into stratified operation cannot take place immediately when starting the engine even though this would be desirable for reasons especially of saving fuel.
The object of the invention is to provide a method for operating an internal combustion engine as well as to provide an internal combustion engine with which the described backblowing is generally avoided and with which especially a starting of the engine is possible with a saving of fuel as great as possible.
The task is solved in a method of the initially-mentioned type in accordance with the invention in that the pressure is determined with which the fuel is injected into the combustion chamber and the fuel is injected during the induction phase when the pressure is less than a pregivable minimum pressure. In an internal combustion engine of the initially-mentioned type, the task is solved in accordance with the invention in that a pressure sensor is provided for measuring the pressure with which the fuel is injected into the combustion chamber and that, via the control apparatus, the fuel is injected during the induction phase when the pressure is less than a pregivable minimum pressure.
In this way, it is ensured that there is no switch into the stratified operation when the pressure is too low. Instead, in this case, the engine is operated in homogeneous operation. Generally, and especially when starting the engine, it is achieved in this manner that a backblowing of combusted mixture from the combustion chamber back into the pressure store is reliably avoided. At the same time, however, and, inter alia, when starting the engine, the pressure in the pressure store is further increased because of the injections in the induction phase and because of the first ignitions of the engine which result therefrom so that the above-mentioned pressure increases in such a manner relatively rapidly and the high pressure, which is needed for the stratified operation, is reached.
In an advantageous embodiment of the invention, the fuel is injected during the compression phase when the pressure is greater than the pregivable minimum pressure. If the pressure in the pressure store has reached the high value in this way, especially when starting the engine, then the engine is operated in stratified operation. This affords the advantage of saving fuel and reducing toxic substances.
It is advantageous when the fuel is injected again during the induction phase when the pressure is less than a pregivable hysteresis pressure. The hysteresis pressure is less than the minimum pressure. In this way, a hysteresis is built up which reliably avoids a continuous sequential back and forth switching between the stratified operation and the homogeneous operation in the region of minimum pressure.
In an advantageous embodiment of the invention, the pressure, with which the fuel is injected into the combustion chamber, is controlled (open loop and/or closed loop) to a maximum pressure. This defines a pressure control of the pressure in the pressure store with which this pressure is limited to the maximum pressure.
It is important that the method of the invention can be generally used for the operation of the internal combustion engine. The use of the method of the invention is especially advantageous during starting of the engine, especially when the rpm of the engine is less than a pregivable start-end threshold. With this use of the method of the invention during the starting of the engine, a start operation can be generated, which is combined from an initial homogeneous operation and a subsequent stratified operation which reduces the disadvantages of the homogeneous operation, especially the high fuel consumption thereof, to a minimum and which simultaneously brings out the advantages of the stratified operation as early as possible.
The realization of the method of the invention in the form of a control element is of special significance. The control element is provided for a control apparatus of the engine, especially of a motor vehicle. A program is stored on the control element which can be run on a control apparatus, especially on a microprocessor, and is suitable for carrying out the method of the invention. In this case, the invention is therefore realized by a program which is stored on the control element so that this control element, which is provided with the program, defines the invention in the same way as the method for whose execution the program is suitable. As a control element, especially an electric storage medium can be used, such as a read-only-memory.