Could Plug-in Hybrids Revive the Rotary Engine? Consider Audi A1 e-tron.

By · November 15, 2011

The Audi A1, the smallest model wearing the Audi badge, was unveiled last year at the Geneva motor show. As you would expect, the manufacturer had many models on its stand. There were production models, and two prototypes: one as a sports version, the other a plug-in hybrid. Not many people looked at it back then, but we should now because Audi has built 20 of them. The Audi A1 e-tron is still far from a production model but at least it has left the shop floor. Those 20 cars are now part of a pilot electric mobility project in Munich.

The unveiling of the Audi A1 e-tron plug-in hybrid in Geneva

The unveiling of the Audi A1 e-tron plug-in hybrid in Geneva

The definition of the car is quite strange. They could have done it as a pure electric car, but they didn't. It would have been the easy choice to use a 20-kWh battery pack, but the German engineers chose a 12-kWh pack with a small internal combustion engine behind it. That brought a huge problem since the A1 is a small car, only 155 inches long. The Chevy Volt is a packaging marvel with both its electric motor and internal combustion engine (ICE) under its front hood, but it was impossible to use that same architecture in the A1. The engineers have designed an electric propulsion system to power the front wheels, and it fits nicely under the hood, but there is just not enough free space there to fit an ICE on its top or on the side. It has to be elsewhere, with the only possible place below the trunk's floor. No production engine from the Volkswagen group's part bin could fit there, so the engineers had to design a new engine specifically for this purpose. That was undoubtedly a tough decision to make considering the necessary budget. It would have been so much cheaper to make a pure EV.

The Audi A1 e-tron plug-in hybrid with a rotary ICE

The Audi A1 e-tron plug-in hybrid with a rotary ICE

The engineers kept on their plug-in hybrid idea, and they didn't hesitate between a diesel or a conventional Otto-cycle ICE. They chose a rotary. You may know those engines from Mazda. They were originally a German-design, from Felix Wankel. Every manufacturer tried the technology in the 1970s, even GM made a Corvette prototype with a rotary, but Mazda was the only one to build them in significant numbers. Compared to a conventional ICE, a rotary shares some virtues with an electric motor. It makes much less vibration, and it can rev at higher speeds, but its poor fuel economy has prevented it from going mainstream. This may change and the rotary may see a revival as a range-extender—because it has a strong advantage of a very compact size. Viewed from the top, the rotary inside the A1 e-tron has about the same dimensions as a laptop computer. That's with the generator mounted at its end, which provides 15-kW at 5,000 rpm. It doesn't rev any higher to maintain decent fuel economy, though we don't have much information about that.

An Audi A1 e-tron plug-in hybrid charging in Munich

An Audi A1 e-tron plug-in hybrid charging in Munich

It will come next year after 12 months of testing. Right now the driving and charging habits of all 20 drivers are precisely monitored, with Audi explaining that the 12-kWh should give a 50-km (31-mile) range. The motor makes 75 kW, allowing 0-to-62-mph accelerations in 10.2 seconds. We note that Audi doesn't overestimate the car's range on electric power—31 miles from a 12-kWh battery seems very realistic. Some drivers may get more from it. More important, the test will show if the rotary has some future as a range-extender. We know now that the fuel economy of the Fisker Karma isn't very good with its ICE running, and the Chevrolet Volt is only average there. Maybe a a low-rev rotary get better results.


· Anonymous (not verified) · 6 years ago

It's a series hybrid, like the Volt (much of the time, when the engine is running) and the Karma (all the time the engine is on), so its fuel consumption is bound to be relatively poor. Series hybrids are at their most effective in city driving, but the battery of a plug-in does the city thing nicely, leaving the engine to power highway driving inefficiently. Conclusion: plug-in hybrids should be parallel hybrids, for most applications. Watch the market reach the same conclusion, painfully, over several years.

· Kei Jidosha (not verified) · 6 years ago

Audi says the A1 e-tron returns a cruising equivalent of 124 mpg on the European cycle. A worthy competitor for the Lotus and BMW range extenders, I’d like to see this unit in other platforms. It's compact displacement and packaging looks like a winner in the RE-EV segment. Until battery technology improves, this is a prudent answer for the impulsive, impatient, or occasional exception to a routine commute. A fleet of 20 A1 e-Trons is about to hit the streets of Munich for evaluation. Let the market decide what is the best solution.

· · 6 years ago

It is good to see someone do something different with the range extension engine again. In earlier days of plug-in hybrid development, such as the 1980's through the 1990's all kinds of engine types were looked at. The interesting thing about a series hybrid is that you can have the engine sit at a narrow range of RPM and that RPM can be where ever the engine is most efficient. This opens the door to engine types that are very efficient but don't have a big RPM range. Most of those engine types are rotory in nature but not necessarily Wankle. Many of them can handle a variety of fuels.

But when GM and Fisker went into production both where in a hurry and so put in engines that where already well developed for car use (both technically and in part supply logistics, etc.).

It is true that any time energy is converted from one form to another that something is lost. And it is true that parallel hybrids convert forms less and thus win there. But being able to use a more efficient engine can more than make up for this.

Plus, we have to step back and look at where we are trying to go with plug-in hybrids. It is not just about the oil that isn't burned today, although that is worthwhile. It is about going somewhere where we won't need to burn any oil at all. Bridging us to the day that we have better batteries that can handle everyone's duty cycle every day. And Serial hybrids do that best by allowing the car to be as much of an EV as possible.

Having said all of that, I am also for parallel hybrids, high MPG ICE and anything else that helps.

· Francois B. (not verified) · 6 years ago

As said in the main article, rotary engine did not go mainstream because of their high fuel consumption. It seems a contradiction to install this kind of engine into a plugin car, as this will impact negatively on the fuel consumption, when not in electric mode.

I would rather guess that this car is a concept car, and will probably never reach mass production, because of such a design flaw.

They rather could go with a 2 cylinder diesel, or direct injection 3 cylinders to furnish the range extender generator.


· jim1961 (not verified) · 6 years ago

The Toyota Prius Atkinson cycle ICE has a thermal efficiency of 38%. Toyota engineers are targeting 45% thermal efficiency in their next generation of hybrids.

A rotary would seem to be a step backward.

· jim1961 (not verified) · 6 years ago

This may sound trollish of me but I love to come to this website and point out the anti-Volt bias. Sometimes the bias is quite subtle.

"...the fuel economy of the Fisker Karma isn't very good with its ICE running, and the Chevrolet Volt is only average there..."

Average compared to what?

TrueCar says that U.S. consumers purchased vehicles that averaged 22.0 miles per gallon in September 2011, compared with 21.7 mpg in August and 21.4 mpg in September 2010.

· · 6 years ago

A serial / series hybrid has several advantages that can improve the efficiency:

* The engine only needs to meet the *average* torque needed over time, since the battery is being used as a buffer. This means the displacement can be quite a bit smaller than if the engine had to be the prime mover of the vehicle. A smaller displacement engine has several advantages: it weighs less, and the cooling system is smaller, the fuel tank is smaller, and it will warm up more quickly.

* It doesn't need a transmission, which saves the friction losses. On a ICE powered vehicle, the transmission means by definition that it only can run at it's best RPM part of the time.

* The engine can be run at it's best RPM all the time, and the load is fixed, so the intake and the exhaust can be optimized for that RPM. Most modern engines have variable valve timing and 3-5 valves per cylinder. A simple 2 valve design can be used on a serial / series hybrid.

* Since the engine is only running part of the time (roughly 1/3rd of the drive time), the cooling system can be closed off, which improves the aerodynamic drag of the vehicle. This can also be used to shorten the warm up time to reach the optimum efficiency more quickly.

* The engine doesn't need to be located so it can be mechanically connected to the wheels. And so it can be packed more efficiently.

The Volt is *not* a serial / series hybrid, any more than the Prius is a serial /series hybrid. They both can run in a series mode, a combined mode, and they both can have the engine directly drive the wheels -- and they both have very complicated transmissions. The Volt has *three* clutches! And the Volt varies the RPM of the engine, and it uses a much larger displacement engine than is required.

All the current hybrids are combined-mode hybrids. We do not have serial or parallel (only) hybrids. The GM Precept was a parallel hybrid:

The Urbee is a parallel hybrid:

My issues with parallel hybrids is that for a typical trip that starts slow, then is at highway speeds, and then ends slow -- it cannot use electricity at the end if it depleted the battery at the beginning. Also, there is only one speed possible for optimum engine operation, and if the battery is depleted, then you must have a multi-gear transmission to finish the trip AND the engine must be large enough to move the car in all situations by itself.

I think that serial /series hybrids have the best efficiency overall -- like 2-3X better. A plugin electric completely trounces all other drivetrains, and adding a serial genset adds the range as efficiently as possible.

Sincerely, Neil

· Charles (not verified) · 6 years ago

Rotary offers a few advantages other than power to weight ratio. It can run on a lot of different fuels. Mazda has been running some on H2 (not that H2 is a viable fuel source). Other Wankel manufactures have concentrated on fuel efficiency and get near Otto cycle economy. The Mazda RX-8 and Nissan 350-Z have about the same MPG (19-20) and about the same 0-62 times (6.0-5.8). So the I do not think the MPG difference is as large as it was in the past.

The interesting thing for Audi to do would be a natural gas option.

· · 6 years ago

One of the points I was trying to make in my comment earlier is that there are many engine technologies around that have been rejected in the past for use in a car because they did not have the required RPM opperating range or because their RPM or torque ranges were too far off to be competitively geared to a drivetrain. But some of these engine types have much higher efficiencies than the types of engines used in cars today.

In a serial hybrid we could and should see these engine types come into use. The reason why we didn't see this right away is that it takes time to get such an engine ready for production use both technically and logistically.

But sitting at a limited RPM range is a game changer for some of these engine types.

And the Wankel engine cycle is just one of many of engine cycles that are "rotory" in nature.

Some of these other engines are rotory, some are piston, some are different enough to be in their own categories. Some are multi-cycle combustion engines with regeneration, a concept that is commonly used in stationary generation to achieve vey high efficiency. The use of computer modeling of combustion processes has allowed researchers to explore numerous new engine configurations. And to optimize those configurations faster than ever before.

We shouldn't accept that a random ICE optimized for a certain set of requirements is the optimal for a completely different and much more lax set of requirements.

· · 6 years ago

The 124 mpg rating from Audi is correct, but there are many ways to measure the fuel economy of a plug-in hybrid. I remember GM once said the Volt would get 230 mpg.

· Anonymous (not verified) · 6 years ago

@ Charles,

There are many drawbacks of Rotary Engine (R.E.) by Mazda. Some of those drawbacks are design related. I'm not applying the rule of generalization here as I don't know anything about the German's rotary engines, but here are some of the issues that Mazda has faced but has never been able to overcome :

(1) R.E. are high maintenance engines. The recommended oil "add" interval on Mazda was (if I remember correctly), every 3 gas fill ups. R.E. burns engine oil at a rate much, MUCH quicker than the "regular" engines.
(2) R.E. are NOT fuel efficient at all. The examples you showed were great illustration. 350Z has a high output 3500cc V6 engine. RX-8 used a 1300cc Wenkel engine. Despite the huge difference in displacement, as well as huge difference in hp (more than 100 hp), fuel economy of the 2 engines were almost identical! However, if you compare a regular engine - a 3500cc V6 from the 350Z to a 1600 I4 of a Nissan Versa, the displacement and hp differences have huge impact to fuel economy.
(3) R.E. need high rpm to get power. RX-8 was a very sluggish car. Thus, in order to achieve quick launch time, it has to rev to a very high rpm at the starting line (much higher than, say, 350Z). This, of course, causes 2 problems: Heat - enhance problem in (1) and engine longetivity, and Noise. Notice how the article is reporting 5000 rpm to generate 15kW of energy. 5000 rpm is "high."

For a R.E. to generate power in the case of Audi (if it uses the same type of rotary engine as Mazda, hence facing same problems above), the advantage of small size/light weight will then be sacrificed by these problem.

· · 6 years ago

@ Anonymous - I have been to the factory floor where the Mazda rotorary engines are made and I am reasonably familiar with them. In my view the main reason for the disadvantages of the currently used Mazda rototary engines is the springloaded metal seal against a metal plate approach that they use. The friction requires lubrication and makes noise and robs efficiency.

If Audi has a different method of dealing with the sealing issue then they might do better in those areas. I don't know if they do or not. Certainly other approaches are possible.

As a side note, despite the comment I made above about the metal seal, the people I have interacted with at Mazda in Japan have a bold rebellious streak to them that would be very useful in developing new types of EVs and plug-ins if the Mazda management would turn them loose with that kind of assignment.

· · 6 years ago

It will be good to get clarification of the 124mpg number -- if it is the mileage during charging mode, they may be counting all of the distance driven on the charge in the battery from the gen set. Or, it might be an average -- they could use a spread sheet like the X-Prize to account for both the charge in the battery from plugging it in, as well as fuel burned.

It would not be surprising if this is a "real" number that includes the electricity appropriately. Remember, the Illuminati 7 got an EPA Combined number of 207MPGe and the Edison2 VLCe got 245MPGe.


· · 6 years ago

While I do appreciate you're efforts to maintain balance against GM and Volt bashers, even GM admits that the ICE in the Volt is just an off-the-shelf beast that was not optimized for efficiency in the regime that it operates in.
I look forward to companies spending some effort to optimize their ICEs for PHEVs in the future. I think we could see some good stuff. I, too, wonder if the rotary might be ok for the constant RPM, constant load operation that a PHEV requires. Does anyone know how much of Mazda's extra lube oil consumption was caused by blow-by as the Wankel worked hard during acceleration? I've heard Rotaries make good stationary engines but am not an expert in ICEs.

Its too bad GM spent all of their money optimizing the Volt's new transmission to make it and the ICE indispensable instead of working on making the engine better.

· · 6 years ago

I know that the variable peak pressure in the combustion chamber (due do variable RPM) makes the oil sealing issue a tougher problem. Also, there are many design elements where compromises are made to be able to service the whole RPM range. Such as how the intakes are designed to be able to accomidate the low RPM region. Even if you took this one engine design (the engine in the R-8) and optimized it for only one RPM the fuel efficiency would go up.

· Herm (not verified) · 6 years ago

They chose this particular engine because it was the only one available at that size that could deliver 15kw, plus the low vibration is a plus.. since the operating rpm is fixed the exhaust and intake can be fully tuned for lowest noise.

I think the genset is made by AVL and they claim a fuel consumption of 260 g/kWh

here is an image:

Fuel economy is not that big a deal in a range extender, the whole idea behind an EREV is that you seldom will use the genset anyways, so make it as light and unobtrusive as possible. Eventually you can replace the 140lbs of the genset with extra batteries.

· Anonymous (not verified) · 6 years ago

Herm says that the fuel consumption of the rotary wankel is 260 gram a KW. Lotus says that their range extender gets 240 to 255 grams per KW depending on size.
Is 20 grams a kw that much of a difference between engine types? I would think that the package advantages more than makes up for 20 grams + low vibration.

· Anonymous (not verified) · 6 years ago

I just want to share my input and clarify some facts on rotary engines because there is some misinformation here. I have owned one or more Mazda rotary powered cars for over 20 years now.

First of all I want to touch on the oil consumption "issue" which really isn't an issue at all, it's intentional. All Mazda rotaries have an oil injection system that, through means of a metering pump, injects engine oil into the combustion chamber along with the air/fuel mixture. This is burned during the combustion cycle and it serves to lubricate the internal apex seals. Another interesting point to note about oil here is that the design of rotary housings are such that oil flowing through its channels does a significant amount of engine cooling. The oil system also has one or more oil coolers.
The combustion cycle on the rotary is incomplete, as some combustion occurs in the exhaust (think afterburner) which leads to high exhaust temps, much hotter than piston designs. So this is where the fuel inefficiency comes into play
Yes, rotaries make their power at higher rpm. That's really not a problem if you consider that they are designed to make their power that way. Lets remember that horsepower is a derivative of torque, and can be made through either torque or RPM. We're not talking about hybrid tow trucks here, so I don't understand the importance of torque, or lack thereof, for this application.
5,000 rpm isn't "high" for a rotary. In fact the torque peak of the current RX-8's Renesis engine is @5,500 rpm, while the horsepower peak is at 8,500 rpm.
As the article mentions, the rotary is inherently smoother than piston engines. There is no converting the up & down force of pistons to rotational force. Also the rotary fully utilizes all of its displacement in each cycle. For every turn of the eccentric shaft, there is a different stage of combustion happening on each face of the rotor. So in a 2 rotor application there are 6 power strokes per revolution. Now think about all of that happening at the rate of 9,000 times per minute, with very little vibration felt by the driver. It truly is something to experience, and a must for any driving enthusiast to feel at least once!

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