Mazda plots Wankel-powered range extenders, HCCI mild hybrids

  • 30-Dec-2013 08:10 EST
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Mazda illustrates how its compact 330-cc single-rotor rotary engine (shaded blue here) compares in size to a piston engine of equivalent power.


The problem with electric drive systems is that they are too electrified. That’s Mazda’s view, judging from the company’s plans to fortify internal-combustion technology so that hybrids and EVs can carry smaller electric motors and batteries, making them lighter and cheaper.

For mainstream products, that means piston engines that feature homogeneous-charge compression ignition (HCCI) and adiabatic design.  For EVs, Mazda is eying a compact, quiet, and smooth range extender using the company’s signature Wankel rotary engine to power a small on-board generator.

HCCI and adiabatic development

Today’s Mazda SkyActiv engines employ non-typical compression ratios—14:1 for gasoline engines and the same CR for diesel. This focus on optimal combustion lays the groundwork to move into HCCI, according to Takahisa Sori, Managing Executive Officer for research and development at Mazda Motor Corp.

These engines and their successors will be normally aspirated rather than turbocharged, because normally aspirated engines’ quicker response is better suited to Mazda’s focus on fun-to-drive cars, he explained. And while reduced-displacement turbo engines can achieve high efficiency on standardized tests, such engines commonly disappoint customers with their real-world fuel economy, he pointed out.

Hence Mazda’s continued refinement of normally aspirated internal-combustion engines.  Because IC engines still waste most of their energy in the form of heat, converting more of that to useful energy is Mazda’s focus for future engines, Sori said.

A lean-burning HCCI engine will help achieve that using a compression ratio that is even higher than today’s SkyActiv 14:1 ratio. Diesel-like throttleless intake reduces pumping losses, and the company aims to trim friction losses by 20%. Together these changes will boost fuel economy by 30% over today’s engines.

HCCI engines are limited in the load range in which they can run in HCCI mode, but Mazda expects to expand that range with improvements to the fuel injection system. Meanwhile, matching the HCCI engine to a hybrid electric drivetrain will let the HCCI gas engine run in its most efficient operating range, letting the electric motor assist as needed.

In this de-emphasized role, the electric motor and battery need not be as large as in conventional hybrids, reducing their cost.

These improvements to the engine reduce the energy loss through the exhaust but increase the heat loss to the cooling system. For SkyActiv 3, Mazda is pursuing adiabatic combustion, using insulation and possibly ceramic materials to minimize cooling-related efficiency losses, Sori said. He did not offer a timetable for the delivery of these technologies.

Rotary redux?

Meanwhile, Mazda is pursuing a new way to preserve the relevance of its signature rotary engine, which was dropped from production in late 2011 due to steadily decreasing volumes (in the RX-8 sports car). The rotary has struggled since the 1990s to meet increasingly stringent emission regulations.

“We engineers at Mazda are very proud of the rotary engine,” Sori said.  “Therefore, we have been continuing research and development of the rotary.”

One benefit of Mazda’s six decades of experience with the technology that yields benefits for today’s piston-engine programs is a thorough understanding of combustion. But rather than viewing the engine in romantic terms, Mazda is taking a hard-eyed approach to its potential return to production. “If it will become profitable, we will use it,” Sori said.

One potential application would be to address driver concerns about electric vehicle range. Mazda2 EV customers list driving range as a top concern, and using a rotary-powered range extender would alleviate that, according to Takashi Suzuki, program manager in Mazda’s powertrain development division.

Aside from Mazda’s desire to find a 21st century role for the rotary engine, the design carries other benefits in comparison to a piston engine of equivalent power. For example, radiated noise, measured from 50 cm (7.8 in) with the engine running at 3000 rpm while producing 25 kW, is 87 dB for a rotary, compared to 92 dB for a gas engine and 96 dB for a diesel, noted Suzuki.

A chart of vehicle road and wind noise at speed closely tracks one showing the noise produced by a rotary range extender while making enough power to propel a car at those speeds. This means that an EV can use a pistonless rotary-powered range extender without the driver ever hearing it run, unlike range-extender hybrids such as the Chevrolet Volt and Cadillac ELR that use reciprocating engines to extend their EV range.

A 330-cm3 single-rotor Wankel can produce 22 kW (29.5-hp) at 4500 rpm or 28 kW (37.5-hp) at 6000 rpm, according to the company. Its compact size means it fits easily beneath the rear load floor of a subcompact Mazda2.

Audi touted a similar solution in its A1 e-tron concept of 2011, which used a 15-kW (20-hp) 254-cc engine. The company never developed the concept for production, however, saying it wasn’t cost-effective. Engineering-services companies AVL and FEV also have shown Wankel-based range-extended EV proposals that are under development. 

Mazda’s design makes a key change. Aside from the slightly larger displacement, Mazda’s approach installs the gas engine and generator side by side, with a belt drive geared to double the speed of the generator. Audi’s design stacked the generator atop the rotary engine, with a direct connection driving the generator at engine speed. The faster spinning generator is 5% more efficient than one turning at engine speed, and Mazda’s generator produces a continuous 20 kW (27 hp), according to Suzuki.

Mazda has survived in a market dominated by much larger rivals thanks to its unconventional approaches to design and engineering, so the company has an established track record of bringing unorthodox solutions like these to market.

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