Still Waiting

Came across the "Why Rotary" thread and thought of something people kept mentioning.

Everybody keeps on commenting "1.3L and it's rated 238 hp". Therefore a conclusion is draw up that it's more efficient because it makes more power per displacement than it's piston counterparts of similar displacement. If that were the case then wouldn't rotaries be the standard engine of choice by now. We all know it's not thermodynamically efficient because tremendous energy is lost as heat to a greater combustion surface area so how can a rotary engine make more power than a slighly bigger piston equivilent? It's not and I think people are getting mislead by the 1.3L annotation.

If we were to compare as close as we can apples to apples the rotary vs the piston in terms of efficiency and operation, then shouldn't the correct displacement be 2.6L in frame of reference to the standard that 4 stroke engines are measured?

I thought about it this way. You have 1 rotor face (0.6L) combusting per revolution of the crank. The other rotor is phased 180 degrees apart and that as well combusts within the same revolution. IF you wanted to compare displacement, wouldn't you want to compare it to the amount of air the engine displaces PER TURN of the crankshaft. A 4 cyclinder is always rated at 2 revolutions to fire all pistons and a Renesis consumes 1.3L for one revolution of the crankshaft, hence 2.6L for 2 revolutions!

OR

If you wanted to rate your 238 HP Renesis at 1.3L, wouldn't it be politically correct to say then an S2000 creates 240 HP with one revolution of the crank consuming 1.0L of AIR?

In the end it's basically how much CFM's the engine consumes (and consequently the correct ratio of fuel) to how much power it will generate. If both engines reved to near 9k to make their peak power, the Renesis will consume more air and yet create out less power than Honda's F20C1.

Heck you can take it a step further and say if you wanted to fire off all 6 rotor faces (3.9 litres of air in 3 crank revolutions) and compared it to 3 crankshaft revolutions of the S2k, you will see that it's engine equates to only 3.0 litres of air!

What do you think and is this a real measure of efficiency?

cLLcLe

I think.. it's cuz three combustions happen in one revolution where as in a piston engine it takes two revolutions for 1 combution... right?

army_rx8

rotarygod had a really nice write up abou tthis a long time ago...hopfully he'll chim ein b/c i have no idea where that thread is:o

Still Waiting

No no... you got it backwards, the rotor "orbits" at 1/3rd the rate of the crankshaft. If the crank is spinning at 9000 rpms, the rotor is doing 3000 orbits per minute.

So to completely fire all 3 faces of a single rotor (an orbit) you would spin the crankshaft 3 revolutions. One face of the rotor fired off (don't know how much sweep during combustion) would equate to one turn of the crankshaft.

Still Waiting

I read that thread a long time ago... and I'm just trying to compare simply in terms of air displaced per turn of the crankshaft and the relation to the resultant power output by the consumed air.

I'm just trying to clarify the 1.3 Liter annotation that is probably misleading to the masses.

Sigma

Actually, depending on who you ask, it can either be 1.3L, 2.6L, or 3.9L.

Technically a 13B is a 3.9L motor if you really wanted to try to compare it to a piston engine measurement. But it's also a 1.3L and a 2.6L motor too.

Still Waiting

well 3.9L if you wanted to fire off all 6 of the 2 rotor faces. But then that would equate to 3 crankshaft revolutions. A piston doesn't have to fire off all it's pistons to be a standard measurement because if you rated it at 3 revolutions then it would be 1 and a half it's displacement.

Just saying if you're going to call a rotary 1.3L and compare it.... the 4 stroke displacement should be halved to keep the "consumed" air constant by the crankshaft revolution. Don't confuse this with the fact that the other 4 rotor faces aren't firing off becuase then half of the pistons aren't fired off as well.

kmg1186

You just opened up a huge can of worms

I remember reading this same discussion in a thread awhile ago. I can't find it, but a quick link could solve this real fast.

RPIRX-8

I'm with Army on this one, try and find that thread where RG talked about this. I believe the gist of the post was there are arguments for the renesis being 1.3, 2.6, and 3.9 liters. All the arguments have technical merit which makes none of them 100% wrong and all of them are technically correct. From a gas mileage standpoint there is an argument for a higher displacement rating. From a size standpoint there is an argument for lower displacement.

Still Waiting

As I said let's put it in the simplest scheme possible. Who cares if it's rotary or piston for the time being.

One engine is rated to combust 1.3L of air per turn of the crankshaft... it's factory rated at 238hp.

The other engine is calculated to combust 1.0L of air per turn of the crankshaft. It's rated at 240hp.

Both will do 9k rpm. You can see that one engine consumes 0.3L more per rev. You can see now why it's not efficient and why 1.3L is a different frame of reference when you need to alter the view point of 2.6L directly to a 4 stroke, 1.3L to half the displacement of a 4 stroke or 3.9L but then you'd have to say 1.5 times the displacement of a 4 stroke. In teh end we are keeping the volume of air consumed PER revolution of hte crank constant to keep it apples to apples.

and YES I UNDERSTAND RG's post. It was just explained from a different starting viewpoint. I'm just trying to let everybody know that 1.3L shouldn't be directly compared to a 4 stroke's standard displacement rating. The pistons rating of displacement needs to be halved (equals to one revolution) or you have to say that the renesis is 2.6L (2 revolutions) if you want to compare directly to a standard rated displacement of a 4 stroke (2 revolutions) piston engine.

This is just to keep the number of revolutions constant (no matter what the engine design) to show the difference in combusted air vs power output.

ZoomZoomH

iTrader: (1)

FWIW, Ward's Auto (the group that gives out Engines of the Year awards) categorizes the RENESIS (and all its 13B predecessors) in the 2.5L-3.0L engine class.

the whole '1.3L, 238hp' is indeed a marketing gimmick, since Mazda doesn't have to explain that SAE specifies engine displacement differently between a rotary engine and a piston engine, but technically it IS in fact a '1.3L, 238hp engine', just that most people don't bother to ask "1.3L of what type of engine"

Still Waiting

The lower MPG has to do with the fact that there is less power being made per cubic volume of air and also less mechanical leverage by design. It doesn't have anything to do with being small and large at the same time. It consumes more fuel because you are generating less power (it's less therodynamically efficient by design) and consequently you have to really work the engine more so than a similar displacement piston counterpart.

Don't get me wrong, I love and hope to own a rotary some day for its other merits, just want to clarify the 1.3L issue in regards to the current 4 stroke displacement measurment process.

Still Waiting

I just stared looking up the 20B specs, I know it's dated technology but it makes me cringe at how less it stands against a similar displacement piston engine. Just some food for thought.

20b in 2 crankshaft revolutions is 4.0 litres of air! That is 6 rotor faces FYI. It has a turbo setup at .7 bar (~10 psi). For sake of simplicity I'll just say that's an extra 50% of atmospheric air packed in 2 revolutions (2.0L). Total air consumed is 4.0+2.0 = 6.0 litres! This is just roughly simplifying, don't have to get too technical. And the output at the time was 280 hp and ~296 lb-ft. Think of what a 6 liter piston engine will put out nowadays?

I know technology today is better for piston engines like the newer Hemi 6.1. This is just theorizing more of the "why rotary" concept.

What would a center perphrial ported rotor sandwiched inbetween 2 mutli side port rotors would put out NA? I think that would be today's 3 rotor technology equivelent.

PaulieWalnuts

Here, digest this and then take some Tums

Section 8

Go over to RX7club.com and search on this topic. It's been discussed quite a bit over there.

KYLiquid

by the way you calculated the displacement (and im not saying its wrong, im not to good with the math) the turbo 3 rotor could do about 300hp and 300tq @ 6 liters, the new vette has 400hp and 400tq for 6 liters. so the vette is making 100hp/tq more but the power bands on the motor are differnt arnt they, doesnt the rotary give a more constant power delivery thru the RPM band?

obviously a 1.3L rotary isnt the same as a 1.3L piston motor, cause all the time you see rotary cars that will run side by side with piston cars that have more peak power, can this be attributed to the was the power is delivered in each motor.

I used to watch my friend with a 500hp RX7 go up against 600-700 hp piston cars of similar weight, and it was always much closer than if it were 2 piston cars of similar weight with that power gap.

Gomez

Paulie had it spot-on. Here is this definitive thread in it's entirety.....https://www.rx8club.com/series-i-tech-garage-22/why-1-3-a-15316/ .

Fill your boots .

therm8

As long as my insurance company keeps on believing that it's 80 cubic inches, I'm happy.

neit_jnf

it's equivalent to a 2.6L 4-cyl engine,,, 2 power pulses per crank/eccentric shaft revolution

check this out:

last 2nd gen RX-7 13B 6-port N/A engine = 160hp

RX8 13B 6-port N/A engine = 238hp

Subaru 2.5L opposed 4-cyl N/A engine = 165hp

Subaru 2.5L opposed 4-cyl Turbo engine = 210 ~300hp depending on model (Forrester to STi)

2nd and 3rd gen RX-7 13B 4 port Turbo Engine = 187 - 200hp (2nd gen) 255 - 280hp (3rd gen)

See the similar power figures?

zoom44

i think its quite amzing that still waiting thinks he understands the whole topic and RG's thread but keeps saying things like


like the rotary doesnt do all 4 "strokes". see now we can start the whole stroke explanation again

shelleys_man_06

I'm sorry I have to jump in. Ya know. :p

This topic has been done to death. You can use 1.3, 2.6, or 3.9 liters. It doesn't really matter. It's almost like a sign convention. Here's a suggestion: don't worry about it. You'll give yourself a bigger headache. Personally, I accept Mazda's 1.3 liter convention, and that's how it stands. KISS.

wedge357

I think I smell a troll...

PaulieWalnuts

^ you mean the S2000 avatar didn't give it away?

rotarygod

While I'm not going to rewrite everything over again, I will add what size piston engine the rotary "uses" air like. It isn't like a 1.3, 2.6, or a 3.9 liter piston engine. In terms of displacement per revolution the rotary compares well to a 2.6 liter 4 cylinder. However it's use of air differs. A typical piston engine uses 7 lbs of air to make 1 hp. A typical rotary uses 10 lbs of air to make 1 hp. When you take this into account you now have an engine that behaves and uses air at a comparable rate to a 3.6 liter piston engine. I have a Megasquirt ecu and have to enter in engine parameters to determine proper fueling. If I enter in 1.3 it won't start. If I enter in 2.6 liters I can get it to start but everythig has to be richened up big time. If I enter in 3.6 liters I just have to smooth out the prespecified map for that size engine and it runs VERY good. That's how it compares.

zoom44

come on give the post about "stroke" :D