Magnesium cylinder heads

[SOAP]

What are the main problems with using magnesium for cylinder heads? It is much lighter than aluminum, and has a thermal expansion close to that of aluminum. Aprilia has made a cylinder head out of magnesium, but no one else seems to want to take advantage of the much lighter weight.

 

[GregLocock]

In a conventional engine the head provides much of the structural rigidity of the assembled unit. So you want to be very careful about dropping its stiffness.

FWIW I know that for one particular engine there is a minimum stiffness we want, and it is somewhere between that supplied by the minimum castable thickness of cast iron, and that of the minimum castable wall thickness of aluminium.

Having said that I can see that in certain cases magnesium would have advantages.

Cheers

Greg Locock

 

[GraviMan]

Hmmm, I presume we are only talking about a structural reinforcer here? Not sure having magnesium in close proximity to a combustion source is that good an idea.

I have seen carbon fibre reinforced aluminium castings - most notably pistons! I would imagine these are cast in the absence of oxygen, to avoid carbon burning up. Any kind of stiff fibre reinforced component (head or otherwise) strikes me as a more practical proposition.

Maybe I'm wrong - who knows...

Mart

 

[sreid]

I recall that there was a problem with magnesium transmission cases due to dimentional changes from creep. This would also be a bad thing for cylinder heads.

 

[WilliamH]

I am familiar with magnesium being used by two OE engine manufacturers. These were engine block castings (investment). Both had serious problems in service. If I remember correctly, magnesium has greater stiffness than most casting aluminum alloys-----at room temperature.

As sreid suggests, creep is a problem with magnesium at elevated temperatures. In our testing of magnesium casting alloys from the 1970's the material begins to lose tensil strength at approximately 100 degrees C. At 120 degrees C creep is a serious factor. Screw threads are at risk and can fail at 120 C. The compressibility of the alloy at elevated temperatures is dramatic. There goes the fastener per-load.

There is a new magnesium alloy (can't recall the vendor) that has much better elevated temperature properties. Supposedly anyway . . . I sure wouldn't use magnesium for a cylinder head until I saw some very convincing life cycle tests!

WH

 

[SOAP]

thanks for all your inputs. I thought creep might have been an issue, but i didn't think that it kicked in until 260 degrees celsius. I might have been using the wrong numbers though.