Magnesium in piston alloys

[JOHNDOE333]

I saw a show the other day about the inventor of the compressed air powered car. The show stated he worked for a forumla one team for a number of years as an internal engine components engineer. They said he was know for first using magnesium in piston contruction to get piston weight in these engines down to around 250 grams. Wow! Does anyone know if this was an alloy and if so what might be the ratio of magnesuim to aluminum?

-Good day

 

[swall]

The common commercial magnesium casting alloys have a mag content of 90% or higher, with aluminum ranging from 4-10%.

 

[JOHNDOE333]

I see. So how did this affect piston to cylinder wall clearance? I am unfamiliar with the expansion rates of magnesium. Also, what about melting points in relation to aluminum? I guess I am curious overall as to the pros and cons of using magnesium. I do know that when I worked with this material in a machine shop setting that we had to keep the metal chips in a container with a cover for safety reasons due to its ignitability.

-Good day

 

[ivymike]

why not try matweb w/ your material questions?

 

[BrianPetersen]

Somehow, to ME, significant magnesium content in a piston would not seem to be a good idea. Magnesium burns (and has a low melting temperature) and if used in a piston, it's exposed to fire.

Magnesium alloys have been used for crankcases and gearbox housings since the air-cooled VW Beetle era.

 

[Lou Scannon]

I've always considered magnesium to fall between aluminum and plastic in the materials world... i.e. in terms of strength, density and ignition/combustion properties. I guess alloying can significantly modify properties, but a high magnesium alloy doesn't seem to be well suited to high performance piston requirements.

 

[EngineNut]

Harley Davidson racing 750cc two cylinder WR and KR model flatheads or the 1940's and early 1950's gave a choice between DOW Metal (Magnesium) and Aluminum for pistons. See piston section on copy of instruction sheet attached.

Ron

 

[tbuelna]

The piston structure is one where stiffness is important, so magnesium is typically less effective than aluminum, even though it is less dense. Forged 2618 aluminum is the material of choice for racing pistons, even in F1.

There was some use of AlBe alloys (AMS 7911?) for pistons in F1, due to its low density and high modulus, until it was made illegal by the FIA.

 

[JOHNDOE333]

I have noticed that with the short stroke of the F1 engines that they are not really running much higher piston speeds than that of a nextel cup engine which seems to be at or around 6000 ft. per minute. This seems to be the limit on most pistons even the 2618 as mentioned with its low silicon content and high expansion rates. Correct me if I am wrong.

-Good day

 

[tbuelna]

JOHNDOE333,

You are mostly wrong. In a high speed recip piston engine, the piston/wrist pin/con rod/rod bearing loads are mostly a function of piston accelerations, not piston velocities.

Piston linear accelerations (and thus inertia loads) are a function of crank and conrod kinematics, which in turn are determined (primarily) by stroke length and rod length.

F1 engine cylinder dimensions are typically way over-square (ie. bore-to-stroke ratio > 2.0). While Cup car engines have bore-to-stroke ratios closer to square (ie. 1.2?) Both types of engines have rules that dictate displacement, no. of cylinders, bore center dimension, max. bore dimension, etc.

Rod-to-stroke ratios in an F1 engine are kept as low as possible to minimize reciprocating mass. But these short kinematic rod ratios also produce very high peak piston accelerations, even though the mean piston velocity may seem low in relation to a Cup car engine. The rules for Cup cars essentially dictate the stroke dimension and block deck height limits that may be used, so the rod lengths are set by those rules. I haven't checked the Cup rules lately, but I believe all of the crank, rod and valvetrain materials must be a "magnetic metal" (ie. steel), so titanium conrods are not legal for race engines.

 

[JOHNDOE333]

I don't think I am too far off in reference to my piston speed data.
(stroke x 2 x rpm) /12 gives 5500 fpm for a cup engine with a 3.50 stroke @ 9500 and I have seen them turn tighter than that. For the F1 engine I am not exactly sure what the stroke is but I figured it was around 2.00" which gives 6000 fpm @ 18,000 revs.

I was mainly pointing out that not many automotive engines go beyond 6000fpm and I think that statement still stands true. As for the rod lenth in reference to rod/piston acceleration, I am well aware. Since most of the engines that I build are for race applications at local track levels (drag & circle track), I ussually opt for a long rod that is slow at top dead center and bottom dead center but runs the short rod down on the straights as I say (in the middle of the stroke). Since they both must wind up at true tdc and true bdc at the same time. I have found that rod ratios of 1.7 or better will give the motor characteristics of a long rod engine and 1.6 or less will act like a short rod motor. But I am an engine builder not an engineer so thank you for elightning me and teaching me a few thanks, that is always appreciated.

-Good day