Use of MMC in engines 1

[ivymike]

Does anyone have information in regard to the applications of metal matrix composite (MMC) materials within piston engines? I understand that MMC (Al-SiC variety) is used in F1 pistons, and perhaps a few other applications.

I'm interested in finding a supplier for this sort of material, and if someone could give me some contact info I'd appreciate it.

I'd also like to know what the material strength and fatigue properties of various MMCs are, so that I may determine their suitability to an application that I have in mind.

 

[GregLocock]

War story alert!

for many years we have struggled with driveshaft whirl, since in the past we've resisted going to a two piece driveshaft. Our driveshaft supplier came up with an MMC driveshaft, which improved the whirl speed (basically root E/rho) by 15%. He also gave us a video showing what happened at whirl. A steel or aluminium shaft flips out, and then twists itself into a knot and breaks in half, which is fairly exciting, at 5000 rpm, but not too bad. The MMC shaft flipped out, and then exploded into a hail of shrapnel.

The supplier was Dana, perhaps they could help you.

Cheers

Greg Locock

 

[ivymike]

hehehe... thanks for the war story! The E/rho of MMC was precisely what interested me in the first place.

Lemme take a guess about your driveshaft, to see if I'm on the right page. The trouble with a 1-piece driveshaft is that it's got a solid center, and thus a low whirl frequency, right?

 

[GregLocock]

No, its a rear wheel drive propshaft, so baically a thin walled tube. As you increase the wall thickness the mass of the system increases, in virtually direct proportion to the stiffness increase, so the frequency doesn't change. So you change material, but realistically steel and alumin(i)um are the only viable options, and of course E/rho is the same for them.

We used aluminium sometimes because it is more highly damped than the steel ones, so it is better for gear noise etc. We could also get it in a bigger o d, which does improve the frequency a bit.

One rather nasty solution is to use a carbon wrapped aluminium shaft - this only buys you about 5-10% or so, because the stiffness mismatch between the aluminium and the carbon results in an inefficient use of teh aluminium. The cost is ridiculous.
Cheers

Greg Locock

 

[ivymike]

I guessed a solid shaft because you called it "one piece," and I was thinking of the endcaps and the tube as separate pieces. My new guess would be that a "two-piece" shaft has a support and perhaps a joint midway between the tranny and the rear diff.

In regard to "roughly proportional," if I remember right, that's true for thin-walled shafts, but not solid ones. For the thin-walled ones, the only ways to bring up the frequency that come to mind are to increase the OD of the tube (perhaps maintaining the same wall thickness), and to reduce the length of the tube. I don't have the eqns in front of me, so please correct me if I'm wrong on that. I assume that something gets in the way if you increase the diameter, which is why you'd strap a bearing in there? A big, long, fat shaft is probably more expensive and prone to dents, dings, and miscellaneous abuse too... Please tell me more about the constraints, if you get a chance.

I once had a client ask why I'd suggested a hollow shaft for his (high-speed) application. He'd expected a thick solid shaft. I said "because it increases the whirl frequency," to which he replied "how can it possibly do that?" I tried to find a polite way to explain the relationship between the area moments of inertia and mass per unit length, and I think I pulled it off w/o embarassing him. Tough to say though...

Anyone else out there - I'm still looking for a MMC supplier (haven't called Dana yet).

 

[GregLocock]

Yes, a two piece shaft has a softly mounted centre bearing, and a UJ to break the tube up. It still has a whirl mode, of course, but at say 600-1200 rpm, where there is less energy to break things. This is the cause of driveline shudder which you get on some cars at around 20 -30 mph.

OD is more or less fixed by clearance to floor (above), exhaust (right) fuel tank (left) ground clearance line (below), and what the customer actually cares about, the size of the tunnel. Cheers

Greg Locock

 

[ivymike]

Split conrods are not uncommon.

I can't (of course) comment on the usefulness of MMC in that application.

 

[ivymike]

um... by "split conrods" I meant the fractured-cap variety.