Abstract: Heat energy retainment systems and methods involve a set of devices configured to restrict or stop an upstream flow and release of heat energy from exhaust gas upstream from an exhaust burner upstream from a catalyst in an exhaust system of an internal combustion engine and a controller configured to proximate to cold starts of the engine, control the exhaust burner to increase a temperature of the catalyst to a desired operating temperature and, during engine off periods, control the set of devices to retain the heat energy in the exhaust system proximate to the exhaust burner and the catalyst, wherein the retainment of the heat energy in the exhaust system during engine off periods decreases a duration of engine cold starts by decreasing catalyst light-off time.