This means the wheels are turned by power coming directly from the gasoline engine, or the electric motor, or both together, as conditions demand. This parallel hybrid drive architecture will serve as the foundation for future hybrid drive vehicles to be developed by Hyundai.

Unlike traditional [power-split] hybrids that trade off highway fuel economy for higher city ratings, the Sonata Hybrid delivers best-in-class highway fuel economy, while still delivering about a 40 percent improvement in city fuel economy compared to a Sonata equipped with the Theta II GDI engine. We think this is a better balanced approach for the majority of car buyers.

—John Krafcik, president and CEO, Hyundai Motor America

Hyundai Hybrid Blue Drive
30kW, 151 lb-ft electric motor Regenerative braking system Integrated starter generator lithium polymer battery pack (5.3 Ah, 270V) Theta II 2.4-liter engine 6-speed automatic transmission with an improved efficiency electric oil pump Weight-efficient architecture coupled with a low drag coefficient (.25 Cd target) Electric air conditioning compressor Hybrid power control unit	Click to enlarge.

Power-split hybrid systems with a planetary-geared Continuously Variable Transmission (CVT) use electric motors that have to power the planetary gear set. The full parallel drive system in the Sonata Hybrid uses the power from the electric motor more efficiently, Hyundai says, to directly control the vehicle, allowing it to be operated at much higher speeds than the competition in EV-only mode. This technology is also a key enabler of Sonata Hybrid’s highway fuel economy.

The Sonata Hybrid. Click to enlarge.

The Sonata Hybrid will offer projected highway fuel economy of 39 mpg US (6.0 L/100km) and city fuel economy of 37 mpg (6.4 L/100km). The Sonata Hybrid can deliver electric drive operation at steady-state speeds of up to 62 mph (100 km/h). Gasoline engine engagement depends on state of charge, acceleration and vehicle speed. Its two propulsion units develop a total output of 209 hp (156 kW) at 6,000 rpm, exceeding all mid-size competitors, and 195 lb-ft (264 N·m) of torque.

The weight-efficient architecture of the new Sonata platform, combined with the lightweight lithium polymer battery pack, make the Sonata Hybrid the lightest vehicle in the segment, at just 3,457 pounds, 263 pounds lighter than the Fusion Hybrid. Combined with Sonata’s best-in-class horsepower rating, Sonata Hybrid has a significant advantage in power-to-weight ratio, a key enabler of both performance and efficiency, Hyundai notes.

Engine. In the Hyundai Hybrid Blue Drive system, the Theta II with multi-port fuel injection (MPI) operates on an Atkinson Cycle. To further improve fuel economy, all of the Theta II major driveline and cooling system components have been optimized to reduce friction, while the crankcase has been filled with low friction oil.

Hybrid Power Control (HPC) management software automatically shuts off the engine when the vehicle comes to a halt, cutting emissions to zero. When pressure is reapplied to the accelerator pedal, the Hybrid Starter Generator (HSG) automatically restarts the engine. This control strategy assures that maximum efficiency is achieved during gentle acceleration and greater power is immediately available during full acceleration. During deceleration, braking regeneration comes into play. Sonata Hybrid also features “smart brake” technology in which braking input over-rides accelerator pedal input.

The high-tech, all-aluminum, 16-valve engine also features Continuously Variable Valve Timing (CVVT) on both camshafts and newly developed engine components to reduce friction. This optimized Theta II engine achieves 10% better fuel consumption over a conventional Theta II engine.

Transmission. Hyundai Hybrid Blue Drive uses the company’s proprietary 6-speed automatic transmission rather than a conventional hybrid’s CVT. Hyundai’s strategy involves an adaptation of the modular 6-speed transmission, replacing the torque converter with an electric motor and high-efficiency oil pump. This technique uses a traditional step-shift 6-speed transmission rather than a CVT to provide a more traditional shift feel that is preferred by customers and sometimes artificially replicated in CVT applications. This saves on cost, making the Hyundai Hybrid Blue Drive system a better value. It is a more robust and elegantly simple solution to a complicated engineering challenge, Hyundai says.

The top three gear ratios in the transmission have been extended to ensure the engine runs at lower RPMs, the electric motor-assisted steering system reduces demands on the engine, and low resistance tires further optimize fuel economy.

Li-poly battery pack. Sonata’s hybrid system stores its electrical charge in a 34 kW lithium polymer rechargeable battery (5.3Ah/270V) from LG Chem. The Sonata Hybrid’s battery pack weighs just 95.9 pounds versus the Camry Hybrid’s 123.9 pounds. The compact battery pack resides in the forward portion of the trunk to maximize cargo space.

Compared with nickel-metal hydride batteries, lithium polymer batteries deliver the same power with 20-30% less weight, 40% less volume and 10% greater efficiency over the nickel-metal hydride batteries found in today’s hybrids. Lithium polymer batteries offer 1.7 times more energy density than nickel-metal hydride batteries, allowing Hyundai engineers to devote less space and weight to the battery pack. Lithium polymer batteries hold their charge 1.25 times longer. Lithium polymer batteries also are more resistant to changes in temperature, which improves cycle life. Additionally, lithium polymer’s self-discharge rate is less than a third of a nickel-metal hydride battery.

Lithium-polymer has significant advantages over other lithium-ion approaches, including higher energy density and lower manufacturing costs, Hyundai says. Lithium polymer is more resistant to physical damage and can handle more charge-discharge cycles before storage capacity begins to degrade. Lithium polymer technology also offers significant advantages in thermal robustness and safety.

Another key engineering challenge for Hyundai Hybrid Blue Drive has been assuring maintenance-free battery operation over the vehicle’s life—at least 10 years, and 150,000 miles—in all weather conditions. Hyundai’s thermal imaging testing shows how much cooler a lithium polymer battery is compared to today’s nickel-metal hydride battery or a conventional lithium-ion battery.