2025/2026 Hybrid Powerunit speculation

[Badger]

I was doing some calculations on con rod thermal expansion using aluminium alloys. The con rod is around 120 mm long per the rules, and aluminium has a TE linear coefficient of 23 x 10 (to the −6) x K−1. So if we assume that the engine is running 100 degrees C above ambient (120 C), we would get a thermal expansion of 12 (cm) x 0,00023 x 100 (C) = 0,0276 cm. 0,276 mm.

To go from 16:1 to 18:1 compression in a 266,667 cc cylinder you need to reduce the minimum volume by around 2 cc. The piston head area is around 50 cm2, so 2/50 = 0,04 cm. 0,4mm.

So if you were to use aluminium alloy for the con rods you'd get 2/3 of the way to 18:1 compression just through linear thermal expansion in the con rod, not even accounting for expansion in the piston or the block.

But given the con rods must be iron or titanium alloy it makes it harder. Iron has around half the linear thermal expansion of aluminium and titanium even less.

C15.7.3 Connecting rods must be manufactured from iron or titanium-based alloys and must be machined from a single piece of material with no welded or joined assemblies (other than a bolted big end cap or an interfered small end bush).

[wuzak]

Won't the requirement for steel pistons also reduce heat transfer to the rods?

[Tommy Cookers]

the pistons will be very hot - that's why they are steel
the block (ie its coolant) is unusually cool - being plain water (and at atmospheric pressure only ??)
ie the rod is hotter than the block

the whole rod length is making an expansion contribution 'raising the piston'
some of the block crank centre height to deck height is making an expansion contribution 'dropping the piston'
and some is making a contribution to raising the piston
there would be an overall favourable expansion .....
certainly with an iron block and high-strength (maraging) 'steel' or Vascomax rods c 9 ppm/deg F and C.15.7.3 compliant
and probably also with a low-expansion Al alloy block

(don't know why Al alloy rods having c 13 ppm/deg F aren't allowed) and ok anything Ti is NBG expansionwise
so we don't have to consider the behaviour of the block and through-studs with clever preload effects (if any)

[vorticism]

I’m inclined to think the compression ratio rumor revolves around the allowed cylinder head “inserts” & “dismountable components exposed to the combustion chamber.” Inserts as in press-fit components within a monolithic part. Dismountable components as in separate parts fastened to monolithic parts. All prescribed and typical ICE parts.

Allowed cylinder head inserts (C5.1.7) are limited to the expected: valve seats, valve guides, and spark plug sleeves. They must comprise ≤3% of the cylinder head volume.

ICE components in general may be no more than 10% inserts by volume (C5.1. and this seems to include the other half of the CC--the piston. A piston has bushings for the wrist pin and if they do not consume 10% of the piston volume presumably an expanding insert could be placed on the piston face as the face geometry is unregulated.

Dismountable components within the combustion chamber (C5.1.9) are limited to: spark plug, fuel injector, poppet valves, a sleeve for the spark plug, and “a single component to replace an in-cylinder pressure sensor.” The lattermost stands out to me. I read that as a plug or blanking device. “In-cylinder pressure sensor” on its own is not in the list. That might just be an oversight in the writing. When are pressure sensors in place and in what share of the cylinders? I had assumed in the prev regs that they were always in place in each cylinder. Maybe they were not and were only used for homologation, and a knock sensor elsewhere in the block was used. Regardless, a plug is permitted and only its outside diameter is spec’d as max 7mm. A free geometry plug might supply the TE displacement needed, imo.

The volume of the CC at TDC and 16:1 will be around 17cc. Rather small, obviously, so a displacement of only 2cc is needed to provide the 15cc of an 18:1 CC. A cube of side 1.4mm. A grain of sand inside of an ⌀80mm crisp.

Ultimately, spectators are now at least talking about the incredible 18:1 compression ratio that was achieved previously. Diesel level--at 12k RPM, or... three times diesel engine speeds.

Note there is no cheating being implied. The FIA CR test is done at room temperature and that’s all they asked for.

[vorticism]

A free geometry plug might supply the TE displacement needed, imo.

For example, TE within a tightly-wound self-contacting monolithic helicoil constrained diametrically would express lengthwise along both the central axis and the helix vector. I think you could get a few mm of displacement at the end of the coil this way so long as binding/stiction allows for it.

Another option could be a cone section acting within a diametrically constrained countersink. Diametrical TE expansion of the cone section would express as axial displacement riding along the countersink as a ramp. Stack several of these instances together to get the required amount of displacement, and make sure to point the ramps the right way.

[diffuser]

How do they get all that to run without ping? Wouldn't the increased compression ratio just decrease the amount of boost you could add from the turbo ?

[Tommy Cookers]

boost is lower, because fuel heat rate is lower
still use knock sensing

[wuzak]

MAP is now limited to 4.8 bar.

Previously it was unlimited, combined with the 18:1 CR.

The 18:1 compression ratio was not part of the original V6 turbo hybrid regulations, being added a few seasons in.

[diffuser]

So the answer I'm reading here is that they need to up the compression, or at least get back to the 2025 ratio, cause we have less fuel to compress. So this Compression restrictions sounds counter productive. Why are FIA restricting the compression ratio? I presume it has something to do with costs ...although they're allowing PU manufactures to use titanium rods... There is also a COST CAP for PU manufactures ....So I non capisco.