Mathematical Modeling of Monolith Warmup Behavior in Variable-Fuel Vehicle Exhaust 932721

The transient, one-dimensional monolith model previously developed for gasoline emission control applications has been extended to study converter warmup behavior in the exhaust from a variable-fuel vehicle (VFV) running on mixtures of methanol and gasoline by including the catalytic oxidation of methanol which involves the formation of stable gaseous formaldehyde as a reaction intermediate. The model calculations show that the aldehyde formation increases gradually at the early stages of converter lightoff (when methanol conversions are low), peaks at ∼50% methanol conversion, and then declines rapidly with a further increase in methanol conversion. Consequently, for all cases of practical interest, the total amount of aldehyde produced during the converter warmup period correlates well with the time to converter lightoff, with lower aldehyde emissions predicted at faster converter lightoff.

In recent years methanol has received considerable attention as an alternative fuel for motor vehicles largely because of its potential for improving air quality in certain areas of the country [1, 2]. Conversion from gasoline to methanol would replace a large portion of the reactive hydrocarbons (HC) in gasoline exhaust with less reactive methanol, and thus can lower the ozone-forming potential of the exhaust. However, methanol-fueled vehicle exhaust also contains significant amounts of photochemically reactive aldehydes (primarily formaldehyde). Previous studies [2, 3 and 4] have clearly shown that maximum air quality benefit from methanol fuel can be obtained provided that exhaust emissions of formaldehyde are kept to very low levels. The California Air Resources Board has recently enacted a 15 mg/mile formaldehyde emission standard for methanol-fueled vehicles, and the requirement that this standard be met for at least 5 years or 50,000 miles of vehicle use presents a challenging emission control problem.

Given the prospect that methanol would not be uniformly available among service stations during the transition period, General Motors has developed and produced variable-fuel vehicles (VFVs) that can operate on mixtures of gasoline and methanol. This will allow the customer the flexibility to select either fuel based on price and availability. Variable-fuel vehicle exhaust contains a broad range of hydrocarbon species as well as unburned methanol and formaldehyde. Catalytic control of exhaust emissions from VFVs poses an interesting problem because many catalysts have been shown to exhibit tendencies to partially oxidize unburned methanol to formaldehyde at temperatures typically encountered during the converter warmup period [5, 6]. Thus, the ideal exhaust catalyst for VFVs must possess both high activity and high selectivity for the complete oxidation of methanol, in addition to the usual requirements related to the control of gasoline-derived exhaust pollutants.

This study was undertaken to mathematically analyze the emission performance characteristics of a monolithic catalytic converter operating in the exhaust environment of VFVs. We focus on the converter warmup period because the first few minutes after a cold start contribute up to 80% of the total exhaust emissions of HC, methanol, and formaldehyde collected over the FTP cycle [7]. Since formaldehyde emissions are of particular concern with VFVs, the primary objective here is to identify important factors affecting formaldehyde emission levels and to gain insight into the mode of formaldehyde production over the catalyst during converter warmup. To this end, it is instructive to consider the simplest of transient inputs -- a cold converter subjected to a step change in inlet exhaust gas conditions. Such approximation, though an idealization of reality, captures the essential features of the converter warmup process and affords better understanding of the relationship between converter design/operating parameters and its cold-start emission performance. It is hoped that the understanding generated from this simulation study would aid in the development of monolithic converters which efficiently and selectively oxidize exhaust pollutants produced by VFVs.