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Old 10-08-2020, 10:26 PM   #1 (permalink)
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Mazda Rotary Range Extender Confirmed

https://www.motor1.com/news/447897/m...n=RSS-all-news

This is pretty interesting. The old Renesis had a best BSFC around 270g/kWh (~31% thermal efficiency), which really isn't worth the space savings of a piston engine.

I've read a bunch of Paul Moller's research papers and it seems that by far the biggest problem with a rotary is unburned fuel. Charge cooling the rotor (aka a warm air intake) and increased rotor temperature dramatically reduce unburned fuel and bring the efficiency more in line with piston engines, but decrease torque (which isn't a problem on a range extender).

Additionally, the oil cooled rotor on the Renesis produced a lot of windage, so roller bearings on the crankshaft can help high rpm efficiency considerably.

I'm guessing this range extender concept probably uses either thermal barrier coatings, a charge cooled rotor, or both, to bring the internal surface temperatures up, and hopefully uses silicon nitride seals to avoid frequent apex seal service. I wonder if customers would fret at having to top up the engine with lubricating oil though.

Additionally, the 16X concept supposedly was all aluminum, having solved the cooling issues, so such an engine would be VERY lightweight (probably <200lbs for a 1.3L 2-rotor configuration, whereas a piston engine producing similar power and torque weighs around 300lbs).

If they can get the thermal efficiency to 36%, that would be pretty impressive. In a smaller car, saving 100+lbs of mass at the engine probably could make up a good chunk of the fuel efficiency gap to a more efficient piston engine. It would make a great motorcycle engine as well.

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Old 10-08-2020, 11:15 PM   #2 (permalink)
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Quote:
...and hopefully uses silicon nitride seals to avoid frequent apex seal service.
My understanding, when rotary engines were introduced was that they had seals that wore excessively but were affordable, else seals that wear well but had unobtanium pricing.

Is silicon nitride expensive?
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Old 10-09-2020, 01:23 AM   #3 (permalink)
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Due to the porting, much like a 2-stroke reciprocating engine, a Wankel has a quite narrow gap on its RPM range where it will operate more efficient. Sometimes I wonder if a power valve system similar to what used to be common on some 2-stroke motorcycle engines are likely to overcome such deficiency, or if Wankel would only make a comeback serving as range-extenders.
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Old 10-09-2020, 03:19 AM   #4 (permalink)
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Originally Posted by freebeard View Post
My understanding, when rotary engines were introduced was that they had seals that wore excessively but were affordable, else seals that wear well but had unobtanium pricing.

Is silicon nitride expensive?
The aftermarket silicon nitride seals for Mazda engines are something like 2400 a set, so yes. I think Mazda would be able to make them cheaper than that though, and I imagine it's possible to produce a plasma spray coated metal part that's much cheaper. Either way, in this day and age, it seems silly to use plain iron apex seals when there are more advanced materials.

As far as I can tell, silicon nitride powder is pretty cheap (you run an arc through silicon powder in nitrogen gas), but sintering it is very difficult. A process that can produce a thick coating on pieces of stainless steel might be very affordable.

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Originally Posted by cRiPpLe_rOoStEr View Post
Due to the porting, much like a 2-stroke reciprocating engine, a Wankel has a quite narrow gap on its RPM range where it will operate more efficient. Sometimes I wonder if a power valve system similar to what used to be common on some 2-stroke motorcycle engines are likely to overcome such deficiency, or if Wankel would only make a comeback serving as range-extenders.
The porting doesn't make it like a 2 stroke (which needs exhaust scavenging), it makes it like a motorcycle engine (4 stroke but no VVT). I believe the Renesis had 3 different intake ports to increase torque in different parts of the rev range, but fundamentally, a Wankel engine is happier at high speeds due to its high heat conduction losses.

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Old 10-09-2020, 10:43 AM   #5 (permalink)
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I'll give it to them. Mazda is not afraid to crash and burn with these odd/bold engineering feats they have been trying to achieve on their cars since they got away from Ford. I agree with their core idea that hybrid cars will be the future instead of pure BEVs, but I have my doubts on this. With that said I wouldn't hold your breath. Mazda can be a little bit too optimistic on their engineering ideas and time to market.
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Old 10-13-2020, 11:18 PM   #6 (permalink)
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Originally Posted by serialk11r View Post
The porting doesn't make it like a 2 stroke (which needs exhaust scavenging), it makes it like a motorcycle engine (4 stroke but no VVT).
I didn't refer to the need for scavenging, I refer to it being more comfortable at a narrower RPM range. When it comes to VVT on motorcycles, IIRC there are some Honda models featuring it, including the JDM 4-cyl CB 400.
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Old 10-14-2020, 07:51 AM   #7 (permalink)
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The relative limit on compression ratio of past Wankel engines has been a problem. But modern technologies should be able to address them:
  • cooled exhaust recirculation - reduces combusion temperature which reduces the NOx problem. Possibly use electrostatic separator.
  • exhaust oil scavaging - a vortex chamber should recover a substantial part of the exhausted oil. A small, electrostatic separator might also help.
  • hybrid - use electric to cover the power ranges that are efficiency death to the Wankel or any ICE.

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Old 10-14-2020, 08:47 AM   #8 (permalink)
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It think the main problems with Wankel engines go away when you just use them as range extenders.

Emissions: You never need to run them in a city.
Maintenance: A range extender in a 200km electric car might not reach 1000h of operation during the cars lifetime.
Efficiency: Not as important, see above.

That said, could a $200 piston engine last 1000h producing 15hp?
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Old 10-15-2020, 02:06 AM   #9 (permalink)
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Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
I didn't refer to the need for scavenging, I refer to it being more comfortable at a narrower RPM range. When it comes to VVT on motorcycles, IIRC there are some Honda models featuring it, including the JDM 4-cyl CB 400.
It's still a 4 stroke, so the main issue is heat conduction losses as it breathes pretty efficiently; it's pretty happy >3000rpm. You can mess with the intake side to get a slightly wider powerband, but at least it's not like a 2 stroke where volumetric efficiency takes a nosedive outside a narrow range.
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Old 10-15-2020, 02:15 AM   #10 (permalink)
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Originally Posted by jakobnev View Post
It think the main problems with Wankel engines go away when you just use them as range extenders.

Maintenance: A range extender in a 200km electric car might not reach 1000h of operation during the cars lifetime.
This is a good point I haven't thought of. If the range extender is only expected to run 1000h, then the seals and bearings don't need to last very long, making it affordable.

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