Nissan bringing new CVT, small engine family to market
07-10월-2009 20:00 GMT
Nissan and Jatco jointly developed the CVT that employs a sub-planetary gear two-speed geartrain that provides a ratio spread of 7.3.
“The concept has been with us—and maybe with others—for a very long time,” confided Hitoshi Suzuki, Senior Development Engineer at Jatco, the specialist supplier of automatic and continuously variable transmissions (CVTs).
The concept refers to a sub-geartrain added to a CVT, thereby widening the transmission’s ratio spread. In this case, for Nissan, the Xtronic transmission is the steel push-belt-and-pulley type. The ratio spread ranges between a smallish 4.5 up to a fraction over 6.0, according to Suzuki. Recently Subaru (which makes its own CVTs) has achieved an impressive 6.3 using a pull-chain-and-pulley arrangement.
The Jatco CVT employs a sub-geartrain, a planetary gear unit, that provides two ranges: low and high. The ratio spread is a 7.3.
Jointly developed by Nissan and Jatco, the unit allows for a compact size and light weight. The two pulleys are positioned side by side vs. the conventional CVT’s stacked-at-an-angle placement. The pulleys are completely out of the fluid sump, eliminating the slushing of fluid altogether.
Frictional losses have been reduced by 30%, Nissan claims, and fuel economy improvement by 10% vs. a comparable, conventional CVT-equipped car.
A Versa hatchback equipped with this transmission was available for test-driving by AEI. The car moves off in the sub-geartrain’s Low with the CVT obviously in the lowest ratio, producing brisk acceleration.
The torque converter locks up fully at 14 to 15 km/h (8.7 to 9.3 mph). At about 20 km/h (12 mph), the sub-gearbox shifts into high and keeps it thereafter. On deceleration and braking, the CVT stays in high to a full stop, fuel economy being primary objective here.
The next-generation Xtronic CVT is best suited to small-displacement engines and will likely be complemented by the company’s dual-injector technology.
Nissan will deploy its next-generation small-displacement gasoline engine family in its small and compact cars, beginning in 2010. The new inline four-cylinder series will likely have a median displacement of 1.5 L and will be equipped with continuously variable valve timing control (CVTC) on both the intake and exhaust camshafts.
The unique feature is that each cylinder is fed by two port injectors. The new injector nozzle has 18 miniscule holes vs. a more common single injector with 12 holes. The injectors are placed in the intake ports closer to the intake valve openings. They inject fuel in wide-angle spray cones, with droplet size reduced from the single 12-hole type’s 70 µm (2800 µin) to 30 µm (1200 µin). This promotes faster and finer atomization of the incoming charge, according to Nissan engineers.
The dual-injector system, developed by Denso, is relatively straightforward. It is simpler and lighter in weight than a comparable direct-injection system, according to Nissan. Injection pressure is port-injection-norm, much lower than that of a typical direct-injection (DI), therefore its hardware is less expensive.
Nissan quotes a cost reduction of 60% vs. DI. Stable combustion allows a generous internal exhaust gas recirculation introduced during a larger overlap by the dual CVTC, which improves fuel economy and reduces pumping loss.
The dual-injector system achieves a 4% improvement in fuel efficiency vs. a conventional single-injector port-injection engine, according to Nissan.
The other significant benefit of the system is that the amount of rare metals in the catalytic converter may be greatly reduced by nearly 50%, as the catalyst would have to cleanse less hydrocarbon emissions.