TORRANCE, CA --- Lexus said its 2010 HS 250h offers a highly efficient engine design, an exhaust heat recovery system, reduction in parasitic losses, a new evaporative emissions system and driver-selectable driving modes. 

The HS 250h is expected to achieve an EPA-estimated 35 mpg city and 34 mpg on the highway for a combined 35 mpg, Lexus said. 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 using regular 87-octane gasoline. 

Lexus said it opted for an Atkinson-cycle engine design for the 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. The vehicle is also augmented by a new system that reduces evaporative emissions to near zero. 

The intake system design 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, Lexus said. 

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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, Lexus said. 

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). A motor speed-reduction planetary gear unit helps provide high power output in this small package. The HS250h also uses a 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. 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. 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 improving 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. 

Acceleration from 0 to 60 mph is rated at 8.4 seconds. 

A new standard multi-media package includes voice recognition, Bluetooth technology, integrated satellite radio (subscription required - first three months free) and USB iPod connectivity operated via steering wheel controls or from the display screen. A multi-information display situated in the instrument cluster keeps the driver informed of the numerous actions controlled by the steering wheel-mounted switches. 

The HS 250h's navigation system is optional and features Lexus' remote touch controller with user-adjustable haptic feedback. The controller, similar to a computer mouse, eliminates the need for touch screen controls, allowing the pop-up navigation system screen to be mounted further up the center stack and away from the driver. The driver benefits with a clear, high-resolution screen, improved site lines and reduced eye movement. 

A new casual-language voice recognition feature, Voice Command, is standard on navigation-equipped vehicles. This new technology, previously only available on the 2010 Lexus RX350 and RX450h, allows the driver to give more flexible, conversational commands for easier access to audio and climate, as well as navigation system functions. 

The HS 250h incorporates a 10-airbag supplemental restraint system with dual-stage airbags and knee airbags for driver and front passenger, side curtain airbags, and both front and rear seat-mounted side airbags to help protect front and rear passengers in certain types of severe frontal or side collisions. The front passenger airbag is a twin-chamber type that further helps protect the front-passenger. 

Additionally, front and outer rear seatbelts incorporate pretensioners with force-limiters to help protect occupants in the event of a severe frontal collision. Also, active head restraints are provided for the front seats. 

An optional Pre-Collision System (PCS) with Dynamic Radar Cruise Control and Driver Monitor uses millimeter-wave radar to measure and maintain a pre-set following distance from a vehicle traveling ahead. The radar sensor can detect certain obstacles in front of the car, and a PCS computer with vehicle speed, steering angle and yaw-rate inputs helps to determine whether a collision is highly possible. In such a situation, PCS alerts the driver with audible and visual warnings, preemptively retracts front seat belts and pre-initializes Brake Assist so that increased braking will be applied the instant the driver depresses the pedal.   

Originally posted on Green Fleet Magazine

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