EFINI_RX-7_RZ

Ok, taking snippets of information from chapter 4, "PERFORMANCE AND COMBUSTION" of Mr. Yamamoto's essay, you can see some indications that low thermal efficiency does, in fact, promote high exhaust temps:


4.1.5"FUEL CONSUMPTION PERFORMANCE":

"In the rotary engine, the surface to volume ration of the working chamber is greater and the time required for one-cycle is 1.5 times longer than those of the reciprocating engine, causing a greater cooling loss. The greater surface area/volume ratio of the working chamber also causes a greater loss of unburned gas."

See Fig. 4.5, the % of heat attributed to unburned gas.

4.5.2"VOLUME OF ROTOR RECESS"

"The compression ratio depends mainly on the volume of the rotor recess. A smaller volume of the rotor recess increases the compression ratio improving the theoretical thermal efficiency. (...)Fig. 4.36 through 38 show the effect of the compression ratio on fuel consumption performance, performance at W.O.T. and exhaust emission. As shown, the smaller volume of the rotor recess improves fuel consumption and performance at W.O.T., but, at the same time, increases the amount of emission of NOx and HC."

In Fig. 4.38 you can see that the higher the compression ratio (and thus its thermal efficiency) the lower the exhaust temps are.

So I guess low thermal efficiency and high volume of unburned air/fuel mixture both contribute to high exhaust temps.

Nihilanthic

That would definitely explain the high BSFC!

KevinK2

".... you can see some indications that low thermal efficiency does, in fact, promote high exhaust temps...

... So I guess low thermal efficiency and high volume of unburned air/fuel mixture both contribute to high exhaust temps."

easy now. I'm not sure you are extracting the right conclusions.

4.5.2 just says a smaller dish (and higher cr) results in more nox and hc. This is due to less natural egr with high compression and small compressed chamber volumes, and is true for boingers too.

4.1.5 is more relevant. I read the "heat balance" graph differently, regarding unburned gases. In combustion analysis, "unburned gases" = unburned mixture of air and fuel, and is considered to be at a much lower temperature than burned gases. Like coolant and oil, it would be considered a heat sink, and as such the graph could read "heat absorbed by unburned gases". By "heat of exhaust", I interpret this as burned gas.

If the quantity of unburned gas is decreased, per the graph, note that all other components increased. Less unburned gasses, more exhaust heat beause more mixture is burned. more effective heat too.

So, I see nothing here that suggests why rotaries have high egt. I'm glad your digging into the document..... a lot there.

EFINI_RX-7_RZ

As I said, check fig. # 4.38, which is related to the quoted text. You will see that, indeed, as you said, as CR goes up, NOx and HC levels go up too, but it shows, too, that exhaust temps, on the contrary, go down. NOx emissions, as I understand on boingers or Wankels or more clearly on diesels, rise due to higher combustion temps, which a higer CR will usually promote, and I say usually because this can be joggled with by the design of the combustion chamber on a boinger. Engineers actually add EGR to bring down high levels of NOx. HC levels, on the other hand, develop mostly due to high levels of unburned mixture, and the mechanics for this is explained in further detail earlier in the document at 4.3.1 "FLAME PROPAGATION". As I understand it, the higher thermal efficiency of a high compression ratio better translates the heat generated from the ignition event to working load and thus exhaust temps go down. So I guess that, if thermal efficiency is improved by other means, like using heat reflecting ceramic coatings to cover all the chamber areas, exhaust temps would go down, too, in the same way as higher CRs get you lower exhaust temps. I remember reading people in here finding that their cermet coated housings and/or ceramic coated rotors did not rise their exhaust temps, but that their fuel mileage improved significantly. Maybe since they were expecting higher exhaust temps from these coatings (if heat wasn’t taken away by the coolant and the oil, then it must go out as high exhaust temps, right?), they didn’t realize that maybe they were actually running lower exhaust temps than before, as more heat went through the eccentric shaft as power.

Yeah, they don’t explain very clearly how to read the graph, as it only shows heat generated as a % and not by total measurement, like BTUs or the like, nor do they say at what moment of the combustion process . But doesn’t “unburned gases” mean mixture that didn’t burn while the combustion chamber was at a position where its burning will translate into work? Doesn’t this unburned gas ultimately gets burned later during the expansion stroke, where it doesn’t help turn the shaft and thus goes out as a very hot exhaust gas?

EFINI_RX-7_RZ

Have to, cuz I've read the later SAE papers on the 13B-T second gen, the 13B-REW and the 13B-MSP (a.k.a. Renesis) and I had to fill some voids left on them as these documents don't explain rotary basics in depth.

KevinK2

4.38, it's not a contrary effect that exh gas temp went down with higher CR. more unburned gas in the power stroke means cooler gas mixing with hot, or cooler exh. Putting this with CR vs power plot: higher CR gets more power, more unburned gas, and lower exh temp., per this paper for the wankel.

4.3.1 does not mention exh temps, but I think you are right on exh temps dropping with high CR.

If the unburned gas was burned, it would not be unburned. I think most of the HC measurements are going on at the exh manifold, not in the chamber.

CalG

I'm no pro, but a pretty good observer. Excuse me if I fall for tricks easily.

Exhaust Temp and piston speeds seem to become entwined in this thread. I do know that limits on piston speeds are more closely tied to friction/lubrication between sliding elements and flame propagation. Have you noticed how "flat" piston crowns are these days?

Well, On to "heat in exhaust". First let's remember there are two aspects to heat. Temperature, which is really a measure of speed of motion (think electron volts)
And Calories. It's like volts and amps, pressure and flow volume.

So, the RX's "could" have hotter exhaust gas temperatures due to many variables (I honestly don't know which ... yet) It's not likely that the gas burned with oxygen in a rotary is any hotter than gasoline burned in a recip, so we should disregard that. If the gas/air mix burns, it will attain some temperature, then it can only cool if it gives up it's heat by the usual mechanisms of Radiation or conductance. Well, there is the third, expansion. (read turbo charger or "extractor exhaust" etc.) It could be that the heat loss to metal in a poppet valved engine is the difference. Exhaust valves burn in short time if they are not allowed to cool by contacting the valve seats. But modern engines have pretty efficient exhaust ports.
Does anyone know if a rotary engine requires more or less water cooling capacity than an equal output recip? The heat would need to go some where.

or. The RX's might put out the same temperature exhaust, Has anyone got some EGT measurement values? only more of it. Heck these simple two bank rotarys have only two holes going out. Maybe there is a clue. The heat from a familiar four (or more) cylinder passing out through two pipes.

Also, I know that I can get any recip to run it's exhaust pipe red hot just by retarding the spark a bit. Forcing some fuel to be burned in the exhaust rather than the combustion chamber. This Has to be an obvious reason, for how else could the "reactor" work and these engines are noted for poor economy and poping in the exhaust.

I conjecture. That the rotor faces just do not allow a geometry useful to complete combustion. Unburned fuel is caught at the edges and the apex areas during the combustion phase only to be expelled into the exhaust port when the time comes.

An EGT and a lamda (Oxygen) sensor would tell us a lot!

An additional note: The exhaust note and intensity has everything to do with temperature of the exhaust gas (velocity) and the SPEED at which the exhaust port opens. Frequency is it's own thing.

Perhaps we could fit marine type water cooled exhaust systems to these RX's and bring the temp down to a point where there are only drips of water and a little puff of "air" comming out the tail pipe. ;-)

Can anyone point me to a theoretical discussion of "tuned exhaust" for these rotarys?
Extraction systems would seem counter productive to economy.

Has anyone an idea what the volumetric efficiency of a rotary engine is? Recips frequently run well over 100%.

What about BMEP.

It's all thermodynamics. The rest is just guessing.

Regards

CalG

EFINI_RX-7_RZ

Me neither

CalG

An excerpt from Paul Yaw's pages



We have already discussed how insufficient exhaust flow reduces volumetric efficiency, but the presence of exhaust gasses in the intake charge (Exhaust gas dilution) causes other problems as well. The rotary engine is known for its poor combustion characteristics. Due to the shape of the chamber, and the location of the spark lugs, a large percentage of the intake charge does not burn in the chamber. The end result is a fair amount of unburned gasses, or hydrocarbons being passed into the exhaust system. This reduces power output, because a portion of the mixture that we tried so hard to put into the engine did not burn. This also reduces fuel economy, and increases emissions. Another effect that is not often realized is excessive exhaust gas temperatures. These hydrocarbons will then burn in the exhaust system raising the exhaust gas temperatures.

The addition of exhaust gasses to the intake charge will reduce the already poor combustion quality. The end result is that the mixture is harder to ignite, and when it finally does light up it will burn at a slower rate further reducing power output. In a turbocharged engine excessive exhaust gas dilution will cause its own unique set of problems.


Regards

CalG

KevinK2

"What about BMEP."

this is a very theoretical energy based valve, for work done on piston/rotor from tdc to bdc. It can be determined by p1, p2, and deltaV. Or, for a given rpm, it's just a function of engine torque and engine displacement, that's all. rotary's have relatively low torque, so this value will be low, compared to boingers, except at high rpm.

"These hydrocarbons will then burn in the exhaust system raising the exhaust gas temperatures."

Yaw has a lot of good stuff posted, but not sure about this. When tuning based on egt, rich mixtures with high unburned hc's tend to drop temps, lean ones raise egt's. This is consistant with Yamamoto's heat balance in 4.1.5. air ratio is lambda, and at .85 slightly less normalized working heat, vs much less exhaust heat. I would expect lower egt too.

""unburned gases" = unburned mixture of air and fuel, and is considered to be at a much lower temperature than burned gases. Like coolant and oil, it would be considered a heat sink, and as such the graph could read "heat absorbed by unburned gases". By "heat of exhaust", I interpret this as burned gas."

I might have been off on this . Other heat balance literature would say the "heat of unburned gas" is just the potential btu value of gasoline that was not burned. The heat of the exhaust is based on total exhaust mass flow and temperature.

CalG

KevinK2

There is a LOT more "heat" in a small mass of Unburned fuel than there is in the same "volume" of spent exhaust gasses. That's what infurnal combustion is all about ;-)

If that heat is released by the mechanism of continued combustion, even though the temperature is lower, the abount of calories (heat) is greater.

The subjective "measurement" of hot exhaust pipes is only the difference of heat in and heat out. So.... more low temperature "heat" could easily manifest as "hotter" than a smaller quantity of higher temperature gas. Hot gasses do not actually carry a lot of "heat". I like to use this analogy: You can place you bare hand into a hot oven and not get burned. Just don't touch anything!"

I consideration, There must be a lot of heat given up to the intruding poppet valve, guide and exhaust tract even in a modern four cycle recip. It is interesting how "cool" a high performance two cycle engine exhaust is.

Perhaps none of this is true, but there is some good stuff here

CalG

KevinK2

"There is a LOT more "heat" in a small mass of Unburned fuel than there is in the same "volume" of spent exhaust gasses. That's what infurnal combustion is all about ;-)"

lbm for lbm, yes.

That is what fig 4.5 shows. At lambda = .85 (12.5 A/F), the energy of unburned fuel equals the hot exhaust energy, and each is greater than the fuel energy that does work.

pluto

what he meant was that if you look at it in terms of velocity, it did stop at TDC or BDC, however, the acceleration is what drives the velocity to zero. It's just an intergral of the forumla. If you take diff eq and visualize a spiral vector, you'll see that the acceleration is constantly changing in both the vector and its angle causing the velocity to be constant but yet with a circular direction.

I think you're thinking of it as acceleration instead of a vector in velocity form.

KDiDP

There is such a device. It was developed and has been in use since the 1950's.

https://en.wikipedia.org/wiki/Turbo-compound_engine
https://en.wikipedia.org/wiki/Napier_Nomad
https://en.wikipedia.org/wiki/Wright...Duplex-Cyclone
Detroit Diesel DD16; Detroit?s Biggest Compound Turbocharged Heavy Haul Engine
https://www.dieselnet.com/tech/engin...pound.php#mech

Shiver77

Hi I'm hoping someone could tell me why there isn't an extra rotor on the motor that the exhaust passes through in order to bring up fuel efficiency? That way the main rotors are turning the extra rotor that only gets exhaust. I think that's what a piston engine does or maybe I'm remembering it wrong.

Why isn't there a turbine engine available? So the blades are an inch thick and the hole in the space pushes the gas then covers the hole trapping the gas between the blades. They are then ignited and a few mm of space opening to the second blade would be how the gas escapes the chamber turning the rotors. It would continue through many of these mashed together. The spacers can contain lots of these holes for the gas to get trapped in that are compressing and igniting the gas and even turn the exhaust around so it goes back the other way in the engine making it very fuel efficient and high torque and hp. Sorry this makes sense in my head but don't know if I'm explaining it properly so you can imagine what I'm saying. A turbine ignites fuel to turn the blades. Place spacers between each set of blades where the air is trapped momentarily then ignited so each set of spacers are igniting the mixture to turn the next set of blades. Directing the gas using the spacers will slow the movement creating better efficiency.

TeamRX8

iTrader: (2)

it’s maybe not as efficient as you think, more like adding additional inefficiency to an already inefficient engine.

https://www.adrianflux.co.uk/influx/culture/gas-turbine

when you find the free lunch, please let me know where the line starts.
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