Mazda is bringing back the Wankel - as a range extender in the MX-30 PHEV. We take a detailed look at the new rotary piston engine and explain the technology.
This is a comeback in the style of the house: Mazda, always known for extraordinary approaches in vehicle technology, has brought the rotary piston engine back to the light of day. The engine, developed by German engineer Felix Wankel and built under license by Mazda since 1967, was further developed in Japan to a certain degree of perfection, although the fundamental problems of this technology, including high consumption and poorer thermal efficiency compared to reciprocating engines, remained until the end persisted. The Mazda RX-8 marked this conclusion. As the last Wankel engine model, it was discontinued in 2012.
Ten years later, Mazda makes it clear with the MX-30e-Skyactiv R-EV that the rotary piston concept was only given a creative break at its headquarters in Fuchū, Japan. The engine with the characteristic triangular rotor celebrates its resurrection. However, it is no longer used to drive a car itself, but as a generator of electricity. In combination with a downstream generator and an electric drive, it serves as a so-called range extender.
Newly developed single-disc engine
Mazda relies on a completely newly developed Wankel engine for the MX-30. The machine has a chamber volume of 830 cm3 (hence the internal designation as engine type 8C) and delivers a maximum output of 55 kW/75 hp. For comparison: the double disc engine in the last Mazda RX-8 had a total volume of 1.3 liters with 2 x 654 cm3 chamber volume. The compression of the new engine is at a comparatively high ratio of 11.9:1, the fuel is allocated via direct petrol injection. With exhaust gas recirculation, catalytic converter and particulate filter, the unit meets the Euro 6d-ISC-FCM emissions standard.
Compared to the previous RX-8 13B engine type, the new machine uses aluminum instead of cast iron for the side housing. According to Mazda, the machine is 15 kilograms lighter than the 13B. And above all considerably narrower. The rotor is only 76 millimeters wide, the entire motor housing is only 8.4 centimeters wide. This means that it can be combined with the generator and the e-machine to form a very compact unit.
Engine housing only 84 millimeters wide
The injection technology used in earlier rotary piston engines posed the problem of incomplete combustion of the fuel-air mixture. The portion of the mixture above the spark plug was partly discharged unburned in the exhaust gas. With the new 8C engine, this problem is said to be solved by direct fuel injection.Injection directly into the combustion chamber in combination with a special flow of combustion air, which flows in via two side intake ports, results in the best possible distribution of the fuel-air mixture in the combustion chamber.
In addition, the compression has been increased to further improve efficiency, it is 11.9:1. For comparison, the 13B engine in the Mazda RX-8 had a compression ratio of 9.4:1. Other improvement measures include new sealing strips on the rotor, which are wider and cause less friction loss, and the new sealing rings should also have an improved service life. In addition, the surfaces of the lateral motor housing have been given a ceramic coating, which also reduces friction losses.
Low speed, very smooth running
The new rotary piston engine reaches its maximum output quite early, the 55 kW/75 hp are at 4,700 rpm. The maximum torque of 116 Nm is available at 4,000 rpm. This suggests that the engine, which is already very smooth-running due to the system, should hardly be noticeable inside the vehicle during generator operation. The engine appears as a generator mainly on long-distance journeys. As soon as the capacity of the traction battery falls below 40 percent, the Wankel combustion engine switches on to generate electricity. The battery capacity of 17.8 kWh is quite high for a plug-in hybrid and should enable an electric range of up to 85 kilometers.
That leaves the topic of consumption, which has not always been the prime discipline of Wankel engines. Mazda does not give an exact consumption value for the new 8C machine, and the WLTP consumption (1.0 l + 17.5 kWh /100 km) also gives little indication of the thirst of the 830 cc engine. Rather, one can deduce something from the stated maximum range of the MX-30 with its 55-liter tank. Ideally, Mazda specifies this as "over 600 km". After deducting the ideal electric range, the theory would leave around 515 kilometers in generator mode to empty the 55-liter tank and, accordingly, a consumption of over ten liters/100 km.
With the new MX-30 e-Skyactiv R-EV, Mazda is once again going its own way. We haven't been able to test the new concept of a serial plug-in hybrid with a Wankel engine as a range extender yet. But the pure technology with the extremely compact new rotary engine is definitely very interesting.