The great F1 compression ratio thing

[hondajack85]

Forgive me for getting rather technical but whats the game here?
As far as I know only a few companies make pistons for F1 engines. Did one of them get the instructions to make some sort of composite piston with an acme expando crown? Did someone leak the info about this.
Maybe its something everyone was already doing for several years and its just off season baiting,dropping a few names out there.
Anyway at least honda , ferrari and co know about my composite acme expando piston now and can get cracking.

[Hustle_]

More likely the conrod that's extending rather than the piston top

[Jazoli]

Hustle_ said:

More likely the conrod that's extending rather than the piston top

Would that not then reduce the stroke?

[RizzoTheRat]

Jazoli said:

Would that not then reduce the stroke?

The stroke will be the same but the compression will be higher.

eg imagine the initial stroke takes a cylinder from 100mm to 10mm, you have a compression ratio of 10:1. If you extend the conrod by 1mm you're now going from 99mm to 9mm, a compression ratio of 11:1

[Hustle_]

I believe that 18:1 was pretty close to what is practically possible so there wasn't much benefit in exceeding it, although I'm sure this was an area of development and in any case the CR cannot be accurately measured under operating conditions...

Now that the regs have pegged the maximum CR back to 16:1 I guess some engineers quickly started thinking about ways to creatively recover that loss.

[The Hypno-Toad]

Back in 2018, I was found myself sitting next to senior member (or so he said) of the Red Bull team at a social occasion. I told him that I worked for Mazda and in the course of a long conversation about the sport said,

"You guys do a variable compression engine don't you? How have you got that to work?"

Could be a coincidence. Could have been just his general interest in engines. Might have been very early thoughts about what's happening now.

I believe that my old buddies at Saab did a patent for a variable compression engine with a variable length conrod a good few years ago. Being Saab it never came to anything.

[FeelingLucky]

RizzoTheRat said:

Jazoli said:

Would that not then reduce the stroke?

The stroke will be the same but the compression will be higher.

eg imagine the initial stroke takes a cylinder from 100mm to 10mm, you have a compression ratio of 10:1. If you extend the conrod by 1mm you're now going from 99mm to 9mm, a compression ratio of 11:1

But BDC would ALSO be 1mm higher

[7Llewelyn]

FeelingLucky said:

RizzoTheRat said:

Jazoli said:

Would that not then reduce the stroke?

The stroke will be the same but the compression will be higher.

eg imagine the initial stroke takes a cylinder from 100mm to 10mm, you have a compression ratio of 10:1. If you extend the conrod by 1mm you're now going from 99mm to 9mm, a compression ratio of 11:1

But BDC would ALSO be 1mm higher

Not if the conrod has a variable length

[Super Sonic]

Jazoli said:

Would that not then reduce the stroke?

Stroke is determined by crank throw

[richhead]

The old GP500 2 stroke bikes had variable cylinder heads for this. Not sure the F1 rules would allow it, and also alot simpler with a 2 stroke.

[tight fart]

Also it needs to work by thermal expansion as its checked at ambient temperature ?

[RizzoTheRat]

FeelingLucky said:

RizzoTheRat said:

Jazoli said:

Would that not then reduce the stroke?

The stroke will be the same but the compression will be higher.

eg imagine the initial stroke takes a cylinder from 100mm to 10mm, you have a compression ratio of 10:1. If you extend the conrod by 1mm you're now going from 99mm to 9mm, a compression ratio of 11:1

But BDC would ALSO be 1mm higher

Yes, BDC and TDC would both be 1mm higher, so 100mm and 10mm from the head becomes 99mm and 9mm. So lengthening the conrod by 1mm has made a 1% difference to the maximum volume, but 10% difference to the minimum volume.

[hondajack85]

So is it a conrod that has stretch at higher rev's?
I imagine even a standard car or motorcycle engine would have some elasticity in the conrods at peak revs.
At some stage did someone think hang on, why are we making these things so stiff? Maybe excessive longitudinal stretch is ok.

[coppice]

The engine produces 530bhp ish , the same as Renault's v6 turbo in 77/78, which had a 7.5/1 C/R. Why does the 2026 engine have such a very high C/R , and up to 4.8 bar of boost , but for 300bhp less than 2025, and equivalent to a near 50 year ol engine with lowwer boost and C/R ?

[Hustle_]

Probably because the 70s car was running some ungodly cocktail and the modern cars are running on wheat chaff or something.

Anyway I doubt the 70s car could produce 500+ bhp pretty much continuously for 300km without refuelling.

Edited by Hustle_ on Wednesday 21st January 08:34

[RizzoTheRat]

hondajack85 said:

So is it a conrod that has stretch at higher rev's?
I imagine even a standard car or motorcycle engine would have some elasticity in the conrods at peak revs.
At some stage did someone think hang on, why are we making these things so stiff? Maybe excessive longitudinal stretch is ok.

My above example was assuming thermal expansion of the conrod, but remaining a constant length once hot. If you assume elasticity then it's going to be a bit more complex. At the bottom of the stroke the momentum of the piston will compress the conrod, and at the top of the stroke the momentum will stretch it, but at the top of the stroke it's compressing the air, which will in turn compress the conrod. So I've no idea if the overall effect would be to increase or reduce the compression.

[ralphrj]

The Hypno-Toad said:

I believe that my old buddies at Saab did a patent for a variable compression engine with a variable length conrod a good few years ago. Being Saab it never came to anything.

They made prototypes and tested them in cars in the 90s and early 00s. Claimed to have the consumption of a 1.6 litre engine with the performance of a 3.0 litre. It worked by having a 2 part block with a hinge that could pivot and change the volume of the combustion chamber (and therefore the compression ratio).

[WTDMM]

coppice said:

The engine produces 530bhp ish , the same as Renault's v6 turbo in 77/78, which had a 7.5/1 C/R. Why does the 2026 engine have such a very high C/R , and up to 4.8 bar of boost , but for 300bhp less than 2025, and equivalent to a near 50 year ol engine with lowwer boost and C/R ?

Because it uses a much less volatile fuel and uses a helluva lot less of it

[thegreenhell]

RizzoTheRat said:

hondajack85 said:

So is it a conrod that has stretch at higher rev's?
I imagine even a standard car or motorcycle engine would have some elasticity in the conrods at peak revs.
At some stage did someone think hang on, why are we making these things so stiff? Maybe excessive longitudinal stretch is ok.

My above example was assuming thermal expansion of the conrod, but remaining a constant length once hot. If you assume elasticity then it's going to be a bit more complex. At the bottom of the stroke the momentum of the piston will compress the conrod, and at the top of the stroke the momentum will stretch it, but at the top of the stroke it's compressing the air, which will in turn compress the conrod. So I've no idea if the overall effect would be to increase or reduce the compression.

You don't want floppy conrods. The engine's power comes from pushing down on the piston into the conrod, so any potential gains from increasing compression at the top of the stroke will be lost when you push back down on the combustion downstroke.

[thegreenhell]

ralphrj said:

The Hypno-Toad said:

I believe that my old buddies at Saab did a patent for a variable compression engine with a variable length conrod a good few years ago. Being Saab it never came to anything.

They made prototypes and tested them in cars in the 90s and early 00s. Claimed to have the consumption of a 1.6 litre engine with the performance of a 3.0 litre. It worked by having a 2 part block with a hinge that could pivot and change the volume of the combustion chamber (and therefore the compression ratio).

Nissan put a variation of this into production in some Infiniti models.

https://www.nissan-global.com/EN/INNOVATION/TECHNO...

This would clearly not be legal in F1.