I've always wondered why aluminum is still used for high performance pistons instead of a much higher strength alloy.
At first glance, using room temperature curves, the specific strength (strength/density) of 2618 and an average steel are about the same. This means that although 2618 has about 1/3 the weight, it's only about 1/3 as strong. But pistons don't operate at room temperature.

I work for an aerospace company and we are basically forbidden from using Aluminum alloys (like the popular 2618) above temperatures above of about 400*F. This has always bothered me because I know similar alloys are used for pistons in a much harsher environment.

It is not uncommon for piston peak temperatures to reach 600*F and even upwards of 750*F in a high performance application.

2618-T651 will have a yield strength of ~11,000 psi at 600*F.
At 800*F, 2618 is about as strong as JB weld at room temperature.....

On the other hand, at 600*F a typical Inco718 will have a yield strength above 120,000psi. This is approximately 11x higher than aluminum. At 800*F, Inco 718 is about 18 times stronger than 2618. Even certain steel alloys will still be above 100,000 psi at these temperatures.

So, I realize steel or inco is going to be about 3x heavier than aluminum, but, they have over 10x the strength at temperature. This is a much different story than at room temperature. It seems to me one could easily design a 600 gram steel piston that would be much much stronger than a forged aluminum piston at temperature...

I must be missing some other reason why aluminum is still used for pistons. Any thoughts?

 

I'm more of a combustion guy and engine systems guy than a materials guy, but what I'll say is this:

The biggest driving factors I've seen in piston and bore materials are going to be friction reduction and improved cooling for knock. So from a systems perspective, I have some target for friction reduction, I have a target combustion phasing (related to spark advance), and I have a target continuous cylinder pressure, and a target peak cylinder pressure.

You'll notice for example that aluminum block engines are slowly moving away from liners and going right to spray-bore technology. With spray-bore technology, you have more room for the water jacket. This relieves knock. You also see piston cooling jets being used on more and more engines to relieve knock.

You'll also see friction-reducing coatings on the piston skirt, rings, etc. Friction reduction helps with wasted energy, improving performance and fuel economy. Then there's the thermal expansion aspect, because you want to minimize blowby and piston slap, especially on forced induction applications.

On the topic of durability: most n/a engines might hit 60-70 bar peak cylinder pressure with enough spark advance, when not knocking severely. Most of the latest direct injected turbo/supercharged gasoline engines are going to hit about 90-100 bar during heavy load normal operation, and 120+ bar if low speed preignition occurs. Clearly aluminum can hold up this, because if it couldn't hold up they wouldn't use it.

 

Simply because there is no real need for much stronger alloys (at least in OEM and basic aftermarket parts). Why put a much more expensive and heavy part when the current one does the job fine?

OEM pistons (even cast ones) can last for a long time and handle some pretty harsh environments. Aftermarket 2618 pistons can handle hp in the thousands.

So what is the need for a piston 18x stronger than that unless you have an application that makes 3000+rwhp? What percent of applications is that the case in?

 

We learned a little bit about Inconel. I know thats a really good alloy for harsh environment (Some engines ise inconel exhaust valves), I know its expensive, and I think its probably trickier to machine. Ti would work well too.
Cool ideas, do racecars still use aluminum pistons? (F1, indy etc)

 

Aluminum is used because it's strong/lightweight and inexpensive compared to alloys.

 

I guess I should have caveated my question with "why use Al in high hp applications"

I too am more of a aero/simulation guy than materials as well, but I end up dealing with materials a lot as well. I do a lot with combustion as well, but we typically measure flow's in lbm/s instead of lbm/min...

I agree with your sentiment that other factors such as friction and especially economics come into play. Though on the friction front, I would think there are off the shelf coatings that could be used on steel that would be comparable to the aluminum coatings.

I have mixed thoughts on wanting to increase the heat rejection from the cylinder. I understand the goal is reduced surface temps, but the more heat you reject, the more you throw away (though I guess this is really only applicable during ~60% of the power stroke or about 15% of the cycle). After the power stroke, any extra temperature buys you nothing, so I guess I can see that justification.

On the thermal expansion topic, this is where I think a steel would be ideal. It has a much lower CTE than Al (though this does vary by alloy) and would allow you to keep the piston-wall clearance to a minimum. At the very least, I would see a steel piston being no worse than a hypereutectic and much better than a forged, low silicon alloy.

I in no way disagree that Al holds up just fine in stock applications. Though in my engine simulation model, I'm predicting peak cylinder pressures of up to about 180 bar for a 1200 bhp build. This is saying nothing of the people making much more hp. Again, Aluminum based alloys can hold up to this, but you start to run on the ragged edge. If you go lean for just a second, or if you get any sort of knock at those levels, your $20k engine is likely done. (I'm presuming that, I don't personally have a $20k, 1800 hp engine so I can't speak to how sensitive they are, but that is the general impression I get).

I guess the biggest concern I could see is that the steel piston will run hotter due to the conductivity difference. Depending on the configuration, this might increase the risk for pre-ignition - which would negate any benefit.

As mentioned above, I definitely agree about the OEM part. And I also agree that 2618 will hold up to a decent amount. But my understanding is that as you get above the 1000 hp level, things become much more delicate and sensitive to variation. I feel like a steel piston would have the potential to be much more resilient.

It would take some work for sure to design a steel piston of similar weight and similar overall dimensions. It wouldn't be easy to reduce volume by 1/3. And things like the skirt length and bore obviously would want to be the same. So you'd have to get away with making the skirts 1/3 as thin, and putting the top rings much closer. But, if steel is 10x stronger at that temperature, then even if you're reducing volume by 3x, it would still be 3.3 times stronger.

I would think that at high hp levels, the extra tolerance to detonation or lean conditions would be a huge advantage?


F1 used to use Beryllium Aluminum alloys I believe. The issue with that is Beryllium is considered hazardous and ended up being banned I think.

 

Also aluminum dissipates heat quickly which makes it ideal to stop hot spots during combustion.

 

That's exactly what I'm talking about, but it depends on just how high of a power density you're referring to. Consider how much loading modern turbo boosted engines have from the factory. Look at the torque per liter. A run of the mill BMW 4 cylinder turbo makes 258 lb/ft of torque at 1250rpm from 2 liters unmodified. Multiply that to get to 6.2L LS9 displacement, and you'd have 800 lb/ft of torque instead of the 604 lb/ft it comes with.

Or from a BMEP perspective, you're at about 23bar peak on the most power-dense OEM applications in production today if you do a little basic calculation from the torque per liter. Typical supercharged engine is more like 17 or 18 bar. So relatively ho-hum production commuter engines are more output dense than some of the highest horsepower production engines.

If the knock limit is holding the engine back on the available fuel, then you've got to raise the knock limit. Also, you want to keep the heat OUT of the exhaust so you don't have to run a rich mixture. That's why you have water-cooled exhaust manifolds integrated into the head (obviously short collector design). There's also cooled EGR like on diesels (Prius also uses that) and exhaust heat recovery systems (Prius uses that too, but not for power). If knock isn't the limiting factor, it's going to be cylinder pressure breaking things.

That may be true. I'd be interested to find out.

180 bar at what BMEP and what combustion phasing? It greatly depends on the spark advance and combustion phasing. MBT is considered to be 6-10 degrees for 50% burn, and peak pressure in say 12-16 degree range. If you've got 50% burn at 35 degrees, you're going to have significantly lower peak pressure than at 20 degrees. Peak pressure can be very sensitive to combustion phasing, often more sensitive than it is to BMEP. 180bar peak pressure is diesel territory.

The piston isn't the only thing you've got to worry about at high pressures. Really anything could go.

Diesels do run steel pistons on heavier duty applications. And diesels have much higher cylinder pressures than gas engines. So at some point it makes sense to run steel pistons. I agree with you.

For a relatively short lifespan application, like a 1500horsepower race engine, withstanding a catastrophic event is going to be very important. I totally agree with you. Diesel technology and processes are slowly trickling down to gas engines, so there's probably a lot that can be learned from studying steel diesel piston design. Think about what those engines for industrial or military applications have to hold up to.

 

Inconel and Titanium Aluminide are used in high end turbochargers.

 

Does inconel conduct heat as well as aluminium? I would think a steel piston would not be able to dissipate heat fast enough, and you would end up with a 1000+ degree glowing red piston.

 

Cost is king, cost is king and cost is king. What makes
us motorheads get wood, doesn't matter to mass
market manufacturers.

 

In the diesel world, aluminum skirt/steel top pistons are used more and more these days.
I'm fairly certain the caterpillar C15 uses a 2 piece pistons that has the skirt and piston "head" secured by the wrist pin (in conjunction with the rod of course)
Mahle actually makes a completely steel piston called the "monotherm" for the B series cummins. Only benefit over aluminum is the ability to accept habitual abuse from extreme heat conditions (1600* EGT's)

 

I believe the primary reason is mass. Yes you can use materials with much higher strength to weight ratio, but as mentioned they weigh much more. The acceleration of the piston as it changes direction at TDC is extremely high, especially at high rpm or high stroke lengths. Force = Mass x Acceleration. Therefore a more massive piston will greatly increase forces on the rods, crank, and bearings. Some diesels use other materials because they operate at a much lower speed and therefore tolerable piston accelerations for using those materials.

 

key points i think is missing is the relationship between torque, horsepower, cylinder pressure and the application. You can have a high horsepower motor but have low torque output simply by spinning it faster. That's essentially how a turbo shaft works, the torque output is quite small only around 200 lb-ft but at 30,000 rpm you have shaft horsepower values over 1000. so if you're making horsepower in a piston engine, what you're mostly concerned about is high rpm which equals piston speed which requires less mass.
the other thing is cylinder pressures aren't that much usually, cranking pressure (when you do a compression test) is usually not more than 200 psi. then add gasoline combustion and peak cylinder pressures at TDC aren't much over 1000 psi under full load, then fall off dramatically at the piston travels downward and cylinder volume increases. from the original post stating yield strengths of around 11,000 psi at 600F, if i know pressures aren't coming anywhere near 2000 psi maximum then what's wrong with using the aluminum/silicon alloy for a piston?

 

 

Okay how about a titanium alloy based piston? That should be lighter and stronger than aluminum REGARDLESS of the cost, would something like this work?

 

First, something to consider, automotive and aircraft applications are way different. Aircraft engines have to be, to use an automotive term, bulletproof. A car engine throws a rod, pull over and call a tow truck. An aircraft throws a rod, or looses a turbine blade, and down you go. Engine failure at 30,000 feet is no joke.

Payload, means just that, for aircraft less weight means more passengers, freight, or bombs. Weight is everything for aircraft.

What make a good piston? Lightweight, strong, able to resist corrosion and high temperatures, ductility, coefficient of expansion, ability to be manufactured at low cost.

Inconel is great for rocket engine nozzles and turbine wheels. It is heavy and a real bitch to machine, not a good choice for a piston. It would offer no performance advantage.

Cast iron pistons are cheap but brittle, good for low power stuff.

Cast aluminum is cheap, easy to machine and is the choice for mass production. The so-called hypereutectic stuff is little stronger but has its own problems with ductility.

Forged aluminum is stronger and can be made lighter, easily machined, and is reasonable in cost. The coef of expansion is higher, so they need more clearance, which makes them noisier than cast, and customers will complain, so manufac. don't really like them, besides the cost.

Bang for the buck nothing better than aluminum.

Titanium? Super expensive, hard to machine, but could work, but why? The cost vs. benefit even for F1 cars is doubtful.

 

F1 teams were experimenting with ceramic pistons and aluminum pistons with an embedded ceramic matrix a few years ago. They were turning those engines well over 20k rpm. The problem with doing anything like this is cost. Remember, 25 years ago titanium rods were were almost unobtainable to regular people. Now you can buy a set without taking out a second mortgage.

 

Actually, engine failure at 30k feet is not really a joke at all, and no you don't just "go down" as soon as you lose power. All that happens is you become a glorified glider.

Engine failure while in the middle of a take off at 1000 feet is no joke...you do go down, engine failure cruising at 30000 feet and 99.9% of the time most experienced pilots will get the plane down on the ground safely given there is a place to land.

 

Bad example . . .

A car in a high-speed turn ( above 100 mph ) blows a rod and knocks a hole in the motor. The oil gushes out and gets on the tires, causing a spin at 100 mph plus . . . High speed rollover and call an ambulance, if you can . . .

See the difference ?