I tried searching for an answer to this question, but I couldn't find any related posts.

How come rotaries have iron side housing rather than aluminum? Is it because of cost or did Mazda R&D decide on iron rather than aluminum side housings? Just looking for the reason why the side housings are iron.

IMO, iron side housings are like iron blocks: really outdated!

On another note, why are the rotors made of iron, too?

 

$$$

 

Besides cost, I believe cast iron side housings offer better heat retention than an aluminin side housing. An aluminin side housing may not be suitable for everday street use. The engine will generate or be exposed to a lot of heat, and aluminum just isn't strong enough to retain/dissipate that kind of everyday heat. Though a good concept, I believe an untreated aluminum side housing would deform under the immense heat transfer between the housings and the surroundings.

As for aluminum rotors, I believe at low-rpm, the rotor would not run as smoothly as the cast-iron counterpart. From what I have heard, lighter rotors do not increase power, however, they do affect how smoothly the engine runs. If Mazda had its way, perhaps they could spray the side housings and rotors with Cermet.

 

Lighter rotors also allow the engine to rev faster- always a good thing, especially when trying to get to/through the power band.

 

Aluminum side housings DO NOT WARP. They are simply used for weight reduction. There is also a benefit of having a slightly better "bearing" wear surface on the side housings.

This is a quote from the RB website about their housings:
"After each aluminum housing is cast, using A356 Aircraft Alloy, it is heat treated to “T-6” hardness and then CNC-machined prior to being flame-sprayed and ground to a final finish. The greatly increased wear resistance is the result of this aerospace originated flame spray (or plasma spray) process that imbeds into the wear surfaces a proprietary carbide material whose durability greatly exceeds that of the nitride surface found on the stock cast iron housings."

I'm not sure about having a greater durability than the nitrided surface on the irons. RB may just be trying to sell parts, but the finish is probably comparable to the nitrided irons for a friction coefficient and hardness. Since the aluminum has a higher heat transfer coefficient than the irons, there may be a small reliability benefit because it will get the heat away from the combustion chamber in less time. But it will probably never be noticed.

Lighter rotors will definately add more power (less rotating mass = less parasitic drag). The engine wont have to use as much energy to make itself move. Probably not a very noticable amount of power though it will rev much faster. There will be more NHV because the smaller mass will have less of a dampening effect on the pressure waves created at each combustion event. But I would guess that it wouldnt be much worse than an old V-6. They also have a combustion event every 120 degrees, but there is alot more friction surface on the rotating parts of a v-6 to dampen the shocks. I would be most worried about the reliability hit you take when lightening rotors.

 

dont forget that the increased conduction of heat from the combustion chamber by the aluminum will lower the engines VE alot, less power/worse milage

 

It will not have more power because of rotating mass, just like a lighter flywheel doesnt give you more power. It will have less reciprocating mass which might yield slightly more power but I doubt it would be alot since there isnt anywhere near the reciprocating motion of a piston engine. I know it is just a technicality reciprocating vs rotating, but I wanted to clear things up a bit.
lighter will rev faster though, just like a lighter flywheel.

 

lighter rotors will give you more power. Same thing with lighter pistons, rods, crank, valves, springs, retainers, etc... just like a lighter flywheel or clutch will let you put more power to the wheels. Same thing with a lightweight driveshaft or even lighter rims. Instead of using energy to move the mass of the engine or drive train from one point to another, the energy is instead used to push the car forward.

 

IIRC aluminum has a stress life. Steel and iron can be pushed on again and again, so long as its not pushed beyond its breaking point. It will retain its shape and strength. Aluminum on the other hand has a stress life. That means that it can be stressed only so many times before it breaks. This has led to some rather nasty aircraft accidents particularly the roof tearing off of a Hawaii Air (I think, not sure) flight back in the late 80's.

The stress life is referred to in cycles, 30,000 cycle life = 30,000 bends and pulls and pushes before its unsafe. For a thicker heavier piece of material, like a housing, I'm sure that is a very large number.

For a daily driver, street car aluminum may not be the best choice because of its heat dissipation. My carb'd 12a can get a bit finicky when I try to fire her cold, if I had more aluminum in the engine it would save even less latent heat and be harder to start.

If any engineers or metallurgists on here could add something to that or tell me I'm totally wrong please do.

 

All materials have what you are calling a 'stress life'. It is called the fatigue limit and depends on the number of cycles to which the part is rated. Aluminum is more sensitive to fatigue that steel but this would not be a factor in a properly designed side housing.

Dokta is incorrect. Lighter parts whether rotating or reciprocating do not cause an increase in power output. At a constant speed the power output would be the same. When ACCELLERATING lighter parts consume less parasitic power allowing more power to be used to accelerate the car.

 

The engine produces a certain amount of power, and it has to do lots of things with that power. Like keep your lights on, run your fuel injectors, pump your speakers, turn the power steering pump, turn the flywheel, clutch, drivetrain, etc... The less work the engine does moving those things, the more power available to make the car go.

Less rotating mass will put more power to the wheels. Ive seen it dozens of times on the dyno. This is first hand experience. The engine is not generating more energy. It is wasting less energy accelerating its moving parts, and putting that otherwise wasted energy where you need it. Making the car move. In otherwords, you have MORE POWER.

Power output at not measured at a constant speed. When I put a car on a dyno, we start at a low RPM then accelerate to redline. That is how power is measured.

tmiked, im not trying to prove you wrong or anything, im just trying to make sure that everyone understands exactly what is happening when you have less rotating mass. let me know if you still dont understand what im saying

nevarmore, your logic is faulty with the heat dissapation in your engine. If you park your car outside and it is 60 degrees out. That engine will be at 60 degrees after 12 hours, no matter if was made of aluminum or iron.

I dont remember what the equation is to find the time for the heat to be transferred. Maybe if someone else knows it, they could figure out what kind of times we are talking about.

 

True. What I was getting at is that an aluminum engine will reach that 'cold' temperature sooner than the iron one.

 

VW successfully used aluminium side plates on their rotary back in the day but pulled the plug due to investors becoming uncomfortable with the whole rotary thing.
The issue isn't aluminium itself but the surface coating. The top of the side plate(intake) is a lot cooler than the bottom(combustion).

 

You are not measuring power and that is NOT how power is measured by any scientific method. Sounds like you run a dynojet.

 

Nevarmor is more correct.

A material's fatigue life is dependant on the level at which it is stressed. The less you stress a material, the more cycles it can handle.

For iron/steel, there is a level of stress below which the part will last for ever. For aluminum, no such endurance limit exists, which is why certain aircraft parts are replaced after X take-offs and landings or Y hours in service. This is also why dude on monster garage was able to get the helicopter stabilizer for $6.

 

I was under the impression that the housings were iron because the rotor were iron. Aluminum would expand differently than iron with heat causing bad things. With both things iron the heat expansion would be about the same. I'm sure the rotors are iron because it makes it run smoother.

Of course, with modern technology they could probably make an all-aluminum rotary engine without much problem.

 

I wonder why they didn't when they created the RX8 motor. I'm sure the chassis guys would jump at the opportunity to knock 50lbs off the front of the car if they could.

ps -Hows the Roc? I grew up there