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In order to determine the main factors causing the mileage-related increase in formaldehyde emission from methanol-fueled vehicles, mileage was accumulated on three types of vehicle, each of which had a different air-fuel calibration system. From exhaust emission data obtained during and after the mileage accumulation, it was found that lean burn operation resulted in by far the highest formaldehyde emission increase. An investigation into the reason for the rise in engine-out formaldehyde emission revealed that deposits in the combustion chamber emanating from the lubricating oil promotes formaldehyde formation. Furthermore it was learnt that an increase in engine-out NOx emissions promotes partial oxidation of unburned methanol in the catalyst, leading to a significant increase in catalyst-out formaldehyde emission.

The second generation methanol lean burn system has been developed. The power unit is a new, 4 valve 1.6L in-line four with compact combustion chambers. Lean misfire limit was extended by using a swirl control valve in the intake port which improves combustion under partial load. Lean mixture control is made by using a signal from lean mixture sensor provided in the exhaust manifold. An EGR system has been newly adopted to reduce NOx emissions and a under-floor type catalyst is also used to reduce formaldehyde emission in the cold transient mode in addition to the manifold type catalyst. Permissible excess air ratio range (PEXARR) was defined and used to indicate the potential for reducing vehicle NOx emissions in engine dynamometer tests to optimize compression ratio, valve timing and swirl ratio and to evaluate the effect of the EGR.