Can just one under-dash HVAC system provide climate control to three seat rows in a full-size crossover vehicle that previously had a separate rear system? Such an approach would seem to require a major compromise in cooling and heating performance. As a result, a single system invariably had been limited to smaller two-row vehicles—until the new Acura MDX.
The MDX is a three-row crossover that for the 2014 model year incorporates a novel approach to a three-zone layout. It uses a partitioned evaporator and heater core in a partitioned under-dash HVAC case that feeds an external multiple rear duct system, where a strategically located second blower motor maintains the auxiliary airflow.
The partitions are for front and rear, with the sizing bias toward the front as would be done with a dual-zone system, but the zoning approach itself is totally new. So although it's a system with just one evaporator and heater core, Honda claims performance is equal to HVAC with a separate rear system, plus with notable advantages.
The A/C evaporator and heater airflow splits are 65% front, 35% rear, explained Kevin Cheung, Senior Engineer for Interior Design at Honda R&D Americas in Raymond, OH. In front there are two HVAC zones, as expected, one for driver and a second for right-side passenger, but with supplemental delivery from each for heated air, via floor ducts to the left and right seats in the second row.
The second and third rows form the third HVAC zone. The controls for floor, upper register, and bi-level, plus blower speed and air mix to regulate temperature for second and third rows, are on a panel at the second-row center location. A third-row floor duct is for heat only.
This system configuration reduces the number of refrigerant line connections by a third, to equal the lower leakage rate of a single evaporator system, which of course it is from a refrigeration system design standpoint. There is 40% more heat transfer area vs. conventionally sized front-only evaporator and heater cores, to provide the thermal performance for rear HVAC. The refrigerant charge was reduced (compared with the previous system that had rear HVAC) by 50 grams to 700.
The U.S. EPA has a CAFE (Corporate Average Fuel Economy) carbon credits program to evaluate new technology that increases A/C fuel efficiency and reduces refrigerant leakage per SAE J2727. This SAE standard features a spreadsheet for estimating system leakage according to the components and sealing technologies used and, where appropriate, with verification with helium leak testing. Low-leakage systems earn credits toward CAFE.
MDX is classified by EPA as a light-duty truck, a category with higher CAFE credits. The maximum EPA credit for a low-leakage truck system is 7.8 g/mi; for high system fuel efficiency, it's 7.2 g/mi. Extra credits may be granted by EPA if a manufacturer can demonstrate that it has an innovative HVAC design that improves fuel efficiency.
There is another example of engineering for A/C fuel efficiency: the MDX uses a smart HVAC control module that incorporates the high-occupancy-rate front seats, which are heated and cooled, in its overall regulation of heating and air-conditioning. The second-row seats are heat-only, passenger-controlled.
The integration concept originated at Honda R&D Americas, noted Bradley Errington, HVAC Manager, and Acura engineering approached potential suppliers with a structure and construction method Honda had patented. This led to a new HVAC supply relationship with Delphi, which developed the heat exchanger partitions and also is supplying its CVC seven-cylinder variable-displacement compressor for the MDX.
As the illustrations show, the integrated system has a complex layout of ducts, both within the HVAC case and out to the second- and third-row seats. A side view of the HVAC case makes it seem as if the rear airflow, which flows in a U-pattern within it, might be restricted. But because rear airflow is only 35% of the total, Cheung explained that "the cross-section is sized appropriately for the airflow volume and does not impose a large pressure drop inside the HVAC case." He added that this was confirmed both by CFD and actual performance testing.
Ducts integrated with dashboard
Within the HVAC case itself, the Acura and Delphi engineering teams were able to provide the needed separation and minimized thermal bleed between airflow passages with specifically designed seals, some of which are covered by patents. Further, there is a major design feature that helps package the in-dash system: integration of much of the HVAC case ductwork with the substrate of the dashboard. That is, the ductwork is C-sectional and is infrared welded to the substrate, which forms the closing side. This not only serves as a major space saver, but it also increases the structural rigidity of the dashboard.
The evaporator's significant mounting angle was a key part of the design of the case to manage airflow and package the system, noted Errington, and it posed a challenge to ensure effective condensate drainage to prevent evaporator icing. This required a careful balancing act with the evaporator fin design and count needed for good heat transfer (evaporator is rated at over 6000 W of cooling power) and the drainage requirements. Honda designed the Acura MDX seal and drain system and holds a patent for it.
Although there is a multiduct system from the integrated front unit, it is lightweight plastic and results in a 10 lb (4.5 kg) overall HVAC saving in weight vs. HVAC with a rear unit. There also is an obvious cost-saving from the integrated system, which Cheung said was 18%. The new design also clears a huge storage space in the console—at 12,500 cm³ (762.8 in³) it has 250% more volume than in the 2012 MDX.