Solving the EV Range Vs. HVAC Dilemma

In an SAE WCX 2021 technical presentation, Mahle Behr experts reveal insights into a vexing issue with electric vehicles.

Every OEM can benefit from engineered HVAC strategies to help maximize EV range. (Nissan)

The loss in range from climate control operation on battery electric vehicles (BEV) has become a source of concern among automakers as many of them begin the transition from internal combustion engines. An AAA study showed an average range loss of 41% to warm the average passenger compartment in an ambient of 20° F (-7° C), and a 17% drop in range to cool the cabin with an ambient of 95°F (35° C). Even with improvements in batteries and larger packs leading to increases in onboard energy storage and greater rated range, many of the BEVs themselves include performance compromises with the climate control systems to maximize range.

Energy consumption of system in all test modes: 100% OSA (0% recirc), 2LF, 50% recirc and 30% recirc, adjusted to same warmup profile. (Mahle Behr)

Two solutions are increased use of recirculating interior air (“recirc”) and modification of the HVAC case to provide two-layer airflow (“2L”) from the HVAC system, an approach that is widely used by Toyota. The two-layer approach splits the airflow, providing 55% outside air for upper registers, including defrost, and 45% recirculated air for the foot outlets.

Mahle Behr, which includes the Delphi Thermal group acquired seven years ago, studied the effects of test settings of “recirc” and “2L” on both BEV range and vehicle glass fogging, an obvious safety issue. It reported on control strategies to prevent fogging without reducing range, in a presentation at the 2021 SAE WCX. The testing showed a small improvement in range (1.9%) was possible without window glass fogging but going for the maximum range improvement would cause a fogging issue.

Studying OSA

Outside air (OSA) has lower relative humidity than air continuously recirculated (“recirc”) in an occupied passenger cabin, where evaporation of normal human perspiration in the closed compartment raises the percentage. So, a system that inhales a continuous OSA flow provides the best way to inhibit fogging. However, OSA may be at much higher temperature in summer, increasing the energy draw of the air conditioning to cool the cabin and thus accounting for loss of vehicle range.

Bolt HVAC case was modified to permit performing all recirc and defog tests. Note partition across midpoint of cutaway and two flap doors at top (small door at left). (Mahle Behr)

Outside air temperature may be much lower in winter, so the energy draw from the highest possible setting on a BEV electric heater to provide adequate cabin warmth, as noted, becomes a major factor in vehicle range. In hot weather, a high percentage of motorists switch the A/C to “Max Cool.” This mode primarily closes the OSA flap door, so the system runs with recirculation of interior air, which already has been cooled by the HVAC system.

X marks the critical view area of the windshield, as seen from the driver’s seat, that had to be cleared in defogging tests. (Mahle Behr)

The earliest use of electronic “recirc” control was in conjunction with a Bosch electronic sensor that detected hydrocarbons in ambient air entering the cabin through the outside air intake, typically when vehicles were close together, and slow-moving in heavy traffic. The sensor would trigger an actuator that closed the OSA door for a preset period, running the system in recirc for that time. However, 100% “recirc” has limits, as the buildup of humidity results in a “clammy cold.” And over time any debris on the A/C evaporator core may stimulate the growth of microbal matter, which can turn malodorous.

“Recirc” also may cause fogging of the windshield, forcing the motorist to turn on the defroster, reducing the A/C performance. In winter, the buildup of humidity with “recirc” also can result in fogging and switching to drier outside air and/or turning on the A/C may be necessary, reducing the warming of the cabin.

In the Mahle wind tunnel

The Mahle test, on a Chevrolet Bolt, was made with a modified HVAC case. This enabled the research team to perform all test functions, including vehicle warmup, two-layer flow and recirc (and therefore the interior air) to chosen percentages of “recirc” – 0%, 30% and 50%. The case was horizontally partitioned to incorporate two-layer airflow (upper for outside airflow to the defroster duct, lower for recirculated interior airflow to the floor outlets). Added were a dual inlet fan with two scrolls of different diameters operated by a single motor and two flap doors for flow control.

Illustration shows locations of relative humidity sensors. (Mahle Behr)

The cabin was instrumented with 10 relative-humidity sensors, covering the windshield, backlight and front side windows, right rear door window and two locations on the floor. Temperature sensors were hung throughout the cabin close on and close to glass areas and passenger breath locations. The upper and lower sections of the HVAC module were equipped with pressure sensors to enable real time airflow calculations.

If elimination of the possibility of glass fogging were the only objective, running the system in 100% OSA would do the job. For many years some cars didn’t even offer recirculation for the heating mode; there was only the Max Cool mode on the A/C, which was a “recirc” position that often provided a small percentage of outside air to attempt to minimize the fogging issue.

The testing was done in a Mahle wind tunnel with the Bolt cold-soaked to three test temperatures: 0° C (32° F), -10° C (14° F), and -20° C (-4° F). To validate heating performance for warm-up curves, the car was started with the HVAC in foot mode and one of the chosen recirc modes – 100% OSA, 50% recirc, 30% recirc and two-level flow. The Bolt was run at 25 mph (40 kph) for 20 minutes, 36 mph (60 kph) for 20 minutes, and finally, left in the BEV-ready mode (equivalent to engine idle) for 10 minutes. The warm-ups were at 100% OSA, 30% and 50% recirculation, and dual level flow, to each a cabin temperature of 75° F. (24° C).

The energy savings were based on electrical consumption of the car’s PTC heater (voltage x amperage). The greatest savings overall, compared with a 100% OSA baseline (across the three test ambient temperatures), occurred at 50% recirculation – about 17%. However, the defogging performance was unacceptable.

The fogging/defogging test began with a three-hour cold soak at -10° C (14° F). Then, with motor and blower off, over a 10-min period the interior glass was fogged with a long-used Mahle laboratory-built steam generator that pumped vapor into the center of the cabin at a rate of 70 g/hr per passenger, for one, two or five passengers. The number is based on calculations derived from an ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) study of human moisture generation that indicated an adult at rest would release 30-70 g/hr.

After the 10 min, the car was started, put into drive at 25 mph (40 kph) and HVAC set in defrost mode, full Hot. Once the critical area of the windshield is clear (2-3 min), HVAC is switched to foot mode and Automatic Temperature Control to 72° F. Note if the windows fog up or not, and record results.

“Recirc” the winner?

The Bolt was run in the wind tunnel at 25 mph and a baseline was determined with the system in OSA (0% recirc), defrost mode, HVAC temperature control in full hot and the blower at high speed. Cabin temperature was maintained at 23-25°C by modulating the temperature control module. When the critical vision areas were cleared, the system was switched to floor mode. This basic test procedure then was repeated for 100% OSA, “2LF”, 50% and 30% “recirc” settings.

The 30% “recirc” and “2LF” were comparable performers in the defogging tests, with “2LF” slightly better with five passengers (but this represents under 5% per the U.S. Dept. of Transportation). The effect on range between the two is close, and the 30% recirc had a very slight edge, providing a 2.64 mi (4.2 km) improvement over the Bolt’s 238-mi (381 km) rating. The location of the backlight makes it the coldest glass in the cabin and therefore the slowest to clear. The 2LF design also is more complex and costly, tilting the balance to 30% recirc.