Method and Apparatus for Adjusting Fuel Supply to an Internal Combustion Engine

The invention relates to fuel management systems for internal combustion engines. More particularly, the invention relates to a fuel injection system for engines in which there are disposed electromagnetic fuel injection valves that are actuated by control pulses that are generated by a suitable control pulse generator. The frequency and duration of the valve-actuating pulses depend on the engine speed, the inducted air flow rate and other secondary variables such as, for example, engine temperature. Additional engine conditions affecting the valve control pulses may be engine starting, the barometric pressure of the ambient atmosphere and the pulse generator may include a provision for suppressing or sharply reducing the fuel control pulses whenever the vehicle is being operated in downhill use, i.e. when the engine delivers negative torque. This condition is detected as a combination of events, namely that the throttle valve of the engine is closed whereas the engine speed rises above a predetermined minimum. The known fuel injection system and its pulse generator may suppress the valve actuation pulses during engine braking and thus entirely suppress the fuel supplied to the engine. In the known system, when engine braking ends, the system reverts to normal operation, i.e. to the generation of normal control pulses. It may happen however, especially when the engine has been operated in downhill operation or in engine braking for a substantial length of time, that the induction tube is cooled off substantially so that the injected fuel is condensed on the cold interior surfaces of the induction tube and the cylinder walls in which case the resulting mixture will not be optimally combustible and the engine will run rough and tend to generate toxic exhaust gas components, in particular hydrocarbon emissions.

It is thus a principal object of the present invention to provide a fuel injection system which includes a mechanism for enriching the fuel-air mixture provided to the engine after the occurrence of engine braking, for example, downhill operation. A related object of the invention is to provide a fuel injection system which causes the engine to operate smoothly and quietly at the end of engine braking, thereby increasing the comfort of the driver and keeping to a minimum any toxic emissions, especially hydrocarbon emissions.

These and other objects are attained according to the present invention by providing a mechanism which senses the condition of engine braking as the occurrence of a closed throttle and a certain minimum engine speed and generates an output signal when this condition is met. This engine braking detector may be part of a known and existing fuel injection system in which the invention constitutes an improvement. The output signal of the engine braking detector is used in normal fashion to diminish or suppress fuel injection during engine braking but it is also used as a provision of the present invention to start a counting process in a digital counter which is originated when engine braking ceases and to use the magnitude of the count to adjust the amount of excess fuel to be supplied to the engine by the control pulse generator after engine braking ceases.

The invention will be better understood as well as further objects and advantages thereof become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawing.

The single FIGURE of the drawing is a simplified block diagram of a known fuel injection system including the post-engine-braking enrichment mechanism according to the present invention.

Turning now to the FIGURE, there will be seen a pulse generator 10 which generates a train of pulses whose frequency is synchronous with engine speed. These pulses are fed in known manner to a first timing circuit 11 which also accepts air flow rate information indicated schematically by the symbol Q and which generates preliminary output pulses t.sub.p. These pulses are fed to a known pulse length correcting circuit which may be a pulse width modulator or multiplier of known construction 12 which normally accepts signals related to engine variables, for example, as shown, a temperature signal .theta.. The pulse width modulator then supplies a corrected output pulse t.sub.i to fuel injection valves 13, possibly via an amplifier stage. In some cases, the known fuel injection system includes an engine braking detector 15 which receives a signal related to throttle valve closure from a switch 20 and also receives the aforementioned preliminary control pulses t.sub.p which are synchronous with engine speed. The engine braking detector 15 uses these two input data to generate an output signal whenever the throttle valve is closed and the engine speed has risen above a predetermined value, thereby indicating engine braking. In order to fulfill the objectives of the present invention, i.e. to increase the amount of fuel after the termination of engine braking, there is further provided a counter 16 which also receives the aforementioned preliminary pulse t.sub.p at a counting input and which has a reset input connected to the output of the engine braking detector 15. The digital content of the counter 16 is fed to the input of a decoder, embodied as a digital-to-analog converter 17 which generates an analog signal whose amplitude is related, e.g. inversely, to the magnitude of the contents of the counter 16. This analog signal is delivered via a line 25 to a correcting input of the control pulse width modulator circuit 12 and is used thereby to increase the length of the control pulses, i.e. to cause an increase of the fuel supplied by the valves 13.

The operation of the apparatus described above is as follows:

The preliminary pulse t.sub.p is generated by the timing circuit 11 on the basis of engine speed and air flow rate Q. This preliminary pulse t.sub.p is corrected on the basis of engine variables in the pulse width modulator circuit 12 and is fed to the valves 13. Whenever the throttle valve is closed, the switch 20 is also closed and if, at the same time, the engine runs faster than a settable speed, the engine braking detector 15 generates an appropriate signal 21. The engine braking detector 15 may be, for example, a frequency-to-voltage converter coupled to a threshold switch and an AND gate.

The output signal from the detector 15 is used as a direct input via line 26 for the pulse width modulator and causes therein a reduction or complete suppression of fuel control pulses. The same signal is used to reset the counter 16 which thus begins its counting process at the termination of engine braking and which counts the preliminary control pulses t.sub.p at its counting input. When engine braking has terminated and counting begins, the decoder circuit 17 generates an analog signal related inversely to the magnitude of the contents of the counter 16 and this signal is used to temporarily lengthen the valve control pulses t.sub.p in addition to the corrections taking place on the basis of other variables.

The counter 16 must include a provision for preventing a renewed (overflow) count prior to the occurrence of the reset signal. This may be done by arresting the counting process when a particular desired content is reached, for example by introducing a gate 24 at the input 23, with control inputs connected to appropriate output lines of the counter 16.

The above-described improved fuel injection system permits a wide variability of the duration of fuel injection subsequent to the termination of engine braking. This is possible because of the presence of the decoder circuit 17 in combination with the timing circuit or pulse width modulator 12. The duration of the fuel injection control pulses may be increased by a constant amount for a number of pulses which depends on the setting of the gate 24. The pulse width may also be increased for a constant period of time subsequent to the termination of engine braking, for example by the inclusion of an auxiliary timing circuit 30.

It is also possible to increase the length of a certain number of control pulses by an amount depending on counter content.

In the simplest case, the counter 16 may be one or a cascade of bistable flip-flops.

The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other embodiments and variants thereof are possible within the spirit and scope of the invention.