The HS 250h features the brand's first four-cylinder gas engine paired with Lexus Hybrid Drive. Utilizing the most recent generation hybrid system, HS 250h takes 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, an 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 HS 250h. 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 a 3.48-in. bore diameter and 3.78-in. 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 250h generates 187 total system horsepower.
It also meets the California SULEV and federal Tier 2 Bin 3 exhaust emission standards and also augmented by a 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, A/C 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 helps prevent 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 luxury utility vehicle, 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 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 (ECVT) 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.
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 drive-mode buttons that allow the driver to select between four modes: Normal, Power, Eco and EV. Each mode changes the rate of throttle opening for a given throttle-pedal angle. Normal mode has an essentially linear throttle response that gives naturally progressive power. Power mode can be selected for a more responsive feel when desired. In Eco mode, air conditioning settings are adjusted and the throttle response is reduced relative to the pedal angle to emphasize fuel economy. Under certain circumstances, the 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. Responsive passing acceleration that benefits from the instant torque of the electric motor and the ECVT's ability to quickly find the optimal gear ratio enhance the driving experience.
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