1. Field of the Invention
The present invention relates to an internal-combustion-engine combustion condition detection apparatus that detects pre-ignition caused, for example, in an internal combustion engine having a plurality of ignition plugs in a single combustion chamber, based on an ion current produced by combustion or on an ignition discharge time. Based on the detection, the internal-combustion-engine combustion condition detection apparatus suppresses pre-ignition so as to maintain the combustion in a good condition.
2. Description of the Related Art
In recent years, a multipoint-ignition internal combustion engine has been put to practical use in which ignition is performed by a plurality of ignition plugs provided in a single combustion chamber so that the speed of combustion in the combustion chamber is accelerated and the combustion can be maintained in a good condition. Additionally, it is tendency that a high compression ratio is employed in an internal combustion engine. Under such a condition of high compression, the temperature of the inside of the combustion chamber increases, and in a locally overheated portion, a fuel-air mixture spontaneously ignites earlier than the normal ignition timing, whereby pre-ignition occurs. The occurrence of pre-ignition causes a poor driving condition, and in extreme cases, melting loss of an ignition plug or a piston may be caused. Therefore, in Japanese Patent Laid-Open No. 1988-68774, a technology is proposed in which pre-ignition is determined by means of an ion current that flows across ignition-plug electrodes.
Additionally, it is known that, by detecting a precursor phenomenon (post-ignition), of pre-ignition, in which, in a locally overheated portion, a fuel-air mixture spontaneously ignites at an early stage after the normal ignition timing, pre-ignition through which melting loss of an ignition plug or a piston is caused can be prevented.
FIG. 17 is a configuration diagram illustrating a conventional internal-combustion-engine combustion condition detection apparatus that detects an ion current. Reference numeral 2 denotes an ignition device having an ignition plug 1, disposed in a combustion chamber. In the ignition device 2, when a transistor 36 is controlled by an ECU (Electronic Control Unit) 42 to be turned off, counter electromotive force is produced across a primary coil 31; in response to the occurrence of the counter electromotive force, a negative high voltage is produced across a secondary coil 32, whereby the ignition plug 1 discharges.
After that, when a fuel-air mixture in a combustion chamber 33 combusts, an ion-current detection device 41 detects an ion current that is caused by the combustion and flows across the electrodes of the ignition plug 1. Next, in order to raise noise immunity, an ion current shaping unit 43 provided in the ion-current detection device 41 applies waveform shaping, such as constant multiplication processing, to the ion current, and then the wave-shaped ion current is inputted to the ECU 42. In order to comprehend the driving condition of the internal combustion engine, the outputs of various kinds of sensors 44 such as an air-intake temperature sensor, a throttle sensor, a crank angle sensor, and a water temperature sensor are inputted to the ECU 42.
The ion-current detection device 41 includes a biasing device connected to the low-voltage end of the secondary coil 32 in the ignition device 2, i.e., a capacitor 45, a diode 46 inserted between the capacitor 45 and the ground, and a voltage-limiting zener diode 47 connected in parallel with the capacitor 45 and the diode 46 that are connected in series to each other. A series circuit consisting of the capacitor 45 and the diode 46 and the zener diode 47 connected in parallel with the series circuit are inserted between the low-voltage end of the secondary coil 32 and the ground so as to configure a charging path for charging a biasing voltage across the capacitor 45 when an ignition current is produced.
The capacitor 45 is charged with a secondary current that flows through the discharging ignition plug 1 due to a high voltage outputted from the secondary coil 32 when the transistor 36 is turned off. The charging voltage is limited by the zener diode 47 to a predetermined biasing voltage (e.g., several hundreds volts) and functions as a biasing device, i.e., a power source for detecting an ion current.
Accordingly, in the case where, with the foregoing configuration, electric charges are accumulated while ignition discharge is performed and then a combustion ion current is detected, it is impossible to detect an ion current while ignition discharge is performed; therefore, the ion current represented by the broken line in FIG. 3 cannot be detected, and only the ion current represented by the solid line is detected. In the case of a driving condition, such as pre-ignition or a precursor phenomenon of pre-ignition, in which the combustion speed is high, most of ion current information may be produced during a discharge time; thus, it is difficult to accurately comprehend the combustion condition by means of a combustion ion current.
Moreover, in the case where, in an internal combustion engine having a plurality of ignition plugs in a single combustion chamber illustrated in FIG. 1, a flame propagation time from a time instant when the ignition plug 11 ignites a fuel to a time instant when an ion current in an ignition plug 12 is detected, i.e., a combustion speed is tried to be measured, it has been a problem that, in the case where both the ignition plugs 11 and 12 concurrently ignite the fuel, respective combustion ion currents, as ion currents 1 and 2 represented in FIG. 3, are produced approximately at the same time due to discharge of the ignition devices, whereby the flame propagation time cannot accurately be measured.