Method and system for exhaust heat recovery

An engine method comprising:

• during a first condition, storing thermal energy from at least a portion of exhaust at a thermal energy storage device, and then flowing exhaust through a heat exchanger;
• and
• during a second condition, recirculating a portion of exhaust from downstream of the heat exchanger to upstream of the thermal storage device, drawing thermal energy into the exhaust from the thermal storage device before flowing the exhaust through the heat exchanger;
• wherein, during each of the first and second conditions, a thermal input from the exhaust into the heat exchanger is maintained.

An engine system coupled to a vehicle, comprising:

• an engine intake manifold;
• an engine exhaust system including an exhaust temperature sensor and an exhaust pressure sensor coupled to a main exhaust passage, a bypass passage coupled to the maintain exhaust passage, across a thermal storage device, and a recirculation passage including a blower for recirculating exhaust from downstream of a heat exchanger to upstream of the thermal storage device;
• an exhaust heat recovery system including the thermal storage device coupled to the exhaust passage, the heat exchanger coupled downstream of the thermal storage device, a bottoming cycle including the heat exchanger, an expander coupled to a battery, a condenser, and a pump for circulating a working fluid through the bottoming cycle;
• a vent line including a vent valve coupled to the recirculation passage;
• a diverter valve coupled to the exhaust passage for regulating exhaust flow via the bypass passage;
• a recirculation valve coupled to the recirculation passage for regulating recirculation of exhaust downstream of the heat exchanger to upstream of the thermal storage device;
• and
• a controller with computer readable instructions stored on non-transitory memory for:
• operating the exhaust heat recovery system in a first mode with the diverter valve closed, the recirculation valve closed, and the vent valve closed to flow exhaust directly to the heat exchanger, bypassing the thermal storage device;
• and
• operating the exhaust heat recovery system in a second mode with the diverter valve open, the recirculation valve closed, and the vent valve closed to flow a first portion of exhaust directly to the heat exchanger, bypassing the thermal storage device, and a second portion of exhaust to the heat exchanger via the thermal storage device while transferring thermal energy from the second part of exhaust to the thermal storage device;
• and
• transitioning from the first to the second mode responsive to an increase in an estimated exhaust thermal energy above a first threshold thermal energy, the thermal energy of the exhaust estimated based on inputs from the exhaust temperature and pressure sensors.