Yamaha Motor Co Ltd unveiled a concept model of a four-wheeled series hybrid vehicle (HEV) at Tokyo Motor Show 2017, which was open to the public from Oct 28 to Nov 5, 2017 (Yamaha to Exhibit 4-wheeled Compact Series HEV).
The HEV, "MWC-4," can be driven with an ordinary motor vehicle license in Japan. The company defines the vehicle as an "ultra-small mobility."
The most distinguishing feature of the MWC-4 is that it is equipped with a "leaning multi-wheel (LMW) mechanism" and runs on four wheels. Compared with Yamaha Motor's two-wheeled vehicles and "Tricity MW125" three-wheeled vehicle, the running stability of the MWC-4 can be more easily improved (See related article).
"The basic technologies (of the MWC-4) are based on those of our three-wheeled vehicles such as the Tricity, but we renovated the shape of the mechanism," Yamaha Motor said.
The previous mechanism consists of "parallelogram links" and "cantilever telescopic suspensions." There are two links arranged in parallel with each other. The center of each link is fixed to an auto body so that they form a rectangular shape when the vehicle is turning.
With this mechanism, the parallelogram links move the right and left suspensions to different positions when the vehicle is turning. The right and left wheels move in different directions (up and down) while being in parallel with each other. The vehicle leans its right and left wheels in a direction in which the driver moves the steering wheel.
This time, Yamaha Motor employed a double wishbone for both the front and rear wheels of the four-wheeled concept, in place of the rectangular link. The stability of the MWC-4 is high even when it is running at a high speed. The new system is being increasingly employed for four-wheeled sports cars.
The company made some original improvements to the concept car. For example, a shock absorber that absorbs impacts from the road surface is fixed while being separated from the vehicle frame.
Because the concept car is expected to be a series HEV, it stores electricity generated by an engine in a battery and runs on a motor. As an engine, Yamaha Motor plans to use a single-cylinder engine designed for two-wheeled vehicles, but the company said, "We are still considering engine performance and so forth."
Yamaha Motor will probably use a new engine developed only for the MWC-4 because it eliminates the need to consider the compatibility with the transmission. The engine is transversely placed due to the limited installation space. The piston moves in the width direction of the vehicle.
The engine is placed under the seats. A fuel tank for the engine and the engine for power generation are located in the front and rear areas, respectively, under the seats. For power generation, it is necessary to rotate the engine at a constant rotation speed. Therefore, a vibration absorbing material is used to reduce the vibration transmitted to the seats.
The battery is located under the floor. When the engine and the fuel tank are detached, the MWC-4 can be used as an electric vehicle (EV).
"In consideration of practical drive range, a series HEV is desirable at this point," Yamaha Motor said. "We will consider making it an EV when battery technologies advance."
While Yamaha Motor is considering developing the MWC-4 as an EV, it is easier for the company, which has a strength in engine technologies for two- and four-wheeled vehicles, to develop a series HEV.
The MWC-4 is a rear-wheel-drive car that uses a water-cooled in-wheel motor (IWM) attached to each of the right and left rear wheels. It does not need differential gears, enabling to reduce weight. By controlling the torque applied to the right and left wheels, it becomes easier to realize swift driving through torque vectoring, etc.
The series HEV is equipped with a linear steering that is similar to the steering of a two-wheeled vehicle, and the steering is used to control the front wheels. As in the case of two-wheeled vehicles, the body of the MWC-4 is tilted by weight shift.
The vehicle measures 2,680 (L) x 900 (W) x 1,480mm (H). Though it has two seats, the rear seat is supplementary, Yamaha Motor said.