2010 Lexus HS250h Review & Test Drive
Hybrid efficiency is at the core of the new HS250h and Lexus’s 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 Atkinsion-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 Time with Intelligence on the intake side. The engine is undersquare, with an 88.5mm bore diameter and 96mm stroke, and has a relatively high compression ratio of 12.5:1. Peak output equals 147hp and in combination with the hybrid drive motor, the HS generates 187 total system horsepower. That is the most of any other dedicated hybrid motor.
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 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 RX450h luxury SUV, 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 lower power demand conditions in city driving. Overall, the system can improve fuel efficiency by as much as 7% in low ambient temperatures.
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