Yes they do, especially when higher combustion heat is involved, many million plus mile truck engines out there.
In all cases of Al in engines is because of the easy to manufacture, and weight is some cases.
Cast Iron Engine Blocks & Heads vs Aluminum
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In all cases of Al in engines is because of the easy to manufacture, and weight is some cases.
truck engines that have a 2000 rpm redline? I've never come across an engine where combustion heat was limited by the pistons, it's often the turbo.
I’m pretty sure most diesel engines are using aluminum pistons.
Weight for weight, aluminum is much stiffer, assuming good design.
My 1st car was a 51 Chevy with PG slush box that put up with a beating. It was a 2 speed with a torque converter.
Yes, and saving weight isn't a bad thing in cars either. Aluminum suspension pieces impress me as those must cost more to make than stamped steel, or cast iron?
Cast steel suspension pieces maybe cost even more than Aluminum, and should be even more durable, but also heavier.
Aluminum needs to flex less than steel to last, and almost anything aluminum in a car is designed so it is never stressed enough to flex above its "fatigue limit" so it should last for infinite cycles in an ideal situation, but also cars see less than ideal conditions, so aluminum parts still fail. But generally aluminum parts are almost always much stiffer, and the first aluminum mountain bike frames and even motocross bike frames were too stiff compared to steel, and felt harsh compared to the steel frames they replaced.
Just like small engines on OPE have had for quite a while.
Aluminum costs more on a per pound basis, and die casting tooling is super expensive compared to sand casting.
Even in VERY high production quantities where the tooling cost is justified, the material cost is higher for aluminum.
There are other permanent mold processes that are compatible with iron. There’s very little correct in this post.
Some of you know that I work in engineering for one of the largest engine makers in the world. I’ll pass along what I’ve gleaned from that experience to answer the OP.
Aluminum is used primarily for weight and (secondarily) thermal properties and almost no other reasons. If the engine has to move (i.e. small engines, cars), especially with CAFE rules that make weight a premium, aluminum will be used. Aluminum is harder to cast properly, but does have superior specific heat and thermal conductivity, so an aluminum block (and especially heads) will cool much better and more evenly. The weight and thermal properties are far and away the reason aluminum is used in racing blocks and heads. There’s no other reasons to use it, really outside oddball considerations like drag racing where the ability to weld-repair a block is quite useful (yes, in everything from sportsman classes to Fuel classes it’s quite common for a block to be TIG weld repaired).
Iron is used where the weight penalty is moot. Stationary equipment or heavy things where a couple extra pounds won’t matter. Iron has much to recommend it. Iron is very easy to cast and make good castings free of porosity and inclusions. Iron has very high natural damping, so it vibrates less and makes it easier to make a quiet engine. Iron has very good stiffness and thermal stability— it expands and contracts MUCH less than aluminum does with temperature swings.
Iron has excellent machinability and is typically free-machining. Your typical A356 sand cast or A380 die cast machines pretty well, but not as nice as iron. Iron has graphite in the microstructure which lubes the cutting tools.
Iron is much stiffer than aluminum and deflects less under load. This is really important for things like main bearing stability in the block and valve stability in the head. it takes a lot less iron to make a block or head stiff enough.
Another major strike against aluminum is the lack of an endurance limit for fatigue. Ferrous metals like iron and steel have a stress value below which they have infinite fatigue life (> 10^7 cycles). Aluminum has no such limit— all cyclic loading creates a fatigue cycle that counts on the clock (This is why aluminum drag racing con rods are strongly discouraged in street engines). This disparity in fatigue life explodes with hotter temperatures, eventually making aluminum far, far inferior in TMF (thermomechanical fatigue).
Aluminum and steel are both cutting edge piston materials. Aluminum is preferred for smaller engines that rev higher, but monotherm steel pistons are strongly preferred for industrial engines (diesel) at lower RPM because they offer superior fatigue life and they fit tighter in the bores because they expand and contract less with temperature.
I’ll close by observing that aluminum and iron are not two materials but two families of materials with literally hundreds of variants of each within those families. At cummins, the cast iron used in the head and in the block are two different alloys. Both are different from cast irons used for brackets and such. While we have materials standards for pearlitic ductile iron, bainitic iron, Austempered ductile iron (ADI), high moly nickel silicon ductile iron (for exhaust manifolds), NI-resist, and MANY other varieties, they are all “cast iron.”
We tend to be less picky about aluminum, where it’s either a sand cast A356 variant or a die cast A380 variant, with the rare 6061 or 7075 making an appearance when needed (generally at Cummins if you show up with a design that needs 6061, it will fail the design review and they will expect you to redesign it to make A356 work). We’ve figured out that premium materials with defects are both inferior to and more expensive than plain materials free of inclusions and casting defects— so we prefer alloys knows for castability and easy of achieving high quality.
** Comments reflect author’s opinion only and in no way reflect official commentary or endorsement of Cummins Inc, suppliers, customers, or affiliates**.
My all aluminum TT V6 makes ~480hp, and routinely hauls my 4400lbs sedan down the drag strip to a tune of 120mph. As long as I keep the timing assembly happy it's extremely unlikely that it will need any kind of overhaul before it sees 250k miles.
It’s the rule, not the exception in modern clean diesels. Temperature limits are firstly the piston and combustion chamber and by a large amount.
Turbos don’t see “combustion heat”, they see exhaust heat, the fraction of combustion heat exiting the cylinder.
We set turbine inlet temps to a typical max of 800°C or so. That’s a good 900C lower than NOx cutoff temp even, never mind peak flame temp.
The more efficient the engine, the more heat is captured by the cylinder and less rejected to exhaust.
You can make pistons run hotter and turbos run cooler just by bumping timing up, for example.
The main reason for hot turbine temps on newer engines is the push for ever-faster response from the turbo. Not only for performance, but for emissions as diesels need more air to avoid smoke and keep PM10 within limits. This means smaller turbines at higher expansion ratios.
Thank you for your clear and comprehensive response. It's refreshing to read (and learn from) such a post.
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Duty cycle, duty cycle, and again… duty cycle…
You do not see industrial engines being built out of aluminum. Because aluminum cannot take high duty cycles as well as cast iron. Aluminum can only expand and contract and take heavy stress so many times before it eventually loses its strength to do the job.
That’s why heavy duty application engines are always built out of cast iron, even to this day when aluminum seems like the greatest thing.
If aluminum was so great, ship engines, heavy duty truck engines, and industrial engines would all be built out of aluminum for weight savings, because they would really like weight savings to save money on fuel, but weight savings is not acceptable when duty cycle matters.
You do not see industrial engines being built out of aluminum. Because aluminum cannot take high duty cycles as well as cast iron. Aluminum can only expand and contract and take heavy stress so many times before it eventually loses its strength to do the job.
That’s why heavy duty application engines are always built out of cast iron, even to this day when aluminum seems like the greatest thing.
If aluminum was so great, ship engines, heavy duty truck engines, and industrial engines would all be built out of aluminum for weight savings, because they would really like weight savings to save money on fuel, but weight savings is not acceptable when duty cycle matters.
And not all industrial engines are made of cast iron. Some are made from steel fabrication.
Alloy steels especially some with Ni are extremely tough and durable
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I’m guessing it’s two stroke emd engine types you’re talking about?
Yes. The distinction between “alloy” and “super alloy” often correlates to nickel content.
Making a material that keeps impressive properties while hot is not cheap. Compare the cost of an A-286 fastener to a regular A193-b16 “high temp” alloy. A286 (if memory serves) as 25% nickel content. Stepping up to Inconel 718 gets you 50% nickel content. And a massive increase in cost.
Yes we didn’t use aluminum or cast iron for our rocket engines!
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