HVAC Air Recirculation Control for Start-Stop Engine Fuel Economy
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Solution Overview
Problem
In vehicles with start/stop technology, maintaining passenger comfort and fuel economy is challenging due to interruptions in air conditioning when the engine is stopped, as conventional systems either rely on costly cold storage or electric compressors, and existing recirculation strategies may lead to window fogging during auto stop events.
Innovation Solution
A method for controlling the HVAC system to switch to a full recirculation position during engine stop events under conditions of low fogging probability, indicated by ambient temperature, windshield wiper state, and sunload, to extend engine stop times without compromising comfort or causing fogging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the engine is stopped to improve fuel economy, then fuel consumption is reduced, but the air conditioning cooling action is interrupted and the passenger cabin temperature increases
Solution Approach 1:
The system performs preliminary cooling of the evaporator and cabin air before the engine stop event, storing coldness in the evaporator thermal mass. This preliminary action allows the evaporator to continue providing cooling effect during the engine stop period without requiring the compressor to run, thereby maintaining cabin temperature comfort while achieving fuel economy improvement.
2Loss of energy
If recirculated air is used during engine stop to maintain cooling, then fuel economy is improved, but window fogging may occur under certain ambient conditions
Solution Approach 1:
The system continuously monitors ambient conditions including temperature, humidity, and sunload to assess fogging risk. Based on this feedback, the control system dynamically adjusts the air inlet door position between fresh air and recirculated air modes. When fogging probability is low, full recirculation is used to maximize fuel economy; when fogging risk is detected, the system introduces fresh air to prevent window fogging while minimizing the impact on fuel economy.
3Temperature
If fresh air mode is selected for HVAC cooling in hot weather, then cooling capacity is maintained, but cooling load to the compressor increases and energy consumption increases
Solution Approach 1:
Instead of using 100% fresh air or 100% recirculated air, the system employs partial recirculation by adjusting the air inlet door to a intermediate position. This allows a mixture of fresh air and recirculated air to enter the HVAC system, providing sufficient cooling capacity while reducing the cooling load on the compressor compared to pure fresh air mode, thereby optimizing energy consumption.
Data Source
AI summary
A method and apparatus controls a recirculation position of a return vent in an HVAC system for a vehicle with a compressor driven by a start/stop engine. A partial recirculation position is set according to a basic recirculation strategy while the engine is operating. An auto stop event is entered at a time with the compressor operating. A full recirculation position is set during the stop event by an HVAC controller if a plurality of low fogging probability indicators are true. The low fogging probability indicators include a sensed ambient temperature being above a predetermined temperature, a sensed state of a windshield wiper being an off state, and a sensed sunload being above a predetermined sunload. The return vent is set at other than the full recirculation position by the HVAC controller according to the basic recirculation strategy if the low fogging probability indicators are not true.