MHI Developing Hydraulic Hybrid Cars

May 25, 2014
Motohiko Hamada, journalist
[Click to enlarge image]
[Click to enlarge image]

Mitsubishi Heavy Industries (MHI) Ltd announced that it has started to develop hydraulic hybrid cars and exhibited a hydraulic pump/motor and accumulator.

The company purchased Artemis Intelligent Power Ltd, which has been engaged in development in this field, and is promoting the sale to Japanese automakers.

Because hydraulic hybrid systems do not require high-priced batteries, they are expected to be used as an alternative to electric hybrid systems. In the past, Isuzu Motors Ltd prototyped a bus using a hydraulic hybrid system.

MHI/Artemis' hydraulic hybrid system uses a technology called "Digital Displacement." In general, if a throttle valve is used to control the amount of oil, it lowers efficiency. On the other hand, if the angle of the swash plate is changed to eliminate the throttle, it complicates the mechanism. This is one of the reasons why not many hydraulic hybrid systems are being commercialized despite high expectations for them.

The pump/motor consists of pistons arranged in a radial fashion and is driven by an eccentric shaft located in the center. Each cylinder is equipped with a solenoid valve to switch on/off the operation of the cylinder.

Suppose that the pump/motor is used as a hydraulic pump and 12 cylinders (star-shaped four cylinders x 3) are controlled with a cycle of one second (A cycle can be freely set). In this case, if the rotation speed is 3,600rpm, it discharges 720 times per second. When it is operated 720 times, it is full-throttle. If the operation is stopped once in every 720 times, the discharge amount becomes 719/720. If it is stopped once in every 719 times, the discharge amount becomes 718/719. In this way, it can be finely controlled. When the pump/motor is used as a hydraulic pump, it is controlled in the same way.

MHI/Artemis uses this control method for its windmills. When the wind is weak, the rotation speed of a windmill is low. In such a case, if reduction gears such as planetary gears whose reduction ratio is fixed are used to increase the rotation speed of the generator, the frequency of output alternating current becomes dependent on wind force. As a result, it becomes necessary to use an inverter to transmit the electricity to a power grid, increasing costs. If a power is transmitted via oil pressure, the rotation speed of the generator can be fixed at, for example, 1,200rpm in accordance with a power grid.

MHI and Artemis have been developing the technology to apply it to hybrid vehicles. Artemis has already driven an actual hydraulic hybrid vehicle developed in collaboration with BMW. Thus far, they improved the EU urban and combined fuel economies of a six-speed manual transmission vehicle from 8.6km/L to 18.2km/L and from 12.6km/L to 17.2km/L, respectively, and its US combined fuel economy from 11.9km/L to 17.9km/L.