Why is rotary exhaust so hot?
 

Does anyone know why, or better yet where I can read an explanation for why rotary exhaust is so much hotter than piston engine exhaust?

Matt

The rotary has to open the exhaust port and dump the exhaust out sooner than a piston motor. Since the exhaust is coming out sooner, the combustion gasses have not been expanded fully. All engines expand a hot gas and convert the heat energy into work. Since the rotary opens up the exhaust and dumps it sooner, then there is still a lot of heat energy in the exhaust, and thus a lot of temperature. This is also the reason a rotary engine will not acheive the gas mileage (thermal efficiency) of a piston engine, it is because there is a lot of wasted energy going out the exhaust of the rotary.

The complex motion of the rotor, eccentric shaft, and gears mandate that the exhaust be dumped sooner on the rotary. If the exhaust gasses are held in longer, then they start to do negative work on the face of the rotor (they push the wrong way on the e-shaft,) meaning you loose power if you try and hold the exhaust in longer by delaying the exhaust port opening timing.

i think it might have to do with the fact that it is in a constant firing cycle, unlike the piston engine with 4 strokes.

Speedturn got it. It's hot relative to a 4-stroke piston engine.

With a piston engine the exhaust gases have to flow around the valve head, then flow around the valve stem, then make a right angle turn past some water jacket to get out.

That's also why a rotary is way louder than a boinger.

Thanks speedturn, you answered it perfectly. I was hoping to get a little of the thermodynamics of the situation along with the answer.

Sureshot, not only do the gasses have a more complicated exit in a piston engine, but since they exit at a lower pressure less sound energy is generated.

Not sure if you guys saw this or not, but BMW just came out with a turbosteamer prototype that uses a steam power circuit heated by exhaust gas to reclaim wasted energy. http://www.speedtv.com/articles/auto...hnology/21159/ It makes thier 4 cylinder engine like 15 percent more efficient. It seems like a similar system would be excellent coupled with a rotary engine.

Matt

Not sure where you guys get your information from but just to let you know that the power stroke of a rotary is 270 degrees vs. 180 degrees in a piston so they're actually keeping the stroke longer than the piston.....

That's geometrically impossible..

I might believe 160 vs. 120.

what's geometrically impossible? rotary or piston?

Plutos answer is actually a trick answer. The E shaft spins 270 degrees per phase (intake, compression, power, exhaust), but the port timing is still worked out in 180 degree increments of rotor phasing. The E shaft spins faster, in relation to, the rotor and its 4 "strokes". This is how you can figure that a rotary opens its exhaust "valve" earlier than a typical piston engine.

Pluto, the length of the power stroke is mostly unrelated to gas expansion when you are comparing engines. An internal combustion engine produces power by expanding a heated gas, the further the gas is expanded, the lower it's pressure and temperature become and the more energy is extracted from it.

Matt

This also has something to do with it. Each rotor exhaust port has a continuious exhaust energy while a 4 stroke piston will have the exhaust pulses come every other rotation of it's crank shaft per cylinder. This leaves a slight cooling off period for the piston engine in each cylinder where as a rotary will have none.

I think the fact that oil is injected raises temps a bit

The noise level as well as the exhaust temperature is just as stated . There is no interrupted flow (obstruction--- Valve and stem) in the exhaust port. It is a straight shot. The valves remove alot of heat from a 4 cycle engine through the seat and guides. The high noise level is for the same reason. Think of a turbo in the exhaust of a 962-935 Porsche race car( Or any tubo) . They were producing up to 800Hp but ran rather quietly without mufflers because the turbo act as a sort of muffler. A second gen turbo rx7 producing 280-300hp with no mufflers is alot quieter than a 300 hp Pport with the same exhaust.

Thats correct sccagt3. It's important to note though that a turbo makes a car quieter not just because it is an abstruction, but it removes energy from the exhaust (which it uses to compress intake air) by further expanding and cooling exhaust gas. Sound and heat are both part of the kinetic energy of the exhaust, the other part is the velocity it is moving at and it's pressure.

Ordinary obstructions are removing energy too, because the exhaust is slowed by them, reducing thier velocity (and overall kinetic energy) and increasing the opportunity for heat transfer (another reduction of the energy of the exhaust).

I'm not trying to be a smartass, just to have an engine discussion based on thermodynamics.

Matt

Didn't you just say that the further the gas is expanded, the lower the pressure and temperature? Isn't longer power stroke allows the energy to transfer into mechanical energy further which in terms create less losses (heat)?

Yes I did, and no. In order to expand the gas farther you have to increase it's volume. Comparing the lengths of the power strokes (in crank rotation) between the 2 engines is not a direct comparison. The rotary power stroke may involve more crank rotation, but it takes more crank rotation for it to change it's volume. The mechanisms are so different you have to compare exactly what the engine is doing to the gas.

Matt

I was only trying to give a simple explanation. I thought an in depth Thermo discussion was not needed. Where did you do your SAE? My son is doing that at Duke when he can afford the time .

damn interesting thread....

Yup.

And the funny thing about a turbo making exhaust quieter... My roommate started an argument with me: that a turbo will make it louder since it is going to be forcing more air through the engine, hence more exhaust, and more sound.

I even brought up the point of a WRX STi (2.5l turbo) with full exhaust (no cats, nothing) compared to my current Tiburon (2.0l NA) with a similar exhaust system, and a single high flow cat, and my car was very noticibly louder. He didn't have a good response to that, but ended the debate with "turbos just make it louder, more exhaust is louder" and left the room.

I think I'll point him to this thread.

Glad to help.

Tell me about it. I just modified my NA S5 exhaust sleeves in my s5 rebuild. I still have the stock exhaust but now it's much louder with defusers removed. At first I thought I had and exhaust leak. Nope. I never expected the db's to raise that much when doing that modifcation.

I was at Kettering University for 3 cars, 2002, 03, 04. I highly recommend it. However, nobody has the amount of free time it requires, you just have to do it and make time. I learned far more doing that than I did in any of my classes.

Matt,
That was my point of argument from the beginning, it wasn't the longer duration of the power stroke that makes the exhaust run hotter. I was correcting speedturn that the exhaust doesn't come out sooner than the piston engine (which is why I stated that it was 180 degrees on piston vs. 270 degrees on a rotary). The reason for higher exhaust temp from my understanding is that the inherent design flaw of the rotary engine where it doesn't burn fuel efficiently. If you look at the length and the surface area of the rotor in respect to how it creates the flame front vs. piston engine for the power stroke, you'll see that it takes much much much longer time to burn the fuel completely (which is why by having a 270 degrees power stroke actually help offset that problem, and also 3 "timed" sparks per power stroke to help burn the fuel more efficiently). Eventhough, most of the a/f mixture were converted into mechanical energy during the power stroke, there're still excessive amount of unburn fuel that exit to the exhaust which in terms will convert into heat creating a higher EGT.

This is an awesome discussion, thought I'd add some of my ideas/theories too.

In reference to how the rotary doesn't as efficiently use the energy of the exhaust gasses, there are ways to reclaim it (similar to the BMW's innovation). First we all know, the turbo which takes the still expanding gasses and transfers that energy to compression, which in turn provides even more compressed gases coming out of the exhaust.

Second, any kind of secondary turbo-like driven electric motor that allows the accesories/alternator to run off of it while not creating too much back pressure and hindering the turbo's operation.

The third would be a similar idea to the second except the accessories would be driven off of a device which harvests pure heat energy from the exiting gasses instead of direct mechanical motion, to convert it into mechanical motion for the accesories.

I beleive mazda has explored this with one of its newer concepts (SENKU?), in mating an electric hybrid type starting motor/generator to the rotary.

"device which harvests pure heat energy"

build one of those, and you will be a billionaire the next day. there is no such device. the only current feasible way to harness heat energy is by expansion/contraction of a fluid. this is why many systems waste large amounts of heat. if there was a way to directly harvest it, almost everything in the world would be way more efficient.

You typically wont see a turbo gasoline engine have a lower BSFC. Especially not a rotary thats going to be tuned even richer than a turbo piston engine by nature. Increasing power is fine, but if youre trying to add efficiency youre really trying to lower the amount of gas needed to make each pony.

Turbo DIESELS that dont even have throttle plates and are never really under vacuum, well, thats a different story. Many diesels never throttle air, and just squirt less fuel to 'throttle' the engine. Hence why they knock and rattle like hell at idle. And, the lack of a throttle means the turbo doesnt need a BOV.

Now, if you meant just to add power to the system, well, boost away.

The inherant GEOMETRY of the 13b as it is now is what contributes to the inefficiency, but one of the ironic quirks is that it actually loses a lot of heat to the coolant (well, the engine itself) becuase of the high surface to volume ratio. If the Engine was wider along the axis of the e-shaft, youd gain more volume than youd gain surface area proportionately, and gain efficiency while increasing displacement.

If I remember correctly, mazdas working on such a rotary. That along with side port exhaust could really help out with EGT issues, so long as the EPA doesnt do to a new rotary car what they did to the RX-8 - require it to run rich so the exhaust gas is cooler to make the CAT last longer :rolleyes:

^ word

then what is a steam engine? it takes "pure heat" and converts it into mechanical motion by means of the liquid evaporating and expanding into a piston chamber or turbine. what other kind of energy does it use to produce mechanical energy?
and yes the guy who invented it was probably a billionare in his day of the dark ages.
any kind of convection device can be used here, thats what i am talking about.:MissileSm

i "apologize" for not being so specific, i didn't think anyone would be that literal when reading this...

do you know how to read? or do you just not realize that steam is a fluid, and it does work by expanding or contracting?

I think this is the main reason for the very hot exhaust.

Both engine types are on the exhaust cycle 25% of the time. But the wankel ports in a housing are shared by all 3 chambers, so exhaust is flowing through those ports 75% of the time, vs 25% for ports in a cylinder head. Less cooling of exh gas before it enters the exh manifold.

Regarding other posts: As I recall, stock exh port opens late, vs piston engine, when you look at cycle timing. Also, running rich tends to reduce egt's.

steam is water in a vapor form, once it contracts to a certain point it condenses into water (like when your engine is cold)

Right.... and steam engines (piston or turbine) don't directly convert heat into mechanical motion, it heats up a working fluid (steam is a fluid) and that does the actual mechanical dirty work. Same general principle, from old wood-fired trains all the way to nuclear submarines.


The main reason the exhaust is *hotter* is because the chamber is so inefficient. As others have mentioned, the rotor has 50% more time for complete combustion to happen, and it's *still* not done burning...

The reason it's *louder* is because of the lack of valving to muffle the sound. Rotaries actually open their exhausts closer to BDC than many piston engines do... even before taking into account the degree skew.

I can see who has been studying thermodynamics and who hasn't.

Speedturn, PE

The BMW heat energy recovery sounds great, but how much does it weigh?
And what might be the capital & maintenance costs?

It might be practical for heavy long haul vehicles like trucks & trains.

i really like this thread...very good read....nice info guys

If the heat and inefficiency was due totally to the fuel not having enough time to burn, then it should only be a problem at high revs. And why can a formula one engine run at 19,000rpm while a rotary can't even finish burning it's fuel at 7000?

Also, it seems that people are comparing change in time, change in volume, and change in crank angle as if they are the same.

I think speedturn answered this question completely on page one and everyone has been making crap up since then.

Basically, rotary engines are automatically less efficient than piston engines due to the action of the mechanism in the engine. So why does everyone on here think they are so sweet?

I think they are neat, I have an FB that I am restoring, but I certainly wouldn't go so far as to claim that they are the best thing since sliced bread and put them in everything I own. What are the advantages to using this motor design in a car?

*Please don't answer if you are just pulling assumptions out of your ass.

Matt

Everyone
Say what?


Now what question was it that you wanted answered?

I just read up some white papers and found an interesting theory put forwards by a study:

Because of how the rotor moves during the power 'stroke' of movement, the leading apex seal of the rotor (the one closest to the exhaust port) side of the rotor moves away from the expanding gas charge faster than the trailing edge of it (the ones closer to the spark plugs, and havent actually went over them yet real early after ignition) and its sort of outrunning the flame front.

If its not pushing on the piston to do work, its not giving up a lot of its heat into kinetic movement. Theres a reason why long stroke piston engines cant rev high, it literally outruns the flamefront! F1 engines get their high revs by having incredibly short strokes and low piston speeds.

Im not exactly sure if hot gas loses heat more if it actually does work or just expands to fill a void, but Im just putting it out there.

Check out http://en.wikipedia.org/wiki/Seebeck_effect

Direct conversion of heat energy to electrical energy - a peltier-device CPU cooler or car-fridge uses the reverse effect to cool.

Simon.

Thermocouples arent as efficient as phase change methods of using heat energy.

Theres a reason you use an AC compressor and coolant, and not peltiers.... and why people would use steam and turbines and or pistons instaed of thermocouples in power plants ;)

Newton's first law is the reason long stroke engines can't rev high.

sccagt3 - I suppose youve never seen high revving Honda VTECs :rolleyes:.

But if you want to exaggerate that a little, long stroke big cube v8s have been hitting high rpms for decades.

REGARDLESS, the issue here is the flame front, not metallurgy, piston speeds and rod/stroke ratios. You just took the thread off topic.

Well, sounds like you felt free to extract substantially from your anal opening. 'everyone' in this thread is not 100% correct, but they all are not making up crap.

I see little specifics in your reply to all others, other than agreeing with speedturn, PE, who mostly elaborated on early port opening as the root cause of hot exhaust gases.

Lets be specific. The 13b starts to open the exh port 71 deg BBDC. That is in crank degrees. Given that the rotor runs at 1/3 crank speed, the rotor is 24 degrees before BDC. Now adjust for the rotor cycle of 90 deg rotation from TDC to BDC in the power stroke, vs 180 deg for a piston engine, that would be the same as just 58 deg bbdc on a piston engine, which is in the typical range for stock boingers at .05" checking clearance.

most everyone breathes and farts, but everyone except you is not wrong.

Sure I have, but they don't qualify as a long stroke engine.

Yes but they don't turn high rpms for long. You also made the F1 comparo, what long stroke engines turn those revs and live? The short stroke F1 engine also seems to be able to do a good job of pumping air even at 18,000 rpm.


I took it off topic? I only responded to the statements in your post.
Newton's first law is the limiting factor on turning a "long" stroke engine high rpm. Maybe you should call John Deere and tell them that if they only used a slower burning fuel they could turn their engines more rpms

Speaking of the Vtec--- Honda increased the displacement from 2.0 to 2.2 litres in the 03-04 model year changes. They accomplished this by increasing the stroke from 84mm to 90.7mm. They also DECREASED the max revs, I wonder why? It had nothing to do with flame fronts. It was mean old Mister Newton!

I'm going to have to side with sccagt3 on the piston speeds. I haven't read much about it lately, but I think maximimum piston speeds are usually similar even between vastly different engines. If I still had my engine design book i would look it up, but I don't.

kevink2 - Sorry if I came off as an ass earlier, I was just trying to spur scientific debate with too many assumptions.

Also, I don't think we really need to worry about going off topic, just take it where it goes. This is where good design ideas come from.

Thanks to everyone putting info in this thread. It's a very interesting read!!

It depends on your definition of "revving high". We probably don't have the materials and technology to make an engine have a piston speed fast enough to do that.

Ironically, for a given cylinder displacement, long stroke engines are more efficient, because the shape of the combustion chamber near TDC can be closer to spherical... the flame front has less distance to go to reach the cylinder walls with a smaller bore, and near TDC the piston is right there by the plug no matter what.

F1 engines reduce self-disassembly with incredibly short strokes. Plus, they need all the bore they can get to cram as much valve area as possible into the cylinder head. Valve area = HP potential.

I know this. I also know that for the typical enthusiast metallurgy and strength of the internals has more to do with piston speeds than flame front.

But I do know a F1 engine has no way in hell of getting 18K rpms with a long stroke, unobtanium or not.

Regardless, I Was simply trying to put forwards an idea I came accross and wouldnt know if it was valid or not. I do know taht the exhaust side of the rotarys peripherial housing retains all the heat, and because heat only moves one way ID figure that would CONTRIBUTE to keeping the heat in the exhaust gas instead of out into the metal - I do know piston engines are more efficient at operating temperature instead of when its too cold and its losing heat to the head and cylinder walls, why would a rotary be different?

Also, I know of 700 hp N/A big cube v8s... you dont need tons and tons of valves necessarily, its by virtue of the fact that FIA keeps lowering the displacement theyre allowed to work with and they HAVE to rev high.

But, well, we've gone off track. heh.

Mattgls23,

The advantages of a racing rotary as I see it:

It makes a great race car engine for production car classes because:
1) compact, short, can be placed far back in a front engine chassis to get good weight distribution
2) all racing bodies classify cars based on engine displacement, and the rotary makes great power per cc for a production engine (can't compare it to a strictly race F1 engine, but those go for a million $ a piece)
3) I can build a great peripheral port race engine for $7000 that is capable of racing with and sometimes beating $20,000 n/a nissan and porsche engines
4) I can run on 93 octane pump gasoline - a rotary likes fast burn fuels, that saves me a lot of money versus expensive $7 a gallon race gas
5) my racing rotary PP engine lasts a very long time, longer than the piston motors I race against. My last 12A PP motor had 26 hours on it when I pulled it to step up to a 13B PP. The 12A PP was still running strong, and I sold it for almost half of what I paid for it.
6) The PP rotary is the only Japanese engine to ever win the LeMans 24 hour race overall - not Honda, not Toyota, not Nissan, not even Chevy has ever been able to do that

Disadvantages:
1) low torque, can be slow off the corners if you don't have an extremely close ratio gear box
2) high fuel consumption
3) many sanctioning bodies discriminate against rotaries, and use either high "equivalency factors" or small intake restrictors to choke rotary horsepower. A rotary is very sensitive to intake and exhaust restrictions. This is why rotaries have not been in the head lines of pro racing since Mazda won LeMans in 1991.

I have been running rotaries since 1983. I have been running PP motors since 1997.