The Arjeplog test facility of Hyundai Mobis, where average high temperatures hover around -7 °C. Its frozen lake used for a variety of vehicle testing measures 1.6 million m². (Hyundai)

Testing for cold-climate comfort

AE goes way north for an inside look at Hyundai’s winter testing of the new Nexo FCV and Kona EV and their unique and critical HVAC systems.

Arjeplog, Sweden sits around 35 miles (57 km) below the Arctic Circle, but this remote city has been a bustling hub for the automotive industry for decades. Since the first winter tests were conducted here in the 1973, the area has become a second home to a dozen OEMs and suppliers during the brutally cold winter months. Engineers take over the small town every winter when average high temperatures hover around 19°F (-7 C) and the average low is a brisk 3°F (-16 C).

To make room for the influx, residents rent out their homes to the auto industry, moving into guest rooms or in campers parked with relatives or friends in other parts of town. These temporary living situations are required as the automotive industry pushes forward to greener powertrains.

Hyundai has a particular impetus to test where it's cold. Battery EVs and fuel-cell vehicles (FCV) have unique cold-climate operating characteristics, and those propulsion types will play a major role in the company's aim to vie for the global eco-car sales lead by 2025. Ensuring optimum HVAC system cold-climate performance for EVs and FCVs is paramount.

Arjeplog, then, has become like a second home to engineer Gunther Frank, Hyundai’s head of Functional Vehicle Development. Automotive Engineering was invited to join Frank and his team in their winter lair in early 2018 as they prepared the new Nexo fuel cell vehicle and the Kona EV for production.

Developing FATC

Hyundai does most basic HVAC development work using a climate chamber at its Namyang Technology Research Center in Korea. Then the cars come to Sweden for on-site field tests and fine tuning. Frank said a car typically spends one to two weeks in cold weather testing, and the same amount of time in hot weather. It's the in-between climate that actually takes longer, maybe three or four weeks, because it's difficult to get the full automatic temperature control (FATC) system just right.

"Mild climate conditions are much, much more complex, not from the performance point of view in terms of cooling or heating, but from the control point of view of the FATC," Frank said. "In alpine regions, when we are at a height of 2,500 meters (8,202 feet) where it is quite cool, and we are driving within an hour down to the valley where we see temperatures of 22 or 23 °C (72-73°F), the controller has to be aware of the changing conditions and react."

FATC is Hyundai's end goal for all of its vehicles, no matter what powertrain they use. The basic idea is that the driver can set a temperature, push the "auto" button, and then never think of the heating or cooling settings again.

"This would be the ideal situation, having just the 'auto' button, but human beings are so different," Frank said. "The way you feel one day in comparison to the other is also so different and you are never going to be able to create a control algorithm which will fit all of them. It's a very nice feature, but you have to make sure that the customer has the possibility to overwrite the system, because finally it's up to them."

The more important challenge for Frank and his team in Arjeplog, though, is getting consistent heat without the waste heat from a traditional ICE. Both of Hyundai's new fuel cell and all-electric vehicles use high-voltage electronic compressors. Unlike older compressors, which needed to run off of engine RPM, the electronic compressors can be run independently. They can also be turned off when the cabin climate is stable, which then reduces the load on the energy source and thus leads to increased efficiency.

The high-voltage systems can also heat up the cabin faster than an ICE, since they don't need a warm engine to work. "From the HVAC point of view, it's a much, much better situation than in the olden days," Frank noted. "If you don't have an electronic heating device, when you start your engine, it takes a very long time until the cabin becomes warm. With our high-voltage systems, we have the possibility to immediately warm up the cabin."

Heating the Nexo FCV

The Nexo is the Hyundai's second-gen fuel-cell vehicle, arriving some time in 2018. It follows the Tucson FCV and has increased fuel cell stack efficiency and performance. All of its components are newly developed. In fact, the Nexo achieves 60% fuel cell system efficiency, compared to the Tucson's 55%. Based on this improvement, along with an increase in the hydrogen storage available on board (three tanks that each hold 6.3 kg (13.9 lb) of hydrogen, the Nexo's driving range could reach over 800 km (497 mi) in NEDC city mode, and over 370 miles on the U.S. test cycle.

These numbers are preliminary, but Nexo senior engineer Sang Ho Yoon said the driving range would be around a 35% increase over the Tucson fuel cell. That vehicle is rated at 265 miles in the U.S.

The same energy used to move the car is needed to heat it, so developing an efficient HVAC system is important to achieving long range. While there are similarities between the HVAC system in the Nexo and a standard ICE vehicle – there is some free heat energy available because the stack needs to be cooled down – the overall process is quite different. The good news is that different also works.

Frank happily proved the Nexo's rapid heating power on a day when the outside ambient temperature was -23°C (-9.4 F). After just three minutes in the prototype, a breast-level temperate sensor showed it was 15°C (59 F) in the car.

"We have an internal target that with an ambient temperature of -20 C (-4 F), the average cabin temperature should reach an average of 18°C (64 F) after 20 minutes of driving," Frank said. "Believe me, it's quite tough to fulfill that with low-capacity internal combustion engine cars. With our full EV cars or with our fuel cell car, we are able to reach that target much, much faster."

Part of that quick heat comes from a 3.7-kW Air Side positive temperature coefficient (PTC)  thermistor. The use of PTC thermistors as heaters in cars has been studied since at least the 1990s and they are used today in some gas cars, especially 1-kW units in 3-cylinder European vehicles. With alternative powertrain models, PTCs have really come into their own, Frank explained.

The first step to warming a fuel cell car's cabin, Frank explained, is to heat up the cooling system of the stack. While this happens, the PTC offers quick heat in the Nexo's cabin. Once the stack is warmed up and able to provide some excess heat, the PTC's power is reduced, even down to zero in the conditions we experienced in Sweden.

"When the car is in a stable condition, it's similar to internal combustion engine cars and we don't need additional electronic power to run the heating system," he said. "In stable conditions, there is no influence on driving range."

Hyundai developed a new membrane electrode assembly and 3D porous flow field in house for the Nexo. "The 3D porous flow field is a new concept for the Nexo stack," Yoon said. "It can improve the stack performance for power density." Yoon said the Nexo uses the world's smallest hydrogen supply system for automotive use because it does away with the hydrogen recirculating pump required in the Tucson fuel cell and only uses an ejector to supply hydrogen for the electrochemical reaction within the fuel cell stack.

A new thermal management system means improved response time to control the coolant temperature of the stack thanks to a two-way and a four-way valve. The four-way valve is a world's first for electric vehicles, Yoon said, and it improves the Nexo's cold-start ability. Cold starts are traditionally difficult for fuel cell vehicles, but the Nexo can start at ambient temperatures of -30°C (-22 F), the same threshold the company's ICE vehicles must pass.

That's not the only way the Nexo will function like an ICE. Thanks to a new, highly durable membrane, a new platinum catalyst in the stack, and a new operating control technology, Yoon said the durability rating for the new fuel cell powertrain warranty will cover 160,000 km (100,000 mi) and 10 years, just like the company's ICE vehicles.

The Nexo's powertrain has a maximum power output of 120 kW and 395 N·m. This will improve top speed by a claimed 10% and acceleration performance by 25%, compared to the Tucson FCEV.

Heating the Kona EV

The Kona EV will come in two flavors, a 64-kW·h model with up to 470 km (292 mi) of range (all range numbers here are just targets, and are based on WLTP homologation). This model offers 204 hp and a 0-100 km/h (0-62 mph) time of 7.6 seconds. The 39-kW·h model will get up to almost 300 km (186 mi) on a charge with a motor that generates 135 hp and hits 100 km/h in 9.3 seconds. Both powertrain versions deliver 395 N·m. On a 100-kW fast charger, the 64-kW·h battery can be charged to 80% in less than one hour. A 7.2-kW, Level 2 charger will take almost 10 hours to fully charge the larger pack.

One challenge with electric vehicles is that the battery temperature needs to be moderated along with the cabin. The Kona EV uses battery packs that are bigger than the ones used in Hyundai's Ioniq and Kia Soul EVs, which are air-cooled. The Kona's battery pack shouldn't get hotter than 40°C (104 F), which is why the battery in the Kona EV uses an active liquid cooling system in combination with a radiator to keep it from overheating. If this is not sufficient, the cabin's AC system can be used to cool down the battery as well.

Like the Nexo, the Kona EV prototype uses an AirSide PTC – this time a 5-kW unit – along with a 2.7-kW heat pump. A more powerful PTC is needed because there's no waste heat from a fuel cell stack to help warm the cabin. The Kona EV does manage to siphon some heat energy off of the electronic components, including the motor.

"We are able to transfer it and use that energy to warm up the cabin and to increase the efficiency of the overall heating system," Frank noted.

The heat pump system uses some heat energy from the air, ideally at ambient temperatures above 0°C (32 F), which then runs the AC drive cycle. At an ambient temperature of 0°C (32 F) and a cabin setting of 23°C (73F), "we are losing about 40% in comparison to heater off when we have a PTC system, but we are able to create 20% more energy with the heat pump system and therefore to increase the efficiency," Frank said.

Those are the numbers that Hyundai's winter test engineers have to crunch to keep drivers of their new powertrain vehicles happy. If the PTC and the FATC and all the rest can work in Arjeplog, chances are they will work in other parts of the world. If not, it's back to the cold for more testing.

 

Frozen-lake driving in a fuel cell car

Driving an early prototype gives only a hint of what the finished vehicle will act like, but spinning the upcoming Hyundai Nexo hydrogen fuel cell and the Kona EV around a frozen lake in northern Sweden prove there's more than one way to tune an eco car.

The Nexo, as seems appropriate for what will almost certainly be the more expensive of the two models, has a softer suspension and an aggressive stability control system that prevented the FCEV from ever really losing its footing, even when speeding around Hyundai's circular ice track. Throttle and steering response were dramatically limited and the system's interventions were early and substantial, which bodes well for the safety of the eventual production vehicle. There was not a lot of oversteer and there wasn't much to do about the understeer in the icy circumstances.

The Kona EV, on the other hand, felt further from production ready, with plenty of noise coming from the back when pushed to the frozen limits. The safety control strategy is also tuned different here, allowing us to mildly drift the car on the lake. This meant more fun on the safe confines of the expansive ice track, and it means the EV might be the more fun of the two models.

Thankfully, the Kona EV's floor-mounted battery creates a low center of gravity that helps keep the car firm locked onto the track. It was difficult to test out the EV's flat-our acceleration on the slippery surfaces, but the impressive torque got the tires spinning on the ice, even when we were already at 100 km/h (62 mph). Even so, pushing the accelerator pedal all the way to the floor didn't do much once the system detected slippery conditions and neutered our input.

Thanks in part to their grippy tires, both zero-emission vehicles kept their composure. The Kona rode on Continental 215-55 R 17 V XL WinterContact TS 850P tires, while the Next wore Hankook’s Winter I-cept Evo2 245/45R19 102V M+S.

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