How are burnt pistons avoided at lean AFR seen in F1 hybrid engines??

[PlatinumZealot]

Ok, it is well known now that these hybrid turbo engines are designed to run in lean conditions to bring about fuel efficiency. More heat transfered to the working fluid, in this case - air, and less to the cylinder walls means the heat is used for gas expansion and hence more work done on the piston. But as I think of it more: despite these efforts to reduce heat loss and these engines are running high boost levels, is it fair to assume that the pistons are still hotter than normal sportscar pistons? And with that high temperature and unburnt oxygen in the chamber, aren't conditions ripe for oxidation of aluminum? In other words high heat corrosion/burning of the pistons?

[Tommy Cookers]

maybe it's unfair to asume the pistons will unavoidably be hotter
as the air is relatively more than in any previous SI engine, and the heat rate (fuel burnt per piston) seems hardly more than with the NA V8s
on compression to any given CR the temp rise will be the same regardless of boost (as others have said)

though presumably the charge temperature is chosen to be rather high as this assists combustion, allowing leaner running

ie if the pistons are hotter it's allowed/chosen by the designers

it was suggested by P.U.R.E /Gilles? Simon that pistons would need to be (part ?) steel for adequate strength at the high temperature
the mandated rpm and stroke seem to allow part or wholly steel pistons

and do we have so much spare oxygen or is there internal EGR from intentional 'underscavenge' provided by the high exhaust pressure ?
research and service pre-combustion/'TJI'-type engines use a true exhaust backpressure of eg 0.5 bar (and fuels burning leaner than gasoline)
though this bp doesn't mean there's actual underscavenge, and u/s will tend to increase piston temperature
u/s seems plausible, as it can be seen as wasting less energy other than to coolant, and benefitting recovery

[godlameroso]

The only rules regarding pistons are that they can't be Ti, Mg, MMC's Tg, intermetallic materials(ie no half and half metals), less than .25% berillium in the alloy. As well as no alloys containing more than 5% by weight of Platinum, Ruthenium, Iridium or Rhenium. Finally the pistons have to be no less than 300g each.

Note that steel is mostly iron and only small changes can have profound effects on the strength, ductility, hardness and heat rejection of an alloy, not to mention that you can have a .8mm coating of whatever you want.

[gruntguru]

maybe it's unfair to asume the pistons will unavoidably be hotter as the air is relatively more than in any previous SI engine, and the heat rate (fuel burnt per piston) seems hardly more than with the NA V8s

Agree completely. "Lean mixture melts pistons" is a myth.

More accurately:
- leaning from a typical full power mixture towards stoichiometric will increase peak combustion temperature.
- leaning typically reduces flame velocity, which reduces useful work on the piston and increases heat rejection to internal surfaces

The good news:
- Leaning beyond stoichiometric reduces peak combustion temperature.
- Advanced combustion techniques like TJI reduce the time required for complete combustion, increasing useful work and reducing heat losses.

[PlatinumZealot]

Ah. Who mentioned melting? Wasnt me! But thanks for that because a lot of people confuse melting and burning/oxidation.

I am interested really in what stops the pistons from oxidizing with al that extra oxygen and heat that is present.

[gruntguru]

Well, chemistry is not my strong point but I know that the surface of aluminium rapidly oxidizes to form a hard oxide layer that is impermeable to oxygen. To go further would require raising the surface temperature of the piston crown to the auto ignition temperature of aluminium (which is higher than the melting point) so the crown will melt before it oxidizes (burns). Conduction to the land and under-crown oil-squirter cooling make sure that doesn't happen.

[Brian Coat]

Even if it was getting a bit warm / pressurised, there are loads of options. Materials, coatings, cooling passages (like last turbo era).

Compared to a bog standard jetliner engine at 50:1 CR and 2000K EGT, this is a walk in the park.

[PlatinumZealot]

Yes makes sense. Though jet engine blades are kept cool by air or steam. That reminds me of another thing. Boundary layer control. Maybe keeping the bounday layer of the air above the piston intact is a priority.

[gruntguru]

Yes makes sense. Though jet engine blades are kept cool by air or steam. That reminds me of another thing. Boundary layer control. Maybe keeping the bounday layer of the air above the piston intact is a priority.

Absolutely! One thing detonation does is destroy the boundary layer (locally). That's how it melts the piston.

[Brian Coat]

GG:"One thing detonation does is destroy the boundary layer (locally). That's how it melts the piston"

Have you encountered much thermal failure directly induced by detonation as a primary root cause?

I've detted one or two pistons to destruction in my time and I've not yet managed this. I find that normally (ha!) the detonation will mechanically destroy top land,/fire ring/other, with comedy thermal consequences served as the second course.

In my experience, direct thermal failures are more commonly pre-ignition-induced.

[Frank_]

i have detted too many ringlands but when i fitted stronger pistons the crown suffered little pock-marks intead !

[gruntguru]

GG:"One thing detonation does is destroy the boundary layer (locally). That's how it melts the piston"

Have you encountered much thermal failure directly induced by detonation as a primary root cause?

I've detted one or two pistons to destruction in my time and I've not yet managed this. I find that normally (ha!) the detonation will mechanically destroy top land,/fire ring/other, with comedy thermal consequences served as the second course.

In my experience, direct thermal failures are more commonly pre-ignition-induced.

Pre-ignition without accompanying detonation produces adverse pressure peaks but not the thermal failures you describe. However pre-ignition normally causes detonation and the detonation is what causes the failure. End-gas detonation usually occurs near the edge of the chamber and accelerates convective heat transfer into the nearest point on the piston crown, by scouring through the boundary layer with intense local turbulence and shock waves. The intense local heating softens the aluminium and soon results in increased blow-by at that location which of course is a positive feedback mechanism producing ever-increasing local heating and catastrophic failure.

[Brian Coat]

Hi, I am 99% sure that first sentence is not correct.

Pre-ignition will hole a piston in the complete absence of knock or high pressure waves.

Some pre-ignition safety margin tests are done on very high octane fuel to ensure detonation does not corrupt the result.

Equally, you can sometimes run very heavy knock at high speed and load without piston melting (or even undergoing much hardness relaxation) but can still break bits off the top land and fire ring due to the high mechanical loads. I was once lucky enough to find one of these during a routine boroscope check - just before the engine ate the evidence ... And itself!

Does this make sense?

[n4rf]

Regarding oxidation: If that would be an issue, I would assume they'd just coat the pistons.
I've recently had a piston+conrod assembly from a Toyota V8 or V10 (can't be 100% sure, since there were some V8 and some V10 parts (apparent by the intake trumpets)) and to my amazement, the entire piston was DLC-coated. Also comparing the piston pins with a series production or normal racing uses was a revelation. Less than 2 mm wall thickness, very short, less than 20mm diameter, that's one hell of a piece of engineering. Conrods on the other hand seemed fairly straight forward.

Regarding lean=hot: They run really really lean and with an inhomogeneous mixture inside the cylinder. As it was mentioned , end of combustion temperatures do go down with leaner mixtures once you're above lambda 1.1 (between 1.0 and 1.1 it depends a bit on the combustion system your running). F1 engines are running way above lambda 2, I would hazard a guess at 3-4 peak(exhaust lambda, not where the combustion takes place). I have talked to an engineer from a piston manufacturer in 2016, and while he obviously couldn't provide any definitive information or details, I did get a feeling it's going in that direction. Also you have to keep in mind, that there is currently development towards homogeneous lambda 2.0 in series production engines, with charge motion being the challenge.

Oh, and yes, they run insane boost pressures. Those compressors are huge and at the same time seriously efficient, which makes it even more impressive. I know for a fact, that they're up to at least 5 bar absolute pressure, probably beyond that in some operating areas.

[Tommy Cookers]

we have seen from Honda via Green Car Congress that gasoline homogeneous 2 lambda is possible ......
only with port injection (charge preparation) combined with charge heating combined with a lot of turbulence, and only at low rpm

unless the F1 fuel has the combustion properties of a small gas molecule how will it combust fast and fully beyond 2 ?
(by stuffing it with 'natural' dissolved propane/ethane as I suggested 2-3 years ago, and maybe the pre-chamber/TJI is mainly such gas ?)