Catalytic converters are incredibly efficient at converting pollutants into benign gases once the catalyst material has reached a sufficiently high temperature. For example, hydrocarbon and carbon monoxide gases in the exhaust are oxidized using a small amount of excess oxygen in the presence of the catalyst to produce water and carbon dioxide. In a three-way catalyst, oxides of nitrogen (e.g. NO and NO2, which are collectively referred to as NOX) are also removed through a reduction reaction to produce nitrogen gas (N2) and water. However, a catalytic converter generally requires heating to a temperature in excess of 200° C. or even 250° C. before becoming effective. At temperatures above 350° C. little if any pass-through of the targeted pollutants typically occurs. The temperature at which a catalyst becomes effective for removal of pollutants can be referred to as its light-off temperature or its minimum target operating temperature. During the heat-up phase, exhaust pollutants pass out of the system untreated while sensible (thermal) energy in the exhaust gas is used to heat up the catalyst until it gets warm enough to catalyze the necessary pollutant-removal reactions. In many cases, these untreated pollutants can constitute a significant (in some cases a majority) fraction of the total pollutants emitted during a drive cycle.
Previously described approaches to the issue of cold catalyst pollutant emissions have focused on pre-heating the catalyst, for example by electrolyzing water to create hydrogen and oxygen, which can be delivered to the catalyst at engine start-up so that spontaneous catalytic recombination of the hydrogen and oxygen can rapidly raise the temperature of the catalyst substrate to a temperature at which the desired pollutant-removal reactions occur efficiently. Such an approach can be undesirable in that an extra energy input is required, which can lead to higher running costs. The added system complexity involved in including a water source and electrolysis apparatus is also generally undesirable.
Another concern with existing catalytic converters is the injection of extra fresh air, known as secondary air injection, in the exhaust manifold, to bring the catalytic converter up to light-off temperature more quickly. When the engine is cold, the secondary air can provide an optimum mixture composition to increase the reactivity of the catalyst on the extra-rich exhaust which is being produced during engine warm-up, thereby generating heat which assists in heating up the catalytic converter. After the light-off temperature of the catalytic converter is reached, the secondary air provides sufficient oxygen to enable the conversion of carbon monoxide and unburned hydrocarbons. However, such a system requires a pump to inject the air in effectively, which can create an extra load on the engine and hence increases energy usage.
A related, co-pending and co-owned application (U.S. patent application Ser. No. 14/274,612) discusses, among other features, adjustment of a fuel-air ratio in the fuel mixture in the combustion chamber of an engine to enhance the amount of hydrogen gas (H2) in the effluent passing to the catalyst under low temperature operations. The resulting hydrogen gas reacts with the catalyst and available oxygen (O2) at a relatively low temperature, and the resulting oxidation reaction releases energy and heat to speed the warm-up of the catalyst, thereby causing the catalyst to reach its effective temperature more quickly and improving the removal of pollutant compounds from the exhaust stream immediately after engine start-up.
It would be desirable to provide an improved way of reducing the time taken for a catalyst or catalytic converter to heat up to an effective operating temperature, for example at engine start-up, which could be used instead of or additionally to chemical energy methods.
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