Story 1
For MIRAI—For the Future
Chapter 4

The Pursuit of Efficiency—Battling Waste

Katsutoshi Shiromaru

Group No. 111
Compressor Division

The Thrill of Acceleration

The previous chapters introduced optimizing the flow of magnetic flux, creating motors that are smaller and lighter, yet with better responsiveness and acceleration. On the other hand, they also worked to increase acceleration performance in the compressing section. What, specifically, did this include? We asked Mr. Shiromaru, involved with this project.

"Increasing the rotational acceleration of the air compressor is essential in order to improve vehicle acceleration. We worked to increase compression power and reduce friction loss, and at the same time also focused on reducing the moment of inertia*13 of the rotor."

Reducing the weight of the rotor (as well as other factors) is effective in reducing the moment of inertia. Accordingly, they started on work in order to reduce the weight of rotor parts, without compromising either performance or rigidity. Reducing the distance between the support points as mentioned in the section about rotor shake also proved successful.

"Reducing the gap between the support points reduced shake, but it also reduced the moment of inertia. Reducing this gap between the supports significantly hollowed out the rotor, also reducing weight. Given that the rotor is quite heavy, this was very effective—this definitely killed two birds with one stone."

While ensuring that rigidity did not suffer, they also used lightening holes in the gears, which was also very effective in reducing the weight of the rotor. However, it was difficult to determine the extent of weight savings possible. Removing too much matter in order to save weight is pointless, as it degrades both rigidity and performance. They repeated testing and simulation, and found a balance between these two conflicting requirements—reducing weight, while retaining rigidity and performance. These efforts resulted in a threefold increase in acceleration.

"Installing the improved prototype with better acceleration in a vehicle, and going for a test drive was fun, and the best part was experiencing the acceleration for ourselves."

Mr. Shiromaru beamed.

Air compressor rotor (CG)

*13: Moment of inertia
The force required in order to start or stop rotation of an item.


Energy Savings in the Air Compressor

As well as improving performance, improving fuel economy was also a major challenge, Mr. Shiromaru explained.

"An FCV drives using motors powered by electricity, which is generated by a fuel cell. However, this electricity is also used in order to drive the motor in the air compressor. So, if the air compressor consumes a lot of electricity in its operation, this will result in less power available to drive the vehicle, and lowered fuel efficiency."

The performance of the air compressor has a major impact on FCV fuel efficiency, and so is an important issue. So how can this be made more efficient, and fuel efficiency improved?

"The performance of the air compressor depends on three factors—compression power, leakage losses, and mechanical losses. Out of these, we considered compression power and reducing leakage losses most important, so we started work on these."

Investigating these two factors requires base figures which we can use when carrying out performance simulations. However, this six-lobe helical root-type rotor is a world-first, and as such these do not exist. Before starting to improve efficiency and fuel efficiency, it is necessary to draw up parameters in order to verify performance.

"Many factors needed to be tested—rotor length, rotor diameter, number of lobes, twist angle, cross-section, aspect ratio (length/diameter), gap between cylinder and rotor… While verifying each and every one of these, we looked carefully for points which would be the most effective at achieving our goals."

This work is numbingly tedious, but has the potential for great things. Namely the MIRAI, a new type of FCV. Throughout its development process, there have been changes to both how the vehicle is used, and to its specifications. Having these base parameters for performance simulations lets the team rapidly respond to these changes.

"During the development process, at first we strive to improve efficiency, however part way through, we also set our sights on guaranteeing quality. As an example, reducing the gap with the rotor reduces leakage losses, however this wears away the surface coating of the rotor, and this residue can end up entering the fuel cell. This could result in lower output from the fuel cell, and so it is important to investigate this from both perspectives—efficiency and ensuring quality."

Performance simulations proved very helpful in this work. These can also rapidly respond to changes in air compressor specifications, and are a powerful tool in helping propose those that are optimal.

Mr. Shiromaru opined, "Usage methods change, and keeping pace with these changes was difficult, but using these simulations to meet customer needs, and utilizing this technology to make better cars has been a great motivation."

Their efforts resulted in contributing to product value, giving the vehicle 650 km range.

Fulfillment in Other Than Development

From the start of development, Mr. Shiromaru has been engaged in various discussions regarding methods for prototyping, machining, and inspection, and he commented about working in this development project.

"I learned so much from this. As well as development work, of course, having design study meetings together with the Production Engineering Department with a view to the assembly process was also important. Engaging in discussions with departments with extensive experience in production was a new experience for me. I rapidly became aware of the importance of collaboration with other departments."

Furthermore, this product is an essential part of the vehicle's drive system, and this department had not handled this before. If the air compressor stops working, then the vehicle will no longer move. With this in mind, discussions were carried out with the Quality Assurance Department as to what kind of quality assurance system was required. Development of this part proved highly stimulating for Mr. Shiromaru in many ways, even outside of the actual development process itself.