As far as time to heat/cool an engine, here are the specific heats and densities of aluminum and iron:
Aluminum .91 J/(g*K)
Iron .46 J/(g*K)
Aluminum 1.00 g / cm^3
Iron 7.87 g / cm^3
Given design differences for the two materials, you'd really need actual engine masses for any sort of decent comparison. Since .91 / .46 = 1.978, you can say that unless the cast iron block is more than about twice as heavy as the aluminum then its thermal inertia would actually be less. In practice, what sort of weight differences are actually realized?
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The biggest difference in construction material is at the cylinder head, not the block. The thermal conductivity of aluminum is much higher than cast iron, meaning less chance of hot spots and pre-ignition. So, an engine with an aluminum head can have a higher compression ratio. Hig compression = lower fuel consumption.
Many engines have an iron block and aluminum heads. High compression is the reason.
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Just an FYI, all aluminum block engines have iron sleeves in the cylinders.
The exception is single cylinder consumer grade engines.
I don't think alluminum is hard enough. Hard metals to resist ware, light metals to hold the whole sytsem together.
As far as temperture regulation, it is your coolant system.
Your coolant only gets cooled IF your termastat diverts it to your radiator.
Every automotive I have ever drive uses temperture regulation to cool the engine, because any 4 stroke engine is a very effective heater.
Shell flex fuel, tried their new stuff, kinda liked the old mix.
Interesting thing I noticed is that a 6 cylinder Honda Accord Engine weighed much much less than a 4 Cylinder Toyota Corolla engine I had in 2000. Honda engines are all aluminum while Toyota at the time was using iron block, aluminum head engines. Both were mid nineties engines.
It also helps that Honda engines and trannies are so much smaller than Toyota engines/trannies.
It's really hard to get a head gasket to seal with an Al/CI combo - not impossible mind you, just really challenging for 200K miles and beyond.
I like Honda because they have been making Al/Al/CIL for longer than most anyone else. The d-series SOHC VTEC engines with three layer metal head gaskets are among the most durable out there.
My Buick 3800 V6 is CI/CI and is OHV (push rods and cam in block). It's as old school as you can get but it works fine. The Buick has 154K miles and seems that it'll make it another 50K (fingers are crossed, though).
Most Kia 4 bangers are still Al/CI. I wanted to look at a 05 Spectra today but was turned off by the head/block combo and changed my mind.
Also, not really thrilled with composite(plastic) intake manifolds. Cast aluminum is much better. A buying requirement for me.
The Geo Metro uses an aluminum (cast-iron sleeves) engine. It is nice because I can actually pick up the engine (about 170 lbs). Can't carry it far, though. Haven't heard of any problems due to aluminum construction.
The Cad has an aluminum block (with cast-iron cylinder liners...think water-cooled Corvair) and a cast-iron head. Supposed to give a lot of problems, but I haven't had any. That was one way they could reduce the weight of the car, get decent power and mileage.
The old Vega with all-aluminum block (no sleeves) was pretty bad...wore out quickly. The real train-wreck, however, was the early Mazda rotary...aluminum rotor housings with steel end-caps and bolts. Run low on coolant one time, the rotor housing expanded and cracked. After that, rotor housings were toast. I know....I did that around '79 or '80.
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GM pioneered the Nica-sil aluminum block in the 63 Oldsmobile F85. It was A V8 OF 215 cubic inches turbocharged with 215 horsepower and weighed 215 pounds.
That technology was licensed to Rover and used for many years in their 3.5 liter engines. It was also licensed to Mercedes and first used in their 380 SL which was a good engine when they abandoned the single row timing chain. Also adopted by Porsche in the 928 and 924 if memory serves me correctly.
Very hard surface of the cylinder walls but no possibility of boring the block for a rebuild.
I agree with the previous statement about using the same materials for block and head, becasue similar expansion characteristics make it much easier to keep the head gasket sealed for hundreds of thousands of miles.