TECHNICAL FIELD

The present invention relates to a fuel injection control apparatus for electromagnetic fuel injection valves via which fuel is to be supplied to an internal combustion engine. More specifically, the present invention relates to a fuel injection control apparatus for electromagnetic fuel injection valves via which fuel is to be injected into cylinders.

BACKGROUND ART

Recent strict regulations on exhaust gases such as carbon dioxide and concerns about depletion of fossil fuels such as oil have increased demand for improved fuel efficiency (fuel consumption rate) of internal combustion engines. To satisfy such demand, modern fuel injection control apparatuses are requested to employ multistage injection control, in which fuel is intermittently or separately injected into cylinders during an intake stroke, during a compression stroke, or during a period from the intake stroke to the compression stroke.

Under the multistage injection control, individual fuel injection amounts are set to an amount obtained by dividing the target amount of fuel injected required for one combustion cycle by the number of separated injections. Alternatively, the individual fuel injection amounts are set to different amounts.

As described above, each of amounts of fuel injected separately is smaller than a regular fuel injection amount. When such small amounts of fuel are injected via electromagnetic fuel injection valves, full lift control, in which valve bodies of the electromagnetic fuel injection valves are fully lifted, may fail to inject small amounts of fuel precisely. Unlike the full lift control, half lift control, in which the valve bodies of the electromagnetic fuel injection valves are stopped at midway points before fully lifted and then fuel is injected in this state, can inject small amounts of fuel precisely.

Herein, the full lift control refers to control in which a valve closing operation is started after a stroke displacement of a movable core that drives the valve bodies of the electromagnetic fuel injection valves is maximized with respect to that when the valves are fully closed. The half lift control refers to control in which the valve closing operation is started before the stroke displacement of the movable core that drives the valve bodies of the electromagnetic fuel injection valves is maximized with respect to that when the valves are fully closed. Examples of a fuel injection control apparatus with a multistage injection control system which performs the full lift control and the half lift control are disclosed in JP 2014-152697 A (PTL 1) and JP 2016-37870 A (PTL 2).

CITATION LIST

Patent Literature

PTL 1: JP 2014-152697 A

PTL 2: JP 2016-037870 A

SUMMARY OF INVENTION

Technical Problem

As described above, the half lift control can precisely inject fuel during an operation state where the valve bodies provided in the electromagnetic fuel injection valves have not yet reached a position at which the valve bodies are fully opened. Current fuel injection control apparatuses are, however, requested to inject further decreased amounts of fuel. When such very small fuels are injected, variations in the amounts of the fuel injected via the electromagnetic fuel injection valves in the cylinders can turn out to be a problem. The variations in the fuel injection amounts may be attributed to individual differences (machine differences) between the electromagnetic fuel injection valves.

When very small amounts of fuel are injected, variations in the amounts of fuel injected are caused by a mechanical difference between electrical characteristics of electrical components provided in the electromagnetic fuel injection valves. Examples of the electrical characteristics of the electrical components include: coil resistances of electromagnetic coils; inductances of the electromagnetic coils; and inductances of harnesses. As described above, the electric characteristics of the electromagnetic fuel injection valves differ from one another. In which case, when very small amounts of fuel are injected, a phenomenon occurs in which energy amounts of the drive currents supplied to the respective electromagnetic fuel injection valves differ from one another, even when time widths of injection pulses and set values of drive currents supplied to the respective electromagnetic fuel injection valves are equal to one another.

Since different amounts of energy of the drive currents are supplied to the respective electromagnetic fuel injection valves, lift amounts of the valve bodies differ from one another. As a result, different amounts of fuel are injected via the electromagnetic fuel injection valves. Variations in the fuel injection amounts caused by the difference between the electrical characteristics of the electromagnetic fuel injection valves become prominent within a half lift region in which the half lift control is performed, more specifically, within a region in which very small amounts of fuel are injected.

To solve such problems, the amounts of energy supplied to the electromagnetic coils of the electromagnetic fuel injection valves, or integrated current values of the drive currents (=electric energy amount) are each regulated so as to become a predetermined target integrated current value. In this way, the machine differences between the electrical characteristics of the respective electromagnetic fuel injection valves are compensated for, and the variations in the fuel injection amounts are thereby reduced.

The above electric energy amount can be determined by multiplying the current value of the drive currents flowing through the electromagnetic coils by an applied voltage and an elapsed time. In the region where very small amounts of fuel are injected, a constant high voltage is applied to the electromagnetic coils in the electromagnetic fuel injection valves in order to swiftly open the valve elements. In which case, the applied voltage can be regarded as being constant with time.

Therefore, the total current amount related to the elapsed time, or the electric integrated current value, is regarded as the electric energy amount. Then, the electric energy amount is regulated so as to become a predetermined target electric integrated current value for each electromagnetic fuel injection valve. In this way, it is possible to reduce the variations in the fuel injection amounts which are based on the machine difference between the electrical characteristics of the electromagnetic fuel injection valves.

Drive states of the electromagnetic fuel injection valves are controlled with injection pulses. More specifically, the drive current flows through the electromagnetic coil in response to the rising edge of the injection pulse (referred to below as the valve opening timing). In addition, the drive current stops flowing through the electromagnetic coil in response to the falling edge of the injection pulse (referred to below as the valve closing timing). Thus, to regulate the above electric energy amount, it is necessary to perform an integration operation of the current flowing through the electromagnetic coil in synchronization with the valve opening timing of the injection pulse.

Then, when the integrated current value reaches a predetermined target integrated current value, the drive current interrupts the drive current flowing through the electromagnetic coil, independently of the valve closing timing of the injection pulse.

As described above, as the valve closing timings, a valve closing timing based on the injection pulse and a valve closing timing based on the current integration control are generated. Therefore, a drive circuit that controls the drive current flowing through the electromagnetic coil is set to logic that validates the valve closing timing applied earlier to the drive circuit.

As described above, the drive current stops flowing through the electromagnetic coil either at the valve closing timing based on the injection pulse or at the valve closing timing based on the current integration control. If the valve closing timing of the injection pulse is generated due to any reason (e.g., the execution of a pulse width correction function) before the integrated current value reaches the predetermined target integrated current value, the drive current to the electromagnetic coil is forcibly interrupted at the valve closing timing of the injection pulse, although the drive current needs to be interrupted at the valve closing timing based on the current integration control. As a result, variations in fuel injection amounts newly occur, which may lead to deterioration of exhaust performance and fuel consumption efficiency.

As described above, there is a request to further suppress the variations in the fuel injection amounts in the very small fuel injection region of the half lift region.

An object of the present invention is to provide a fuel injection control apparatus that can suppress variations in fuel injection amounts in a very small fuel injection region within a half lift region.

Solution to Problem

A feature of the present invention is that a fixed injection pulse generator and a drive current interruption unit are provided. The fixed injection pulse generator generates a fixed injection pulse, as an injection pulse for use in driving an electromagnetic fuel injection valve. This fixed injection pulse has a valve opening timing at which a drive current is supplied to an electromagnetic coil in the electromagnetic fuel injection valve and a first valve closing timing at which the drive current supplied to the electromagnetic coil are interrupted. Within a period in which the fixed injection pulse is generated, the drive current interruption unit determines an electric energy amount of the drive current supplied to the electromagnetic coil in response to the valve opening timing of the fixed injection pulse. When the determined electric energy amount reaches a preset, predetermined target electric energy amount, the drive current interruption unit generates a second valve closing timing of the fixed injection pulse different from the first valve closing timing and interrupts the drive current supplied to the electromagnetic coil before the first valve closing timing.

Advantageous Effects of Invention

According to the above, the first valve closing timing of the fixed injection pulse is generated only after the electric energy amount given to the electromagnetic coil reaches the preset, predetermined target electric energy amount and the second valve closing timing is generated. Therefore, the electromagnetic fuel injection valve reliably shifts to the valve closing operation at a time when the electric energy amount reliably reaches the target electric energy amount, thereby suppressing variations in fuel injection amounts to improve exhaust performance and fuel efficiency performance.

