Kia Motors Corporation premiered the hybrid version of its new Optima sedan at the Los Angeles Auto Show. Created primarily with the US market in mind, the Kia-developed powertrain delivers an estimated combined cycle fuel economy is 38 mpg US (6.2 L/100km) (city 36.0 mpg, highway 40.0 mpg).
Arriving in Kia dealer showrooms early next year, the 2011 Optima Hybrid is the brand’s first hybrid for North America and uses a full parallel hybrid system to deliver a 40.7% mpg improvement compared with a regular Optima. Acceleration to 62 mph (100 km/h) takes 9.2 seconds, and the top speed is 121 mph (195 km/h).
The 2.4-liter Theta II gasoline engine is mated to a small electric motor and drives the front wheels via a six-speed automatic transmission without the traditional torque converter. A special clutch is fitted between the engine and motor, enabling the gasoline engine to be de-coupled from the powertrain so that Optima can be operated in zero-emissions, full-electric drive mode from standstill up to 62 mph.
The system’s configuration does not require a high-capacity electric motor and generator, saving weight and cost.
When setting off, the Optima Hybrid operates in Electric Mode. As vehicle speed rises, the Hybrid Starter motor/Generator (HSG) starts the gasoline engine and the clutch is closed, allowing the engine to take over the task of propelling the car. The electric motor switches into hybrid operation and serves as both a secondary engine (during full acceleration and hill-climbing) and a generator to recharge the battery pack as necessary. Whenever the car comes a stop for more than a few seconds, the gasoline engine automatically shuts off to completely eliminate emissions.
When braking, the electric motor converts kinetic energy into electricity for storage in the battery pack. The 8.5 kW HSG also functions as a generator to re-charge the battery pack if the state of charge is low and when the car is stationary. Throughout the entire driving experience, in everyday motoring, under all conditions, the entire hybrid operation is automatic and seamless.
The Optima Hybrid’s 30 kW Interior Permanent Magnet (IPM) synchronous electric motor produces 40.7 ps (30 kW) and 205 N·m (151 lb-ft) of torque from 0-to-1,400 rpm in electric mode. The combined (electric motor plus gasoline engine) hybrid powertrain has an output of 209 ps (154 kW) and 265 N·m (195 lb-ft).
Described by Kia engineers as a Transmission-Mounted-Electric-Drive (TMED), the 30 kW IPM electric motor sits within the car’s extended transmission casing (between the gasoline engine and the automatic gearbox) and is an oil-cooled system.
Kia’s Optima Hybrid employs a lithium polymer battery array developed in partnership with LG Chem. The power and energy density of this battery enabled Kia engineers to create a lighter, more compact power pack with a 34 kW output and weighing just 43.6 kg (95.9 lbs) designed to minimize intrusion of the Optima’s trunk space.
The complete hybrid system is controlled by the Optima's Hybrid Control Unit (HCU) which acts as the car’s mastermind and integrates the smooth operation of the engine’s ECU, transmission’s TCU, battery management system (BMS), low voltage converter (LDC). This brain also gives the Optima a Fail-Safe back-up capability and a Limp-home mode.
Optimizing the Theta II gasoline engine. Taking advantage of the instant and continuously available torque from the electric motor, Kia modified its popular 2.4-liter Theta II engine to operate on an Atkinson cycle, raising the compression ratio by 20% to maximize its efficiency and achieving a 10% fuel saving over a regular Theta engine. This cycle generates a little less torque, but the electric motor compensates for any loss and consequently, the Optima Hybrid’s overall power and torque outputs are greater than the regular model.
For the Optima Hybrid, Kia has modified the six-speed automatic transmission found on the conventional Optima for hybrid application. The traditional torque converter is replaced with an electric motor and a high-efficiency oil pump. This new gearbox allows EV mode operation, maximizes regeneration of kinetic energy during braking and also provides a more responsive drive than a CVT system.
To maximize the Optima Hybrid’s performance and minimize its fuel consumption, special attention has been paid to its aerodynamics. The car is lowered by 5 mm and new features include an active air flap in the front grille, smooth underfloor panels, low-drag wheels and low rolling resistance tires. The drag coefficient is lowered to Cd 0.26.
Energy and fuel-saving technologies features in the Optima Hybrid include a regenerative braking system, electric—rather than hydraulic—assisted power steering, and an electric air compressor capable of delivering high air-conditioning performance.
Two independent liquid cooling systems are fitted to the Optima Hybrid powertrain. The standard high-temperature circuit manages engine cooling and passenger compartment heating. The second low-temperature circuit cools the hybrid starter-generator and the power electronics. The trunk-mounted battery pack does not require liquid cooling and its temperature is controlled by a simple ventilation fan beneath the rear parcel shelf.
