Anyone have any ideas as to why Mazda decided on the 1.3Liter size? Is that the size of the engine in the RX7?

 

yep

 

That just makes me sit here wishfully thinking..."what if they scaled it to 1.5Liters.....", but we all know that'll get you nowhere......

I wonder if the Renesis engine is easily scalable?

 

yeah, that's an idea for the next move up in power, and it's been done before; the easiest way to upscale the wankel engine is just to make the rotors wider... that decreases your maximum port area to volume ratio, but is the easiest way to do it.

 

Isn't it true that the same 13B engine block is used in the Renesis as with the RX-7 FD ?

Maybe it was a cost saving decision, re-using a successful and proven engine block.

 

the size is the same, and most of the bits are extremely similar, but beacause the 13B-MSP has those Multi Side Ports, it really is actually a different block.

anyhoo, yes, part of the reason is that it's a cost saving measure, the other being that this displacement seems "right" for some reason (i have no idea why, learn japanese and ask them).

 

First of all you may just want to print this as it is really freaking long!

I wrote this last summer for the other RX forum so here it is reposted here.

The rotary engine is a 6 stroke internal combustion engine. I know, people will probably start screaming at me for this so lets get into a little explanation as to why and how typical mathematical formulas for piston engines don't work.

First of all, lets get the terms "stroke" and "cycle" defined (Some of you get your heads out of the gutter!) since everyone commonly gets these terms interchanged. They are not the same thing. Every internal combustion engine whether it is a 2 stroke, 4 stroke, diesel, gasolines, propane injected, etc. is a 4 cycle engine. Why? All of these engines take in air (intake), compress the air (compression), ignite the air whether by spark plug or glow plug (ignition), and expell it out the tailpipe (exhaust). There you go 4 cycles. Simple isn't it. The term "stroke" in this context refers to how many times the crankshaft or eccentric shaft makes a piston go up or down to complete the cycle.

The connecting rods and pistons are just an extension of the offset lobes of the crankshaft. This is also true in regards to a rotor and eccentric shaft. When the lobe rotates upward, the piston goes up. When the lobe rotates down, the piston goes down. Every time it moves one way is considered a stroke. In a 2 stroke engine, all 4 phases or cycles of the combustion process are completed in only 2 strokes of the piston, 1 up and 1 down. This is only 1 complete revolution of the crankshaft. In a 4 stroke engine, it takes 4 strokes of the piston, up, down, up, down to go through the complete combustion process. This is 2 complete revolutions of the crankshaft. It's all a very simple mathematical relationship.

Now lets go look at the workings of a rotary engine. If we look at a rotary engine eccentric shaft and compare it to a piston engine crankshaft, we see essentially the same piece. Both have lobes and because of this both engines will have a stroke length, even the rotating rotary. It doesn't matter if it is a piston going back and forth or a rotor going round and round. The crankshaft motion remains the same. On a rotary engine, the rotors are spinning at exactly 1/3 the speed of the eccentric shaft. From the time that the air entering one chamber goes through the combustion phases to the time it leaves the engine from the same chamber (rotor face), the eccentric shaft has gone around 3 complete times unlike a 4 strokes 2 times or a 2 strokes 1 time. If we do the math we see that the lobes of the eccentric shaft must have gone up and down 6 times (up, down, up, down, up, down). Since it does this process the exact same way every time for every rotor face, it is a 6 stroke engine. Thats right the rotary engine is a 6 stroke! Do not confuse these strokes with the 4 internal cycles that every engine has!

Let's sum this up in a simple chart to visually explain how this works.

2 stroke engine (up,down) - 1 complete crankshaft revolution

4 stroke engine (up, down, up, down) - 2 complete crankshaft revolutions.

6 stroke (rotary) engine (up, down, up, down, up, down) - 3 complete crankshaft (eccentric shaft) revolutions.

See a pattern? All of these engines though are still 4 cycle engines! They are different stroke engines though so the amount of work they do per time is very different. A 2 stroke engine does twice the work per amount of time that a 4 stroke does. Don't believe me? Go race 2-80cc motorcycles, 1-2 stroke and 1-4 stroke and see who wins! This must mean that the rotary engine does the least amount of work per time than both other engine types. Yes it does. But, unlike a piston engine, it uses 3 sides of it's piston (rotor) at a time. In reality it makes no difference if we have 1 rotor with 3 usable faces or 6 rotors with 1 usable face each as in a piston engine.

Here's a little info on how to properly figure out displacement on a rotary engine. Everyone argues that it is really a 1.3 liter while others argue that it is really a 2.6 liter engine. They are both wrong! If we look at how a piston engines volume is calculated we arrive at a displacement based on total swept volume of every piston added together. It is not based on rpm. On a rotary, displacement is figured using one rotor face in one complete revolution then multiplyed by 2. This only leaves the total for 2 combustion chambers though and the rotary has 6! Since the volume of a 13b rotary is rated at 1.3 liters (only 2 combustion chambers) it really adds up to 3.9 liters!!! I can hear it now, "...but we only have 2 rotors!" So what. Like I said it makes no difference if there are 2 rotors with 6 faces or 6 rotors with one face each. the total is always 6 and the base numbers are only based on 2 chambers. The rotary merely does 3 times the work in a package 1/3 the size. It's just a 3.9 liter engine crammed into a 1.3 liter body. Just so none of you start a fight over this, I will explain this later so don't chastise me yet!!!

In case anyone is curious I did some math to determine what the 13B rotary would be sized at if it were a piston engine. The results are pretty neat. First of all the rotary would be a 3.9 liter, 6 cylinder engine. It would be a 6 stroke. Each cylinder would be 6.54" across (damn big piston!) but the stroke length would only be 1.18" in length peak to peak. Not much there. Interesting isn't it. Now just imagine a way to make all this work with only 2 intake runners!

In all fairness to the terms I have used, the word "stroke" can be interchanged with the word "cycle" since both technically have the same definition. The terms "periods", "quarters", or "phases" can also be used correctly. I merely wrote it the way I did to get a certain mental picture going.

I have already dealt with why the rotary engine is really a 6 stroke engine and why displacement is really 3.9 liters and not 1.3 liters. Now I need to explain why the rotary engine doesn't have the torque or horsepower of a good 3.9 liter engine or why it doesn't get the gas mileage of a 1.3 liter engine. The world has always wondered so here's why.

Remember that I stated that the true displacement of the rotary engine, if figured out according to the way piston engine volumes are calculated, is according to the total number of rotor faces and not the number of rotors, nor does it have anything to do with rpm. This added up to 3.9 liters for a 2 rotor 13B engine and not the published spec of 1.3 liters. They just crammed all 3.9 liters into a 1.3 liter body. If the engine is really a 3.9 liter engine then why doesn't it have the low end torque of a 3.9 liter engine? This has a very simple answer. Lack of leverage. OK, what the hell does that mean?

First of all we must figure out what a lever is. It is a device that multiplies mechanical advantage over an object to do the same amount of work with a smaller amout of effort. Another way to look at it is to do a greater amount of work with the same amount of effort. It's the same thing. Let's look at leverage differences as an example in a piston engine.

What happens to a piston engine when we make it a "stroker"? Ignoring a host of other variables, it gains torque. It also gains horsepower but they are both a fixed mathematical ratio between each other and you can't increase or decrease one without the other. Why did it gain torque? Greater mechanical advantage or leverage over the crankshaft. The reason being is that on a stroker crankshaft as opposed to the stock crankshaft, the lobe centerline is farther out from the rotational centerline of the crankshaft. This increases the leverage that the piston has over the crankshaft. Don't believe me? Try this. Get a short pole and hold it at the end straight out away from your body. Attach a 10 lb weight to it exactly 1 foot away from your hands. The weight is exerting exactly 10 ft. lbs. of torque on your hands. Now move that weight out away from you to 2 feet away from your hands. Now the same weight is exerting 20 ft. lbs. of torque on your hands. You have just in essence made a "stroker". Now let's get back to the engine.

Now we know that the greater the stroke length, the greater the engine torque. As I stated, the rotary engine only has an effective stroke length of 1.18". My weedeater has that! There is not very much mechanical advantage over the eccentric shaft. This still doesn't explain everything though.

Remember, I stated that if the 13B rotary were a piston engine it would have pistons 6.54" across. Now we just discovered another enemy of efficiency, flame front speed. When the spark plug ignites the mixture in the engine, it doesn't just ignite everything all at once. The spark ignites at the plug and then has to travel outward away from the plug at a certain rate of speed. While this only takes milliseconds, this amount of time gets more critical the higher the rpm gets due to the shorter amount of available time. The result is that as rpm's rise efficiency decreases. The larger the area of the piston, the farther the flame front has to travel and the greater the chance that all of the mixture does not get ignited when it should. Just can't go far enough fast enough. Todays rotaries have 2 sparkplugs per chamber to help combat this problem. Varying their ignition time in relation to each other even helps somewhat with power and emission. That's right they don't necessarily fire together even though they are in the same chamber. This can get complex so I will not deal with it at this time. Some race engines even have 3 plugs per chamber to improve efficiency and ignition wave front speed. On piston engines, Mercedes has capitalized on this and uses 2 plugs per cylinder in some of their higher end cars. Do they know something others don't?

There is also one more aspect that affects it. Remember that the rotary is a 6 stroke engine. A 2 stroke engine does twice the amount of work per amount of time that a 4 stroke engine does. A 4 stroke engine does 50% more work per amount of time that a 6 stroke does. The rotary engine does less work per eccentric shaft rotation than your typical 4 stroke counterpart. All of these characteristics combine to make an engine that has relatively little low end power and needs to be revved up to be truly powerful.

I make it sound like we should have less torque than a 1.3 liter engine due to the above reasons. This isn't true though. Remember that we still have a 3.9 liter engine even though it only uses 2 lobes on the eccentric shaft. We should not expect to develop the torque numbers of a 1.3 liter engine. It should settle in somewhere around 50% less than a 3.9 liter engine which would put it around equal to a 2.6 liter engine in power.

These traits of the rotary engine are also why the engine gets worse gas mileage than your typical 1.3 liter engine. Hell it gets worse gas mileage than your typical 2.6 liter engine. Another aspect that affects this is port timing and duration. If we had a piston engine of 2.6 liters in size that had the same intake and exhaust timing as the rotary then it would get comparable gas mileage to the rotary. The 12A/13B rotary though have much more exhaust duration than intake duration due to the peripheral exhaust port location. This contributes to several factors which decrease efficiency. Exhaust gas dilution is one of them. For each stroke there is a small amount of overlap. The exhaust ports and intake ports are open to the same chamber at the same time for a short amount of time as measured in degrees of eccentric shaft rotation. The higher the rpm's the less important this becomes since air velocity will generally keep the gasses where we want them to go. At lower rpm's though the intake and exhaust air velocity is not very high. This will cause some exhaust to go back through the combustion chamber again. When this happens volumetric efficiency decreases and there is less room for fresh air to fit inside the combustion space. Also this recirculated exhaust gas is very hot. A hotter air molecule is larger than a cold one which means a fewer number of molecules can fit in the same area per amount of pressure exerted on them. Another aspect of the rotary's peripheral exhaust port configuration that contributes to less low end power and greater fuel consumption is its incredibly long duration or time it is open for. Unfortunately when we make the port bigger we also change it's timing. We don't have the luxury of being able to mill out a head to accept a larger valve while still being able to use the same cam. The timing is really only optimized for high rpm use. We are leaving it open for too long which gets back to the whole overlap problem. Again, all of this is just a generalization and can be affected by how well the intake and exhaust flow and how well they can scavenge. The affects of scavenging, intake design, helmholtz effect, and proper exhaust design are all out of the scope of this article. So just assume it is an even world.

Luckily there is a cure for this. It is called Renesis! It is the new 13B based rotary engine in the new Mazda RX-8. The exhaust ports are no longer in the periphery of the chamber but have rather been moved to the side housings. This allowed the designers to more appropriately optimize the port timing duration. The location also allows more port area leaving the engine. So now we have more area to flow air out of faster. This new location also completely got rid of the port overlap. There is actually 64 degrees of dwell. This amount of dwell was originally greater in the early test engine called the MSP-RE since it had the intake timing of the '84-'91 n/a RX-7's 6 port engine. However dwell is only useful if you just have enough to get the job done but not so much that you are getting losses from it. Because of this Mazda engineers learned that they could open the intake earlier than previously and still maintain all of the other good aspects of the new exhaust layout. A bigger intake port = more time for air to enter and a greater cfm rating through the port. Less turbulence through the port as well. Less overlap gives us less dilution of the intake air and a cooler intake charge. More available room for incoming air. Volumetric efficiency increases. Since efficiency goes up, our use of gas gets more efficient. In other words it takes less fuel to do the same amount of work. The result, better gas mileage. With todays gas prices this is a very welcome thing. The efficiency increase also means that emissions characteristics are also improved. Another bonus with todays laws concerning air quality.

So after reading this you are probably wondering why in the world anyone would want to use one of these engines. First, and most obvious is size. They crammed a 3.9 liter engine, or more appropriately a 2.6 usable liter engine into a 1.3 liter body. Second, it is just such a simple design. There are only 3 moving parts. Fewer moving parts have less frictional losses. Also fewer moving parts have less chance statistically of failure. The more it moves, the more chances you have for failure. Third, nothing moves back and forth. So what? A piston stopping and changing direction exerts alot of stress on everything from the crankshaft to the connecting rods, to the pistons, to the wristpins, etc. Lets not also forget the stresses on the valves for being slammed open and shut as well as the temperature extremes they see during the combustion cycle. A body in motion tends to stay in motion. It is a very unnatural act to change direction suddenly or at all for that matter. A rotary just spins away in the same direction. Yes the lobes of the eccentric shaft do see stress but remember that we don't have very much leverage over them. The rotors are also exerting some of their rotational stress on the stationary gears as well so some stress is never transmitted to the eccentric shaft from the rotors. The lack of stroke length and pure rotational motional do make it very naturally adapted to high rpm use. If we look at really high horsepower piston race engines, their stroke length has been shortened to reduce the stresses to all of the engine components at high rpms. The last and most important reason why the rotary engine is still a popular engine despite it's shortcomings is because it is different. There is always something to be said for individuality and uniqueness. If you own a piston engine it doesn't matter how big it is or if it is made by Chevrolet or Honda. It is still the same device.

Just to shoot down right now any arguments on displacement think about this.

The 13B rotary engine is a 1.3 liter. Yes.
The 13B rotary engine is a 2.6 liter. Yes.
The 13B rotary engine is a 3.9 liter. Yes.

Notice that all of these statements are TRUE!!! That's right there is a truth to all of those statements. Go read the whole thing again. To understand why this is so, lets define truth. Truth can be defined in a couple of ways: Anything that is not false (none of those statements is) or it can be defined as: One's individual interpretation of presented facts. This herein is the source of our debate. We can't change the facts no matter how hard we try. Arguing won't do it. What is debatable however, is each individual's interpretation of facts. If your interpretation doesn't match someone else's, you argue about it.

Here are the facts: The rotary engine as rated by Mazda is 1.3 liters because each individual rotor, following one face of one rotor through the complete cycle, has a swept displacement of 654cc or .65 liters. Multiply this times 2 rotors to achieve 1.3. Since this only accounts for 2 of the total of 6 rotor faces, we multiply our answer by 3 to get an actual displacement of 3.9 liters. However since the rotary engine is a 6 stroke engine and not a 4 stroke engine since it takes 3 complete eccentric shaft revolutions to fire all faces instead of the typical engine's 2, it only does 66% the work of a 4 stroke 3.9 liter engine. Calculating for this we divide 3.9 by 1.5 to get a total of 2.6 liters equivalent work to a 4 stroke piston engine. All of this from a 1.3 liter in physical size package.

No one can argue that this is not correct and any response saying otherwise will have been explained by what I just said. Any debate will only focus on one aspect and not the total facts.

Just to put a cap on this whole thing: If at any time you try to calculate proper sizing for a turbo, intake manifold runners, intake plenum size, exhaust size, etc, and you try to use the 1.3 liter number in your equations, you will be way, way, way off!!!!!!!!! There are only 2 ways to flow more air: increase displacement or increase rpm. A 1.6 liter Honda engine doesn't flow anywhere even remotely near what a 13B (1.3 liter) flows per the same rpm. Just some food for thought.

Rebut that! I need the entertainment! Hehe!!!

If you got this far, give yourself a cookie! My carpal tunnel syndrome is starting to get bad

 

Leave it to Professor rotarygod for an expense-free education! Can't believe I actually understood most of that.

 

Rotary god

Great writte up ,thanks.

 

Are you affiliated with www.performancescene.net ?

I noticed an article on their website:
http://www.performancescene.net/?page=rotary

which looks like it may have been written by you (in fact it's word-for-word what you just put in this forum) - is that where you posted it last summer?

Simon.

 

Yep the Performance Scene is where I first posted it. I just used the good old copy and paste feature so I didn't have to retype that whole thing all over again.

Believe it or not people still try to argue with me about that writeup! Trust me. I know how the damn thing works. :D

 

Rotary that was kick ***.

 

Rotarygod your oldpost is great but I have another questionfor you about this Renesis engine. Canyou explain the reasons for increasingthe compression ratio to 10.0:1 instead of keeping the 9.0:1 ratio the FD had?

 

The answer to that one is quite simply, power. If you used 9:1 compression rotors in the Renesis you would have much more resistance to detonation but you would have less power everywhere and even lower fuel economy. The Renesis should actually be capable of high 20's in gas mileage if properly tuned (which Mazda didn't do). My old 2nd gen RX-7 got 24 mpg after I did some intake, exhaust work and tuned it properly. Eamon Hurley of Hurley Engineering in England has gotten 30 mpg on a properly tuned 12A rotary and 24 mpg on a properly tuned naturally aspirated 3 rotor! Remeber that the older rotaries have less volumetric efficiency than the Renesis. Renesis when properly tuned should out do the older motors. The 3rd gen RX-7 also needed the lower compression as a resistance to detonation due to it's turbo system. Even that car with the pathetically restrictive little turbos could get the mileage of most RX-8s on here and still be able to run extremely fast. Something many people don't realize in relation to knock on a naturally aspirated engine is that it isn't going to blow your engine up. You just don't want to keep your engine running here since it isn't efficient anymore. Remember that while knock is not a good thing by any means, when an engine is under boost and the temperatures and pressures are much higher in the engine, the engine is obviously more vulnerable to breakage. The most critical point for detonation in an engine is at it's torque peak. I have quickly gotten out of the scope of your question. One more thing though. In an e-mail chat with Eamon Hurley a few years ago I asked him if he has ever tried customizing some rotors to raise the compression for more power. His response was that yes he has but anything over 10:1 didn't seem to give any more benefit just more detonation and he recommended that I stay with the 9.7:1 rotors of my 2nd gen RX-7. Obviously that was back when that was the highest out there but it interesting to note the number he stated back then and compare it to the Renesis.

 

24 mpg on a 3-rotor... i buy one for 100K if i had the money. hahaha...

 

rotarygod

 

too

 

much

 

in

 

for

 

ma

 

tion! - Just Kidding!!! Thanks!

 

Quite enjoyable rotaryg,
Just wanted to point out that I have gotten close to 27mpg in my 90 convertible and would speculate that the 8 would see much improvement if it used a different final drive ratio.

 

The final drive ratio is a neccesity in the RX-8 due to the much greater wheel height over the previous RX-7s. 6th gear is basically the same as an RX-7 5th gear and with the rear end ratios and wheel heights of each car, the RX-8 engine is turning at roughly the same speed that it would in the RX-7 at the same mph.

 

Rotorygod-

Your talk on displacement was great!

I have some comments about the fuel econ statements however.

When you make the comparison of the wankel as a piston engine with 6.x" bore and 1.x" stroke, I do not question your numbers. I do think you missed an important part of thermal losses with the increased surface area with a chamber with those dimentions. A sphere is of course maximized volume with minumal surface area, and for trapozoidal similar shapes, the square is the winner. The less square or more elongated an area becomes, the more surface area is needed to supply a fixed volume. The temperature differnece between the container for the hot gas and the hot gas greatly effects the curve of gas pressure after cumbustion. Then, in a wankel, it moves the hot gasses around this wall even further maximizing the contact with cooler areas. Idealy, an engine would convert its heat from burning fuel into mechanical energy and make zero waste heat and need no cooling system. In real life, we can only try to minimize the amout wasted into the cooling system by creating cumbustion chamber shapes to minimize surface area. The wankel, as a side effect of its design inherently maximizes this area, reguardless of where the ports are located.

The cause of the inability to have higher compression is pretty simple, the more elongated the cumbustion chamber, the higher pressure the end-gass is under before getting the chance to burn. And this combined with the extreamly long dwell periods makes for a very knock prone situation with compression increases, and compression increases have huge effects on the amount of fuel burned vs the mechanical energy released.

From the above statements, it shows a trend that wankels like to use a bunch of fuel and air to heat there cooling systems (oil/water) rather then create mechanical energy, and reguardless of port layout on the 13b-msp none of the above factors change.

You speak of the zero overlap as a fuel economy benifit. For engine speeds where scavenging velocities are low, this is true. For about 80% of my driving, i am in ranges where exaust port velocity is great enough to assist with scavenging because i also prefere engines with tq peaks at higher engine speeds. You made an exellent point about exaust gass occupying space that could otherwise be filled with useable mixture. Without overlap to "vaccum flush" the chamber at the engines speeds we use when we want power, wont we be in a continueal state of re"burning" the volume of exaust gas occupied by the cumbistion chamber?

I find this perticuarly relivant with an engine with a powerband that needs higher engine speeds to be potent.

You speak of the fewer moving parts creating a less likely to fail system, and in a racing application, i totally agree. However, for street cars, i would like to ask you the ratio of factory equiped wankels at the junk yard with 200k miles and the ratio of small piston engines with 200k miles. I see toyotas with 250-300k miles at junkyards with factory equiped engines often. I see 150k miles Fb's with there second engine failed. I am not saying that a wankel cant do 200+k miles, just that it seems a whole lot of piston engines do, and i have only seen a few wankels do it.

Just curious what you think about these points.
thanks
-Luke