The new Lexus UX 250h compact crossover is the result of a 5-year development programme that involved hundreds of engineers and technicians and among the many advanced features is the new, fourth generation self-charging hybrid powertrain, each element of which benefits from innovations and improvements in design and performance.

To start with, the 2-litre 4-cylinder engine is one of the most efficient Lexus has built, achieving 41% thermal efficiency. Its advanced features include laser-cut intake valve seats and D4-S fuel injection which combines both direct and secondary port injectors. A high 14:1 compression ratio supports overall efficiency while an exhaust heat recirculation system helps the engine warm up quickly. Electric intelligent variable valve-timing (VVT-iE) enhances driveability while, at the same time, reducing emissions.

Then there’s the new front hybrid transaxle which connects the engine to two electric motor/generators (MG1 and MG2). To make it more compact and to improve driving performance, an innovative multi-axial layout was devised, replacing the previous arrangement in which the motors were located on a single axle. With overall transaxle losses reduced by a quarter, the MG2 is now a higher-speed motor, yielding better performance and efficiency.

The new power control unit (PCU) is 20% more powerful yet 10% lighter than the previous version. Size and weight, critical for both design and safety considerations, were reduced by developing a new, compact circuit board, DC-DC converter and power stack structure. Together these reduced power losses by 20% and improved fuel efficiency. Lexus engineers also introduced a two-in-one power card that is smaller and has double-sided cooling efficiency. Furthermore, optimisation of the output filter has reduced noise levels.

The PCU is mounted directly above the front hybrid transaxle, which contributes to the compact size and reduced weight of the entire system and helps free up more space inside the engine compartment. This in turn allows for a low, aerodynamic bonnet and better protection for pedestrians in the event of a collision.

The UX 250h’s new nickel-metal hydride (NiMH) battery not only makes for better fuel efficiency and EV performance but also takes up less space. Its smaller size has been made possible by a revised construction and more compact cooling system. The battery pack (pictured below) has 180 1.2V cells, giving a total voltage of 216V and output of 24kW. It is located beneath the UX’s rear seat, minimising intrusion in the load space and contributing to the car’s optimised front/rear weight distribution and low centre of gravity.

Finally, there is the new E-Four all-wheel drive system which uses a dedicated electric motor/generator integrated in the rear differential. Compared to previous Lexus E-Four systems, the rear hybrid transaxle has been significantly reduced in size and weight, which improves both luggage space and the car’s handling stability.

The front/rear drive torque split can vary from 100:0 to 20:80. E-four also ensures stable driving on uphill slopes or snow-covered roads, while fuel consumption is said to be lower in comparison to vehicles equipped with conventional, heavier all-wheel drive systems.

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[Chips Yap]

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