Being a neophyte with rotary engines I was wondering how come the rotor side housings are cast iron instead of aluminum?

It seems to me these housings have only intake and exhaust ports, oil passages, water jackets and journal surfaces for the rotors and some sort of side seals on the rotors. I would think that a high silicon aluminum (like most Honda cylinder liners) or nickle plating (Nikasil aluminum cylinder liners) would be able to handle the heat and wear associated with this environment. If not there are coatings available that bond well to aluminum and easily meet the other environmental demands.

Of course these alumium side housings would cost more than cast iron. But if the housings are monolithic and one can get away with the high silicon or nickel plating the cost increase would be small compared with the significant weight reduction of maybe 50 lbs.

Think about this; A 1.6 liter two rotor (+23% displacement) with aluminum side housings, the simplified lower cost intake of the low power Renesis and the same power band as the low power Renesis. We would then have have a motor with:

1. About the same weight or slightly less weight due to aluminum side housings and simple intake system.

2. Nearly equal or slightly higher cost due increase from aluminum side housings but a decrease from the simplified intake.

3. 260 hp and 200 lbs-ft torque.

With this engine the most fun to drive four seater would now be perfect. Then stick this engine into a new RX-7 that weighs another 200 lbs less. Yes!!! Hey it's 4:15 AM. I can dream, can't I.

 

Racing Beat currently manufactures aluminum side housings for non-Renesis engines. Expensive, ultra light and extremely durable. They have a 'flame sprayed' coating that gives a very hard and durable surface. Search their website. www.racingbeat.com

I'm sure that they will eventually offer them for the Renesis. They have an RX-8 in house and are developing products for it now.

 

yeah, i had the same question long ago, and wondered if that's how the RENESIS became so much lighter... but then i relized that they were comparing it to the 13BREW (not the 13B), and it's obvious it'd be lighter without the TT's...

anyways, yeah, i don't know why the side housings are iron... eccles??

oh, and btw... when thinking new RX-7, think 600-700 lbs. lighter...

 

That would make the RX-7 lighter than a Miata. I doubt this can be done using the RX-8 chassis. What we are looking at when making an RX-7 from the RX-8 chassis is the removal of the rear doors, a section of the chassis about 70% of the length of the rear seat area (need room for a reconfigured fuel tank) , possibly some thinner sheet in the floorpan since this area is shorter and probably the addition of a hatch. Couple this with with a shorter drive shaft, PPF, exhaust, hydraulic and fuel lines and I think we are looking at about 200 lbs.

 

hey, all i'm saying is that the target weight is 1000kg, okay??? just what i hear ... from "people"...

 

Some facts:

1. The production 13B rotor housings are FORGED! All the factory Mazda racing housings are CAST! Mazda production rotor housing are the strongest & are designed to last. CAST it's to save money & why bother forged, since they are replaced often.
2. Racing rotary engine parts serve their purpose just to last if not less than 3000 miles (extreme high figure for racing) of racing.
3. Aluminum side plate housing will only serve for the purpose of racing ONLY... meaning rebuilt will be needed & they will definetly go out of spec.
4. Race rotary engine parts WILL not be as reliable as factory production engine part.
5. You always sacrifice riability vs. performance.

The RENESIS it's much lighter cos the rotors are use of stronger material...stainless steel vs. aluminum. And, of course some major 13b innovations i.e.: composite intake.

 

Are you sure? I had heard that 1000 kg is the target weight for the NC Miata. I guess Mazda could make a Miata and an RX-7 at 1000 kg but then they would probably be the same car except for engine. Heck I doubt they can make either at this weight if using the RX-8 chassis. Never the less I will be peachy if either car comes in at 1000 kg.

 

Why will a hard surfaced aluminum side housing go out of spec? There are several hard coatings available that exhibit much better wear resistance than nitirded cast iron.

 

an interesting thought... maybe both the RX-7 and Miata would share the same modified FE platform, maybe even some shared suspension bits, with different body work, a few different curves, totally different engines, different trannies, one's a hard top, the other a convertable... hmmmm... not a bad thought.

knowing that the flip ups are staying on the next gen 7, i wonder if the Miata would keep the projectors to further separate it from the 7 in a situation like that... ???

 

As, some of you might know the rotary engine itself flexes (housings, plates) due to factors of vibration, thermal expansion, & thermal efficiency. Using aluminum will just not cut it on the long run, maybe just for racing. If I can remember it correctly hard surfaced Aluminun have more heat expansion than the cast iron plates.

Also, if they were to make those RENESIS side plate housing of aluminum, the side port hi-temp exhaust gasses will expand the port surface more than the other surface of the engine which could cause uneven wear with the side rotor seals & the side plate housing & maybe even stress cracks... Even thou it would be nice to have an all Aluminum rotary engine, but there are factors that will limit it's strengh design & the type of aluminum used.

 

amgtotroise,

I don't know any other way to put it except that your statements are incorrect. It seems to me that your knowledge of material science and structures is limited. In response to your last comments:

All engines (otto, diesel and wankel) are subject to vibration with the wankel being the lowest. Vibration is a non-factor.

Thermal expansion of the side plates has no bearing on the performance of the engine. If the side housings grow the rotor sealing is not affected. The only effect is the long *** bolts that clamp the engine together will stretch a tad more. If the stretch is too high for the current bolts the stretch can be compensated for by design or by a material change to a higher tensile strength steel or low modulus material such as titanium.

What the hell does thermal efficiency have to do with this topic?

Now there has to be a logical reason why Mazda uses cast iron side housings. Probably cost. I wish we could discover the real reason.

 

... me too.

 

Not to discredit anyone, but in common sense...

To be honest! It's not about cost. Did you know that the 13G 13J and the R26B uses the same process as 'CAST' iron side plates as well & while each engine cost 220,000 to 1.2 million at the time it was produced in the mid 80's & early 90's (Teams could only lease it at the time from Mazda)! Mazda could have easily used Aluminum side housing plates, but why not? it's not because of cost...it's because they didn't last for riability issues in either racing or production.

I just don't think that Mazda after researching for many years in rotary engine, some company like Racing Beat can re-engineer side plates out of aluminum. IT doesn't work that way. But, if you want, try them... & I'm sure your engine will not perform any better & your engine will not last or hold together.

And, yes they rotary engine by itself flexes... not like many people think... & please I welcome anyone to do your research.

 

amgtortoise -
In my original post I mentioned a 'flame sprayed' coating. Very hard vapor deposition coatings make the Racing Beat aluminum side housing VERY durable. RB produced some very high horsepower engines that had little to no wear on the side housings. There are racers using these with good results also. And yes, they are expensive.

http://www.racingbeat.com/resultset....rtNumber=11072

I'm not sure if the coating technology was there in the 80's, which would be a good reason for Mazda not to use it.

The notion that the best ideas and parts can only come out of the factory is very flawed. Racing Beat is a very established and long time rotary development company. Pure racing and even the performance aftermarket is much different than developing products for the mass market.

 

Well it's been a year since this topic came up. Does anyone know why Mazda continue to use cast iron side housing instead of aluminum?

Cost is probably one of them. Aluminum side housing from Racing Beat is like $1200 each! I searched for the side housing for the old 13B and they are in the $350 range. You would think that mass production would drive that cost down enough for viable production. At least for a Mazdaspeed version or the next RX-7?

I don't think heat is really the problem. Like someone else already said there are coatings available to withstand the environment.

 

FWIW: Heat can play a role. The cast side housings and the aluminum rotor housings have different thermal exmansion rates. While not enough to affect normal and sometimes even hard use, this could surface on an overheated engine. It is very bad to overheat a rotary. Even once could be catastrophic. This is because the rotor housings themselves have been known to warp just slightly. Nothing you can see visually. However it is enough to cause the water o-rings to not seal well and the engine leaks water from between the housings. When you open up an overheated engine you can tell if it has ever been overheated or not. The housing surfaces are usually highly discolored.


Accountants have almost as much to do with the final product as the engineers do. Cast would be so much easier than aluminum. They'll sacrifice a little weight to save some money. Especially if they can hit their target vehicle weight with the easy way. If they really wanted to make the lightest car they could, it would be much more expensive but definitely possible.

Several years ago one of the major airlines (American?) found that removing 1 olive from every salad in first class only would save them over 2 million dollars a year! Guess what they did? A few pennies adds up big time when you have a huge production run and I guarantee that changing to aluminum would cost more than an olive per engine. The rotor housings have been aluminum for 40+ years so who knows why they aren't cast too.

 

While we discuss materials, I think Mr. Tortoise is also wrong about the rotor construction - I don't think it is even stainless, just a cast iron. All the pieces I have seen are a little 'rusty'.
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It is definitely not stainless.

 

So would the aluminum side housing help with the warping issue since the thermal expansion rate of all the housings would be the same?

I know this would probably cost too much money for the amount of weight it would save. I'm just thinking about what a future rotary would be like. Well people have paid more money for less weight savings so maybe the aftermarket will pick up on this.

 

it already has, for quite a long time now... those gold-plate-priced RB ones are an example.

 

Thermal expansion - which part of the side housing is hottest? From the L plug to the exhaust port. Continuously heat up and cool off such a small area of an alloy housing and it will warp or crack in time, even the super expensive race ones.

Torsional rigidity - Pistons go up and down, a rotor goes round-and-round. Not only must the housings resist crazy twisting forces, they must also withstand lateral forces from combustion trying to push them apart. There are twisting forces in all sorts of directions on a rotary engine, an alloy housing would just crack up.

You can of course make super expensive, super strong intermediate/end housings, but they will still crack up after time (they are not designed for 100,000 kms of city driving!). There really is little advantage in developing an alloy housing when iron does so well. The rotary already has a huge weight advantage, the engine is already tiny, it already outputs the power of a 2.0 liter...

And besides, who knows the most about building rotary engines? Not Beat Racing, but Mazda...

 

Beat Racing? You mean Racing Beat!

 

Why couldnt we make it so that instead of having 3 side housing we had six so that the center housing was two pieces and the end housing were to so you could just replace the wear areas and keep the parts that dont wear You could make the housing out of aaluminum and replace them as needed.

 

...how would that make a difference??? if you think through your idea again, i think you'll agree with me that it's a rather silly proposition.