Nissan's Serena minivan is based on a new, front-wheel-drive platform.
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Nissan's original Serena was a workhorse of a minivan, being derived from a Japanese "one-box" configuration with its engine placed under the front seats and driving the rear wheels. It was produced in Japan, and continues to be made in Spain for the European market. Nissan has revamped the Japanese version using a new MS (medium and small cars and variants, including the latest Sunny/next-generation Sentra) front-wheel-drive platform with its drive-train placed transversely up front. The tall minivan still qualifies as a "small" vehicle by Japanese classifications, with its overall width held under 1700 mm (67 in) and a gasoline engine displacement under 2.0-L. (There is no limit for the diesel, which allows a 2.5-L inline-four engine in the Serena without incurring an annual tax increase.) Other vital statistics include a 2640-mm (104-in) wheelbase, 4520-mm (178-in) length and 1825-mm (72-in) height.
The Serena features two sliding rear doors, with roll-down power windows on upper models, and an upswinging tailgate, and accommodates up to eight people in three rows. The interior has a completely flat floor and versatile seating and cargo-carrying configurations. The Serena's body is of all-steel, unitary construction. The vehicle has been subjected to a series of the Japanese mandatory crash tests as well as the European ones, including a 40% offset crash against a deformable barrier at 64 km/h (42 mph) and a 55-km/h (34 mph) side impact.
The chassis is an amalgamation of Nissan practices. The front suspension uses a MacPherson strut from the larger Presage minivan. A space-saving, multi-link, independent rear suspension is common with Nissan's recent station wagon models. Steering is by a power-assisted, rack-and-pinion system. Front ventilated disc and rear drum brakes provide retardation, with ABS, electronically controlled brake distribution (EBD), and brake-assist fitted as standard equipment.
The Serena's gasoline engine is an SR20DE dual-overhead-camshaft, 16-valve, 2.0-L, inline four-cylinder producing 108 kW (145 hp) at 6000 rpm and 186 N•m (137 lb•ft) torque at 4800 rpm. This engine is combined with Nissan's Hyper-CVT continuously variable transmission employing a torque-converter with electronically controlled lockup clutch and steel-belt-and-pulley transmission. The CVT provides variable internal ratios between 2.326: and 0.434:1, combined with a final drive ratio of 5.743:1.
The diesel is the YD25DDTi unit displacing 2.5 L and having dual overhead camshafts, 16 valves, direct injection, and turbocharging/inter- cooling. The engine operates in what Nissan calls "M-fire combustion" mode when NOx and noise emissions become critical. One of the intake ports is fitted with a swirl-control valve. Air comes in from the high-swirl port with this valve and port closed, generating powerful swirl motion, and fuel is injected in two delayed stages. Thus moderated, combustion temperature is much lower, reducing NOx emission and clattering. Furthermore, a huge dose of recirculated exhaust gas is introduced to lower combustion temperature. Once out of the Japanese 10/15-mode urban test cycle, and on higher operating load, the engine reverts to normal combustion mode with both intake ports opened.
The diesel's variable-nozzle turbocharger and air-to-air intercooler allow the YD25DDTi engine to produce 112 kW (150 hp) at 4000 rpm and 279 N•m (206 lb•ft) at 1800 rpm. This engine is mated to an electronically controlled four-speed automatic transmission. The drivetrain is mounted in the chassis using Nissan active control mounts (ACMs), which generate frequency vibrations opposite to the engine's by means of a magnetic-coil-excited plate within the fluid-filled mount unit. Two such mounts are used to locate the drivetrain.
The Serena is offered in front- and all-wheel-drive configurations, the latter employing a new torque split and transfer system, called ATC (automatic torque control), placed immediately ahead of the rear final-drive unit. The ATC is essentially a hydraulic system, providing driving torque to the rear wheels as demanded by road and driving conditions.
The ATC is a compact and lightweight stand-alone system whose input is from the propeller shaft. The cylindrical casing is attached to the propeller shaft, and within the casing is a disc cam with four wave lobes. Nine pistons are mounted on the rotor attached to the output shaft to the final drive. A closed hydraulic circuit and a small accumulator are located aft of the rotor. When the ATC housing (attached to the propeller shaft) and the output rotor (to the final drive) rotate at the same speed, the disc cam does not act on the pistons, thus no torque transfer occurs. When a difference in front and rear wheel speed occurs, as when the front wheels begin spinning on a slippery surface, the pistons are pressed against by the cam lobes, generating high fluid pressure. Hydraulic pressure reacts and presses the pistons against the cam disc, generating variable rotational force that transmits torque to the rear wheels. The amount of torque transferred to the rear wheels is dependent on the difference between front and rear wheel speed. One of the system's unique benefits is that at low vehicle speeds, when slogging through difficult terrain, more torque is transmitted to the rear wheels. At higher velocities, in slippery conditions or sudden cross-wind situations, an excessive amount of torque transfer may cause vehicle instability. However, the ATC proactively (by a centrifugal sensor/device) optimizes the amount of torque transfer according to prevailing conditions.
Jack Yamaguchi
