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Over-design means an oversized cooling pack and fan that lead to higher drag and lower fuel economy.

Dual-use heater core contributes to better vehicle fuel economy

Imagine a vehicle towing a 3000-lb (1360-kg) trailer up a 6% grade on a 110°F (43°C) day. This scenario describes less than 1% of driving time, but 100% of vehicles have cooling packs designed around this contingency. This over-design means an oversized cooling pack and fan leading to higher drag and lower fuel economy for the everyday driver. A recent study concluded that a 10% incremental aerodynamic drag reduction gives a 1.5% improvement in vehicle fuel economy for midsize vehicles and 3% improvement in trucks.

CSEG has designed, and filed a patent for, a more elegant solution that would allow for the extreme driving condition scenario while downsizing the cooling pack, increasing fuel economy, and reducing costs for the OEM as well as the customers who do the driving. This design uses an additional vent door in the HVAC airbox in conjunction with the heater core to make better use of existing components, optimizing efficiency.

The heater core is capable of removing 10-20% of total engine waste heat because it is already plumbed into the engine with constant ample coolant flow. The heater core could be used to supplement engine cooling, on-demand, so that the front-end cooling pack can be optimized for regular driving, which would improve fuel economy. The trouble is, when it is hot outside and the engine needs extra cooling, the driver is also hot and unlikely to turn on the heater.

To address this, the HVAC airbox is modified by adding a vent door, called the “cooling door,” near the heater core. This cooling door vents hot air from the heater core back out of the cabin so the cabin comfort would not be affected.

During extreme operating conditions and when the driver has A/C on, the blend door would allow some air to go to the heater core while maintaining the cool airflow to the passenger cabin. This diverted airflow passes through the heater core to provide supplemental engine cooling. The cooling door opens, and the hot air from the heater core is vented back into the underhood. The blend door would be positioned such that cabin comfort is not affected. The amount of airflow through the heater core will determine the amount of supplemental engine cooling that the heater core will provide. The blower will need to be operated at a slightly higher speed to deliver enough airflow to the passenger cabin and to the heater core. The cooling door position and the blower speed would be controlled automatically by the ECU depending on the engine cooling and cabin comfort requirements.

When passenger cabin heating is also desired in addition to supplemental engine cooling, the engine cooling door would be positioned such that some of the hot air from the heater core is sent into the passenger cabin and the remaining is vented into the underhood, providing both passenger cabin heating and supplemental engine cooling. When supplemental engine cooling is not desired, the engine cooling door would remain closed, mimicking the HVAC air box operation of today.

The efficacy of this design was computationally evaluated using Flowmaster, part of the Mentor Graphics Mechanical Analysis Division. For every CFM that was sent through the heater core, there was a 1.5x CFM reduction in required front-end airflow. This was because the heater core operates at a much higher effectiveness than the radiator.

There is another variable to consider. When the A/C is on and additional air is sent through the evaporator, the heat load of the condenser goes up. This, in turn, puts additional heat load on the cooling pack. The effect of the condenser load was evaluated computationally. In one scenario, when additional 150 CFM airflow was sent through the evaporator, the condenser heat load went up by 40%. When additional 200 CFM was sent through the evaporator, the condenser heat load went up by 80%. However, when full simulation was performed to account for this added condenser heat load, there was still a very clear benefit. For every CFM that was sent through the heater core, there was a 1.2x CFM reduction in required front-end airflow.

The key benefit of this technology is an optimized front-end cooling module for everyday driving, leading to better fuel economy. The size of the front-end cooling module and the size of the cooling fan could be reduced. If the front-end cooling module size and architecture were fixed for legacy reasons, then adding this technology would give the vehicle a higher trailer-tow rating because the engine cooling capability has been increased. In addition, this technology enhances the operation and benefits of other fuel-saving technologies such as automatic grille shutters and exhaust gas recirculation by enabling them to operate closer to their optimum efficiency points.

Sudhi Uppuluri, Principal Investigator, Computational Sciences Experts Group (CSEG), wrote this article for Automotive Engineering.

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