4 ms·

Haven't rotary (Wankel?) engines proven to be unreliable? I love the RX-7 (my dream is to own a FD one day), but IIRC even the more refined Renesis engine on t

Haven't rotary (Wankel?) engines proven to be unreliable? I love the RX-7 (my dream is to own a FD one day), but IIRC even the more refined Renesis engine on the RX-8 seems to have been proven to be ornery. I think Mazda has a rotary acting as a range extender in their MX-30 EV. I wonder how that will fare.

ilyt 3y ago
Kinda, kinda not, they have been proven to be vulnerable to mishandling. They have few quirks and require a bit different handling than normal engine, which means that someone treating it as any other car. But at some point it will need to have engine out job and replace apex seals. In motorsports it proven to be very reliable for example. > IIRC even the more refined Renesis engine on the RX-8 seems to have been proven to be ornery. Renesis was evolution in performance but IIRC had some quirks that weren't solved because the car stopped being produced. IIRC the modern apex seals with modern materials do a pretty good job. It also have oil metering pump which just...stole the engine oil and put it into fuel (which was required to oil the seals), which means if you were not on top of filling oil up, you could end up starving the engine. It wasn't a good idea either as 2 stroke oil was better for that use case, so many people removed the metering pump and just added some 2 stroke to the fuel tank every time they filled up
- 20after4 3y ago
  The oil metering pump was a simple mechanical device on Mazda Wankel engines up to ~1991 (the first and second generation rx-7, and I presume other models in Japan.) This was replaced by an electronically controlled pump on all the later models. The advantage of electronic control is that it conserves oil at low RPM. Unfortunately, the reliability of the pump (and the wiring) became a problem. With the RX-8, they had to dial back the oil supply to meet modern emissions regulations and this lead to premature wear to a lot of engines. I believe this contributed heavily to dwindling sales and ultimately to the decision to discontinue the RX-8 in 2012.
  - ilyt 3y ago
    Yeah, I wonder how they deal with it in the liquid piston engine, emission control gotta be harder with no valves that can be controlled.
bluGill 3y ago
They were easy to modify for more power, but you trade reliability when you do that. People who ran them stock and followed the maintenance plan generally found them reliable.
20after4 3y ago
What killed the rx-8 is emissions. Wankel engines are very similar to 2-cycle piston engines. Both 2-cycle and Wankel burn oil in order to lubricate their seals. Very difficult to do that and also meet current emissions standards. The second generation NA 13b engine was probably the most reliable / longest lasting wankel. Easily could go 200k miles without a rebuild. The Rx-8 rarely makes it past 150k due to insufficient lubrication which was required to meet ~2010 emissions standards. Source: Experience. I've owned five rx-7s and two rx-8s, maintained them myself for the past 23 years.
- post_break 3y ago
  "Source: Experience. I've owned five rx-7s and two rx-8s, maintained them myself for the past 23 years." Would you describe the past 23 years as self torture or did you actually enjoy the process? The only rotary I'd want is some ridiculous 4 router putting down enough horsepower to make me wet myself, and even then I'd probably sell it before I'd need to do maintenance on it.
  - 20after4 3y ago
    My 2nd gen RX-7s were a joy to own and not at all difficult to maintain. They lack all creature comforts and practicality, they are loud, not fuel efficient but a lot of fun to drive. The RX-8 is much more refined and actually quite practical except for the even worse fuel efficiency - 16-18mpg just doesn't make sense when my Hyundai gets 45-52mpg.
MilStdJunkie 3y ago
Wankels . . are a sort of Lovecraftian fever dream that seems to set in upon the brow of mech eng greybeards every now and again. There you are, comfortable mid-career at LockBoNorthRay, but then you have to go open some weird geometry called NECROWANKELCOM from a Forbidden part of the vault and before you know it you're ranting about how four cycle engines are a Jewish plot. Ok, hyperbole, a bit. But for reals, check out the man himself. It's pretty wild. From an engineering perspective, there's some hard limits. Heat is going to always be a problem for Wankels due to the low thermal mass of the thing, and, of course, the fact that there's always a power cycle. The actual rotational efficiency is pretty bad, combustion impulse goes towards the center of rotation, some of it goes backwards. Compression is inherently limited from the necessities of spinning a triangle in an circle. It's nearly impossible to lube the thing effectively; early on they tried to intermix oil and fuel only to create a high-revving Soviet-era ecological disaster because Wankels tend to spit fuel before it's done combusting. I love, love, love cutting Gordian Knots like this, which makes me want to love the Wankel, but at the same time, sometimes Gordian Knots are tied for a reason. High flow, high RPM, low weight, yes all super awesome, but . . honestly, that's called a turboshaft. I dig this Wankel Rebooty from the Liquid guys, but they also seem to have killed the one solid advantage of the rotary design: its low weight. The thing's a brick. Also, that wobble it has when the rotor engages, do you see the back and forth? That's some weird oscillating vibration there, and I couldn't help noticing what looks like some wicked counterweights. Of course that could be some other doodad I'm ignorant of.

ilyt 3y ago

Kinda, kinda not, they have been proven to be vulnerable to mishandling. They have few quirks and require a bit different handling than normal engine, which means that someone treating it as any other car. But at some point it will need to have engine out job and replace apex seals. In motorsports it proven to be very reliable for example. > IIRC even the more refined Renesis engine on the RX-8 seems to have been proven to be ornery. Renesis was evolution in performance but IIRC had some quirks that weren't solved because the car stopped being produced. IIRC the modern apex seals with modern materials do a pretty good job. It also have oil metering pump which just...stole the engine oil and put it into fuel (which was required to oil the seals), which means if you were not on top of filling oil up, you could end up starving the engine. It wasn't a good idea either as 2 stroke oil was better for that use case, so many people removed the metering pump and just added some 2 stroke to the fuel tank every time they filled up

20after4 3y ago
The oil metering pump was a simple mechanical device on Mazda Wankel engines up to ~1991 (the first and second generation rx-7, and I presume other models in Japan.) This was replaced by an electronically controlled pump on all the later models. The advantage of electronic control is that it conserves oil at low RPM. Unfortunately, the reliability of the pump (and the wiring) became a problem. With the RX-8, they had to dial back the oil supply to meet modern emissions regulations and this lead to premature wear to a lot of engines. I believe this contributed heavily to dwindling sales and ultimately to the decision to discontinue the RX-8 in 2012.
- ilyt 3y ago
  Yeah, I wonder how they deal with it in the liquid piston engine, emission control gotta be harder with no valves that can be controlled.

bluGill 3y ago

They were easy to modify for more power, but you trade reliability when you do that. People who ran them stock and followed the maintenance plan generally found them reliable.

20after4 3y ago

What killed the rx-8 is emissions. Wankel engines are very similar to 2-cycle piston engines. Both 2-cycle and Wankel burn oil in order to lubricate their seals. Very difficult to do that and also meet current emissions standards. The second generation NA 13b engine was probably the most reliable / longest lasting wankel. Easily could go 200k miles without a rebuild. The Rx-8 rarely makes it past 150k due to insufficient lubrication which was required to meet ~2010 emissions standards. Source: Experience. I've owned five rx-7s and two rx-8s, maintained them myself for the past 23 years.

post_break 3y ago
"Source: Experience. I've owned five rx-7s and two rx-8s, maintained them myself for the past 23 years." Would you describe the past 23 years as self torture or did you actually enjoy the process? The only rotary I'd want is some ridiculous 4 router putting down enough horsepower to make me wet myself, and even then I'd probably sell it before I'd need to do maintenance on it.
- 20after4 3y ago
  My 2nd gen RX-7s were a joy to own and not at all difficult to maintain. They lack all creature comforts and practicality, they are loud, not fuel efficient but a lot of fun to drive. The RX-8 is much more refined and actually quite practical except for the even worse fuel efficiency - 16-18mpg just doesn't make sense when my Hyundai gets 45-52mpg.

MilStdJunkie 3y ago

Wankels . . are a sort of Lovecraftian fever dream that seems to set in upon the brow of mech eng greybeards every now and again. There you are, comfortable mid-career at LockBoNorthRay, but then you have to go open some weird geometry called NECROWANKELCOM from a Forbidden part of the vault and before you know it you're ranting about how four cycle engines are a Jewish plot. Ok, hyperbole, a bit. But for reals, check out the man himself. It's pretty wild. From an engineering perspective, there's some hard limits. Heat is going to always be a problem for Wankels due to the low thermal mass of the thing, and, of course, the fact that there's always a power cycle. The actual rotational efficiency is pretty bad, combustion impulse goes towards the center of rotation, some of it goes backwards. Compression is inherently limited from the necessities of spinning a triangle in an circle. It's nearly impossible to lube the thing effectively; early on they tried to intermix oil and fuel only to create a high-revving Soviet-era ecological disaster because Wankels tend to spit fuel before it's done combusting. I love, love, love cutting Gordian Knots like this, which makes me want to love the Wankel, but at the same time, sometimes Gordian Knots are tied for a reason. High flow, high RPM, low weight, yes all super awesome, but . . honestly, that's called a turboshaft. I dig this Wankel Rebooty from the Liquid guys, but they also seem to have killed the one solid advantage of the rotary design: its low weight. The thing's a brick. Also, that wobble it has when the rotor engages, do you see the back and forth? That's some weird oscillating vibration there, and I couldn't help noticing what looks like some wicked counterweights. Of course that could be some other doodad I'm ignorant of.