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Driven: Mazda’s range-extending rotary

Rotary club: Mazda’s compact rotary-powered range-extender provides a surprisingly compact recharging unit for future generations of electric vehicles that could spin off from the Mazda2 EV.

The rotary engine could live on as a flex-fuel generator for Mazda’s electric cars


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25 Nov 2013


ASK Mazda about the long-term future of its rotary engine, and the answer has always been vague.

Short of consigning the fuel-thirsty, emissions-heavy engine to the automotive scrap heap, the Japanese car-maker has consistently said the Wankel-cycle rotary still has a place in its future.

What has never been clear is how it will play a part in Mazda’s future. However, GoAuto’s quick drive of a prototype vehicle that may one day be earmarked for production suggests it will live on, just not as we remember it.

Mazda’s future application for the rotary relies on our fear of the unknown.

The roll-out of battery-electric vehicles throughout the world has given rise to a new phenomenon known as range anxiety, where electric car drivers worry about how far their batteries will carry them, and fret that they will run out of juice short of their destination.

Takashi Suzuki, Mazda’s program manager for advanced powertrains, said that since the launch of a battery-powered version of the Mazda2 in Japan last year, seven out of 10 customers said they were either “dissatisfied, or very dissatisfied”, with the car’s 200km range, despite the availability of quick recharging services.

“That is a problem with many EV users today,” Mr Suzuki said. “We needed to solve the problem from the automotive side we could either increase the amount of battery storage, or think about range extenders.” Those fears have already spurred a generation of long-range plug-in hybrid electric vehicles such as BMW’s forthcoming i8 sports car and Mitsubishi’s Outlander soft-roader, and the Holden Volt hatchback, as well as range-extended electric vehicles as BMW’s i3 city car.

The idea for cars such as BMW’s i3 is to add a small petrol motor that acts as a generator, recharging batteries on the fly to double the range of the vehicle compared with just battery power alone.

While BMW has fitted the i3 with a motorcycle engine, Mazda has taken a less conventional approach – fitting a compact, horizontally mounted rotary engine to a generator that can sandwich into a tight space.

According to Mazda, the beauty of the 330cc rotary engine, aside from its compact form, is the engine’s ability to run on a number of different fuels such as butane and propane, the key ingredients in LPG, and not just petrol. It also has fewer moving parts, making it cheaper to manufacture than a conventional engine.

Pointing more towards its future application, the car-maker has run a fleet of rotary-engined RX-8 sports coupes for almost a decade using hydrogen as a fuel, and producing only water as a by-product.

Now, it has expanded the technology to a one-off Mazda2 concept car called the TPEV.

Mazda has taken a plug-in battery electric version of its city-friendly Mazda2, hooked the rotary engine up to a 20kW alternator, and hung it off the chassis where the fuel tank would normally sit.

The idea is that the plug-in Mazda2 will run just like an electric vehicle until the batteries fall to a certain level of charge. Once that threshold is reached, the rotary engine will fire up to recharge them, doubling the range of the Mazda2 to about 400km.

In theory, that’s not all that fuel efficient. The current car has a nine-litre fuel tank, which means the average fuel economy over the 400km range is an impressive 2.25L/100km – although nowhere near the Holden Volt’s official 1.2L/100km average.

However, this is a prototype engine, and Mazda says the production version of the range-extended Mazda2 would need to either match or better the BMW i3’s 0.6L/100km average.

To look at, the range-extended Mazda2 is surprisingly normal. It sits on tiny 14-inch alloys clad in Yokohama Aspec rubber.

Instead of a fuel flap for the petrol tank down the rear, the Mazda2 EV features a flap on the front guard that opens up to a standard electric vehicle recharging plug.

Down the rear is a bit different, though. Instead of a factory diffuser below the rear bumper, engineers have patched on a low-hanging black plastic panel that features a block of small holes drilled out of it to provide ventilation for the rear-mounted recharging unit.

Jump in behind the wheel, and once again there’s much that is familiar about the Mazda2. Apart from a big, red button marked “EMERGENCY” mounted high on the dash, it looks like any other road-going car.

For testing purposes, Mazda has set up the rotary recharger to kick in at speeds above 10kmh – handy given that our test loop is two laps of a 100-metre length of narrow tarmac squeezed in between a building and a chainlink fence.

Setting the Mazda2 in motion is the same as any other car foot on the brake, put the gear selector into drive, release the handbrake and then roll on some throttle.

The joy of electric motors is that they are well suited to accelerating, taking very little time to reach maximum torque. It’s the same in the electric Mazda2.

The car builds speed quickly, reaching 10kmh within seconds. There’s a slight shudder, and then a noise like a leaf blower from down the back of the Mazda2 as the rotary-engined generator fires into life.

It’s a bit of a surprise, because Mr Suzuki had told us the rotary engine was an ideal candidate for the role based on its traditionally quiet, smooth operation.

According to Mr Suzuki, the electric motor only needs to use about 80kW of electric power when accelerating. However, at other times the motor was using far less power, so all the range extender had to do was provide the average range of power, about 20kW.

To account for the system’s baseline need, the 330cc rotary produces 22kW at 4500rpm, stepping up to 28kW at 6000rpm.

Because the engine is mounted horizontally in the 100kg recharging unit, Mazda was able to use softer mounts to isolate more noise and vibration from the cabin.

Accelerating in the Mazda2, once the shudder of the rotary’s start-up has gone, the range extender’s operation is smooth to the point of unnoticable.

However, instead of the noise from the EV’s normal operation disguising the noise of the generator unit, as Mr Suzuki suggested it would, the rotary provides a constant drone akin to a leaf blower.

Slow down again, and as the speed drops below 10kmh the rotary engine cuts out, producing another slight shudder as the batteries become the only source of power for the motor mounted between the front wheels.

Mazda’s plans for the range-extender run beyond just its automotive application. Mr Suzuki said the unit could become a source of electricity in the event of a natural disaster, and could even make its way into commercial portable power generators thanks to its compact form that gives huge weight advantages over existing units.

Even better, Mr Suzuki said because the range extender was a bolt-on unit that fed to the batteries via a couple of cables, it would be possible to retrofit it to existing Mazda2 EVs.

Mazda is well aware that the rotary engine celebrates the 50th anniversary of its introduction of the technology under the Japanese car-maker’s bonnets.

Mr Suzuki admits the rotary range extender is still years away from production, even if it ticks all the boxes in terms of being fit for purpose.

Mazda may be right in saying that it has not killed off its rotary engine. The telling point, though, is that it is unlikely to reappear under the bonnet of future Mazda-badged models, but rather as the flexible-fuel saviour of electric vehicles.

It’s one way of future-proofing almost half a century of development.

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