SAE 2009: FEV proposes Wankel rotary as range extender
As more automakers are working on extended range electric vehicles, the issue of NVH is becoming even more of an issue than in existing parallel hybrid vehicles. With vehicles that operate predominantly in electric mode minimizing the impact of starting up the range extender or auxiliary power unit is critical. Companies like General Motors are working hard to make sure the range extenders operate as quietly and smoothly as possible, but small piston engines inherently have some degree of vibration.
Engineering services company FEV has been studying the problem and has decided to propose an unusual solution. FEV has built an experimental range extended electric version of the Fiat 500 that uses a small Wankel rotary engine as the APU. The Wankel has a number of potential advantages, including much higher power density than piston engines and virtually vibration free operation. The high specific power output means that a very small engine can be used that reduces weight and adds packaging flexibility. The downside is that Wankel's traditionally aren't very fuel efficient. However, FEV feels that most ER-EVs will be driven predominantly in EV mode and the limited use of the range extender will limit the effect of the lower engine efficiency. New Wankel engines can also be made more efficient through the adoption of technology like direct fuel injection. It remains to be seen if anyone will adopt a Wankel range extender in a production vehicle, but it's an interesting idea.
Reader Comments (Page 1 of 1)
Frank Olson 7:17PM (4/22/2009)
The Wankel engine produced by Freedom Motors http://www.freedom-motors.com/ for the Moller Skycar seems like a good fit for an ER-EV.
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Unni 5:34PM (4/22/2009)
Wondering , why not Atkinson cycle rotary engine ? From the above i understand, the problem is the degree of vibration for piston engine.
http://en.wikipedia.org/wiki/Atkinson_cycle
As its Atkinson cycle, it can be tuned for better efficiency and if they use direct injection and other new advanced technologies, the efficiency can be tuned better ( yes i agree the power wont be same as Wankel but for EREV the need is to drive a generator and not wheels), so for me this looks to be a better option.
Wankel is an excellent idea for power with very less moving parts but power is not the question we have.
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DaveB 6:46PM (4/22/2009)
FEV should be checking out the Moller wankel engine. The Moller flying car may never fly but their wankel engine has the size, smoothness, low emissions, AND the fuel economy that makes it the ideal range extender engine. And it runs particularly well on E100. And a compound version of their engine has the potential to be more economical than a modern turbo diesel.
So what's stopping people from using it? Just the classic NIH (Not Invented Here) problem.
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BoneHeadOtto 10:59PM (4/22/2009)
Great idea.. because Wankels are fuel efficient.....Just look at the RX8
What we need for range extenders is the opposing free piston engine. They just oscilate back and forth generating electricity. No conversion to rotation just electricity generation
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Dave 9:21AM (4/23/2009)
I think the idea here is to build a generator that is cheap and lightweight so that manufacturers can more easily provide a longer EV-only range.
A ten or twenty percent loss of efficiency in the generator is not important if it is only used in rare cases.
BoneHeadOtto 9:30AM (4/23/2009)
Here is the engine that should be used instead. They have very few moving parts and are far more efficient than standard gas engines when generating electricity. They would be cheap, but are long (but very narrow). I fully expect to see these used exclusively as range extenders. They are by far teh best option available now.
http://www.treehugger.com/files/2008/10/free-piston-engines-efficient-plug-in-hybrid-cars-electric.php
randomdude 2:47PM (4/23/2009)
The engine was chosen, as written in the article, due to NVH reasons and space needed.
- It's build into a Fiat 500: after adding the Li-Ion Battery there isn't too much space left and this engine is very small (http://tinyurl.com/cnouwq).
- It's a range extender: it could be quite annoying to hear (or feel) it after it was started.
randomdude 3:01PM (4/23/2009)
The engine was chosen, as written in the article, due to NVH reasons and space needed.
- It's build into a Fiat 500: after adding the Li-Ion Battery there isn't too much space left and this engine is very small (http://tinyurl.com/cnouwq).
- It's a range extender: it could be quite annoying to hear (or feel) it after it was started.
Nateb123 1:12AM (4/23/2009)
Good idea. Agreed that an Atkinson cycle Wankel would make more sense. A 2-rotor is still going to be like a V6 tuned for emissions with a larger housing 1-rotor (ie half the Mazda 16x) being equivalent to lots of I4 motors now. Could someone please point me towards a published paper explaining why the Moller engine is so great? The Freedom Motors site has crap all and I'm starting to smell BS. Kind of like everything to do with the skycar and other scams for investor money...
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Kevin 3:45AM (4/23/2009)
Freedom motors has been out for a while already. Their engines do cost a bit more but they've already been installed in a number of applications ranging from jet-ski's, motorcycles, cars, aircraft. Next year it's looking like they will have applications in the trucking industry.
Moller only owns the tech to freedom motors. You can order the motors today. But from freedom, not Moller.
Nateb123 11:43AM (4/23/2009)
Well it's nice to know that they work but that doesn't really answer my question. Where's the proof of concept? Hell, what IS the concept behind these motors that seem to have magically solved the emissions problems of the wankel rotary engine? If it weren't a load of crap, I imagine Mazda would be copying them and launching nearly their entire line with Wankel engine options like in 70s. There's a lot I know about Wankel engine design that's telling me their numbers aren't as great as the singular emissions graph on their site might claim.
Chris M 3:43AM (4/24/2009)
Atkinson-Miller cycle achieves higher efficiency by making the exhaust stroke effectively longer than the intake stroke, extracting more energy from the expanding hot gas. The downside is somewhat less torque.
Atkinson-Miller cycle would be relatively easy to implement in a Wankel engine, just a matter of altering the intake ports and slightly modifying the rotor.
Ian 10:01AM (4/23/2009)
I have been psuhing for this a lon g time. Makes sense as noted. I also note that present wankel engines in Mazdas are only heavy users of gasoline IF they are reved highly. Of course in the RX8 this car begs for the driver to use those revs. As an aucuxiliary power unit a rotary engine would cruise at lower revs and therefore be very fuel efficient. It's also easily converted to CNG. Next generation mazda rtary engines are expected to increase fuel efficiency by up to another 15-20%.
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Nateb123 11:49AM (4/23/2009)
@Ian
Sorry but Wankels aren't fuel efficient at any revs. Don't know where you got that idea but a 4 stroke piston engine still can be made to knock the pants off of a rotary emissions wise. Surface area to volume ratio is too big for the wankel and the lack of a 3rd spark plug per housing makes it even dirtier. Drive it as conservatively as you like, it won't make a difference.
Ian 1:12PM (4/23/2009)
@Nateb
Sorry but you are wrong and as an 11 year (current) owner of a 1993 RX7 twin turbo I think I have some insight!
Users of rotary engined sports cars find them so smooth and the rev range so enormous and torque comes in later that they tend to delight in using the entire power band. Read high revs7-8.5K!+ Plus note I say that a rotary can easily be tuned to accept CNG which cuts emmisions by a clear 50% easily over gasoline. Then you have the low cost of producing the engine, the fewer parts, the reduced weight and size ...... If a rotary engine is used either as a back up, or preferably at a constant speed say 3-3.5K rpm to produce electricity to recharge the batteries on the move, it is an IDEAL solution and relatively low emmissions. The next generation mazda rotary will make even more progress.
Nateb123 1:38AM (4/24/2009)
I've got plenty of experience too. Got an FC sitting in my parking space, a chemistry degree and a mechanical engineering degree. Rotaries are my life. Read the SAE paper on the Renesis for insight into actual BSFC numbers. Emissions can of course be cleaned up (I'm sure you've heard of a rat's nest after owning an FD) but a thirsty engine isn't the greatest idea for a car that has no room for a big fuel tank.
Chris M 3:38AM (4/24/2009)
I'd have to agree that Wankels are not as efficient as pistons, it has to do with the odd shape of the combustion chamber, leading to too much heat loss through the combustion chamber walls. For that reason, my first impulse on hearing that idea was a negative, but then I realized that, when used as a "range extender for a plug-in, it wouldn't be used often the advantages of small size and light weight could easily overrule the disadvantage of reduced effic andiency.
Ian 11:59AM (4/24/2009)
Chris gets in and Nateb does not...
The idea IS to use a small, light weight, relatively inexpensive engine as an auxiliary, NOT primary power unit. In these circumstances the smooth rotary IS ideal.
It can be used a LOW rpm (read also good mpg) in a car powered 100% by electrical motors as an efficient means to re-charge the batteries while driving and thereby extend mileage greatly. i.e. the rotary combustion engines does not work 100% of the time and when it does it does so at only medium rpms. Alternatively if it's a hybrid drive the rotary can kick in to assist the electrical motor. This will still give good gas mileage, just not as good as running only electical motors as in the first example. The reason being that more rpm from the roatry engine will be used to maximise on torque. But it's still a good overall part time power solution given all its inherent advantages.
Alternatively a reasonably small CNG tank can be used as the secondary fuel base as rotary engines are easily configurable to run on alternative fuels such as cng, hydrogen etc. cng would further reduce gas emmissions versus a regular gasoline fuel by around 50+%.
Overall I stand by my contention that the rotary engine is an ideal secondary engine for forthcoming electrical cars. Far better than a regular combustion piston engine. Research on the new mazda rotary engine which will replace the current genesis version also shows further strong improvement in efficiency.
Nateb123 11:28AM (4/25/2009)
I see where you're coming now however I think you've simply decided if you throw all the enviromental tech you've heard of into one car it'll makes it some super-eco car. It just doesn't work that way and I'll show you what I mean. Rest assured it takes MUCH more refinement in clean technologies than we have now.
First, a rotary means an airpump, emissions valve system, reactive exhaust manifold and multiple cats. Lots of weight. Also the rotary engine that you and I have in our cars suffers from the aforementioned surface area to volume ratio being very big. As you increase displacement, this problem goes away (as in the 16x) but if you decrease it you get a thinner, lower-volume combustion chamber. It's not just that a flame has to travel through this now tiny, twisting combustion chamber which means less torque either. It also means tons of the fuel sticks to the working chamber walls meaning its not atomized and it doesn't burn well. Direct injection will help a bit but only at low RPM so you're forced to run it where there is very little power.
So now we have significantly less torque, lower RPM and worse emissions than a 13b and you want to throw in a CNG tank. Now you've added weight of a hefty fuel tank. Using CNG will be further decreasing torque since it requires a higher compression ratio like diesel to make full use of the fuel's energy. You're also getting less than your claimed 50% emissions decrease (CNG doesn't burn THAT cleanly but okay) because CNG requires a higher compression ratio which just can't be done without turbocharging (more weight). You're now at much TONS less torque (due to low revs) and proportionally the same emissions as a 13b. You've also cut into your range significantly with all this inefficient burning of fuel and using gas instead of liquid. And on top of all that, you'll be constantly topping up oil.
It's not a winning combo mate. If someone redesigns the apex seals to be much stouter then turbocharging could really be used. You could make fantastically powerful and efficient diesel (and other alternative fueled) rotaries but they would still need to be big. Think trucks with rotaries. Small cars, not so much. Might as well just put in a two stroke.
Fred Nurk 4:08AM (4/27/2009)
Sorry, Ian, but I'm on Nateb123's side on the Wankel question.
Let me throw this one in:
What about some kind of tiny little gas turbine directly driving a generator? I remember that back in the 1960s gas turbines were the future, but the future never arrived. I think it might have been that the revs were so high and the gearing would have to be so tall that changing speed would be accompanied by an unacceptably long lag. For a range extender, this wouldn't be an issue. Actually, this would be a lot similar to the APU on a jet aircraft, wouldn't it?
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