only about 70% efficient, thus decreasing its power flow while increasing the overall performance of the transmission. The second planetary gearset is extended with a second carrier and sun gear to a ravigneaux-type gear with four shafts, two of which can be held still alternatively by a brake/clutch. The GS 450h and LS 600h systems utilized rear-wheel drive and all-wheel drive drivetrains, respectively, and were designed to be more powerful than non-hybrid versions of the same model lines, while providing comparable engine class efficiency. Third Generation Toyota CEO Katsuaki Watanabe said in a February 16, 2007 interview that Toyota was "aiming at reducing, by half, both the size and cost of the third-generation HSD system". The new system will feature lithium-ion batteries in later years. Lithium-ion batteries have a higher energy capacity-to-weight ratio compared to NiMH, but operate at higher temperatures, and are subject to thermal instability if not properly manufactured and controlled, raising safety concerns. Fourth Generation On October 13, 2015 Toyota made public details of the Fourth Generation Hybrid Synergy Drive to be introduced in the 2016 model year. The transaxle and traction motor have been redesigned, delivering a reduction in their combined weight. The traction motor itself is considerably more compact and gains a better power-to-weight ratio. Notably there is a 20 percent reduction in mechanical losses due to friction compared to the previous model. The Motor Speed Reduction Device (a second planetary gear set found only in the Third Generation P410 and P510 transaxles), and which connects the traction motor directly to the Power Split Device, and thereafter to the wheels, has been replaced with parallel gears on the Fourth Generation P610 transaxle. The 2012– Prius c retains the P510 transaxle. The P610 transaxle employs helical gears rather than the straight-cut spur gears employed in the earlier transaxles, and which run more smoothly and quietly, while also accommodating higher mechanical loads. With the Fourth Generation HSD, Toyota is also offering a four-wheel drive option, dubbed "E-Four", in which the rear traction motor is electronically controlled, but is not mechanically coupled to the front inverter. In fact, the "E-Four" system has its own rear inverter, although this inverter draws power from the same hybrid battery as the front inverter. "E-Four" began being offered in Prius models in the United States in the 2019 model year. "E-Four" is an integral part of the RAV4 Hybrid models offered in