催化转化器诊断

tiancai

Strategies for monitoring catalytic activity
A three way catalyst is only effective when the air/fuel ratio is carefully controlled to oscillate around the stoichiometric point (a ratio of about 14.6:1 on a weight basis) at a frequency of 0.5 to 1 times a second. When operating in the lean portion of the cycle CO and HC present in the exhaust are oxidised by the oxygen available in the exhaust. At this time the catalyst also stores oxygen. When operating in the rich portion of the cycle CO and HC are oxidised by the oxygen absorbed/stored within the catalyst. The ability of the catalyst to operate efficiently is therefore dependent on it maintaining a sufficient oxygen storage capacity (often known as the OSC) and the engine’s control system providing both the correct air/fuel ratio and varying this by a few percent at the right frequency.

The above forms the basis for the most widely used strategy for monitoring catalyst activity. Fluctuations in the feed gas air/fuel ratio downstream of the catalyst will be damped by the catalyst’s OSC. Comparison of the pre- and post-catalyst oxygen sensor signals is used to derive the OSC of the catalyst.

— 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.

STRATEGIES FOR MONITORING CATALYST ACTIVITY
In Section xy in the main body of the report it was noted that the ability of the catalyst to operate efficiently is dependent on it maintaining a sufficient oxygen storage capacity and the engine’s control system providing both the correct range of lambda and at the right frequency. This forms the basis for the most widely used strategy for monitoring catalyst activity.

Fluctuations in the feed gas air/fuel ratio will be damped by the catalyst’s OSC downstream of the catalyst. Comparison of the pre- and post-catalyst oxygen sensor signals is used to derive the OSC of the catalyst. The problem is obtaining sufficient correlation between OSC and catalyst efficiency under normal driving conditions.

The reason for this problem is that catalyst deterioration is due to either

Loss of OSC (mainly due to thermal ageing).
Loss of surface area (mainly due to poison deposition).
The two mechanisms cannot be directly correlated and the quantification of OSC does not measure directly loss of activity due to poisoning.

Studies have shown that increases in HC emissions correlate better to loss of catalyst surface area than OSC. However, the correlation of OBD index with HC emissions has high uncertainty and depends on

The type of emission control system.
The type of catalyst.
Vehicle history (thermal loading).
Fuelling (poison build up from Sulphur and additives).
Accuracy and repeatability of OSC measurement (response and deterioration of λ sensors)
Accuracy and repeatability of HC emissions measurement in Type Approval test.
Clearly, there is scope for improvements in the diagnosis of catalyst activity. One strategy being researched is the use of thermal sensing to measure the temperature differential caused by the exothermic reactions in catalytic converter. The attraction of this approach is that it measures catalytic conversion directly. However, these sensors will need high stability and short response times for this application. Cost and durability are issues at present.

Another possibility is to use HC sensors based on HC ‘s ionising in contact with hot metallic surfaces. As yet this sensor technology is also at the R&D stage. There are practicality, cost and durability issues to be addressed before this could be widely used.

— 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.

ding2001817

三元催化转化剂只有在空然比被准确控制........

大哥，你是不是毕业设计的英语翻译啊？自己翻译吧，要对得起4年的大学啊