What better automotive adventures than to spend the last days of the year behind the wheel of a Toyota Mirai? None, we would say. After having enjoyed the smooth and refined qualities of this Mirai – which is Japanese (未来) for future- we are truly convinced that Toyota has fully succeeded to build a thoroughly stunning, reliable, useable fuel cell vehicle, which offers everyday practicality, comfort and a unique driving experience. In this first part, we show and explain its stunning technology.
Hans Knol ten Bensel
Indeed, we have become totally addicted to how Toyota’s electrified vehicles drive and handle. Their refined smoothness is something which grows on you, and gives a unique “zen” quality to your driving experience. As you have read in our columns, we have just bought ourselves a Lexus CT 200h, totally smitten as we are by the unique velvety character of its hybrid system.
Here you see the overall positioning of the elements, with the fuel cell stack (in green) tucked under the front seats, and the two yellow hydrogen tanks also clearly visible. Atop the aft hydrogen tanks sits the Ni-MH drive battery, containing 34 cell modules.
But more on this in the second part. Here we lift a further lid on its technology.
The Mirai takes here everything a (giant) step further of course, with its fuel cell system. It is the so-called Toyota Fuel Cell System, which includes proprietary Toyota-developed components, i.e. the fuel cell (FC) stack, FC boost converter, and high-pressure hydrogen tanks. Note here that as of today, Toyota solely owns approximately 5680 hydrogen-fuel-cell-related global patents. Approximately 1970 licenses are related to the fuel cell stack, about 290 to the high-pressure hydrogen tank and about 3350 to fuel cell system control technology. Do we need to say more?
The Power Control Unit (PCU) (see photo above) decides when to use stored energy from the battery or to draw energy directly from the fuel cell stack. It is based on the proven Toyota hybrid PCU found in the Prius…
Of course, the Mirai uses portions of Toyota’s Hybrid Synergy Drive technology including the electric motor, power control unit seen here above and other parts and components from its hybrid vehicles to improve reliability and minimize cost. The hybrid technology is also used to work together with the fuel cell. At low speeds such as city driving, the Fuel Cell Vehicle runs just like any all-electric car by using the energy stored in its battery, which is charged through regenerative braking. At higher speeds, the hydrogen fuel cell alone powers the electric motor. When more power is needed, for example during sudden acceleration, the battery supports the fuel cell system as both work together to provide propulsion.
We explain your here further how it all works. The Power Control Unit (PCU) (see photo above) decides when to use stored energy from the battery or to draw energy directly from the fuel cell stack. The component that optimally controls both fuel cell stack output under various operational conditions and charging and discharging of the drive battery. This is part of what makes Mirai so energy efficient, and is based on the proven Toyota hybrid PCU found in the Prius. You see it when you open the hood, and it sits nicely under the cover with the “fuel cell” lettering.
Then there is the permanent magnet AC synchronous motor, which sits right under the PCU, between the front wheels.
You see it here in the diagram coloured here in green. It has a maximum output of 113 kW (154 DIN-hp) and maximum torque of 335 Nm.
The boost converter, which is a four-phase boost converter, sits behind it, actually almost at the height where the centre console is of the dashboard.
This unit brings voltage to 650 volts. Driving at a higher voltage makes more efficient use of the motor, giving the Mirai a power output equivalent to other hybrids in Toyota’s portfolio. On the photo below, you also see this converter coloured in green, together with the fuel cell stack, which is mounted right behind it, almost under the front seats, we would say. You also see here the yellow hydrogen fuel tanks, of which more later.
This fuel cell stack generates power by combining hydrogen with oxygen from the outside air. This is why the Mirai has such large air intakes in the front grille, as it needs this air for the chemical process of combining hydrogen and oxygen, which generates electricity, and emits as by-product only… water!
This is Toyota’s first mass-production fuel cell, featuring a compact size and top level output density. It is a polymer electrolyte fuel cell, and has a volume to power density of 3.1 kW/L (which is world top level…) Its maximum output is 114 kW (155 DIN hp) Its humidification system is provided by an internal circulation system. The number of cells is 370, and cell thickness is 1.34mm. The whole unit weighs only 56 kg, and has a volume of 37 L. The picture here shows this fuel cell stack, which you also saw above in green.
Beneath the rear seat and behind it are the hydrogen tanks. These hydrogen tanks are produced in-house and specially designed for Mirai. Toyota’s history in the loom-weaving business helped their engineers design the carbon-fiber weaving of these tanks, improving production efficiencies and helping to optimize the weight to-storage ratio.
The nominal working pressure is a high pressure level of 70 MPa (700 bar). The tank internal volume is 122.4 L (front tank: 60.0 L / rear tank: 62.4 L).
Last but not least there is the nickel-metal hydride (Ni-MH) battery, shown here on the photo below .
It is, as the diagram pictures show, positioned behind the rear seat above the second hydrogen tank, and has 34 cell modules. Its power output is 244.8 V (7.2 V x 34 cell modules).
So much for the technical side of the drivetrain, which gives the Mirai quite zesty, refined and silent performance. 0 tà 100 kmh takes 9,6 seconds, top speed is 175 km/h. But more about driving the Mirai in part 2…
Hans Knol ten Bensel