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Begging For Knowledge : Lighter Cast Iron Replacement

Hello folks.

I am not a materials engineer. I have been reading books on solid mechanics and materials science to help improve my understanding though. I am faciniated by machines and also my hobby of amatuer motorsport. I truly love machines and my favourite machine is the rotary internal combustion engine. Here is a webpage describing the engines basic operation:

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I am serious about either trying to convince other people to help me tackle this problem, or to pursue what I can myself. One of the great things that limits the maximum sustained RPM of this internal combustion engine is the significant inertial loads of, in the case of Mazda's 13B design, two cast iron rotors weighing close to 4kgs each.

As this engine design does not require a complex valvetrain, and for the simple fact that unlike piston engines where the rods and pistons go through full cyclic stress reversals twice for every crank rotation, the rotary engine *SHOULD* be able to rev significantly beyond what a piston engine can.

The rotary performance community has been suppressed by a lack of real aftermarket research and development. Yes some progress has been done and its relatively easy to modiy a 13B turbo for 500 flywheel horsepower on

98RON pump fuel, but I know more is there to be had.

What happens at high boost and high RPM is that the mass of the two rotors, being close to 4kgs each, elastically deforms the eccentric shaft. Then what happens sometimes is that the rotors themselves make contact with the epitochroid shaped housing, then it's all over the engine goes into critical failure.

The rotors themselves are some sort of cast iron. I dont know the exact alloy here used.

Q: How much does it cost to have a component analysed for its material specification? Is there any near enough way to find out without involving a lab?

I know that the rotors are cast, the molten metal being fed centrally into a mold for three rotors - Mazda's new process as they said it is better for "uniform metal distribution and more precise rotor balance". There is corner on each of the three rotor angles, and these corners are induction hardened using a pin point automated induction hardening process that won the designed of that a 2002 award of excellence. You see the wear character of the three slots at each rotor point is important as otherwise the apex seals and the corner seals that provide engine sealing will wear the rotor prematurely leading to engine failure. As it is now, rotors still wear in this area.

Now the eccentric shaft (the output shaft) on which the rotors are geared to through the center of the rotor, it is well balanced. There is an aftermarket eccentric shaft that has a larger middle support bearing and other features to help stiffen it up. The rotors themselves due to the gearing rotate at one third the RPM of the eccentric shaft. One of the issues though is that this movement is eccentric and occurs in a planetary sort of motion.

There is people who have tried lightening the factory rotors. This helps but is not of any significant gain.

Under high boost there is cases of the rotor face collapsing in on itself. The chamber pressures obviously plastically deforming the rotor.

One modification is to snap ring the rotor gear to help prevent it from walking away from the rotor itself under high load.

Interally, there is a complex shaped web of cooling fins that directs engine oil throughout the rotor. It was felt that casting is the easiest method, but perhaps two billets could be machined and then joined into one component.

What I have been striving for is rotors that say using aluminium alloy or magnesium alloy etcetc are significantly less heavy, stronger to stop the combustion face collapsing problem and more druable for greater rotor life. There is modified GTR Skylines now doing 12000rpm using six clyinder piston engines! Surely with the basic engine physics to the rotary design this could be exceeded.

A market does exist for such a part. It is nowhere near as large as the piston aftermarket, but its there. Typical uses would be for experimental aircraft and performance rotary engined cars.

Q: Does anyone have a suggestion on a possible material spec?

If anyone would like to contact me, my email removing the nospam is snipped-for-privacy@yahoo.com.au

Thanks.

Reply to
Bathrone
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In your reading of mechanics and materials, surely you have come across tables of strength and stiffness to density.

You can then use those tables to try to find lightweight materials with high strength and specifically attempt to see if Mazda (who isn't completely dense and unknowing) might have managed to make a reasonable materials selection?

If you haven't found those tables yet, then you should do some library or internet research to find some tables on the subject.... take advantage of the brains and organizational efforts of others.

However, watch out for overly simplified tables and tables that emphasize Ultimate Tensile Strength vs Yield Strength and which of these is more critical for your application?

I suspect that the materials selection is much more involved than just strength and density.

Maybe someone will run some calculations for you.

Surely someone will attempt to suggest composites of some kind. Or surely somebody will mentin titanium. Or some of the best wrought or precipitation hardened aluminum alloys.

Reply to
jbuch

Yes thanks JBuch.

I have researched that area, and given aluminiums strength vs weight it is attractive. Magneisum also.

However the complication I see with aluminium is one of thermal expansion. The rotor needs to deal with the heat of the combustion process and not get out of tolerance when hot. The other issue I forsaw was one of cyclic endurance - will an aluminium part suffer a cyclic failure in longer term testing?

I very much acknowledge the complexity of the material selection you mention of, it is complicated and the more I research the more I realise there is many issues.

It has to be strong, durable, light and operate at elevated temperatures. The temperatures Im not sure on, but given oil cooling they wouldnt be heaps.

Im interested in all possibilites. berrylium? I was also reading about composite structures where in some space components boron was added to an aluminium casting in a lattice or similar.

It is fallacious to assume that mazda know all when they arrived at their material specification. Mazda is faced with a broader and different range of issues than the aftermarket. The design goals are for mass produced engines. They are not designing their parts for maximum RPM, maximum power specialist applications in low volume production.

I am committed to either convincing other people that if they could produce this product they would have a market, or to do enough research myself to be able to give a solid shot at it.

Reply to
Bathrone

Aluminum and magnesium and Titanium will all have higher thermal expansions than steel.....

Ferrous alloys have the advantage of a "fatigue limit" where exceptionally long life in fatigue is possible at stresses below the "fatigue limit". Aluminum requires more control of long term cyclic loads to avoid fatigue failure.

Thought ONE

Thought TWO

------------------------------------------------------------------------ I find myself with almost nowhere to go..... It is complicated, you admit, but then those who were successful in the first place are downrated -- because they are "commercial".

Have you read Carroll Smith's "Engineered To Win"? It is about the selection and use of materials in high performance auto racing design.

. . . . . . . . . . . . . . . .

Is this really a new problem? Guys have wanted to really torque up the high end performance of the rotary engine for over 25 years. So, is this interest NEW or NEW TO YOU??

There is probably some real good practical knowledge lying around in the minds of some racing engine folks who have known about rotary engines since the 1960's.

I would try to find that knowledge, more so than asking for help making a materials selection from the basic information that you have.

History.... it reminds me of what happened. What people thought. What people knew. What people did. Why.

Reply to
jbuch

Thanks JBuch

Yes I own and have read all of Carroll Smiths book, shame about his passing on.

Im not downrating Mazda, but the simple truth is that their design is not geared towards high power, high rpm, high boost specialist applications. They successfully mass produce the 13BT for mass produced mild sports cars.

The interest is new to me, but the industry in America has been stunted through no real motorsport class like the old IMSA. In Australia there is various fields of competition classes which has lead to the need for better products. We also have a stronger drag racing presence with rotorary engines.

I have amassed many of Toyo Kogyo (old mazda) technical submissions and papers, some good info.

I am now thinking

  1. Carbon / carbon rotors
  2. Metal matrix composite rotors

Honda and Toyota have had alot of wins with using aluminium metal matrix composites in their engine blocks and other components. Carbon / carbon would be ideal but expense?

Reply to
Bathrone

I've seen titaium pistons in IRL. I don't think that Al and Mg will stand up in racing environments. Ductile iron has many good characteristics such as low cost, easy to machine, good vibration damping, good strength... Ti is expensive and difficult to machine parts with.

Your part is most likly ductile iron or variant of it. You can test the existing parts chemically with a OE spectrometer (arc/spark quicker and cheaper, less accurate on as cast ductile).

You have gone down a long and expensive path...if there was an easier way to do it mazda would have probably done it already.

Good luck.

temperatures.

Reply to
Wild Turkey

You need to choose a market to go after. It sound like you like to go after the high performance crowd with the light weight and high revs.

High revs are your limiting factor. The lubrication of the rotor seal gives up the ghost as you pass 12 grand. With exotic materials for seals, designs that make replacement easy and the best in lubrication you might be able to get 18,000 or better and a few hundred hours of life from the seals. Make the surface the seal runs against cheap and easy to replace or as hard and tough as Wall Chromalloy can get it. I see cast iron and stainless steel in the engine for the lower expansion at high temperature. There are alloys of stainless that coefficient of thermal expansion of zero and you will need so some low expansion parts.

The other rout is to build a throw away engine. That may not be a bad choice in some cases. If you can make one that has a reasonable life at 11,000 rpm but will run 2 races at 18,000 rpm it might be a winner all around.

Pick a path and proceed. Fast light cheap you get two.

Gordon

Reply to
Gordon Couger

The usual limitation or worry with carbon/carbon is oxidation and strength loss. Essentially, one has an expensive fiber reinforced charcoal bricquette.

It is common for people to state that oxidation is not a problem if the temperature is less than 700C, 600C and some say 500C and less.

My informatin on oxidation of carbon/carbon is not recent, however.

I think it is still true that there isn't a well perfected oxygen barrier coating to defeat oxidation.

However, that information too is dated.

Reply to
jbuch

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