Why 1.3?
 

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 ass.

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

You make some valid points. You try typing for that long and not miss something!

I didn't really want to go too far into the shape aspect of the chamber vs a round chamber. I am well aware that efficiency is affected. The whole comparison to an equivalent piston engine piston size was merely for reference purposes only.

As far as the longevity issues goes. I'll go by the cars me and my friends have owned. Between me and my 2 best friends, we have cumulatively owned 19 different RX-7s within the past 10 years. Every generation. The ones that had the worst problems were also the cars that were taken care of the least. Only 2 of the cars had bad engines. One was a turbo and the other was a first gen car that had been raced many times. Both of those cars looked like a cowboy should have put a gun to them and ended their misery. The first gens seemingly run forever. 150000 miles on a first gen is purely pathetic. They will easily top 200000 with no issues what so ever provided that the owners take care of them. This is true for any car though. My friend Dave has a mid '80s 626 that currently has somewhere around 280000 miles on it. The car was originally turbocharged but it died. The car is just a nonturbo now. This same friend has also owned 2 first gens with 200000+ miles and a 2nd gen with that many. I have yet to see a turbo RX-7 get this though. Obviously both can go this long but you do make a good point. The biggest issue is the engine tolerance for stupid lazy owners like I was as a kid. I had many issues with my old 2nd gen back when I didn't know a thing about cars of maintenence. I learned everything by doing it so I messed up pretty bad sometimes. Lets just say that I had the ceramic from 2 seperate spark plugs break and run through the motor. I lost a small screw into the intake manifold and started the engine. I overheated it twice. I ran it low on oil and out once when an oil line broke. Yes I was driving the car at the time. I was also bad about oil changes. I pulled that engine at 140000 miles not because it was dead, but because I had a new ported engine that I had built and wanted to drop onto it. The old engine still ran great and when it was removed it was the fastest that that car had ever been without a turbo. The thing about the rotary is that it is rev happy. Many people that own them will rag them out pretty bad. You also have to realize that the rotary is not as tolerant to lack of oil changes and almost not at all to overheating. Admittedly many piston engines are more tolerant of poor maintenance habits but the truly low maintenance engine is a fairly new thing. We only just heard within the past decade about cars that only need spark plugs every 100000 miles. Ford will even admit that the Tempo engine was designed to run for only about 100000 miles. We had one and that car started to fall apart after about 80K or so. Until the Renesis emerged, the rotary engine was relatively the exact same design that it was 35 years ago. For its time it was at the forefront of durability. A rotary will actually just plain wear out after about 250000 miles. The seal tolerance just gets too high and it doesn't seal as well anymore. Yep there are piston engines that can top that I'll admit it. My basis for reliability simply comes from only having 3 moving parts. Statistically they are much less liely to have problems than say 60 moving parts. We can't break a valve or drop one, throw a rod, etc. I'd love to see someone throw a rotor sometime! A failure for a rotary is usually a water seal failure, bearing failure, or an apex seal breaking and the occasional oil injector failure. The piston equivalent is breaking a piston ring. It also may have the other problems as well as the problems I listed above. I personally have never had to replace a head gasket on my RX-7. The problems lie within the material selection for the engines. Sometimes I think that there are Mazda engineers with their heads up their asses just for the warmth.

Mazda really needs to listen to some of the creative rotary owners out there that make them truly durable. They need a center bearing, all of the engine housings need to either be made out of aluminum (preferred) or steel. The current mix and match system is why overheating is so bad. The aluminum and cast iron heat up and cool down at different rates and develop sealing problems between them. Aluminum is less tolerant but lighter so I vote for it since I take care of my cars (now, not as a kid!). Mazda needs to use a seperate oil reservoir for oil metering that uses 2 stroke oil. Just have a reservoir with a low level sensor on it and if it drops too low the car goes into fail safe mode (limp home mode). I am also waiting to see the results of using the much lower 5W20 oil in the RX-8 vs 10W30 or 20W50 in the older rotaries. They need full length dowel pins. Many people custom machine new larger dowel locations and then stregthen the engine by adding. The simple thing is to use just 2 dowel rods the full length of the engine. The holes go all the way through so why use 4 total pins with 2 end to end on each side? This makes no sense and is a reason why the engine can flex slightly when under severe stress. Mazda race engines used braces under and over the engine that prevented this from happening. Mazda knows what needs to be done but the financial department sometimes has a bigger say in the product design than the designers do. Mazdatrix has a neat picture of what happens when an engine gets so well built that the typical failures don't happen. On a turbo rotary it is possible to dent or break a rotor from detonation if the seals don't break. While still bad, that is impressive that an engine can be built tough enough from basically factory parts (rotor housings, rotors, bearings, etc.) that the weakest link is a rotor. The aftermarket has always made stronger engine parts for piston engines. The bastards!

As far as overlap and scavenging goes, from an economy standpoint overlap is irrelevant on a turbo car because of backpressure from a turbo, and if the exhaust is of a poor design on a non turbo it may not help there either. The 13B engines in the 2nd gen RX-7 have a box for an exhaust manifold. The engine does not scavenge this way. The 13B also has overlap. while a properly designed header system can promote scavenging and help the intake suck in more air, a poor system can actuall have the exact opposite effect and stop some of the air from entering due to greater exhaust pressure. When I installed a header and removed the cat on my n/a 2nd gen, I gained power everywhere not just up high. Gas mileage got better and low end was even improved dramatically. While the RX-8 exhaust isn't as bad as the RX-7 was the point is still valid. As long as there is a cat and muffler in the way to restict the exhaust, the benfits of scavenging will always be reduced. The center exhaust port messes everything up anyways. The lack of overlap and siamesed center port are also the reasons why no exhaust is giving impressive gains anywhere compared to the older motors. Overlap as you said is important for efficiency at higher rpms but only if the exhaust is open enough and designed to take use of this. Most street cars aren't. When an engine is at 100% efficiency, there is still 10% of exhaust that gets recirculated back into the intake. You would think that this would cause the air to keep progressively get hotter and hotter but this isn't so. When the air gets heated during combustion it only goes so high. The old gas that was recirculated have been diluted pretty heavily by the incoming air and consequently cooled down alot. When combustion happens again, this amount of gas doesn't combust while the other does and the whole thing heats back up. Basically it is the same as a slight dilution and temperature rise befor combustion. Even your good scavenging overlap engine will still have some exhaust recirculate. while some gas does get trapped and cariied back to the intake side in the Renesis, it also does in the other rotaries as well. The difference is that it is typically the NOx within the exhaust in the Renesis that goes around again where it gets expelled in the 13B. Others do go around again though including some but not as much of the NOx. These get reburned which results in a cleaner emission. The epa has a huge say in what gets built.

You did make some great points and I thank you for making them. Hell, I'm just glad that people actually read that huge post and this one for that matter!

I'll add everything in if I ever publish a technical book on the rotary engine. Hopefully by then I'll learn how to type without the grammar and speeling errors too!

ROTARY GOD

I finished reading your epic very interesting why dont you get it translated to japanese and send it to the head engineers at mazda japan , maybe they will listen to some of the rxowners views and do something constructive towars the improvemts that you guys have pointed out.

keep up the good work.

michael

I have a hard time not writing small books when I post. This response was a bitch to do! :D

Hey, just so you know that people appreciate your efforts, I've read all your long posts, rotarygod (as well as liveforphysics' there). Thank you both for those well-thought out and clearly explained posts :)

Rotary g and Mr Physics,
great posts-both of you.I,d like to go on record as being the first to suggest the separate oil tank (for metering) on this forum in a previous thread.Of course I could be completely wrong.
Another point of note is that the combustion shape in the wankel,
or area of flame propagation is the worst of any internal combustion engine ; long, thin and partially divided. For this reason fuel efficiency will never be at the level of it's recipricating cousins.
Mazda's engineers are pretty bright although you are correct in stating they;like anyone else are limited by budgets.Some of the recomendations made by RG may be more applicable to rotary race cars and some are presently being applied (center bearing etc.)The added dowels; long or short,are not sufficiently necessary in normally aspirated motors or low boost applications.
Back to the center bearing: It's advantages are beyond 10k rpms.
Mr Physics, you are really correct about the reliability advantages of rotaries being more apparent in racing than on the street. The last normally aspirated rotaries did not regularly see as much as 120k miles.
Whether in racing or production, Mazda will spend what time and money they see as necessary to achieve specific goals.

Rotorygod-

I can not deny your testamonials reguarding your experiences with durability. I can only say that i see many more lower milage wankels in the junkyard then i see japanesse piston engines. Anyone is welcome to draw what ever conclusions they like from that. I have no doubt that much of the wankel failiers are from neglect, I think most people forget an oil change now and then, or even dont notice they are low on coolant once or twice, if your engine is forgiving, it sure is nice. If the basis for there duribility is basied on the fewer moving parts, i ask what about a toyota 4ag head? or a honda b16a head? Both are performance heads that take huge abuse for 200k miles and still operate like the day they are created. I have only heard of these heads EVER failing when people incorectly install cams, or try to "improve" them. Both of these units easily have over 50 moving parts in them. My feelings on the statement that something is less likely to because of less moving parts only hold true in a theoretical world. From my experience, they thing with a weak point always fails first, whether it has 10moving parts or 10000. If you want to compare the durability of a 'built' wankel, i think its only fair to also compare it to a similarly 'built' piston engine. Once you have forged H beam rods, forged ceramic coated pistons, moly-rings, forged steel crank, and stainless valves; you have a package that is pretty much indestructable when operated at a sensible output level. Yes, you can break any engine if you run it at 110%, but if you build it to take xhp and operate it at .7x hp, you have nearly an invincable piston engine. Throughing rods, dropping valves, ect... just stops. Anyways, a disscussion on durability is never going to end up getting anywhere due to 1000 factors from maintenece to climate that effect it greatly.

Scavenging......... I think you may want to re-think most of that paragraph, espically the following lines.

"As far as overlap and scavenging goes, from an economy standpoint overlap is irrelevant on a turbo car because of backpressure from a turbo"

scavenging is not only relivant on a turbo car, but infact so crucial that turbos can increase scavenging. Some engines even use turbo scavenging where the turbo is only an exaust turbine just to help an engine scavenge. Turbos reduce port reversion as well when in a situation with a bit of overlap. I think you may want to research that a bit more before making statements like the above.

"As long as there is a cat and muffler in the way to restict the exhaust, the benfits of scavenging will always be reduced."

Scavenging has long compleated its task before a muffler. I think you are confused with these things that do increase back-pressure haveing a negitive effect on scavenging. It is based on timing the ports high velocity burts to create a low pressure area from the last cylinder fireing to draw to next pulse with less resistance. Cats and muffs much after the collector can increase back pressure, but have a very mild effect on scavenging. Yes, they reduce the speed the pulses get to escape with, but the fact remains, they still get velocity, and scavenging still is a hugely vital part of flushing the chamber to reduce the amount of exaust gases reburned. This is also much the job of anti reversion valve angles in a piston engine, another effect that a wankel does not get to employ. Scavenging also reduces pumping losses on an engine and increses exaust gas velocity by not having such a hard low pressure area that the pulse has to 'escape' from.

The cumbustion chamber design has effectively been skirted, this is ok, it wasnt about the CCs design, but i find it hard to have any discussion where efficiency is mentioned with out looking at the piece that decides 80%+ of the efficancy of an engine. I honestly can not think of a worse design for an area to burn fuel then a wankels CC. Its a knock promoting, heat absorbing, delayed burntime having, slow flame front propagaing, end gass trapping abortian of a shape to burn fuel in. I find it amazing that even with SO much less paracitc losses in mechanical drag on the engine, that they can still be so poor at the conversion of chemical potential energy into mechanical energy.

I have an idea for people interested in modding wankels for lower emisions and improved power and fuel economy. How about ceramic coating the face of the rotor so that you dont waste quite as much heat energy in warming that oil, and more will go to the wheels. This would also make the engine more durable as well as lower NO emmsions from having less fire contact with metal which is the cause of the gas.
-Luke

I dont mean to sound like i am a wankel hater, there are applications where i think they are perfect. If i had to make a tube chasis road race car, it would get a wankel, also for drag i think a turbo wankel is a awsome powerplant.

I dont want to come off as bashing, just want to get some intelectual conversation going on.

I do actually like to brag about wankel durability. In race applications they have been unparalled for longevity.How about in excess of 100 race hours without a pulldown.This is what my friend Rick Engman has gotten out of some of his engines (R26B).
His team owner Jim Downing ;some of you may have heard of him,ran Daytona 24, Sebring 12hour,24hours of le Mans and over 40 test hours on an engine without taking it apart. When he did the corner seal springs didn't exist anymore. The apex seal springs were shot but the ceramic seals;from which I take my name were practically unworn.So also were the surfaces they operated against: housings, apex seal grooves etc. At this point you begin to see new things fail as one of you pointed out. Cracks in the rotors, cracks in the spark plug region of the housings etc.
As iI've pointed out before (To much argument the reason for the relatively short engine life of the 89 to 91 NA engines is poor oil metering. Those driven at the lowest revs (automatic, lady driven;no offense) saw the least apex seal life.The pump employed then didn't put out enough in low throttle situations.
Result: cleaner HC emissions and in extreme situations 70k mile engine failure.

I don't take anything personally. I didn't say that a piston engine can't and many time doesn't outlive many rotaries. In fact I agreed with this comment by stating that the rotary as we know it today in terms of durability isn't really any better than it was 40 years ago. I just disagree with the comment that they typically don't live to 200000 miles. Any engine will if you take care of it. Yes a built engine is tougher be it piston or rotary, I also agree with that statement. Something that many rotary owners don't realize is that just because then THEY can't get the engine to run, doesn't mean that it is bad. Many many people get rid of their rotary cars because they think the engin ehas blown when many times it hasn't. Several of the cars we have bought and fixed up were from non rotary enthusiasts that owned them and figured that the engine was dead or dying. I've picked up a few of these for a few hundred dollars and had to spend next to nothing to get them running perfectly. Lots of people don't know how temperamental they can be and most mechanics don't know any more or hardly anything for that matter as to how to fix or even work on them. I'm sure a few of them in the junkyards are truly dead but I'm willing to bet that a huge majority of them can still run for a long time with the proper maintenance.

I don't need to rethink a single thing about scavenging. You can not honestly claim that a turbo that is a several psi restiction in the exhaust is going to promote scavenging. How? There is not a factory turbocharged car available that does this. A very custom setup can be made to work well though and if done properly can even have less exhaust backpressure than positive intake pressure. Most of the cars out there don't have this though.

The ability of an engine to scavenge effectively is most certainly affected by backpressure. If there is a restiction, yes it will still scavenge as designed but the effect isn't as great. Backpressure and scavenging need to work together to have a perfectly working system.

For the durability side, i totally agree with everything you wrote up there. I bet many junked cars would be saveable for piston and rotory, but i bet more for rotorys.


Look up turbo scavenged engines.

A turbine wheel acts like a one way valve, it is hit by exaust pulses that transfer a portion of there velocity into energy to spin the turbine wheel. When reversion tries to occur, and the low pressure area behind the exaust pulse tries to suck the pulse back, the turbine wheel is like a 1 way valve. Its of course much more complex then this, but i am trying to be brief. A pressure value recorded in a properly tuned turbo manifold is from a frequency of high pressure pulses. This is true with any exaust setup. Some are very much worse then others, but exaust is not a flow, its pulses. Its almost like trying to measure AC power with a DC guage. One is sustained pressure, one is high/low fluctuations. Both will make the guage move and record "backpressure". Exaust is honestly such an advanced thermo-compressable-fluid-dynimics topic it will boggle the mind. Espically as one learns the the best exaust systems taper smaller as they go, unlike the exaust you see on every car.

I am happy to see this can maintain intelectual disscussion.
Please check out turbo scavenging, its a facinating topic.

Actually I do have to concur somewhat in relation to scavenging in turbo engines. The problem is that there are so many terrible design exhaust manifolds out there that don't really direct the energy around very well. Big common chambers, unequal lengths, improper locations for a wastegate, too small of an exhaust wheel on the turbo, etc get used everyday and cause problems of their own. There are so many bad designes out there that have pressure waves, harmfully in terms of performance, interacting each chamber to another before they even get to the turbo. A simple log manifold is not going to do anything to help scavenge the engine. However a proper set of headers that collect at the proper angle and have the proper length feeding the turbo can help the engine spool up faster. In some ways a properly scavenging engine can help the turbo rather than the turbo helping to scavenge the engine.

Marcus Williams had a 3rd gen RX-7 drag car with a bridgeport. A bridgeport has high overlap and needs all the scavenging it can get. A set of good headers on a naturally aspirated engine is a must. A very bad setup can actually result in less power than a much smaller port with less overlap. Backpressure plays a big role in this as well. Since scavenging is using the gas velocity of one chamber to help suck the gasses from the other chamber and therefore more air into the engine, even a turbo should be able to do this. A small turbo or one with a heavy exhaust wheel will not do this. On Marcus' car, he wanted some mega power. He was using a big single turbo. He designed everything right. He had a very free flowing exhaust wheel and housing for as little pressure as he could get. He ran into a brick wall at about 20 psi of boost. The car wouldn't make any more power with added boost on top of this. He ran a pressure probe before the turbo exhaust housing to see how much pressure he had. At 20 psi of boost he had 23 psi of pressure in the exhaust. The exhaust pressure was holding back some of the incoming air and recirculating alot of exhaust back through the combustion process again. One rotor was definitely not helping to suck the gasses out of the other side of the engine through scavenging. He changed his turbo setup to use 2 smaller single turbos instead of one big one. The smaller wheels start to spin faster and the turbos also flow more air this way as we are no longer mixing all the exhaust energy from 2 housings into one unit. The result was far more power and much faster spoolup time. The exhaust pressure probes revealed that at 23 psi of boost he now only had 19 psi of exhaust backpressure. If this is the phenomenon that you are calling scavenging in a turbo engine then I can see how it could happen. Too many of the designs on the road right now do not just automatically promote it though.

I guess I can see the point if technically there is always scavenging going on but the effects of backpressure can cancel out the benefits if great enough. Is this what you are referring to?

You obviously do "live for physics"!!! I have a couple of very good books that are over the heads of most people but I'll bet that you could understand them easily. The first one is an older book and while a little technical is by no means difficult to understand.

It is called: Scientific Design of Exhaust and Intake System.

The other two are very technical and are probably an engineering textbook at some school somewhere. They are:

The Internal Combustion Engine in Theory and Practice Volumes 1 and 2. Volume 1: Thermodynamics, Fluid Flow, Performance. Volume 2: Combustion, Fuels, Materials, Design.

These are some interesting books. The first books is about 260 pages or so but the other 2 books are over 500 pages each. You'd probably thrive on these or maybe you already have them.

Do you have a book about turbos and scavenging? I'd love to pick it up and see what the technical wordings of all the effects are called. Maybe I really do know what is going on but just don't have the technical explanations for them down. Who knows? I already have a problem with putting what I am thinking into words sometimes anyways.

I'm always ready to have good technical chat back and forth. I don't take anything as hostile and I hope you don't either. If you do then it wasn't my intention.

Exellent post!!

I agreed with all of it.

Proper setup has always been, and will allways be crucial to makeing anything perform correctly. I was just assuming this would be considdered.

The books you mentioned, I have most of them within 10ft of me. I also have many more, i am sure you do to. Its been good to discuss with you. If you crave very very advanced engine disscussion topics, i recomend you check out
www theoldone .com and click BBS
Its got some seriously sharp minds on it, and nearly everyone does about 99% lurking and just makes a few posts. The quanity of posts is quite low, but the quailty is very high(generally) and most people do post a small book ;)
Just a caution, they are open minded, but wankels are generally frowned apon due to the cumbustion chamber design. Also, if a post lacks content, it is generally deleated by the mods.

Its a great place to lurk the archives, i learn there everyday.

-Luke

PS, check out that xylene disscussion, it got f%$*ed into outerspace!!!! I have never seen a thread get loaded with soooo much BS!! I tried to straighten it out a bit, you should help too!

Nice to see some posts that lack the b.s. factor. I would like to ask our two resident sages if they would like to switch the conversation to the torque/horsepower generated by rotary engines, how it compares to recip. piston engines, and what it all means in the "real world". I have some thoughts of my own, but I want to offer the topic up before I barge in the room. Thanks.

C.R.

When I have some more time I'll amswer that in depth. I'm pretty tired right now and I tend to be very long winded.

Fyi, I've had 4 rotary motors and three of them exceeded 250k until something else (accident, loan to my bro who blew it up, water pump failure-repairable, but sold to buy the '8). I felt the one my bro blew up could have gone another 100k - easily. The day before I loaned it to him it was turning 60lb oil pressure at idle. The next day, it turned 15 lb. A month later, it smoked, big time. The one that didn't last was a cheap rebuild that never ran quite right and ended up handgrenading when an oil line gave way (would have blown any motor) at 80 mph.

very good info!!

Refreshing to see some technical info. Nice!

This is a beautiful thing my friends... *now joining theoldone*. I love it when technically minded people get together :D.

SOme good info here, but also some ignores engineering and manufacturing basics.


1st: Why 13B? The main reason is Japan requires engines be certified by the government. Mazda first had the 10a, then 12a, and when they made the 13B in 1974 they wanted to note the tremendous changes so used the "B". Many people refered to the 74 and later 12A as a 12B, but officially it was stiill a 12a because of the government certification. The RX8 should be the 13C, but again can't because it is still a 1.3L engine so 13B. The 12a was used in the RX7 until 85.

2nd. The machine to make the rotor housing is VERY special. It is not like buying a cylinder boring bar. The shape is called a epitrochoidal curve (modified to allow for the diameter of the end of the apex seal). As such it has been cheapest and easies for Mazda to change the width for each engine. The 10a is 60 mm wide, 12a 70 mm, and 13B 80mm wide.

3rd. IS heat disapation. The rotary, we can all agree, makes a lot of power in a small package. As such it also makes a lot of heat. The primary way to capture and disapate this heat is by the cooloing water around the housings and the oil sprayed into the rotor. This heat is disapated by the surface area exposed to the water or oil. When you increase the width too much you do not get enough additional surface area to disapate this heat, so problems. Mazda found this in early work and did a lot of testing to determine the size they use. SO larger is not easy...

4. The rotary is bad on gas milage because it has too much surface area for the contained volume. A spherical combustion chamber, like a bubble, has the least surface area while a rotary is really a long thin retactangle. With present day materials the surface must be much cooler than the burning gases. As the flame approches the cold surface it actually stops burning because it is so cold and leaves some unburned fuel. Gas turbins use material that can run real hot, so don't have this problem and both cost a lot and are very efficient.

5. Theroritacal efficiency of a auto cycle (gasoline sparked) engine is primarily determined by the compression ratio. But high ratios also cause more heat and possible detonation - exploding instead of burning fuel. High octan fuel actually has less chemical energy than low octane fuel, but requires more heat to explode. The compression make more additional power than the lost power of the high octane fuel. But if the engine does not require it you will get more power and better fuel economy from the lowest octane fuel your engine can run on without detonation. With 10:1 the 8 does make more power, but will get better milage if you use regular and just live with the 5500 to 6500 loss of power when the ecu retards the timing because of detonation.

6. The ENGINEERING METHOD to determine displacement is the swept volumn in one rotation of the output shaft. PERIOD. But as above there have been many arguments, especially in racing, that the rotary is really a 4 or 6 stroke engine. Mazda uses the accepted engineering method.

There are many publications by Mazda from back in the 1970s (before the www). There was even some machinest that read about the rotary and made his own 7 or 9 lobe (don't remember) rotary engine that ran and was later shown in Popular Science.

Finally in the above post TRZ750 answered the original question of WHY 1.3L - and nicely done too.

However since someone mentioned 'doing the math' I'd like to see the differential formulas for calculating the minimal and maximal areas between the curves described by the epitrochoid/rotor... From there it's trivial math but if someone is going to do the math - then let's post the formulas.


As TRZ750 said better than I - As opposed to 1.2L or 1.4L or whatever... The overall size of the housing is fairly set so they generally only vary displacement in design by widening or narrowing the rotor as he said. The rotor housing can only be widened so far before inefficiency tradeoffs in combustion and mass creep in and ~80mm seems to be the generally accepted optimal tradeoff point. From 80 mm we get 1.3L (or multiples thereof if you like)

Just my $0.02 worth - really just want to call attention to TRZ750's excellent and quite correct answer to the original question posed.

I answered the question last year and even put a link to that thread in one of my responses to this thread.

Why is the rotary the engine of choice to begin with, other than the historical context? Seems that the 220hp 3.0 V6 in the Mazda 6 would give it better low end and performance overall, especially with a little tweaking. The 6 runs to 60 in roughly the same time as the RX8 and hauls nearly 300 extra pounds. Why not give it a V6 and let the gas mileage and perfromance numbers go up?

Let's start with its physical size and smaller number of moving parts compared to a V6 of similar HP.

rx8cited

Size? Just how much smaller do you think a renesis is vs a Mazda V6? I can GLADLY live w/ a 51/49 vs 50/50 for the added power, dependability, cleanliness, reliability, startability and MILEAGE of a V6. Just think - I could actually drive across NV w/o fear of running out of gas w/ only Bill the cow skull to keep me company. I could start the darn thing and immediatley turn it off and not worry about the dread startup issue. After the warranty expires I won't be sweating how I'll 'squeeze' it past the annual emission test. I won't watch big block Chevelles cruise by while I'm gassing up my tiny tank at the pump. And I won't be in the same class as Excursions and Maybachs w/ a guzzler tax. Oh yeah, w/ 4 seats a little torque would be nice too.

Yawn... 10 or 20 years ago I used to make the exact same brag on my beloved 12As vs reciprocating engines of that time. And ya know what? In reality it means DIDDLY.

Either the niche Rx8 will choke on it's thirsty smoggy 'tradition' as little more than a flash in the pan from a company w/ a poor history of supporting it's niche cars or they'll wise up and shoehorn a V6 in it w/ a hood bulge and go mainstream w/ an otherwise nifty car before other makers effectively copy the form factor w/ more realistic powerplants.

Then again a new C5 w/ a $15K end of model discount is awfully tempting w/ C6s already rolling.

First off the engine's lower center of gravity in relation to a piston engine makes packaging a little different. It's not only how much something weighs that counts but also where you put the weight. The rotary engine is also somewhat of a tradition with Mazda sports cars and racing. Is it the best thing out there? Probably not.

My advice to you is that if you think the rotary is so inferior, you should spend more time with other forums talking to people who sympathize with you rather than wasting your time here. You won't get us to say it is a bad engine since it isn't. Go buy a Vette or some other piston powered car and be happy. The rest of us not content on following the crowd will remain happy here. Thanks for stopping by, now go away since you are trying to start a war here.