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From what I can tell by reading, aluminium engine blocks are more thermally efficient than cast iron blocks - but it also seems that aluminium is more thermally conductive than iron.

That seems counterintuitive to me. How come an engine that conducts more heat, is also more efficient? Wouldn't you want to contain the heat in the combustion process rather than warming up the surrounding metal?

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    I'm not sure if you read this answer I wrote, which may provide some insights of aluminum vs. iron.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 3, 2017 at 16:35
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    its not at all clear what it even means for the block to be "thermally efficient". Can you point to where you read that?
    – agentp
    Commented Dec 3, 2017 at 16:57

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The use of aluminum is related to power and fuel octane satisfaction. You need to increase compression with aluminum heads to get back to the same power level as you had with a cast iron head. Increase CR more and you gain power and efficiency but are suseptable to spark knock. The ability to conduct heat away from the combustion chamber helps reduce tendency toward knock. The relationship feeds on itself. Increase compression ratio to gain power and recover lost economy due to heat loss. Conduct enough heat away from the combustion chamber to prevent spark knock or pre-ignition. Aluminum allows for lighter vehicle weight. Getting more power out of small engine necessitates increasing compression ratio. Greater vehicle fuel economy is the result of engine downsizing. Some engine systems run the coldest coolant through the (hottest) heads before circulating it to the block in order to make more power. The heat lost from the heads helps heater performance in the winter. Some of the coolant heat is also used to warm the transmission fluid in some applications, reducing warmup friction and improving economy. The auto companies would also like to see high octane fuel to be the standard so they could increase compression ratio or turbo boost even further than where they are today, and thus downsize engines further.

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Perhaps you should consider the effects of weight on efficiency as well ie the over-arching goal is fuel efficiency as opposed to volumetric efficiency or thermal efficiency.

Also to be considered is not only the conductivity of the metal but to take into account the surface area that the combustion gases are in contact with.

A point to consider is how much heat is lost to the block compared to the amount disappearing down the exhaust : this may be a more interesting area for efficiency improvement...

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  • I see, but factors as volumetric efficiency and exhaust aside - is it correct to say that aluminium blocks are thermally more efficient than cast iron blocks? Or is it the opposite, and the reason for the all-around better efficiency is because of other factors as mentioned previously?
    – Erik
    Commented Dec 3, 2017 at 16:06
  • If you define efficiency in terms of rapid warm-up yes...
    – Solar Mike
    Commented Dec 3, 2017 at 16:09
  • Defining efficiency here apparently isn't as easy and straight forward as I pictured it before asking my original question. But no, I don't define it as rapid warm-up, but rather to have most of the heat be used to expand the gases in the cylinder and not dissipate through the surrounding metal. It seems that aluminium isn't the kind of thermally efficient as I thought, and the alu engines achieve their overall efficiency on other factors
    – Erik
    Commented Dec 3, 2017 at 16:13

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