24 / 1 / 2019
IMAGINATIVE THINKING USED TO ENGINEER THE LEXUS UX 250h SELF-CHARGING HYBRID
Hundreds of Lexus engineers and technicians spent nearly five years developing the new UX 250h crossover. Working to an incredibly demanding brief, they left no stone unturned in creating one of the most advanced vehicles Lexus has ever built.
As well as perfecting breakthrough manufacturing processes and lightweight composite materials, the team used all its imagination and two decades of hybrid experience to develop a new fourth generation self-charging hybrid powertrain.
2.0-litre Petrol Engine
The UX 250h’s new 2.0-litre inline four-cylinder petrol engine is one of the most efficient ever built by Lexus (41% thermal efficiency). Advanced features include laser-cut intake valve seats, a high compression ratio (14:1) and D4-S fuel injection combining direct and secondary port injectors. In addition, while exhaust heat recirculation enables quick engine warm-up, Variable Valve Timing-intelligent Electric enhances drivability and at the same time reduces emissions.
Front Hybrid Transaxle
The UX 250h features a newly engineered front hybrid transaxle which connects the highly efficient 2.0-litre petrol engine to two electric motor/generators, achieving class-leading fuel consumption, quietness and driving performance.
To make the new transaxle more compact and to improve driving performance, Lexus engineers specified an innovative multi-axial layout in place of the conventional arrangement where MG1 (power generation) and MG2 (drive) electric motors are arranged on the same axle. Compared to the previous Lexus compact car transaxle, the unit’s length has been reduced by approximately 45 mm. These compact dimensions allow the steering gear more room to achieve the UX’s class-leading turning radius (10.4 metre curb-to-curb diameter). In addition, with overall transaxle frictional losses reduced by 25%, MG2 is now a higher-speed motor, yielding better performance and efficiency.
Important for smoothness and the long-term reliability of the UX 250h’s self-charging hybrid powertrain, Lexus engineers also developed a new system for returning cooled oil to the transaxle and to the MG1/MG2 motor/generators.
Power Control Unit
The UX 250h’s compact new Power Control Unit (PCU) is 20% more powerful yet 10% lighter than previous units. Critical for both design and safety considerations, the PCU’s size and weight were reduced by developing a new compact circuit board, DC-DC converter and power stack structure, which together resulted in a 20% reduction in power losses and improved fuel efficiency. Lexus engineers also introduced a 2-in-1 power card which is smaller and features double-sided cooling efficiency. Meanwhile, optimizing the output filter has further reduced noise levels.
The new PCU is mounted directly above the front hybrid transaxle, contributing to the compactness and reduced weight of the entire system and creating more space inside the engine compartment. This in turn allows for a low aerodynamic bonnet while providing improved pedestrian protection in the event of a collision.
The UX 250h’s newly developed nickel metal-hydride (NiMH) battery not only makes for improved fuel efficiency and Electric Vehicle performance, it also saves on space. Benefitting from a new electrode material, its reduced size has been made possible by a revised construction and a more compact cooling system. Featuring 180 cells of 1.2 V each for a total voltage of 216 V, the UX 250h’s hybrid battery produces 24 kW and is located below the rear seat, minimising intrusion in the load space and supporting the UX’s optimized front/rear weight distribution and low centre-of-gravity.
The new E-FOUR all-wheel drive system on the UX 250h uses a dedicated electric motor/generator integrated into the rear differential. Compared to previous Lexus E-FOUR systems, the rear hybrid transaxle has been greatly reduced in size and weight, enhancing rear luggage space and improving the car’s handling stability. Information sent from various sensors (wheel speed sensors, G sensor, yaw rate sensor, steering angle sensor, etc.) is used to precisely control the front-rear torque distribution in ratios between 100:0 and 20:80 when accelerating, cornering, or driving on slippery surfaces. E-FOUR also ensures stable driving on uphill slopes or snow-covered roads with lower fuel consumption than conventional, heavier all-wheel drive systems.
To improve the UX 250h’s efficiency and performance, the engineering team set out to reduce weight wherever they could. While the UX 250h’s 112 kg petrol engine is one of the lightest in class, the engineers saved a substantial 27 kg by using aluminium instead of steel for the doors, wings and bonnet and composite materials for the tailgate. The adoption of an aluminium internal tailgate frame with Super Olefin Polymer outer and inner panels was another Lexus first. These weight-saving measures resulted in the UX having the lowest centre-of-gravity (594mm) of any vehicle in its class and the agile handling normally associated with a dynamic hatchback.
The UX 250h is now on sale in Europe. In our next blog post we examine the Japanese traditions that inspired the design and craftsmanship of this new crossover. www.lexus.eu/ux