The new UX250h luxury compact crossover has a new, fourth generation self-charging hybrid powertrain based on a two-litre I4 petrol engine. In contrast, hybrid versions of the redesigned RAV4 use a 2.5-litre I4.
2.0-litre petrol engine
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Lexus claims the two-litre in-line four-cylinder engine is one of the most efficient it has built, achieving 41% thermal efficiency. 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. The unit is equipped with electric intelligent variable valve-timing (VVT-iE) which enhances driveability while at the same time reducing emissions.
Hybrid transaxle
A new front hybrid transaxle 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. The transaxle's length has been reduced by about 45mm, giving the steering gear more room to deliver a claimed class-leading turning diameter (10.4m kerb-to-kerb). With overall transaxle losses reduced by a quarter, the MG2 is now a higher-speed motor, yielding better performance and efficiency.
Power control unit
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By GlobalDataThe 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. 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.
Hybrid battery
The UX250h's new nickel-metal hydride (NiMH) battery delivers better fuel efficiency and EV (electric vehicle) performance and also takes up less space. Its smaller size has been made possible by a revised construction and more compact cooling system. It also benefits from a new electrode material. The battery comprises 180 1.2V cells, giving a total voltage of 216V and output of 24kW. It is located beneath the rear seat, minimising intrusion in the load space and contributing to the car's optimised front/rear weight distribution and low centre of gravity.
E-Four all-wheel drive
The new, optional, E-Four all-wheel drive system uses a dedicated electric motor/generator integrated in the rear differential. Compared to previous 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 system uses sensors to monitor wheel speed, G-forces, yaw rate, steering angle and other factors, applying the data gathered to precisely control front-rear torque distribution when accelerating, cornering or driving on slippery surfaces. 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 lower in comparison to vehicles equipped with conventional, heavier all-wheel drive systems.
Lightweight materials
The engine is one of the lightest in its class, weighing 112kg, but a further 27kg were saved by using aluminium in place of steel for doors, wings and bonnet, and composite material for the tailgate. The use of an aluminium internal tailgate frame with outer and inner panels made of Super Olefin Polymer is a Lexus first.
These weight-saving measures result in Lexus claiming the UX has the lowest centre of gravity in its class and the kind of agile handling normally associated with a hatchback.
