Nihilanthic

I'm sure the rotary vs piston arguement has been beaten to death, but this isnt that kind of thread.

I came across one thread, about how rotaries can make more power with lower boost than a piston engine of the same displacment, and can't find it again! Also, the search engine is down, spitting out error codes, so I figured I'd make a thread.

ANYWAY, to get back on topic, one of the odd things about rotaries is the long combustion chamber, and that apparently theyre not as heat efficient as a undersquare piston engine will be - heat goes into the coolant or exhaust gas isntead of the eccentric shaft. But, that exhaust gas heat can be used to spin a turbo, and Mazda obviously realized that making the TurboII and the FD RX-7.

So, I know that rotaries make gobs of power at lower boost than the equivilant piston engine (if you do the 1.5 x rotary displacement to get a figure to compare to a piston engine, @ 1.95 liters displacement, a FD RX-7 makes insane power compared to say, the turbo 2L youd see in an eclipse or evo at the same boost. Imagine a 13B @ 19 psi!) but I dont know why!

Does anyone know the thermodynamics of this enough to be able to explain it?

Also, something else I kind of thought up - the bathtub combustion chambers that the rotors have arent as efficient as say, a pentroof/hemi head w/ dished pistons, but if you had direct injection (right next to the sparkplugs?) and a specially shaped chamber inside the rotor (the housing itself cant be a wacky shape because of the apex seals, right?) couldnt you improve that?

I saw some honda design where it could run at stoichiometric AFR, or a SUPER lean one where it injects directly into a cavity inside the piston head really close to TDC during the compression stroke, and ignites it inside that small cavity, but the rest of the air in the cylinder is still there, giving an effective 60:1 afr. For the 14.6:1 it injects towards the end of the intake stroke.

Sorry for the BIG question, but I just dont know where else to ask. Thanks in advance!

ronbros3

I believe the chamber you are refering to is called the MAN type developed in germany in the 1940s I read about it somewhere ! it was about multi -fuel engines, also the piston head was force feed oil cooling to cool the chamber, dia gram showed it to be almost a complete sphere in piston head. thanks RON

Nihilanthic

Figures Germany did it decades ago... lol. I guess someone coudl take two scoops out of the combustion chamber of the rotary and get injectors for it, and sell it to mazda. I would if i had the equipment and education, lol.

Anwyay, any idea about why rotaries make so much power at relatively tiny boost compared to piston counterparts?

Kenku

Well, part of it is because a rotary isn't equivalent to a 2L engine. Looking at it in terms of how much air is moved per revolution of the output shaft (and gods, I don't want to start an equivalency argument...), they're equivalent to a 2.6L.

Also, the biggest efficiency loss area is actually quenching the burning air-fuel mixture at the trailing apex. Also, incidentally, the biggest emissions problem in the peripheral exhaust engines, because that unburned fuel gets dumped straight out. On the side-port renesis type engines, the unburned fuel gets recirculated again.

maxcooper

The answer to your riddle is that the 13B flows the same amount of air as a 2.6L piston engine, and that is why it makes more power at low boost than a 1.3L piston engine would. This is my round-about way of saying the 13B has a displacement that is equal to that of a 2.6L piston engine, without upsetting the "it's 1.3 cause Mazda says so!" crowd.

-Max

Nihilanthic

OK, 2.6 then! Thats probably better in the long run anyway!

But what I still dont get is why it makes such power at lower boost than a piston engine. Supras are 3.0 liters... and run twice what I see FDs running here on the forum, but not twice the power. EDIT: Why not just compare to the RB26DETT? Every ricer on earth knows about it, and its 2.6 liters...

Whats the deal? THATS what I dont understand. Whats the thermodynamic issue there? Is a hot-running rotary somehow able to flow more air at lower boost? Thats counterintuitive!

rotarygod

iTrader: (1)

This will only serve to confuse you more but an average (that's the key word) piston engine needs about 7 lbs of air (not boost pressure) to make 1 hp. The average rotary needs 10 lbs of air to make 1 hp. That's a big efficiency difference.

Nihilanthic

I know the BSFC of a rotary is higher... heh. But, I still dont get how it can flow more air at lower boost. Is it because of how air flows into the rotor housings vs the plenum and intake manifold of a piston engine? Its moving more air at a lower boost... so something is going on there.

But, well, thanks for mentioning that. I guess when classes start back in the fall I should corner a physics professor and beat this out of him :/

maxcooper

30 psi of boost is not twice as much air as 15 psi of boost. Add 14.7 psi to both of them to get the absolute pressure, and that will give you more useful numbers to compare.

-Max

Nihilanthic

So, I get 29.7 vs 44.7 absolute pressure. About 50% more, not 100% more. Hmm. (lets just simplify it as 30 vs 45, easier to factor)

The real thing that I'm doing here is focusing too much on the pressure and not the actual FLOW. But, when I do that, I'm left scratching my head even more, because youd think it would be much hotter for a rotary, meaning youd have less mass unless you pressurized it, because hotter air is thinner - unless I just severely need to opne up a book and study gas laws...

But cant you basically look at the power the engine makes and deduce the airflow just from that? So, I get the deducation that the rotary engine w/ turbos is somehow able to flow more at less pressure. Given how heat inefficient a rotary is relative to a piston engine, (although that heat would help the turbo, right?) how does a rotary pull that off, and why? I'm thinking it has something to do with how air intake into the chambers is done differently than pistons... or is this about combustion, too?

peejay

Except that it's 2x, not 1.5x.

Nihilanthic

Max already told us

I still dont know how it can flow more at lower pressure, though. This is really making no sense. I'd expect the heat to make it require MORE psi for the same amount of air due to heat expansion.

Nihilanthic

Ok, I hit up www.wikipedia.com...

And, I checked out Boyle's and Charles's laws. Anyone remember highschool physics?

To maintain the constant during an increase in pressure of a gas, at fixed temperature, requires that the volume decrease. <- Boyle's law.
To maintain the constant during heating of a gas at fixed pressure, the volume must increase. Conversely, cooling the gas decreases the volume. <- Charles's law.

So uhhhh doesnt that imply that somehow the air coming out of the turbos cooler? Or is the velocity higher so the pressure is lower, which is implied by Bernoulli's Law?

Actually, now that I think about that, in a piston engine the air flows into the plenum, and only some of the pistons are open. With rotary, the rotors are always spinning and taking in air, right? Maybe that might be why.

Bottom line is someone on this forum should know, I just hope they see this thread. And I know what classes to take in the fall.

Eggie

You need to forget about power and consider torque per liter and BMEP.

BTW, I'm not at all convinced an equivalent 2.6L piston engine makes less power than a rotary. Examples?

Nihilanthic

Edit: I'm reading up on BMEP now, and while I dont yet know whats going on that might have something to do with it. Plus, how do you compute that for a rotary?

Also, I never meant to say the 2.6 liter piston engine is gonna make less power - just that it would boost higher to do the same.

One thing I'm afraid of, though, is that I made a improper observation and this is all just in my head

Nihilanthic

Right, found a thread on it...

http://www.engineersedge.com/wwwboar...ages/1352.html

apparently at peak power for a 168 non-turbo 13B @ 7000 rpms... BMEP = 39.7.

Isnt that low as hell? lol

EDIT:

This forumla is good for any 4-stroke engine (which a rotary engine is):

bmep = 792000 P/(VN)

where P is power, V is total engine cubic inches, and N is rpm. Bmep is psi.

Displacement for a rotary engine is the displacement of one chamber (654cc for a Mazda) times the number of rotors, times 2 to make rotary displacement equivalent to reciprocating displacement. Don't forget to convert to cubic inches.

You should get in the area of 80-120 psi for a non turbocharged engine. This is the range for reciprocating engines, but I think a rotary engine will fall toward the low side because of port limitations and less efficient cooling.


I got that off of http://www.eng-tips.com/viewthread.cfm?qid=36873. Turns out its lower for rotary engines. So, where does that take me?

turbojeff

iTrader: (30)

Thermo wasn't my strong point but here goes.

Heat energy from the exhaust is used to spin the turbine of the turbo. As the exhaust goes across the turbine the temperature and the pressure of the exhaust both drop. In school we fully instrumented a 6.5L turbo diesel, the exhaust temp dropped 300F across the exhaust turbine. I forgot the pressure drop.

So yes it is cooler coming out of the turbo. The heat energy is "recovered" and used to power a shaft that powers a compressor forcing air into the engine rather than have it draw a vacuum to intake air.

Of course when your just cruising down the highway the throttle plates are almost closed, there is a lot of vacuum on the face of the piston/rotor that the engine has to work to overcome. The turbo doesn't help. On a highly loaded engine the turbo should increase efficiency vs. a larger displacement engine putting out the same power. That is why a lot of boats, trucks, etc have turbos.

For the answer to your power questions, look at the amount of air ingested. A 2.6L piston engine running 10 psi should be able to make ~255hp (FD runs 10psi and makes 255hp). Suby runs a 2.5L at what (?) boost levels in the Legacy and Forester and make about 250hp.

The old TII made 182 hp on 5-6psi.

Nihilanthic

Oh, I know that all, exhaust-side wise...

But what I was trying to make sense of was the perception that I've noticed that somehow rotaries use less pressure to get the same power. So, that would imply air coming out of the COMPRESSOR to the intake would be cooler, so that it could flowt he same mass of air @ a lower pressure.

But, I guess its all just in my head (and maybe a few others) and its functionally a 2.6 liter displacement engine when you measure by two crank revolutions, lol.

ultradef

I am by no means a physics expert so I'm not going to add too much to this thread. BUT - I was wondering how you came to the conclusion that "rotaries can make more power with lower boost than a piston engine of the same displacement"? From what I have seen, rotaries often make LESS (or at least equal) power than an equivalent piston engine (lets say that "equivalent" means a 2.6L piston engine for arguments sake) given equal boost, turbo, mods, etc. Rotaries are not very efficient engines at all (although I still love them ).

Nihilanthic

I know. I picked up a perception some on this forum share that somehow rotaries do just that.

Maybe its just a misperception, I dont know. Mostly its from rx-7 FD vs supra threads and the rx-7 is running lower boost bla bla bla - but I supposed its also making less power, right?

Or, I suppose, theyre comparing it to a 4G63 which runs much higher boost but makes lower power, but not realizing its 2.0-2.6 comparison. Now, I'm starting to think it really is just something I came up with in my mind, lol.

But hey, at least all that heat it shoves out the exhaust gas can help spin up a turbo pretty good, right?

ultradef

Haha...in the never-ending battle of superiority between FDs and Supras, the FD owners like to claim that can make less power and still outrun a Supra (mostly due to the rather large weight difference). Generally speaking (from what I have seen), a 2JZ will make more power than a (stock ported) REW given the same boost/turbo/mods. Granted, the 2JZ is also a 3 liter engine.

But yes, a rotary definitely has strong exhaust pulses and can spool a nice big turbo. If only it was more efficient and could make better use of all that turbo...

Nihilanthic

*scratches head* sooo we basically have a rb26dett (blablabla) under the hood except its smaller. Good for us! lol.

Porting increases the volumetric efficiency like a hotter camshaft would, right? As far as heat goes, though, the only way to change that I could think of is what we do already (use a turbo) or maybe new materials for the internals to keep all that heat inside and out of the coolant.

However, that would kill reliability, wouldnt it?

dubulup

I got the chance to tune a very strong motor up to 19psi of boost and it was amazing...then we roasted the clutch in lower gears.
It was absolutely nuts

Nihilanthic

What turbos? What kind of dyno graph? lol

And another thing.. I go to the ray hall turbo matching applet page.

I put in 2600 cc, 7K RPM, 14 lbs boost (pretty typica, right?)... and I get
MAKE TYPE HP Rating TURBINE HSG A/R
Garrett GT25 400 0.8200000000000001

I see GT35R used here on the forum. GT25 is WAY too small for a 13B, right? So are rotaries just able to use a bigger turbo?

BTW, stats it spat out were 36 lbs/min airflow, and 392 hp at the flywheel estimated.

Do we use bigger turbos for a reason? Btw, I change out to 8000 rpms max and (assuming the ports could breathe) it just has more power being made...

Project84

In this comparison between a 2.6L piston engine and a rotary engine, are you guys assuming that the compression ration and volumetric efficiency is the same between the two? Or is this just a comparison of displacement? Just wondering because I didn't see anyone mention compression ratio and it is a variable that must be accounted for along with volumetric efficiency.

Now I'm no Physicist, but even though VE can exceed 100% in a turbo application, you have to think of VE in relative terms.

VE on the rotary (assuming a 2.6L displacement) could be 400 CFM while on a 2.6L piston engine, its 500CFM (just to use even numbers). If the turbo on each vehicle bring VE up to 150% relative to each engine, then the math will show that 150% of 400 is 600 CFM while 150% of 500 is 750CFM, a much higher flow of air from an engine with the same displacement. To gain equality in CFM under boost, the rotary engines turbo will have to increase VE beyond 150%. But now turbos are not equal.


If VE are equal at 500CFM on both compared applications, what about compression ratio? A 2.6L engine can take in the same displacement, but compress it less, resulting in a less powerful combustion.

Another question. How many cylinders are in this hypothetical 2.6L piston engine? Won't a 2.6L 4 cylinder engine at 3000 RPMs have less power than a 2.6L 6 cylinder engine at the same RPM because the 6 cylinder engine has more power stokes during that minute? I'm pretty sure thats what I heard while watching something about why Formula 1 engines rev up to 19,000 RPMs. VE is not the greatest up there, but to have that many power strokes on all those cylinders in a minute is like having a lot of kids rowing a boat vs a couple of stronger adults. <-- not 100% sure about the 4cyl vs. 6cyl scenerio, just a thought.

Something else that I think makes it hard to compare rotaries to piston engines is length of stroke. On a piston engine it is determined by the length of the rod. Now look at the lobe on a rotary engines crank shaft which. The leverage of the rotor against the shaft is not that good, resulting in lower torque. And Horsepower is a measurment of torque over time (RPM X Torque/5252). So the advantage is to thepiston engine, even though it has the same displacement, same VE, and same RPM as a rotary engine.

I just think there are too many variables. There has to be an advantage in favor of 1 engine or another in each of the categories, else the engines would be the same. If any of the following; RPM, VE, Torque, displacement, compression ratio and stoke are not the same, the loss in one category has to be made up by an advantage in another, which yeilds 2 inequalities. If not, then the engine are equal.

But, if the engines are equal, what about thermal efficiencies? Heat is a by-product of combustion, a form of energy wasted in an internal combustion engine in terms of transforming potential energy (fuel) into kinetic energy to turn the crank. So which engine has the potential to perform more efficiently as a measure of thermal efficiency? Where can a gain in efficiency come from? Less friction? The heat resistant properties of the materials used to make the engines like steel vs iron vs aluminum vs alloys etc..... What about sprung weight? Do all the moving parts from the piston engine weigh the same as the moving parts in the rotary engine?

It seems like it could go on for ever. Like i said, I'm not super engineer, but if 1 thing is different about the engines, then that difference can only be compensated for by making something else different about them. This would equalize the result, but not the engines in there entirety. The only other option is to take away the advantage of the better engine. But then you go in reverse. All things being equal but a rotary engine has the better turbo, and you take away that turbo, the chain reaction goes the other way......

This is just my $.02