[Eternal Student (OP)]

Hi.

    I'm not an engineer.  My question is:
  Why did the Wankel Rotary Engine disappear?

[Image from  https://en.wikipedia.org/wiki/Wankel_engine]

    It seemed to have a lot in it's favour:   Just one moving part for example.
I've heard there are problems but honestly weren't there as many problems when the standard cylinder engine was first developed?   They were inefficient, friction wore out all the tight gas seals like piston rings quickly, getting the combustion mixture and ignition timing right was a problem so that they were subject to "knocking" that rapidly damaged the engine etc.   
    It took some years to perfect and properly develop the standard cylinder I.C.E.   If the same amount of R&D had been applied to the rotary engine, wouldn't they be excellent by now?

   Just look at the design shown in the image above,  isn't it simple and eliminates almost all moving parts?   There's no valves to bounce up and down, no need for a cam shaft,   the whole thing just goes round and round instead of thumping violently up and down so there's less vibration, no stress on the piston rods or the point where they bear onto the crankshaft.   This makes the whole thing potentially much higher revving than a standard cylinder engine.   It also produces power on every single rotation rather than a typically 4-stroke cylinder engine, so you've got the potential to get more power out of a smaller, lighter engine.

 Anyway... it just seems like a good basic design to me and yet it's barely found in anything these days.

Best Wishes.

[Halc]

If the same amount of R&D had been applied to the rotary engine, wouldn't they be excellent by now?

They're still being made, but apparently the shape of the combustion chamber led to incomplete combustion, costing fuel efficiency. The ICE piston rings were eventually improved enough to maintain seals for the life of the engine, but the seals on a rotary engine were prone to leakage. That's a major area for improvement, but one tends to burn oil quickly until that's solved.

[paul cotter]

The chief difficulty is, as Halc has said, is the seals. On a regular ice the piston rings and cylinder liners are made of cast iron and are very resilient. To give an example of this I recently had to relinquish my 19year old Honda civic with 250,000 miles on the clock due to the inability to source a clutch slave cylinder- I never needed to top up the engine oil. The only way to seal the rotor apex on a Wankel to deal with thermal expansion is to use spring loaded flaps recessed into the apex and even this is suboptimal. The need for lubricating oil in the combustion chamber must lead to degraded emissions though I don't believe I have heard this mentioned. Another problem is heat: in the regular ice the combustion chamber is cooled by a fresh charge of cool air during the induction phase but in the Wankel the combustion chamber receives hot compressed air.

[alancalverd]

Small rotaries are used in drones and some microlight aircraft. I think there is a problem of cooling large ones  but according to Wikipedia there are (or have been ) 4500 hp Wankels.

[Eternal Student (OP)]

Hi.

   Thanks.  That's various bits of information that I didn't know.   Here's some follow up questions.  They're not important and I could probably just spend my own time and Google for answers but someone might already know.

....Another problem is heat: in the regular ice the combustion chamber is cooled by a fresh charge of cool air during the induction phase but in the Wankel the combustion chamber receives hot compressed air....

Question 1:  Has there ever been a diesel version of the rotary engine?   I think diesel burns adequately as soon as it is compressed without needing to be too hot.

.....apparently the shape of the combustion chamber led to incomplete combustion, costing fuel efficiency....

 
Question 2:   Has any mathematician tried to develop new shapes for the rotar and/or the chamber that might work better for purging the gases?

Question 3:   A 2-stroke cylinder engine has similar problems.   Google suggests  2-strokes may be 15% efficient while 4-strokes can reach 25%.  None-the-less, since 2-strokes produce power on every stroke, they still provide a power-to-(engine) weight ratio that can't be beaten by the 4-strokes,  as such they are still very useful for some applications.   (? Pause...  not sure....  I wonder why Formula 1 racing cars don't use 2-stroke engines, perhaps just because they must consider the weight of fuel for multi-lap races... I don't know.)
  Sub-Question 3.i :    This might be an AI problem.     Google's AI is telling me that Wankel rotary engines can reach 30% efficiency - which would make it better than a 4-stroke?   So I have no reliable figures.
     I suspect their efficiency is somewhere between the 2-stroke and the 4-stroke  and the point or question I would be going for is that they only need to be better than the 2-strokes in order to be better for any situation where  power-to-weight ratio is important.   This links to @alancalverd 's comments....

...rotaries are used in drones and some microlight aircraft.....

Question 3 (again):   Is this because rotary engines have a better power-to-weight ratio than anything else?

    Everyone has basically said that rotary engines have problems making good gas seals and this is one of the issues frequently mentioned in some of the articles I've seen about them.  Specifically, they scuff and wear away the metals where the rotar contacts (or almost contacts) the chamber very quickly so that the engine becomes useless after a few thousand miles.
     I understand that, similar to 2-stroke engines, oil is directly sprayed into the chamber with the intake of the fuel-air mixture to provide some lubrication.  So you have direct and unavoidable oil consumption by the engine with corresponding exhaust emissions that will be higher in unburnt hydrocarbons (soot and stuff).

Question 4:   Isn't this where the chemists can step in?   Instead of trying just to prevent or reduce wear of metals, why can't we inject something (better than just oil) that will form a slightly longer lasting thin layer of stuff on the chamber walls all the time and just have this wear away?  I'm thinking of something like those products that were around a few years ago and claimed to build a thin layer of Teflon on the walls of your cylcinders that would last a few thousand miles.  We might also need something in the exhaust to collect and remove this substance rather than just throwing it out with the exhaust and into the environment, we don't want to poision the world.

Best Wishes.

[Halc]

Has there ever been a diesel version of the rotary engine?

The shape of the chamber does not allow sufficient compression, so changes need to be made to overcome this (like supercharging)
There were a few prototypes, but nothing ever put into production.

]Has any mathematician tried to develop new shapes for the rotar and/or the chamber that might work better for purging the gases?

Purging gasses was never a probem AFAIK. Compare this to a 2-stroke which has inherent mixing of new/used gasses

I've never heard of a rotary engine getting as high as 20% efficiency. So citation needed for the 30 thingy.
Race cars need to be at least reasonably fuel efficient, else the pit stops will kill you. Weight perhaps isn't a huge issue since weight=traction.  Heavy car maybe wears the tires more quickly. Dunno, I'm just guessing..

I suspect their efficiency is somewhere between the 2-stroke and the 4-stroke  and the point or question I would be going for is that they only need to be better than the 2-strokes in order to be better for any situation where  power-to-weight ratio is important.

Hah!  Say hello to my Wankel chain saw!

Specifically, they scuff and wear away the metals where the rotar contacts (or almost contacts) the chamber very quickly so that the engine becomes useless after a few thousand miles.

Why don't cylinder walls in ICE have the same problem?  What's different?

I understand that, similar to 2-stroke engines, oil is directly sprayed into the chamber with the intake of the fuel-air mixture to provide some lubrication.

Do they?  I just remember mixing in oil with the gas.

So you have direct and unavoidable oil consumption by the engine with corresponding exhaust emissions that will be higher in unburnt hydrocarbons (soot and stuff).

Also destroying your catalytic converter.

[alancalverd]

[Quot from: alancalverd on Today at 10:40:37
...rotaries are used in drones and some microlight aircraft.....

Question 3 (again):   Is this because rotary engines have a better power-to-weight ratio than anything else?[/quote]

Power/weight ratio is favorable, as is the inherent simplicity of the Wankel, but AFAIK it has never made an impression in the "certified" aircraft market which is exceptionally conservative. Problem is that whereas a car engine cruises at about 10% of its rated power, an aircraft engine is required to deliver 75% power continuously, so wear and cooling are critical considerations.

The question of relative wear rates between piston and Wankel engines hinges on the single point of contact of a Wankel seal, which has to tolerate the combustion temperature and carry out both compression and oil control duties - these functions are shared between two, three or more piston rings in a conventional reciprocating engine, each of which can have an optimal shape and composition for its job.

[paul cotter]

Eternal Student's question #1. A diesel engine is a compression ignition engine as opposed to a petrol(gas) engine which is a spark ignition engine. To achieve quick ignition of the fine spray produced by the injectors the air in the cylinder must be quite hot and this requires a considerably higher compression ratio in the diesel than in a petrol(gas) engine. Roughly speaking the pressure at tdc in a diesel will be approximately twice that of a petrol(gas) engine and assuming adiabatic compression and ideal gas laws it will be twice as hot in a diesel. On the subject of two stroke engines a well designed diesel can perform very well. General Motors' "Detroit Diesel" is a two stroke turbocharged supercharged two stroke engine legendary in the generator business and this machine will go from zero load to full load in one hit, a parameter called load acceptance.

[Eternal Student (OP)]

Hi,

Thanks for all the additional replies.

Why don't cylinder walls in ICE have the same problem (rapid wear)?  What's different?

Exactly the sort of question I would be asking myself.

(i)    @alancalverd 's reply about pistons having multiple rings seems relevant.
(ii)    After a moments thought I would also suspect / speculate on the following:     Reciprocating engines don't actually push the pistons into the chamber walls, they just push them straight up and down the cylinder walls.   They'll wobble about a little but they're not being directly pushed into the walls.   Meanwhile, a look at the animated diagram in post #1 suggests the edges of the rotor (e.g. the point labelled A) are taking a curved and somewhat eccentric  path and there's a lot of  (what we can sloppily call "cenfrifugal") force against the chamber walls.   If the apexes of the rotor have spring loaded flaps (as suggested by @paul cotter ) then the chamber walls are going to be the only thing those flaps can get their centripetal force from.

Best Wishes.

[Halc]

Reciprocating engines don't actually push the pistons into the chamber walls, they just push them straight up and down the cylinder walls.

That can't be right. The connecting rod to the crank pushes at a significant angle, putting a lot of stress on the same side of the cylinder each time (one side on compression stroke, opposite side on power stroke).  The rotary on the other hand need not push at all since its motion is restrained by that internal gear. It is physically incapable of touching the wall, but it stays close the whole way around, only the spring-loaded 'wipers' actually making contact, and without any thrust stress like the ICE.  It seems the rotary has the advantage here. What am I missing?

Perhaps I'm missing the fact that the picture in the OP is 2D. We also have to worry about the rotor being sealed at the sides, not depicted. That's not a problem with the ICE.

and there's a lot of  (what we can sloppily call "cenfrifugal") force against the chamber walls.

Yes, the center of gravity moves around, so you get vibration. Best to have 2 or 3 of them to counterbalance, but one will do if the internal shaft has a counterweight such as those they put on crank shafts.

[paul cotter]

Halc is correct here and a cylinder liner will get worn out-of-round eventually. As regards the apex in the rotary engine even if it does not touch the wall and maintains an infinitesimal separation it will suffer from flame erosion. Off topic, it is such a pleasure to have a sensible productive discourse like this one as opposed to the endless pointless arguments I get dragged into on the wacky side of the forum, ie new theories.

[vhfpmr]

I used to work with a guy who rallied an RX-7, not a particularly easy life for an engine, but I don't recall him saying anything about it being unreliable (unlike the NSU Ro 80).

[Petrochemicals]

My guess is the vector is not as efficient, a right angled tangent to the explosion piston would be n!o good at all.

Halc is correct here and a cylinder liner will get worn out-of-round eventually. As regards the apex. in the rotary engine even if it does not touch the wall and maintains an infinitesimal separation it will suffer from flame erosion. Off topic, it is such a pleasure to have a sensible productive discourse like this one as opposed to the endless pointless arguments I get dragged into on the wacky side of the forum, ie new theories.

This will be exacerbated by the force direction

[Eternal Student (OP)]

Hi.

Thanks for more replies.   I'll write more about some of those when I get a few more minutes.

    I've just done some more reading about Rotary Engines.    Apparently temperature differences are a problem, leading to expansion that causes poor gas seals.

Brief explanation:    Stop looking at the rotor for a moment and just look at the chamber walls,  they just sit still all the time.   The combustion is always happening on the right hand side (in the diagram in post #1).   The differences in temperature between the right and left hand sides of the engine can be 159 deg. C.  This is quite different from a standard reciprocating cylinder engine where the whole cylinder generally changes temperatures uniformly and we can attempt to have pistons, piston rings and engine blocks all with similar expansion.   In the rotary engine, the seals on the right hand side just deteriorate as the engine gets warm and there's no way you can compensate for the expansion of the right vs. the left by changing the shape of the chamber because then the seals would be far too tight when the engine was starting from cold.

     The engine housing has vastly different temperatures in each separate chamber section. The different expansion coefficients of the materials lead to imperfect sealing......

[sub-section "Rotor Sealing" from   https://en.wikipedia.org/wiki/Wankel_engine#Mechanical_disadvantages ]

   At least it seems that some R&D has been done on that issue - some engines have been constructed with heat pipes just to elevate the tempertaure of the left-hand side of the engine (although that had its own problems and lead to decreased efficiency).

Best Wishes.

[Eternal Student (OP)]

Hi.

...The connecting rod to the crank pushes at a significant angle, putting a lot of stress on the same side of the cylinder each time (one side on compression stroke, opposite side on power stroke)...
   [ + supporting comment from @paul cotter ]

    Yes.  I hadn't really thought of the force coming from underneath the piston, only that coming from the combustion and expansion above the piston.

I used to work with a guy who rallied an RX-7, not a particularly easy life for an engine, but I don't recall him saying anything about it being unreliable

     I've never had one but the internet is suggesting that maintenance is a problem.   Chat on the owners and fanatics websites is filled with people who just don't look upon a complete engine rebuild as too much work or expense and still conisder the car to be great.   I suspect your rallying friend is in that category.

   Here's an example of some chat where one person gives 3 options for buying an Mazda RX7, none of which seems to avoid a complete engine rebuild.
     For the engine I'd recommend either buying one which has had a recent rebuild from a reputable dealer or alternatively buy one in the knowledge that depending how long you intend to keep it, at some point you will need to get it rebuilt. Third option is to buy one with a knackered engine for cheap and spend the money you've saved getting it rebuilt properly.

[From:   "Pistonheads",  https://www.pistonheads.com/gassing/topic.asp?h=0&f=71&t=1136259

Best Wishes.

[paul cotter]

I have been doing some reading on the subject myself as my previous knowledge was from a long time ago. It does seem the main pro is a high power to weight ratio and the main con is a short engine life. On all the images I could find there is definitely some device recessed into the apices rather than direct contact.

[Eternal Student (OP)]

Hi again,

    What is the proper name for the dynamics of flame propagation through a (moving) fluid?   I don't know so I'm going to say "Flame Fluid Dynamics".

   Anyway, there are significant problems with the flame fluid dynamics.   Apparently it's best to have the chamber walls moving away from the epicentre where combustion starts, much as in the standard cylinder engine.    Meanwhile, the rotary engine always has one apex that is moving towards that epicentre.

      ....Flame travel occurs almost exclusively in the direction of rotor movement, adding to the poor quenching of the fuel and air mixture, being the main source of unburnt hydrocarbons at high engine speeds: The trailing side of the combustion chamber naturally produces a "squeeze stream" that prevents the flame from reaching the chamber's trailing edge....

[Taken from   https://en.wikipedia.org/wiki/Wankel_engine#Thermodynamic_disadvantages ]

    There was some research done on improving the efficiency of combustion by injecting fuel only towards the leading edge of the rotor and leaving the trailing edge effectively un-used.   However, this reduces the overall internal capacity for combustion to occur and trades the engines main strength  (high power-to-weight ratio) for the improved fuel efficiency.
    There are also some versions of the engine (specifically the Mazda 6B design) that obtained significant improvements in complete combustion throughout the chamber just by using 3 spark plugs well spaced over the ignition side of the engine to create multiple epicentres for ignition.

- - - - - - - - - -
    Anyway, I think I've spent enough time looking at Wankel rotary engines to satisfy my curiousity about why they haven't become more widespread.   I still think many of the problems are solvable and we could have developed an engine that was superior to the 4-stroke reciprocating engine.   For some reason(s) the same amount of research and development that we had for reciprocating engines just wasn't done on the rotary engine.  That may have a human explanation - the consumer and industry are naturally un-willing to take a giant leap backwards to start with.   What the consumer wanted was an engine that was good now not one that was only going to be good in fifty years.   
   But.... I don't know.  It may have been some conspiracy.   Existing I.C.E. manufacturers have connections with the large oil companies and if there's any research they want to suppress then they have demonstrated significant ability to do so.

   [ Example:    Research on the harmfull effects of lead additives in petrol was suppressed......
     ...... General Motors, duPont and Standard Oil were making billions of dollars worldwide from selling the lead formula which they had patented while paying for and controlling the research into the health effects for more than 40 years. The research always favoured the industry's pro-lead views or was suppressed, .....
     - Taken from an article in the Guardian newspaper,   available at  https://www.theguardian.com/environment/2000/jul/13/uknews  ]

     However, there's no need to devolve this discussion into a conspiracy theory.   For whetever reason, the same amount of R&D just wasn't applied to the rotary engine and I suspect that's very unlikely to happen now.   Current interest is on developing electrical engines and solutions that do not utilise fossil fuels.

   Thank you again to everyone who has made some replies.

Best Wishes.