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The insight is designed to operate at a very lean air fuel ratio during light throttle cruise conditions. The mixture can be as lean as 22:1 if conditions allow. This provides very good fuel economy, and low HC and CO emissions. However, the lean mixture also causes a rise in combustion temperature, and high NOx production is the result.
In order to prevent high NOx tailpipe emissions during lean burn operation, the Insight uses a dual catalyst system that includes an extra NOx storage/NOx reduction catalyst.
The 3-way catalyst is closely coupled to the exhaust manifold to minimize heat-up time, thus reducing emissions after a cold start.
The lean NOx catalyst is in the conventional location, underneath the car.
Lean NOx Catalyst Functioning
The lean NOx catalytic converter is comprised of a ceramic A1203 substrate, a platinum (Pt) catalyzing surface, and a titanium-sodium (Ti-Na) NOx storage surface.
During lean burn operation, the exhaust gas contains a larger percentage of oxygen (O2) and NOx, the NOx being primarily nitrogen monoxide (NO). The platinum catalyzes the O2 and NO to produce nitrogen dioxide (NO2), which is able to be stored on the Ti-Na surface.
When it determines that the Ti-Na surface is saturated, the Insight temporarily richens the mixture. This action decreases the NOx and O2 in the exhaust, and raises the levels of hydrocarbons (HC) and carbon monoxide (CO). The platinum is then able to use the HC and CO to catalyze the NO2 (that has been stored) into harmless nitrogen gas (N2), carbon dioxide (CO2) and water vapor (H20).
Catalytic Converter Deterioration Detection / Replacement
The Insight uses 2 oxygen sensors to monitor the condition of the 3-way catalyst as required by OBD-II and EU2000 regulations. However, the detection system is not able to directly monitor the condition of the lean NOx catalyst.
If the ECM sets a code for a deteriorated catalyst, both catalysts have to be replaced. It must be assumed that the same conditions that caused the 3--way catalyst to deteriorate also deteriorated the lean NOx catalyst.
Exhaust System Weight Saving Measures
The Insight's exhaust system incorporates several weight- and space-saving measures made possible by the IMA engine's reduced size. For example, smaller-diameter, thinner gauge pipe is used, thereby saving the weight that would be required for a larger engine. In addition, the system's smaller weight and the elimination of the exhaust header (the header is now integral with the cylinder head) allowed it to be constructed as a single unit, thus eliminating the additional weight of interconnecting exhaust flanges.