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失火监测的主要原理
In the main body of the report is was described how an engine misfiring for about 2% of the time can raise emissions by up to 50% whilst if engine misfire occurs in excess of about 17% of the time catalyst damage will occur. This accounts for the importance of identifying misfire to keep emission low (both short and medium term). Five strategies for achieving this were listed, and will not be described in more detail.
Crank speed fluctuation
The presence of a misfire results in the absence of a power stroke. This temporarily interrupts the provision of power by the engine to the vehicle and causes a discontinuity in the crank shaft’s velocity (starting with a deceleration). The detection of this can form the basis for misfire monitoring. The technique has its challenges, vis:
- the accuracy is very dependent on the algorithms used for signal analysis,
- detection is very difficult for light load high engine speed, and
- monitoring is disabled during poor road conditions or rapid clutch engagement.
Crank Angle Sensor, picture from Mitsubishi
Ionising Current Monitoring
In this strategy the spark plug is used as an electrode and the ion current is monitored. If no ion generating flame is produced by the spark, no current flows through the measurement circuit during the “power” stroke of the cycle. This ion current/time trace is quite different from that observed for a cycle when normal combustion occurs. This strategy is reported to have proved 100% effective at monitoring for misfires on dynamometer based tests.
Exhaust pressure analysis.
this involves using a pressure sensor in exhaust manifold combined with Fourier analysis as the first stage of the signal processing. It can detect single misfires and it is possible to identify which cylinder is misfiring. This strategy has been demonstrated to detect all misfires up to 6000rpm for all engine configurations, loads, and fuels. It uses a ceramic capacitive sensor which has a short response time and good durability.
Cylinder pressure sensing.
This potential future technology for misfire detection gives good information on knock, misfire, peak pressure location, and combustion quality. However, the sensors are expensive and, as yet, are not sufficiently durable for production.
Magnetostrictive sensing analysis.
This possible future strategy induces an electric field in the engine crankshaft to measure engine stress fluctuations. It is at the start of its development and at present the response is poor, with a noisy signal.
— Appendix 2, An In-Service Emissions Test for Spark Ignition (SI) Petrol Engines – PPAD 9/107/09 Phase 2a Report Evaluation of the significance of OBD/OBM.
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