At some crank angle in an engine cycle after the beginning of combustion in a cylinder, cylinder pressure reaches a maximum, or peak, value. Typically, that peak pressure exists for only a minute fraction of the engine cycle, and in a trace of cylinder pressure vs. crank angle, the duration of peak pressure would appear essentially as a single point of the trace, as shown for example in FIG. 3 of U.S. Pat. No. 4,397,285. It is known that the area under such a pressure trace at least approximates the engine power output. Prolonging the duration of peak pressure could be an effective way to increase the area under the pressure trace, and hence power output, if that extended duration does not cause offsetting power loss due to post-peak decreases in pressure.
The nature of a diesel engine and its fuel injection system allows fuel to be injected even after combustion has commenced within a cylinder. The fuel injection process during an engine cycle is some generally described in terms of phases, such as pilot injection, main injection, and post-injection. Any particular fuel injection process typically always comprises a main fuel injection, with pilot and post-injection being optional possibilities.
Contemporary fuel injection systems allow injection pressure, injection rate, and injection timing to be fairly accurately controlled. The inventor has recognized that such control capabilities can be used to accomplish the goal of prolonging the duration of peak pressure for increasing the power output of a diesel engine.
U.S. Pat. No. 5,522,359 describes a method of fueling a diesel engine in a manner that limits peak pressure in order to limit in-cylinder temperature rise for the purpose of limiting NOx formation. A single pulse is used for an injection, and its timing occurs so as to cause combustion to occur during the downward expansion stroke.
Principles of the present invention can be embodied in fuel injection control strategies that use more than one discrete injection pulse, in particular by implementing such strategies in the processing system that processes various data to control the operation of the fuel injectors that inject fuel into the engine cylinders.
Controlling fuel injections in a manner that properly controls the rate at which fuel is being injected as a function of crank angle can be effective to accomplish the desired goal. Exactly how fuel injections will be controlled in any given engine depends on specifics of the engine, the fuel injection system, and the processing system that controls the fuel injection system. For example, fuel must obviously be injected at a pressure greater than in-cylinder pressure, but control of injection pressure by itself may not necessarily control the instantaneous rate of fuel injection because of other factors that are present, such as the design of the particular fuel injectors or the crank angle at which fuel actually beings to combust and release energy into a cylinder.
For certain engines, fuel injection systems, and processing systems, the control of fuel injection pressure and the number of fuel injections, including the timing and duration of each injection, may be sufficient to at least approximately define a desired trace of rate of fuel injection vs. crank angle that will result in prolonging the duration of peak cylinder pressure in a way that increases per-cylinder power output.
Because an engine that powers a motor vehicle will run at different speeds depending on how the vehicle is being driven, the crank angle at which peak pressure occurs may change with engine speed. Hence, the task of successfully prolonging peak pressure at different engine speeds depends on the capacity of the processing system to store a number of data maps that will store data that can be used in the calculation of fueling data to assure proper control of the fuel injection system within each of multiple speed ranges. The data stored in the maps is obtained by mapping a base engine for various combinations of engine speed and load to develop the data that needs to be stored in the processing system for use in calculating engine fueling data. The engine fueling data will cause the engine to be fueled in such a way that the rate at which the combusting fuel is releasing energy will cause the peak in-cylinder pressure to be prolonged for a greater fraction of an engine cycle than has heretofore been possible.
It is to be understood that the mapping of an engine that achieves this goal results from investigation and analysis of many factors. Knowledge of the combustion chamber design and of the capabilities of the fuel injection system and the associated processing system forms a starting point in mapping an engine. As long as the fuel injection and processing systems are capable of providing the necessary degree of control accuracy, the goal of prolonging peak in-cylinder pressure becomes attainable, albeit that different factors might be used for control in different systems depending on specifics of those systems.
With knowledge of how a given fuel injection system and associated processing system perform, and with knowledge of how combustion occurs within the cylinders of a particular engine, it becomes possible to correlate in-cylinder pressure resulting from combustion of injected fuel with the rate at which the combusting fuel is releasing energy. A peak pressure can be prolonged for a limited fraction of the engine cycle in an engine cylinder by controlling the rate at which additional fuel is injected into the cylinder once fuel already in the cylinder has started to combust and is releasing energy.
For any given speed and load the data entered into a map can be obtained through from engineering analysis and/or actual data from a running engine.
For certain speeds and loads, only two discrete injection pulses, each of suitable timing and duration, may be sufficient for initiating combustion within a cylinder to the point of maximum pressure and prolonging that maximum pressure for a limited fraction of the engine cycle in a way that increases the power output. Such a combination of pulses may be a main pulse that is preceded by a pilot injection pulse or a main pulse that is followed by a post-injection pulse.
For other speeds and loads, more than two discrete injection pulses may be required, such as one or more pilot injections and/or one or more post-injections accompanying a main fuel injection, may be needed in order to cause the combusting fuel to reach maximum pressure and then prolong that maximum pressure for a limited fraction of the engine cycle so as to increases the power output. Moreover, each such injection, whether it be a pilot injection, a main injection, or a post-injection, must be defined by timing, duration, and fuel pressure.
Consequently, the task of mapping an engine requires a developmental effort where multiple solutions become entirely possible due to the number of variables and possible combinations of those variables. However, once a base engine has been mapped and appropriate solutions selected as a result of the development effort, the mapping may be used in the mass production of the engine to the benefit of end users of the engines.