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前言
Engine misfire describes a lack of combustion in the cylinder of a spark-ignition engine. It can be due to the absence of a spark, poor fuel metering such that the fuel system fails to deliver a combustible mixture, poor compression or other causes. It results in high HC emissions into the exhaust manifold. These can be consumed by the catalyst, generating heat which damages the catalyst both physically (it can melt) and chemically (by reducing activity). It is also likely to result in excess HC emissions from the tail pipe.
Experience has indicated that an engine misfiring for about 2% of firings can raise emissions by up to 50%. If engine misfire occurs in excess of about 17% of the time catalyst damage will occur.
Strategies that are currently used to monitor for misfire include:
- looking for crank shaft velocity fluctuations,
- monitoring the ionising current using the spark plug as an electrode, and
- analysing the exhaust pressure.
Further technologies are being investigated. These include:
- in-cylinder pressure sensing, and
- magnetostrictive sensing analysis in the engine crankshaft.
— Page 13, 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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