Dual converter engine exhaust system for reducing hydrocarbon emissions

An exhaust system for the conversion of hydrocarbons in an exhaust stream from an engine comprising:

• first catalytic converter means for substantially converting hydrocarbons in a hydrocarbon-containing engine exhaust stream to water and carbon dioxide, said first catalytic converter means having a light-off temperature;
• second catalytic converter means, situated farther from the engine than said first means, comprising (a) molecular sieve means capable of adsorbing hydrocarbons from said engine exhaust stream and further capable of having said hydrocarbons desorbed therefrom upon heating to a desorption temperature, and (b) at least one catalyst for the conversion of hydrocarbons to water and carbon dioxide, said catalyst having a light-off temperature;
• and
• one or more conveying means for selectively conveying said engine exhaust stream within said engine exhaust system, whereby said conveying means operate to
• (i) convey, during a first period of time prior to said first catalytic converter means attaining its light-off temperature, substantially all of said engine exhaust stream through said first catalytic converter means and then through said second catalytic converter means;
• (ii) convey, during a second period of time subsequent to said first catalytic converter means having attained its light-off temperature and prior to the molecular sieve means having attained its desorption temperature, (a) a portion of the exhaust stream discharged from said first catalytic converter means through said second catalytic converter means, and (b) the remainder of the exhaust from said first means to the atmosphere, bypassing said second means;
• and
• (iii) convey, during a third period of time subsequent to said molecular sieve means having attained its desorption temperature, (a) a portion of the exhaust stream from said first catalytic converter means through said second catalytic converter means and then recycle substantially all of the exhaust discharged from said second means back to either said engine or to said first means, and (b) the remainder of the exhaust stream from said first means to the atmosphere, bypassing said second means.