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2010 Lexus HS 250h Overview Part I: Engine, Aerodynamics, Fuel Economy

Lexus HS 250h, World’s First Hybrid-Only Luxury Vehicle, Offers Sustainable Innovation

With the introduction of the world’s first hybrid-only luxury car, the 2010 HS 250h, Lexus brings the promise of sustainable luxury to the forefront of the auto industry. In the process, the brand also elevates luxury transportation with innovative engineering and intelligent, driver-inspired design that offers excellent roominess and nimble maneuverability.

Each new generation of Lexus hybrids brings with it new gains in efficiency. With the HS 250h, designers and engineers took the opportunity to create a truly unique luxury car that maximizes fuel economy while further shrinking its impact on the planet by using ecologically advanced materials, clever packaging of people and components and eradicating waste wherever possible. They also found fresh ways to make driving easier and more efficient, with the premium quality and luxury expected from a Lexus.

Engine and Hybrid System
Hybrid efficiency is at the core of the HS 250h and Lexus’ first four-cylinder gas engine and latest hybrid system take advantage of the company’s continuous engineering advances to maximize fuel economy, minimize emissions and deliver driving pleasure. A highly efficient engine design, an exhaust heat recovery system, major efforts to reduce parasitic losses, a new evaporative emissions system and driver-selectable driving modes all make notable contributions. Significant efforts were also made to minimize engine noise.

An Atkinson-cycle engine design was chosen for its high efficiency and compatibility with hybrid operation in the new HS. The twin-cam engine uses four valves per cylinder and Variable Valve Timing with Intelligence (VVT-i) on the intake side. The engine is undersquare, with an 88.5-mm bore diameter and 96 mm stroke and has a relatively high compression ratio of 12.5:1. Peak output equals 147 horsepower and in combination with the hybrid drive motor, the HS generates 187 total system horsepower. It also meets the California SULEV and federal Tier 2 Bin 3 exhaust emission standards, emitting 70 percent fewer smog-forming emissions than conventional vehicles; it is also augmented by a new system that reduces evaporative emissions to near zero.

Careful design of the intake system reduces engine pumping losses and its twin resonators minimize intake noise. The cooling system uses a modular construction that combines an aluminum engine radiator, hybrid system radiator, AC condenser and fan into a single assembly for minimal weight. Plus, the electric fan speed is controlled via Pulse Width Modulation to reduce energy consumption and noise. Careful ducting of the cooling system prevents hot engine air recirculation that tends to occur at low speeds and while idling, further enhancing efficiency.

One of the keys to lower fuel consumption involves heat. Similar to the Lexus RX 450h, the HS employs an Exhaust Heat Recovery System that captures the heat of spent exhaust gases to speed engine coolant warm-up and allow the hybrid system to stop the engine earlier and more often in the driving cycle when it’s not needed, for example in low power demand conditions in city driving. Overall, the system can improve fuel efficiency by as much as seven percent in low ambient temperatures.

Using a two-motor hybrid system, the HS 250h uses a 141 hp drive motor that combines with the 147 hp gas engine (Note that the sum of the engine and motor power are not equal to the total system power as the operation of the engine and motor cannot be maximized in the same operation range). A second motor in the hybrid system serves as both engine starter and generator to charge the hybrid battery pack.

The efficient two-motor system also uses an electronically-controlled continuously variable transmission (CVT) that is a model of efficiency, simplicity and compact size with extremely smooth and quiet operation. A motor speed-reduction planetary gear unit helps provide high power output in this small package. The HS250h also uses an easy-to-operate shift-by-wire system that eliminates the need for bulky shift cables, saving space and weight while allowing much more design freedom in placing the shift lever in the interior.

An electric transmission oil pump eliminates another source of parasitic loss to further increase efficiency. Electronic operation allows a control strategy that invokes the oil pump only when needed, and since the oil also flows over the drive motor, overall efficiency of the motor and transmission are improved.

The HS 250h also incorporates a drive-mode switch, an important element in improving efficiency that involves driver and car interaction. The driver can select among four modes: Power, Normal, Eco and EV. Each switch position changes the rate of throttle opening for a given throttle-pedal angle. Normal mode has an essentially linear throttle action that gives naturally progressive power. In Eco mode, the throttle actuation on the engine and drive motor is reduced relative to the pedal angle to emphasize fuel economy. While Eco mode assists the driver in achieving improved fuel efficiency, overall the automatic A/C and hybrid battery coolant control helps achieve maximum fuel efficiency. Power mode can be selected for a more responsive feel when desired. Under certain circumstances, a new driver-selectable EV mode can allow the vehicle to be driven short distances using only the electric motors.

Aerodynamics
The HS 250h body design received considerable attention to aerodynamics. Ambitious goals were set requiring designers to work closely with aerodynamicists from the outset to create a low-drag form in an attractive, spacious, yet compact package. Their work yielded a top-in-class 0.27 coefficient of drag (Cd), which makes a considerable contribution to fuel economy, especially at highway speeds.

The low nose encourages airflow while also helping enhance vehicle dynamics. Smooth transitions along the front fender and A-pillars avoid disturbance of the airflow. Subtle front- and rear-bumper spoilers and a rear-lip spoiler contribute to low drag and minimize lift. The body sides taper from the center pillar to the rear where they meet the slightly kicked-out tail lamps, helping reduce drag-inducing wake turbulence. Also, a gull wing-shaped roof panel enabled designers to lower its profile to reduce air resistance and with a shape that is naturally stiffer than a conventional curved roof, engineers were able to reduce its thickness for further weight reduction.

Several measures were also taken under the car to promote airflow and reduce drag. Airflow resistance in the engine compartment was improved by optimizing the duct profiles around the radiator. Careful positioning of numerous underfloor covers plus added diffuser fins on the rear covers further reduce drag while enhancing straight-line control.

Acceleration and Fuel Economy
All of this focus on efficiency means that performance benefits as well, not only from gains in the engine and hybrid powertrain, but also from the HS 250h’s excellent aerodynamics (a product of frontal area, body drag and airflow management) and weight reduction efforts throughout the car. Outstanding fuel economy with confident, responsive acceleration is the inevitable result.

The HS 250 is expected to achieve an EPA-estimated 35 mpg city and 34 mpg on the highway for a combined 35 mpg. Careful drivers can use the Eco mode in conjunction with a new hybrid system monitor plus eco-driving indicator to improve on these numbers in certain conditions. The expected fuel efficiency is more than 20-percent better than the most fuel-efficient model currently in the Lexus lineup, while earning a SULEV emissions rating utilizing regular 87-octane gasoline.

Acceleration from 0-to-60 mph is rated at 8.4 seconds. Responsive passing acceleration that benefits from the instant torque of the electric motor and the CVT’s ability to quickly find the optimal gear ratio enhance the driving experience.

2010 Lexus HS 250h – Hi-res. Images

2010 Lexus HS 250h
2010 Lexus HS 250h
2010 Lexus HS 250h
2010 Lexus HS 250h
2010 Lexus HS 250h
2010 Lexus HS 250h
2010 Lexus HS 250h
2010 Lexus HS 250h


Sustainability
Underlying the HS 250h’s forward-thinking interior design is one of the most extensive implementations of bioplastic material in an automobile to date. Known more formally as Ecological Plastic, the material uses plant sources as raw material and its use for parts of the interior upholstery in the HS 250h is a world first.

Ecological Plastics is also used in a number of injection-molded, foam and board components throughout the car, including trunk compartment trim, cowl side trim, door scuff plates, seat cushions and the package tray. Overall, approximately 30 percent of the interior and trunk are covered in Ecological Plastic, resulting in a reduction of carbon emissions by approx. 20 percent over the estimated life of the vehicle.

2010 Lexus HS 250h Overview Part II: Technology
2010 Lexus HS 250h Overview Part III: Design
2010 Lexus HS 250h Overview Part IV: Comfort and Convenience
2010 Lexus HS 250h Overview Part V: Safety

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