The Return … of the Rotary Engine.

More than a decade ago, Audi introduced twenty A1 e-Tron models with a rotary Wankel engine as a range extender at the Geneva International Motor Show.  The circle was finally complete because NSU, the first car company to market a rotary Wankel engined car, had been absorbed by Audi in the 1970s.  The prototypes were driven by all the automotive journalists and the unanimous verdict was that it was ready for production.

But nothing came of it.

Mazda has been talking about a similar application in their MX-30 EV model for several years and promised to debut it last Friday at the Brussels Motor Show – and I almost didn’t believe them.   But it did happen and the rotary Wankel engine is back!

The MX-30 R-EV will have a very small 18 kWh battery pack with only 80km of range but a single rotor 830 cc rotary Wankel engine (8C) weighing only 100 kg and 8 cm thick with a conventionally sized 50 L gas tank giving a further 600 km of range by strictly operating to drive a generator and recharge the battery pack.    The generator can also supply 1500W of pure electricity for those who need power off the grid (camping, natural disasters, power failure)


The rotary engine is direct injected into the center of the combustion chamber with peak power at 4700 rpm and compression ratio of 11.9:1.  Not only is there better atomization of fuel because of the high pressure delivery, Mazda admits that in the past fuel injection into the ports resulted in a lot of the fuel air mixture situated poorly in the posterior region of the combustion chamber and not being combusted.   There are two side housing intake ports, the secondary port can be opened to prolong the intake stroke under high power demand situations.  The location of these ports on the side housing and not the rotor housing confirm that the engine will not be used under high rpm situation but spend its time in the low end and running under conditions of maximum fuel efficiency and minimum emissions production.   Exhaust gas recirculation is used again to reduce thermal efficiency losses due to the inherently large surface area of the rotary’s combustion chamber.    Side housings are now fully aluminum allowing 15 kg of weight savings each and sprayed with a plasma ceramic coating.  Apex seals are now 25% wider at 2.5 mm for better wear resistance.  

The rotary engine eccentric shaft is connected to a generator on one end and a small electric motor on the other end which will deliver positive or negative torque to the spinning rotor.  In effect this will give variable timing to the engine under varying degrees of load to keep those emissions under control.  Rotary engines tend to produce more exhaust hydrocarbons than typical internal combustion engines owing to a long combustion chamber with a relatively large surface area. At low rpms there is positive assist torque (to shorten intake timing) and at high rpms there is negative assist torque (to prolong intake timing).    Interestingly, the actual December 2022 patent application also describes an even more elegant solution to timing change by having the generator control rotary engine rotation speed itself.  By momentarily reducing the electrical load on the generator, the resisting torque of the spinning coil is also momentarily reduced and the rotary engine is allowed to spin faster and causing a shortened intake timing.  However it is unlikely that this will be implemented in production. 

This complexity would not be needed if the rotary could be run at one optimum constant speed but there are many scenarios where the power output of the engine needs to be variable.  Certainly a cold engine at start up should not be run at high rpm and battery charge rate depends on the level of charge, a battery near full charge must be charged slowly.   And when climbing an elevation you want the ability to access maximum rotary engine output to provide additional electrical power above and beyond that provided by the battery to boost the output of the electrical drive motor.

Some automobile critics have scoffed at the reintroduction of the rotary engine.  It introduces complexity in a system that should be simple.  It deprives using the rotary engine in a manner that demonstrates its virtues – the ability to produce prodigious horsepower at extremely high rpm and ear shattering noise as well.  But as someone well acquainted with the Mazda corporation, I clearly understand their goals.

Mazda must remain committed to their path of creating a comprehensive car driving experience to distinguish their products from other companies in order to remain profitable and independent (from corporate takeover).   By minimizing the mass of the battery pack, they can create a lighter car with improved driving dynamics like no other EV.  The use of a rotary engine means a small footprint that can be unobtrusively packaged in the chassis without compromising the cabin experience.  In fact it appears that the synchronous electric motor, generator and rotary engine will be packaged as one inline unit mounted transversely in the front of the car.  Its vibration free operation at low rpm/low noise will be harmonious with the EV experience.  During inner city driving, the car can operate strictly as an EV and plug in your car every night to recharge.   During highway operation when bursts of acceleration are required, the additional electrical power to the drive motor can be supplied by bringing the rotary engine online to generate that power.  And if traveling say from Toronto to NYC, you don’t have to rely on planning your route around charging stations because now you have 600km of range plus instant range restoration at any conventional gas station.  The success of this model will depend on how seamless Mazda has made the transitions between the three different modes of driving.

The MX-30 R-EV will be available in Europe & Japan first and then possibly California.    In North America the priority is the introduction of the new CX-90.   But Mazda has an older relationship with Canada than the US and availability to Canada, if it happens, will be nationwide and MSRP is expected to be $45k.  It will be interesting to see if the rotary engine is used to provide heat in the cold Canadian winter during a cold start to both bring the battery pack to proper operating temperature and to warm the cabin.

I think Mazda may be on the right path.  In Canada, we have the challenge of long distance driving and the winter cold (which reduces range) to contend with EV ownership.  It is nothing for a Canadian to get up and drive 20 hours to Florida from Toronto in one go – I’ve done it many times.  And this trip often occurs in the winter, when the climate in Florida is actually pleasant.  Unless both the supercharging infrastructure and the EV range increase substantially, this is a way of life that cannot be replicated.   Unlike other parts of the developed world, North Americans are uniquely tied to car ownership and the personal freedom that entails.  This is partly a result of geography which influenced the design of our cities and our decision to abandon train development.  Consequently a car is still the best method to access our large cities. 

I used to think hydrogen was a better alternative to rechargeable batteries but I’ve recently learned hydrogen production is currently extracted from methane or natural gas with the problem of CO2 containment that is also produced in the process.  Electrolysis of water would be the best method but most countries do not have green sources of electricity and solar and wind based electricity is not a consistent source to produce the quantities of hydrogen needed to fuel all the future cars.  And the platinum and iridium needed for the proton exchange membranes vital to the fuel cell are even more rare than lithium.   If commercial fusion technology can actually be accomplished in 10 years, that will likely consume all hydrogen production.  Incidentally, rotary engines trump piston engines when burning hydrogen instead of gasoline.  Hydrogen burns very easily and is prone to preignition, which is very damaging to a reciprocating piston but not so much to a unidirectionally rotating rotor.  And the intake phase of a rotary engine is inherently cooler than in a piston engine (as well as longer allowing better fuel air mixing) since intake physically does not occur in the same region as combustion.     Burning hydrogen of course cannot yield CO2  or CO products and unburnt hydrogen doesn’t yield any HC emissions either.  Only some nitrogen oxides result in the emissions. 

Throughout my childhood, I was told that peak global oil production would soon be reached and from then on we would be living on less and less.  Well, that point still has not been reached.  I suspect we will be living with fossil fuels for quite sometime.

Below is a fascinating video showing a transparent single rotor engine running and allowing one to see the actual combustion process and the 4 phases of the Otto cycle taking place.

Addendum:   April 1, 2023.

An exciting new development for rotaryphiles, Mazda has been seen testing its concept RX-GT3 sports car in public.




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