3D-Printing Wankel Engine From Mazda’s Beloved “Rotary Rocket”

Although there was briefly a company called Rotary Rocket, the term is much better known as a nickname for the Mazda RX-7 — one of the few cars that used a Wankel, or rotary, engine. If you ever wondered how these worked, why not print a model? That’s what [Engineering Explained] did. They printed a 1/3 scale model and made a video explaining and demonstrating its operation. The model itself was from Thingiverse, created by [EricThePoolBoy].

One thing we really liked about the model was the use of lights to show the different stages of combustion. Cool air intake is a blue light, hot air is red, and so on. It really helps visualize what’s happening. You can watch the video below.

If you haven’t seen a Wankel before, it is a clever design. It has very few moving parts and offers very smooth power transfer and high power to weight ratio. The downside, though, is that the engine deliberately burns oil to lubricate and seal, so it is difficult to meet emission standards and requires a lot of oil. The fuel efficiency of current designs is not very good either, especially since manufacturers will often trade fuel efficiency for better emissions.

If you’d like to read more about the Wankel, check out our earlier post (and the 165 comments attached). We also looked at — or rather through — another Wankel earlier this year.

30 thoughts on “3D-Printing Wankel Engine From Mazda’s Beloved “Rotary Rocket”

  1. always wanted an rx7 when I was growing up. that was one of the cool cars to have, back in the 70’s and 80’s.

    I didn’t know that its that bad for emissions. we really do need more choices than pistons and e-cars. shame those wankels never really caught on.

    1. I don’t think we do need another ICE, it’s been perfected for a long time. Many would say that reliability has gone backwards as emissions standards forced redesigns to ecu control, overhead cams, timing belts, catalytic/dpf filters. If we had a non polluting motor then efficiency wouldn’t need to be mandated and vehicles could be used long term instead of being scrapped at 100K like the japanese legislate.

      1. Hence the drive towards EV. Still the range issue needs to be worked out, but hybrids stand a better chance, than an ICE needing to meet all needs instead of a narrower, “charge battery”.

        1. Not all of us have a place to plug in an EV, or live in an area with charging stations. I think that ICE powered cars will probably be around in some form longer than is currently predicted, as the infrastructure for electric vehicles is far from complete.

          1. heh, I work for an EV company and they have free chargers out front for the employees, but I don’t own a house and I’m not ready to go EV until I can home-charge.

            many of us are stuck like this.

            ICE is not going anywhere, anytime soon. sadly, but for realistic reasons.

        2. Range isn’t a fundamental problem with EVs. The fundamental problem with EVs is that using them for road trips won’t scale.

          I’ve driven EVs for 5 years now, and for 99% of our needs, they’re fine. Where they fall apart is taking a road trip longer than the one-way range of the vehicle. This necessitates charging en-route, and that’s where the scaling problem rears its ugly head.

          Filling a gas tank is an energy transfer of somewhere of around 6-10 MW. And that’s *one* pump. A station on the interstate is going to have perhaps a dozen of them. There’s no way that an equivalent throughput can happen unless each station comes with a nuclear power plant.

          “But wait,” I hear you cry, “EVs are 3 times more efficient than regular cars!” Fine. You can call the problem a 1-3 MW problem instead of a 6-10 MW one. Tesla Superchargers are right now at a tenth of that level of power. And when you start ramping up to the MW power transfer level, you gotta wonder whether the batteries will be able to keep up.

          So you make up for it in time. You’re there for 30-60 minutes instead of 5-10. But realize that that means the throughput is going to bog down and there will be lines. It’s better if there is a major city en route, so there are more opportunities to charge, but right now the only EV that can go from Northern California to Southern is a Tesla, and I gotta wonder how bad the backups are Thanksgiving weekend at Harris Ranch and Tejon Ranch. I don’t see how an all electric transportation system will be fundamentally capable of allowing people the freedom of individual travel we’ve become accustomed to since Horatio Nelson Jackson took his famous ride.

          As for me? I like EVs because they’re hella-fun to drive will all that low-end torque, and they need very, very little maintenance until it’s time to replace the battery pack (which is not my problem because I lease).

        1. What? Like a steam car? I’m here for it, that sounds awesome, but for some reason I don’t see any serious developments coming up in that field.

          Ever see that English guy with his steam bicycle? It’s pretty great.

    2. Manufacturing a car creates as much carbon as the vehicle itself will create during it’s lifetime. Wankel engines have thousands fewer parts. Wankel engines might offset their operating emissions by the simple fact that emissions were not generated by production and transportation of thousands of other parts which they don’t have.

      1. It’s not just about carbon (CO2) emissions; these engine have some inherent flaws, mostly total-loss lubrication and a highly sub-optimal shape of the combustion chamber. This causes harmful emissions like carbon-monoxide, unburnt fractions of fuel, and nitrous oxides. Depending on the type of oil and additives used, other harmful compounds can be emitted, just like two-stroke engines typically do.

        Also, a car with a Wankel engine will get about 50-75% the fuel efficiency of a conventional internal combustion engine car, while the engine itself is only a small part of the whole car, which means the emissions saved by using a lighter engine are quickly offset.

      2. The concern when we’re talking about burning oil is not carbon emissions, but particulates and other nastiness that make smog– making urban life unpleasant and wrecking peoples’ lungs.

    3. Back in the day… … …I was ‘loaned’ a ’93 RX-7 for about 2 and a half years, part of my maintenance regimen was to take it up the highway and red-line it ‘to blow the carbon out…

      One of the failures of the emissions standards is that they really don’t take overall efficiency into account… yeah… by taking a general sample for composition… not the cleanest emissions, but when factored with fuel in-power out taken into consideration.

      I think it’s amazing that all these years later people are building monster 4-rotor engines (stock is a 2-rotor) on YouTube.

  2. “The downside, though, is that the engine deliberately burns oil to lubricate and seal, so it is difficult to meet emission standards and requires a lot of oil. ”

    Seals have always been the issue from side to tip. That’s why all the alternative designs. Although one would think that piston rings would leak some too.

    1. Piston rings do leak. However, in a properly-maintained piston engine the leakage around the piston rings is small compared to that of a typical eccentric rotary engine.

      By the way, that’s one heck of a model.

  3. Had a 1986 RX-7 with the new improved rotor tip seals and it was still crap. Mine blew at around 60K miles, at which point I had to replace the engine. While waiting at the dealer for the new engine to arrive, the radio/CD player, amplifiers, and sub woofers were stolen. The rotary engine I had was not really very powerful (no turbo on it), had lousy low end torque, and poor gas mileage. The rest of the car was great, being the best handling car I have ever owned. Oh – I had a plastic model of the engine that I purchased and glued together. It has an electric motor in it so you could see how the engine worked. It was pretty cool.

    As I recall, the tachometer in the car gave the RPMs of the gears that the rotors drove, not the RPMs of the rotors in the engine. The rotors were actually turning at a much lower rate. So much for being a high RPM motor. And it was always confusing to me how the rotors made those central gears/drive shaft turn, because the rotors rotate around those gears. Weird.

  4. Great model, and possibly the best description and illustration of how the Wankel engine worked.

    That LED indication of the stages is genius. For bonus marks, there should be a flash when the spark plugs fire

  5. After going to wikipedia for a little bit: I knew the RX-7 was rotary, I did NOT know it was a ‘Kei car’. A Miata is already pretty crazy, how bad was the thrust to weight ratio on this thing?!?

    1. There is a video on YouTube highlighting the one left-hand-drive SpecA and they mention MazdaUSA explicitly telling them that the car doesn’t have any kind of stability or traction control and that if they are midturn when the second turbo spools it will go sideways real quick.

      I always smile a bit at that part when I’m watching.

  6. The Wankel rotary engine has an interesting characteristic in that the rotor sweeps the mixture past the spark plug(s). This means that it has/had great potential to be used as a “stratified charge” engine where just enough fuel is injected directly into the cylinders as combustion takes place to maintain a stationary flame front. That prevents pre-ignition/detonation as there is no unburned air/fuel mixture in the combustion chamber. This allows much higher compression ratios and efficiency. I always felt that a turbocharged, stratified charge, ceramic coated (to help address the innefficiencies due to the large surface area of the combustion chamber) Wankel engine could be a real competitor to conventional ICEs but I suspect that the opportunity has passed now.

    1. I got the Renwal motorised visible Wankel engine for my brithday, back in the mid 70s: https://picclick.com/Vintage-Renwal-The-Visible-WANKEL-Operating-Rotary-Engine-372437824762.html
      It had little rice grain (or whatever they are called) lightbulbs for the sparkplugs.
      I never really thought it was an exciting kit to build or operate, the Airfix James Bond Aston Martin DB-5 with the ejector seat and rotating number plates was far more interesting to build. My Airfix(?) Scharnhorst had a bolt for ballast weight taped to the bottom of the hull and was subsequently blown up with crackers whilst sailing across a friend’s swimming pool. It sank in about a second or two, not very realistic.
      Wish I still had (any of) them now :)

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