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2011 Sonata Hybrid Delivers The Best Highway Fuel Economy Of Any Mid-Size Sedan On The Market

2010.06.21 00:00:00 No. 129


2011 SONATA HYBRID DELIVERS THE Best Highway Fuel Economy of Any Mid-size Sedan on the Market 


Sonata Hybrid Introduces New Powertrain Technology and Hybrid Blue Drive Architecture 


FOUNTAIN VALLEY, Calif., June 21, 2010 – After moving to the top of the U.S. Corporate Average Fuel Economy rankings in 2009, Hyundai remains firmly committed to leading the auto industry in fuel efficiency. With the introduction of a powertrain lineup exclusively consisting of four cylinder engines and six-speed transmissions, Hyundai's 2011 Sonata sedan became the most efficient vehicle in its class with an EPA rated highway mileage of 35 mpg.

The next stage in Hyundai's efficiency drive is the North American introduction of the Hybrid Blue Drive system. The Sonata Hybrid will debut in winter 2010 with a powertrain entirely developed in-house by Hyundai that includes several technical firsts. These innovations allow the Sonata to provide performance and efficiency improvements not available in other hybrids.


System Configuration of Sonata Hybrid



The first production application of Hyundai Hybrid Blue Drive actually debuted in mid-2009 on the Korean domestic market Elantra LPI mild-hybrid.  As implemented in the 2011 Sonata Hybrid, Hybrid Blue Drive is now a full parallel hybrid system. The Sonata Hybrid can be driven in zero emissions, fully electric drive mode at speeds up to 62 miles per hour or in blended gas-electric mode at any speed. When the car comes to a stop and the electrical load is low, the engine is shut down to completely eliminate idle fuel consumption and emissions.


The heart of Hyundai’s breakthrough Hybrid Blue Drive technology is its remarkable lithium-ion battery pack.  Hyundai is the first automaker in the world to incorporate this remarkably efficient battery technology into production vehicles.  Automotive duty cycles, with temperature ranges from -40 to 120+ degrees Fahrenheit, and 10-year-and-beyond longetivity requirements render the lithium ion batteries used in consumer devices unsuitable. Lithium polymer is the next generation of lithium ion technology and is ideally suited to automotive applications thanks to a robust and reliable chemistry.


The lithium polymers cells, developed with our partner LG Chem, use a manganese spinel chemistry that provides an excellent balance between power delivery, energy density and thermal stability. Thermal stability is critical to ensuring durability eliminating the need to replace the battery pack during the normal lifespan of the vehicle. The electrodes in older lithium ion chemistries expand and contract with the heating and cooling that occurs during charging and discharging. This thermal expansion causes cracks in the electrodes which ultimately reduces the cell's ability to hold a charge. Manganese spinel lithium polymer cells have much lower expansion rates and are thus able to go through tens of thousands of charge cycles even without having to use a heavier, liquid cooling system.

The power and energy density of this new battery type allowed Hyundai engineers to create a lighter and more compact battery pack that maximizes Sonata cargo space. The Sonata Hybrid’s 1.4 kilowatt-hour battery pack weighs in at just 95.9 pounds versus the 123.9 pounds for the nickel metal hydride pack in the Toyota Camry Hybrid.  In addition to the 20-30 percent mass reduction, the lithium-ion battery is 40 percent smaller in volume and 10 percent more efficient.

That improved efficiency is important because it means that more of the recovered kinetic energy and charging energy from the engine will be available to propel the car when needed. That allows the Sonata to provide electric driving boost more often and for longer periods of time.  Lithium polymer also has less of the self-discharge characteristic inherent in most rechargeable batteries. The Sonata's battery will hold its charge 25 percent longer than hybrids with nickel metal hydride batteries. As a result, the battery is more likely to have usable energy in it when the car is started even if it has been sitting for several days. This allows more electric starts and drive-aways, cutting both fuel consumption and emissions.

The construction of the LG Chem lithium polymer cells is also much better suited to automotive applications than the cylindrical cells typically used in laptop computers. Because of their shape, cylindrical cells inherently leave more empty space between them when packaged for use in a vehicle. The smaller, common “18650” cells used in laptop computer batteries can be tightly packed but their small size means that many hundreds or thousands may be needed in order to provide enough capacity for use in a car. All of those cells need to be connected together which dramatically increases manufacturing complexity and cost. Using larger cylindrical cells reduces the assembly complexity but also cuts the packaging efficiency further.


The "prismatic" cells used for the Sonata have a flat, rectangular shape like a thin stack of paper. Rather than wrapping the electrodes around each other and filling the cell with a liquid electrolyte, the LG Chem cells have flat, stacked sheets of electrode material surrounded by a polymer gel electrolyte. Even if the cell casing is pierced the electrolyte remains contained and the cell resists thermal runaway. The flat packaging allows the lithium polymer pack to be 20 percent smaller than a similar capacity pack with cylindrical cells. The arrangement of the cells also improves the air flow paths and exposes more surface area for cooling.


The Hyundai Hybrid Blue Drive powertrain is configured with a unique architecture unlike any current system on the market. In contrast to the more familiar power-split hybrid systems such as those from Toyota and Ford, Hybrid Blue Drive uses a Transmission-Mounted-Electric-Drive (TMED) layout where the motor is separated from the transmission gear-set. This modular layout offers Hyundai a number of advantages.



Most importantly, the TMED configuration provides flexibility to mix and match different sized electric-motors and transmissions to suit different vehicles. For example, Hyundai has three different six-speed automatic transaxles in its portfolio, any of which could be utilized in a hybrid application.


The mid-sized transmission in the Sonata is used in conjunction with a 30 kW motor to provide full hybrid capability including electric drive at speeds of up to 62 mph. Other possible future combinations include the use of even more powerful motors and higher capacity batteries to create plug-in hybrids similar to what was shown recently in the Blue-Will concept.

Each application can utilize an existing, off-the-shelf transmission, dramatically reducing the engineering investment required to introduce additional hybrid vehicles. With a power-split hybrid transmission, additional transmissions would have to be developed to cover a full range of vehicle types and sizes.



The Hybrid Blue Drive system is among the first full hybrid systems on the market to use a "conventional" step-ratio automatic transmission. Earlier power-split, full hybrid systems such as the General Motors Two-Mode system have generally relied on Electronic Variable ratio Transmissions (EVT) with integrated motor/generators. While this approach facilitated blending of drive torque from the traction motor and engine for seamless output, it brings along other potential problems. The transmission is far more mechanically complex and expensive to produce than a conventional unit. This approach also requires developing multiple unique transmissions to meet the requirements of different vehicles.


The Hybrid Blue Drive modular architecture developed by Hyundai facilitates the combination of different motors, transmissions and batteries with common hardware interfaces.  This allows Hyundai the flexibility to roll out a range of mild and full hybrid powertrains to meet consumer demands.

The extremely compact six-speed automatic (designated A6MF2H) that debuted on the 2011 Sonata is carried over to the hybrid largely unchanged. For the hybrid application an external, electrically driven oil pump has been added to provide the hydraulic fluid pressure needed to keep the clutches engaged when the Sonata is in idle stop mode.



The Sonata hybrid is equipped with a 30 kW permanent magnet synchronous electric traction motor that produces 151 pound-feet of torque from zero rpm. In an ingenious example of value-focused systems engineering, the motor itself is hard-coupled to the input of the transmission and completely replaces the torque converter.  A multi-disc clutch pack sits within the inner circumference of the traction motor and is used to de-couple the motor from the 2.4-liter Theta II inline-four cylinder engine for idle stop and electric drive modes. This layout allows the entire package along with a torsional damper to fit within virtually the same volume as the traditional torque converter.

Separating the motor from the gear set has several functional advantages over more heavily integrated systems. In addition to providing the flexibility to accommodate different applications, it also is one of the main factors contributing to the Sonata's superior highway fuel efficiency.



The internal power-split configurations typically have the motor tied to the sun gear of a planetary gear set. The result is that the motor ends up rotating at higher speeds where its efficiency drops off significantly. This increased motor rotational speed is a key reason why maximum electric drive speed in most other hybrid applications is limited to speeds well below the 62 miles per hour maximum achieve by Sonata’s Hybrid Blue Drive system. The greater motor speed also causes increased levels of audible motor whine.

Another downside of power-split systems such as those developed by Toyota and Ford is that all of the tractive power-flow has to drive the motors at all times.  These systems typically contain two motor/generators and because of their mechanization only one motor is used to provide tractive effort to propel the car in electric mode. When driving in electric mode, the second motor is actually used as a generator to provide a reverse torque that balances against the engine to prevent it from spinning up. This results in a loss of overall efficiency. At lower vehicle speeds, this loss is relatively small but as speeds climb it becomes increasingly significant. This is one of the main reasons that such systems typically have lower fuel efficiency on the highway than they do in urban driving.

The power-split EVTs also lose efficiency through the ratio control mechanism. In addition to providing tractive effort and battery charging, the motor/generator is also used to manage the rotational speed of the planetary gear set and change the effective output ratio. Doing this means that extra electrical energy has to be expended just to control the transmission.

The hard-coupling of the traction motor to the transmission input reduces its rotational speed. The use of the clutch to completely decouple the engine and motor allows the Sonata to drive electrically at speeds up to 62 mph without spinning the engine or causing excessive noise levels. A step-ratio transmission also has mechanical efficiency of greater than 90 percent contributing to overall vehicle efficiency.

Directly coupling the motor to a step-ratio transmission did provide Hyundai engineers with some technical challenges to overcome. On a non-hybrid vehicle the output torque is automatically reduced by the engine management system when a shift is about to occur to prevent sudden jolts as clutches engage and disengage. These jolts are both uncomfortable for passengers and have a negative effect on powertrain durability. Managing engine torque is a fairly straightforward process of reducing the fuel-flow, spark advance and throttle position.

Unlike an internal combustion engine, an electric motor has essentially constant torque delivery over much of its operating range. One potential solution to managing the drive torque delivery during shifts is to retain a torque converter between the motor and transmission. While this does provide a torque dampening effect, it adds weight, complexity and cost to the system as well as increasing the length of the powertrain. In a transverse-mounted powertrain configuration such as the Sonata and most other mainstream cars, this extra length simply cannot be accommodated.

Hyundai powertrain engineers have done extensive work on the control logic for the electric motor to manage the output so the entire system can function seamlessly in combination with Sonata's six-speed automatic transmission. The result is that no torque converter is required for the Hybrid Blue Drive system.




Another unique aspect of Hybrid Blue Drive is the Hybrid-Starter-Generator (HSG). This 8.5 kW starter motor-generator is belt-driven off the Theta II engine. However, it is not to be confused with integrated starter-generator (ISG) systems used for so-called micro or mild-hybrid systems from other manufacturers, such as GM’s “Belted-Alternator-Starter” system. Those ISGs typically operate at either 12 volts for micro-hybrids or 100-120 volts for mild hybrids. In the case of the mild hybrid systems, the ISG can also provide a small amount of power boost back to the engine.

The Hyundai HSG operates at the same 270 volts as the electric traction motor and the lithium-ion battery, but it does not provide any tractive effort to the vehicle. Instead, the HSG is used only to start the engine and then to charge the hybrid battery.

Because the primary traction motor is coupled to the transmission it cannot be used to charge the hybrid battery when the car is stationary. If the battery state of charge is low and the car is stationary, the HSG will automatically restart the engine if it is off. The engine will then drive the HSG to charge the hybrid battery directly.

When the driver releases the brake and presses the accelerator, the Sonata will pull away smoothly and silently in electric drive mode with the clutch still disengaged. In this mode the Sonata will briefly behave like a series hybrid. Once the car is in motion the clutch will engage, again sending engine tractive effort through the transmission. Using this strategy the Sonata hybrid is able to achieve exceptionally smooth starts.


In order to charge the 12 volt battery, Hybrid Blue Drive includes a low-voltage DC converter (LDC) in the power electronics. The LDC steps the 270 volt output of the lithium-ion battery down to the 12 volts needed to charge the lead-acid battery and power all of the vehicle accessory systems.


One of the main benefits of the hybrid electric powertrain is the ability to recover kinetic energy lost during braking and store it for use during acceleration and cruising. When the driver applies the brake pedal, the engine is decoupled from the traction motor so that it provides no drag on the vehicle.

The motor is then used as a generator to recharge the hybrid battery. Converting the kinetic energy into electrical energy rather than thermal energy through the friction brakes also reduces the wear on the brake linings so that they last longer.  The extra reliance on regenerative braking also allows the brakes to work better when needed for emergency stopping or driving hill.  

An electronically controlled brake system is used to determine the driver's braking demand based on the degree of pedal application.  This is converted to a total brake torque requirement. The hybrid system control then looks at the current battery state of charge, vehicle speed and other parameters to determine how much regenerative braking is possible under the current conditions. The maximum regenerative braking level is passed over to the brake control system which manages the friction braking torque so that the total net torque (friction plus regenerative) equals the driver's request.



 Like all other 2011 Sonatas, the hybrid uses an Electric Power Assist Steering (EPAS) system that provides boost instantly and only when it is needed during steering maneuvers. This reduces the overall parasitic load compared to an engine driven hydraulic assist. Because the EPAS is independent of the engine it also provides assistance when the engine is not running such as during idle stop or electric drive mode.

The Sonata hybrid also features an electrically driven air conditioning compressor so that climate control can be maintained during engine off driving. Like power steering, the electrically driven air conditioning compressor allows for more precise on-demand control reducing the overall load on the powertrain, cutting fuel consumption.


Because the electric traction motor provides 151 pound-feet of instant-on and continuously available torque, Hyundai's powertrain engineers have also re-calibrated the Theta II engine to operate on an Atkinson cycle. The Atkinson cycle uses late closing of the intake valves to produce an effective compression stroke that is shorter than the expansion stroke. This helps to reduce the pumping losses and improve the overall thermodynamic efficiency of the engine. However, that improved efficiency comes at a cost of low-end torque response.

Even running on an Atkinson cycle, the Theta II still produces more power and torque than any other vehicle in the mid-size hybrid sedan segment. The traction motor is used to fill the gap in the engine's torque curve relative to the base Sonata so that the net result is improved overall power and torque response. With the Sonata also being the lightest car in the segment, it will provide sporting performance to go along with its outstanding fuel efficiency. In fact, the weight-efficient architecture of the new Sonata platform, combined with the lightweight lithium-ion battery pack, make the Sonata Hybrid the lightest vehicle in the segment, at just 3,457 pounds, 263 pounds lighter than the Fusion Hybrid.



The Sonata hybrid features two independent liquid cooling circuits for the powertrain. The standard high temperature circuit is used to manage engine temperatures at up to 190 degrees Fahrenheit and provide passenger compartment heating needs.


A second low temperature cooling circuit is incorporated to manage the temperature of the hybrid starter-generator and the power electronics. The low temperature circuit utilizes an electric pump to circulate the coolant along with a separate heat exchanger to keep the electronics between 86-95 degrees Fahrenheit.

The favorable thermal properties of the lithium-ion battery mean that liquid cooling is not necessary for the energy storage system. A simple duct draws air in from the passenger compartment through the rear package shelf and exhausts it out the rear of the car.


Beyond the innovative powertrain, Hyundai's engineering team have addressed all aspects of vehicle efficiency including the most visually obvious one, aerodynamics. The Sonata hybrid shares the same "Fluidic Sculpture" design language with all other 2011 Sonatas but features a unique exterior that sets it apart from its siblings. The combination of a re-shaped front and rear fascias with a deeper air dam, extended rocker panels and lower drag wheels allow the air to flow around the body with less resistance. The drag coefficient for the Sonata Hybrid is an exceptionally low 0.25 that compares favorably to the Porsche 911 GT2 RS (0.34) and the 2011 Nissan GT-R (.27).  


The ability to easily slip through the air combined with low rolling resistance tires, along with the sophisticated Hybrid Blue Drive powertrain with engine decoupling, significantly reduces the amount of energy required to maintain highway speeds, resulting in the remarkable highway fuel efficiency of the Sonata Hybrid, the best of any mid-size hybrid sedan in the market.




Hyundai Motor America, headquartered in Fountain Valley, Calif., is a subsidiary of Hyundai Motor Co. of Korea. Hyundai vehicles are distributed throughout the United States by Hyundai Motor America and are sold and serviced through about 800 dealerships nationwide. All Hyundai vehicles sold in the U.S. are covered by the Hyundai Assurance program, which includes the 5-year/60,000-mile fully transferable new vehicle warranty, Hyundai’s 10-year/100,000-mile powertrain warranty and 5-years of complimentary Roadside Assistance.

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