When the Hyundai Sonata Hybrid debuted in MY2011, it was met by criticism for the behavior of its hybrid drive system. Aggressive regeneration on throttle lift caused the car to slow more than intended and low-speed braking was plagued by intrusive regeneration, too, compounding the difficulty of bringing the car to a smooth stop. Accelerating was little better, as the car’s response to throttle input seemed inconsistent and transitions between gas and electric power were coarse and obvious.
Hyundai recognized two things about this situation: 1) critics were right that the car drove poorly and 2) there was nothing inherently wrong with any of the car’s hardware. All it needed was a little more midnight oil to be burned by engineers to refine the software controlling the hardware.
“We got some customer feedback that they weren’t happy with the transmission’s feel,” reported Product Planning Manager John Shon. Rather than the CVT employed by many hybrids, the Sonata Hybrid uses Hyundai’s own six-speed planetary automatic transmission. “Especially in cold weather, the shift shock was a bit harsh,” he said. The company responded by programming the transmission’s clutches to slip and warm the cold transmission fluid more quickly.
Hyundai’s programmers also addressed deceleration. “Regeneration is more linear than before,” Shon said. “The tuning logic for the hybrid clutch control is smoother than before.”
Additionally, the company turned up the power on the electric drive system without changing the electric motor/generator or the battery. The retuning focused on lower-rpm real-world driving conditions rather than on maximum power at redline.
Now the electric motor contributes a maximum of 35 kW at speeds between 1630 and 3000 rpm, compared to 30 kW at 1400 to 6000 rpm. The hybrid starter/generator’s maximum power holds steady at 8.5 kW, but its continuous power is boosted to 8.0 kW from the previous, more conservative 6.4 kW.
Similarly, the battery pack’s output is boosted from 34 to 47 kW, as programmers tapped more of the battery’s reserve power. There is no change to the 1.4-kW·h battery pack. The battery’s container and its cooling system are revised to shave five pounds of weight and reduce intrusion into the trunk storage space. Now there is partial access to the rear seat’s ski pass-through, and luggage space grows from 10.7 to 12.1 ft³ (303 to 343 L).
The gasoline engine sees changes, too, with a reprogrammed ECU that shifts the engine’s powerband to lower revs, seeking to beef up power at the speeds drivers use. That comes at the expense of peak power, which drops by 7 hp (5 kW) to 159 hp (119 kW) for the Atkinson cycle 2.4-L four-cylinder engine.
Net system power for the combined hybrid system is a maximum of 199 hp (148 kW), down from the previous peak of 206 hp (154 kW), but at lower revs the gasoline engine’s flatter torque curve and the increased power of the electric motor give the Sonata more power in most everyday driving situations.
In recognition of the lower powerband, the rev limit is 500 rpm lower at 5500 rpm. With lower engine speeds, Hyundai engineers made a single hardware change to the gasoline engine: softer valve springs for improved efficiency.
The result of all the changes is an increase of 2 mpg on the EPA city cycle and 1 mpg on the highway test, for scores of 36 mpg city and 40 mpg highway.
Outside, the 2013 hybrid is distinguished by a slightly revised grille and new aluminum wheels from Hyundai’s Mobis wheel-making affiliate. The hybrid has nearly flush spokes to reduce aerodynamic drag, but buyers complained that the previous wheels were too plain. For ’13, the car gets new wheels with more aggressive surface contouring and a two-tone paint color to exaggerate the topographical differences on the face of the wheels.
While the appearance and few physical items that are new for 2013 obviously can’t be had with a simple flash update, the new car’s software can be. That means owners of 2011 and 2012 Sonata Hybrids will be able to get the improved bits and bytes for their cars once Hyundai devises a delivery strategy, according to Shon.
As the company’s first try at hybrid engineering, it shouldn’t be terribly surprising that there was a learning curve, he said. “Sometimes the first iteration of a product can feel like a science project.”