im curious what properties of aluminum make aluminum connecting rods expand with heat and stretch (reversibly) when pulled as opposed to steel which apparently does not.
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You're talking about the "coefficient of thermal expansion."
There are actually several ways of interpreting this. One can either refer to how much an object changes in LENGTH as temperature increases, or you can refer to how much it changes in VOLUME. The so-called "linear coefficient", and "bulk coefficient" in engineering jargon, respectively.
In the case of a connecting rod, it's useful to talk about changes in length. The linear coefficient of steel is around 11-13 ppm / degree C, whereas for most aluminum alloys it's around 23 ppm / deg. C. In other words, aluminum expands roughly twice as much.
Let's say the connecting rod is 20 cm long, or 200mm. Room temp is about 30 deg. C, while normal running temperature of the engine oil (and hence the crank and connecting rods) is about 120 deg. C. for a difference of 90 degrees.
So for aluminum, the change in length is 90 * 200mm* 23/1,000,000 = 0.414 mm.
Half a millimeter may not seem like much, but it may be enough to, for example, put a bearing out of tolerance.
For steel the change in length is 90 * 200mm * 12/1,000,000 = 0.216 mm.
As a side note, the linear and bulk coefficients are not fixed constants, they generally increase in value as temperature increases.
There are actually several ways of interpreting this. One can either refer to how much an object changes in LENGTH as temperature increases, or you can refer to how much it changes in VOLUME. The so-called "linear coefficient", and "bulk coefficient" in engineering jargon, respectively.
In the case of a connecting rod, it's useful to talk about changes in length. The linear coefficient of steel is around 11-13 ppm / degree C, whereas for most aluminum alloys it's around 23 ppm / deg. C. In other words, aluminum expands roughly twice as much.
Let's say the connecting rod is 20 cm long, or 200mm. Room temp is about 30 deg. C, while normal running temperature of the engine oil (and hence the crank and connecting rods) is about 120 deg. C. for a difference of 90 degrees.
So for aluminum, the change in length is 90 * 200mm* 23/1,000,000 = 0.414 mm.
Half a millimeter may not seem like much, but it may be enough to, for example, put a bearing out of tolerance.
For steel the change in length is 90 * 200mm * 12/1,000,000 = 0.216 mm.
As a side note, the linear and bulk coefficients are not fixed constants, they generally increase in value as temperature increases.
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i dont know...