In an engine fuel system having a plurality of fuel injectors, it is typically desirable that all of the injectors have substantially identical injector operational characteristics. In the past, it was recognized that each injector should deliver approximately the same quantity of fuel and approximately the same timed relationship to the engine for proper operation. If the timing of the injectors diverged beyond acceptable limits, problems would be encountered due to the generation of different torques between cylinders. Further, knowledge that such variations in injector characteristics are inevitable require engine system designers to account for this variability. Accordingly, many engine systems are designed not for peak or maximum cylinder pressures or output, but rather, are designed to provide an output equal to the maximum theoretical output less an amount due to the worst case fuel injector variability.
One approach for solving these problems in unit injectors is the so-called select fit manufacturing process. Generally, a common procedure involves flowing fluid through each unit injector nozzle and pumping mechanism and categorizing each nozzle and pumping mechanism accordingly. During assembly, each nozzle is matched with pumping mechanisms known to be compatible, depending on the category under which the nozzle is categorized. The disadvantage associated with this approach is the relatively high cost involved with sorting the nozzles and pumping mechanisms and maintaining these groupings for the duration of the manufacturing and assembly process.
Another approach for solving these problems involves extremely rigid manufacturing procedures for achieving the high manufacturing precision necessary to meet the desired design specification. Such high manufacturing precision has the disadvantage of increasing the manufacturing costs, including the costs involved in manufacturing precision components and subassemblies and the cost related to the subsequent assembly process. Further, neither of the above mentioned manufacturing-oriented solutions satisfactorily controls rejection of completely assembled injectors that fail to fall within the timing and delivery tolerances of the design specification. Thus, excess scrap remains a problem with these manufacturing-oriented approaches.
With the advent of increasingly sophisticated electronic control a new approach to the problem of timing and delivery variations has emerged. In known electronic fuel injection systems, especially diesel-cycle internal combustion engine systems, the timing or start of injection, as well as the end of injection, are controlled by an electronic control, which controls these parameters for all of the engine cylinders.
An early attempt at using an electronic control to compensate for individual injector timing and delivery variation in an engine system involved measuring the flow characteristics of a particular injector at a single operating condition and comparing the flow characteristics to an ideal fuel injector. The results of this comparison were used to modify a nominal control signal to compensate for the measured variation. This approach has proven unsatisfactory because it does not take into account the fact that timing and delivery variations exist not only between injectors, but also as a function of the particular operating condition at which the injectors are operated. Thus, this approach has failed to provide a reduced injector to injector and injector-to-nominal performance variation necessary to meet today's increasingly strict emission standards.
Others have tried to compensate for variation in the start of injection characteristic of individual injectors in an engine system by designating a proxy for the timing or the start of injection characteristics of the injector. In general, these methods first electrically detect the closure of a valve controlling the start and duration of fuel injection in response to an injection command. These methods further assume that the time between valve closure and the start of injection is fixed. Given these two time intervals, the injection command can be modified to compensate for variation in the time between the injection command and valve closure. The problem that remains with this type of approach is that the detected valve closure does not precede the start of injection by a fixed time period. Thus, this approach does not eliminate injector-to-injector and injector-to-nominal variation due to variations of the valve closure to start of injection time interval.
Shinogle et al. U.S. Pat. No. 5,634,448 discloses a method and structure for controlling an apparatus, such as a fuel injector, using electronic trimming. The method includes the steps of measuring the resultant characteristics of the apparatus at a plurality of operating conditions, such as timing and delivery characteristics of a fuel injector, and adjusting a control signal as a function of the measured characteristics. This adjustment may be accomplished by adjusting base timing and duration of a fuel delivery command signal for the fuel injector. The fuel injector is thereafter controlled in accordance with the adjusted command signal to reduce performance variation. The method is implemented utilizing an electronic control module having a memory for storing trim signals for each injector which are used to effect the adjustment of the command signal.