16:1 to 18:1

[maddim]

There seem to be a story about Mercedes and RB trains could use something (of a grey area from FIA) that can run their 26 engines at a higher combustion (18:1 as it was with the past hybrid engines) instead of the 16:1 that were supposed to run under the 2026 regulations.
The FIA said that there is no way to measure this under the current measuring method. That has been raised by the other 3 manufacturers at the first place. So what do we know and what we do not know?
1. If it was initiated by the other 3 that means surely they know about it and probably how to implement it as else they wouldn't have known it. By asking FIA, they are now assured that it is legal for the beginning of 2026 at least. Or they are not? Because they can fight it?
The engines are not yet homologated so everything can happen. This is the most intense time period.
2. This is not good for the sport happening before the season has even started.
3. From the technical side of things, that means around 1.8cc of combustion engine decrease. No thermal expansion can meet these numbers and also no way of controlling it inside a combustion chamber. So, what is this mechanism that can allow them to run on 18:1 during running, measured 16:1 and not having any moving parts? Interesting...
Do moving parts cover the injector or the igniter coil (spark plugs)? I am wondering if there is this loophole there?
4. 18:1 means surely heavier fuel to gain from it, isn't it? With the new fuels that is bigger task. Will they be reliable?
5. Why did FIA reduced the combustion ratio in the first place? What is behind of this? Did they just wanted to be less demanding on the new fuels?
6. Will all 5 of them now implement this on their PUs as it is deemed legal? Probably yes if they think it is a gain. They all know it, as it seems. It was never a mystery.

[.poz]

5.6.1 The cylinder bore diameter must be 80mm (+/- 0.1mm)
5.3.2 Engine cubic capacity must be 1600cc (+0/-10cc)

single cylinder volume 266,67 cc

it's a oversquare engine so a little difference in piston rod lenght makes a big difference in volume.. less than 0,2mm for 10cc

[Tommy Cookers]

... No thermal expansion can meet these numbers ...
So, what is this mechanism that can allow them to run on 18:1 during running, measured 16:1 and not having any moving parts? ...

np problem ...
if the rods expand more than the block

the engine spec is conservative and could manage with eg aluminium alloy or titanium/aluminium alloy rods
maybe even iron blocks

[wuzak]

... No thermal expansion can meet these numbers ...
So, what is this mechanism that can allow them to run on 18:1 during running, measured 16:1 and not having any moving parts? ...

np problem ...
if the rods expand more than the block

the engine spec is conservative and could manage with eg aluminium alloy or titanium/aluminium alloy rods
maybe even iron blocks

Rods have to be made from iron or titanium based materials:
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).

And pistons have to be made from one of a few iron based materials:
C15.7.1 Pistons must be produced from one of the following iron-based alloys: AMS 6487, 15cdv6, 42CrMo4, X38CrMoV5-3.

[BassVirolla]

What about a bowing / straightening connecting rod when the inertia of the piston pulls at TDC?

I think probably is more related to thermal expansion but I was wondering.

Outside of pure geometric changes, hydraulic tappets with a clever valve shape (relying on cilindrical fit rather than conical, while still being "standard" valves) could lead to fill part of the combustion chamber with such cylindrical valve when filling the tappets with pressurized oil, but being perfectly under 16:1 while there is no oil pressure. Makes sense to have tappets with pneumatic springs?

Or just as I was thinking now... A cylindrical fit valve with a specific opening stroke length could have some stroke with no opening at all, just filling the chamber with more "cylinder of valve".

[Cold Fussion]

As a non expert, why wouldn't this have been an issue in past? Is there some test they can do to measure the compression ratio under operating conditions?

[gruntguru]

Perhaps the 18:1 Cr that was permitted in the past was close to optimal and there was little gain in exceeding it. Now they have to reduce the CR two points from the 18:1 they have been working with for a number of seasons.

[maddim]

What about a bowing / straightening connecting rod when the inertia of the piston pulls at TDC?

I think probably is more related to thermal expansion but I was wondering.

Outside of pure geometric changes, hydraulic tappets with a clever valve shape (relying on cilindrical fit rather than conical, while still being "standard" valves) could lead to fill part of the combustion chamber with such cylindrical valve when filling the tappets with pressurized oil, but being perfectly under 16:1 while there is no oil pressure. Makes sense to have tappets with pneumatic springs?

Or just as I was thinking now... A cylindrical fit valve with a specific opening stroke length could have some stroke with no opening at all, just filling the chamber with more "cylinder of valve".

I think all these parts are spec parts so no way of making them different.

[maddim]

As a non expert, why wouldn't this have been an issue in past? Is there some test they can do to measure the compression ratio under operating conditions?

That is a very good question. How do we know if it was also be done for earlier seasons also. If they were not able to measure it.

[maddim]

Perhaps the 18:1 Cr that was permitted in the past was close to optimal and there was little gain in exceeding it. Now they have to reduce the CR two points from the 18:1 they have been working with for a number of seasons.

That is my thought also, but mind that we had also the MGU-H past seasons and regularized fuel. Maybe the fuel could not handle more compression as there is the limited factor.

[wuzak]

As a non expert, why wouldn't this have been an issue in past? Is there some test they can do to measure the compression ratio under operating conditions?

That is a very good question. How do we know if it was also be done for earlier seasons also. If they were not able to measure it.

The 2025 rules do not specify that the compression ratio is measured at ambient temperature. So it applied at all times.

[saviour stivala]

The compression ratio under operating conditions is the dynamic compression ratio (DCR), It is not measured directly while the engine is running, but is calculated using engine specification and valve timing data. The standard ''static'' compression is a fixed value based on engine physical bore, stroke, combustion chamber volume. The dynamic compression is always lower than the static ratio because it accounts for point at which the intake valve actually closes, which is typically after the piston begun its upward (compression stroke. This late closing allows some of the air/fuel mixture to be pushed back out of the cylinder, reducing the effective compression.

[Cold Fussion]

But isn’t there a line in the 2025 and 2026 rule set that is to the effect ’”the rules apply at all times”? Practically how could they have enforced the compression ratio limit under operating conditions?

[BassVirolla]

What about a bowing / straightening connecting rod when the inertia of the piston pulls at TDC?

I think probably is more related to thermal expansion but I was wondering.

Outside of pure geometric changes, hydraulic tappets with a clever valve shape (relying on cilindrical fit rather than conical, while still being "standard" valves) could lead to fill part of the combustion chamber with such cylindrical valve when filling the tappets with pressurized oil, but being perfectly under 16:1 while there is no oil pressure. Makes sense to have tappets with pneumatic springs?

Or just as I was thinking now... A cylindrical fit valve with a specific opening stroke length could have some stroke with no opening at all, just filling the chamber with more "cylinder of valve".

I think all these parts are spec parts so no way of making them different.

The valves are stipulated to be poppet valves, but the geometry of the valve is not strictly stipulated, I seriously think that "after" the conical seal, the valve can have whatever profile you want.

[Greg Locock]

I'm not going to participate in the wankfest, but a while back there was some research that indicated that high CRs weren't especially efficient, above 20:1 the theoretical thermodynamic advantages were outweighed by mechanical inefficiencies.