# Could Wankel engines make a comeback?
**URL:** https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526
**Category:** In My Humble Opinion
**Created:** [October 30, 2021, 3:43pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526 "2021-10-30T15:43:57Z")
**Posts on this page:** 15
**Page:** 1
### Author:  [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)
#### Post date: [October 30, 2021, 3:43pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/1 "2021-10-30T15:43:57Z")
This article popped up on Facebook today.
> **[Why the Rotary Engine Is Perfect for Hydrogen Fuel](https://www.roadandtrack.com/new-cars/car-technology/a25684786/how-wankel-rotary-hydrogen-engine-works-mazda-rx-8/)**
>
> The Wankel rotary has a lot of drawbacks as a gasoline engine, but they become benefits when you switch to hydrogen fuel, as Mazda did with a limited-production RX-8.
Unfortunately it seems the information is in a video, and I’m not in the mood to watch videos. So I found this article:
> **[Why Rotary Engine Is Perfect For Hydrogen Fuel?](https://carfromjapan.com/article/car-maintenance/rotary-engine/)**
>
> The rotary engine has the power to produce rotary motion through the revolving parts that allow this process to happen. Experts say that rotary engine has too many drawbacks, but when one switches to the hydrogen fuel, this drawback turns into a...
Not much more information, but the two articles indicate that Wankel engines are ‘perfect’ for running on hydrogen. The second article says the car has a 100 km range on H, and can switch to gasoline. Would it be possible or practical to store H in a car such that it has a 300 to 400 mile range? (i.e., Could enough hydrogen be stored for the range, such that the car isn’t completely filled with hydrogen tanks?) Obviously, we don’t have hydrogen stations. Would propane work?
---
### Author:  [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)
#### Post date: [October 30, 2021, 4:09pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/2 "2021-10-30T16:09:53Z")
Wankel engines are very efficient at burning gaseous fuels like hydrogen (as opposed to atomized liquid fuels). However, hydrogen is a terrible transportation fuel for many reasons including its very low mass energy density, tendency to leak from even the tiniest gap, causing embrittlement with many alloys, and the fact that most hydrogen production is from coal gasification and steam reforming of natural gas which is actually less efficient and a bigger carbon footprint than just burning the gas directly for thermal energy. Hydrogen is really a storage medium and for fixed installations using excess electricity from some sustainable and renewable source like solar or wind it could be produced by electrolysis (the efficiency is not great but if solar panels are cheap it may still be preferable from a carbon emission standpoint) it may be viable it is very poorly suited for mobile applications.
Stranger
---
### Author:  [@bump](https://boards.straightdope.com/u/bump)
#### Post date: [October 30, 2021, 4:57pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/3 "2021-10-30T16:57:52Z")
Yeah, hydrogen is something that only starts to make sense as a motor fuel at the point when we already have very abundant clean energy from some other source- likely nuclear or something along those lines. Otherwise, it’s almost always cleaner to just use the fuel directly if it’s natural gas or petroleum.
As best I can tell, the reason hydrogen (or any gas really) works well in Wankel engines is because metering it into an air-fuel mixture is easier, it burns faster, and emissions aren’t so much of an issue with hydrogen. All of those things are issues with using gasoline in Wankel engines, and hydrogen obviates them all.
But that doesn’t mean that it’s necessarily any sort of wonder technology; AFAIK natural gas would work the same way as hydrogen in a Wankel engine, and we don’t see a lot of Wankel-powered natural gas generators, cars, etc… even though natural gas fuel is commonly used in things like municipal buses and stationary generators.
Maybe if we start shifting to hydrogen as an energy carrier (i.e. carrying energy from some other process like nuclear or solar), then Wankel engines might make some kind of sense, but I’m inclined to think that if they’re not already getting serious use or scrutiny in the existing gas fuel applications, it’s unlikely to happen just because hydrogen becomes popular.
---
### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [October 30, 2021, 5:12pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/4 "2021-10-30T17:12:27Z")
[Here](https://www.liquidpiston.com/how-it-works) is a new engine in development by a company called LiquidPiston. They claim it’s not a Wankel. I don’t know much about these things, but it sorta looks like a Wankel to me.
---
### Author:  [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)
#### Post date: [October 30, 2021, 5:34pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/5 "2021-10-30T17:34:48Z")
> [@bump](#):
>
> Maybe if we start shifting to hydrogen as an energy carrier (i.e. carrying energy from some other process like nuclear or solar), then Wankel engines might make some kind of sense, but I’m inclined to think that if they’re not already getting serious use or scrutiny in the existing gas fuel applications, it’s unlikely to happen just because hydrogen becomes popular.
If you actually had some cost- and energy-efficient way to produce hydrogen, and some way to store it with enough density for long-distance transportation applications, it would almost certainly make more sense to use it in a fuel cell application with an electric motor than in a thermodynamic heat engine running at variable RPM. Wankel engines “make sense” in the the context of high specific output but they are still limited in terms of thermal efficiency and a relatively narrow operating band where high specific power is available, notwithstanding the material issues that have always plagued rotary engines at the edge seals anyway.
> [@Crafter\_Man](#):
>
> [Here](https://www.liquidpiston.com/how-it-works) is a new engine in development by a company called LiquidPiston. They claim it’s not a Wankel. I don’t know much about these things, but it sorta looks like a Wankel to me.
It is correct that it is a rotary engine but not strictly a Wankel cycle; the exhaust is vented into a chamber into the piston which alters the cycle giving a longer expansion duration, similar to an Atkinson cycle in reciprocating piston engines. How this achieves “increase efficiency by 30% thanks to the company’s patented thermodynamic cycle” I cannot tell since they don’t offer any figures (there is a P-V diagram [here](https://www.liquidpiston.com/hehc) but no scale on the axes) but they seem to be making the comparison to a standard Otto cycle. FWIW I don’t think you can actually patent a “thermodynamic cycle” since that is not a human construct but rather basic principles of physics but you could patent a specific type of mechanism that produces that cycle.
Stranger
---
### Author:  [@Mallard](https://boards.straightdope.com/u/Mallard)
#### Post date: [October 30, 2021, 8:57pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/6 "2021-10-30T20:57:17Z")
The rotary wankel engines were going to be used by GM in the mid 70s until they determined they were gas guzzling oil burners and that project came to a quick end.
---
### Author:  [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)
#### Post date: [October 30, 2021, 9:38pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/7 "2021-10-30T21:38:55Z")
So what I gather is this: If it were more efficient to produce hydrogen, and if hydrogen could be compressed densely enough to give a decent range, then… yeah, it would be viable. But it’s not efficient to produce hydrogen, and it can’t be compressed enough. And even then, it would be better to use it in a fuel cell.
So off on a tangent. As a teen, a friend had a clapped-out '70s-vintage Mazda. It had a problem and needed the ‘thermal reactor’ replaced. (IIRC, it was something like $300… for a teen… in the early-'80s.)
1. What is a ‘thermal reactor’ on a Mazda?
2. How cool is it that my friend’s car had a Thermal Reactor? Sounded very science fictional.
---
### Author:  [@Mallard](https://boards.straightdope.com/u/Mallard)
#### Post date: [October 30, 2021, 9:55pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/8 "2021-10-30T21:55:47Z")
It sits right beside the carbon-tredder which rotates on the reamestran bushings. 🤔
---
### Author:  [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)
#### Post date: [October 30, 2021, 10:11pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/9 "2021-10-30T22:11:50Z")
> [@Mallard](#):
>
> It sits right beside the carbon-tredder which rotates on the reamestran bushings.
I believe it is actually attached to the para-orthogonal lattice aligner and connected to the therimin junction.
Seriously, it is a primitive type of catalytic converter that is supposed to ensure more complete combustion of exhaust products. Apparently not available anymore and you have to retrofit a proper cat converter for replacement.
Stranger
---
### Author:  [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)
#### Post date: [October 30, 2021, 10:25pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/10 "2021-10-30T22:25:32Z")
But you have to remove the turboencabulator to get to it.
[ ](https://www.youtube.com/watch?v=61R2P20Ca2U)
---
### Author:  [@Mr\_Downtown](https://boards.straightdope.com/u/Mr_Downtown)
#### Post date: [October 31, 2021, 4:00am UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/11 "2021-10-31T04:00:40Z")
Hmmmmmmmmmmmmmmmm.
---
### Author:  [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)
#### Post date: [November 1, 2021, 2:38pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/12 "2021-11-01T14:38:31Z")
My understanding is that the major problem with Wankel engines was that they were extremely rough on the seals between the three sections, which abraded very rapidly and lead to incomplete combustion and lower efficiency.
Does using hydrogen instead somehow avoid this problem, or have tougher seal materials been developed since the 1970s or something?
---
### Author:  [@Do\_Not\_Taunt](https://boards.straightdope.com/u/Do_Not_Taunt)
#### Post date: [November 1, 2021, 3:35pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/13 "2021-11-01T15:35:18Z")
> [@Stranger\_On\_A\_Train](#):
>
> hydrogen is a terrible transportation fuel for many reasons including its very low mass energy density
Surely you must mean its very low _volumetric_ energy density? On a mass-weighted basis, the energy density of hydrogen is quite good (around 120 MJ/kg, compared with 44 MJ/kg for gasoline.)
---
### Author:  [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)
#### Post date: [November 1, 2021, 4:29pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/14 "2021-11-01T16:29:11Z")
> [@Do\_Not\_Taunt](#):
>
> Surely you must mean its very low _volumetric_ energy density? On a mass-weighted basis, the energy density of hydrogen is quite good (around 120 MJ/kg, compared with 44 MJ/kg for gasoline.)
Yes, you are correct, my error. Even under cryogenic liquification (less than 33 K) the volumetric energy density is 2.36 kWh/l; by comparison, gasoline at STP is 8.76 kWh/l. Even theoretical metal hydride storage only gets diatomic hydrogen to 3.18 kWh/l at the expense of an almost two order of magnitude decrease in mass energy density.
Stranger
---
### Author:  [@RitterSport](https://boards.straightdope.com/u/RitterSport)
#### Post date: [November 1, 2021, 5:54pm UTC](https://boards.straightdope.com/t/could-wankel-engines-make-a-comeback/953526/15 "2021-11-01T17:54:49Z")
> [@Johnny\_L.A](#):
>
> If it were more efficient to produce hydrogen, and if hydrogen could be compressed densely enough to give a decent range, then… yeah, it would be viable
Not really. Hydrogen car engines today convert the hydrogen directly to electricity for use in electric motors.
Also, see here:
> [@Stranger\_On\_A\_Train](#):
>
> …it would almost certainly make more sense to use it in a fuel cell application with an electric motor than in a thermodynamic heat engine running at variable RPM.