2025/2026 Hybrid Powerunit speculation

[BorisTheBlade]

Wouldn't the same be realized if they reduce throttle pressure, for example to 50kW and then the rest can harvested?

Seems like it would be more intuitive but maybe harder to get correct every lap.

There is rule C5.2.5, which limits fuel energy flow at partial load:

At partial load, the fuel energy flow must not exceed the limit curve defined below:
EF (MJ/h) = 380 when the engine power is equal to or below −50kW
EF (MJ/h) = 9.78 x engine power (kW) + 869 when the engine power is above −50kW

Basically, at 50kW you ony get 1358MJ/h, so ~45% of the maximal fuel energy flow. Assuming a 400kW ICE and that you can achieve same efficiency at reduced fuel flow (which is probably not possible), you get around 180kW out of the ICE. This allows you to recover 'only' 130kW from the MGU-K. (This all assumes that the engine power in the rule refers to the PU power, which I think most people here assume.)

Exactly! I made a chart about this prescribed behavior here: viewtopic.php?f=4&p=1311253#p1311253

[johnnycesup]

Wouldn't the same be realized if they reduce throttle pressure, for example to 50kW and then the rest can harvested?

Seems like it would be more intuitive but maybe harder to get correct every lap.

There is rule C5.2.5, which limits fuel energy flow at partial load:

At partial load, the fuel energy flow must not exceed the limit curve defined below:
EF (MJ/h) = 380 when the engine power is equal to or below −50kW
EF (MJ/h) = 9.78 x engine power (kW) + 869 when the engine power is above −50kW

Basically, at 50kW you ony get 1358MJ/h, so ~45% of the maximal fuel energy flow. Assuming a 400kW ICE and that you can achieve same efficiency at reduced fuel flow (which is probably not possible), you get around 180kW out of the ICE. This allows you to recover 'only' 130kW from the MGU-K. (This all assumes that the engine power in the rule refers to the PU power, which I think most people here assume.)

Exactly! I made a chart about this prescribed behavior here: viewtopic.php?f=4&p=1311253#p1311253

In the "definitions" part of the technical regulations document (page 193), the word "engine" refers only to the ICE:

“Engine” (“ICE”): The internal combustion engine including Ancillaries and actuator systems
necessary for its proper function.

[gruntguru]

As I posted in the other thread;

V8 drag race and circle track builders set P-H around .040in (1mm) with steel rods which in practice is near zero Piston to Head clearance at redline (the goal is to run around 0.1mm running clearance). This isn’t out of the realm at all.

For a typical racing US domestic V8 with a 4.25in bore, 64cc chambers, and a 10cc dome, that’s a change from to 12:1 to 13.9:1.

This has been a “thing” for decades in race engine building. No fancy degrees or materials needed and certaintly not an industry secret.

All engines are limited by piston to head clearance. You can’t modify this, at some point, they meet, and it’s expensive. Now depending on the rods being used, it may matter. For example, on an American V8 drag race engine, aluminum rods may require 1.50mm p-h clearance ambient vs the 1mm for steel rods. Both will end up around .1mm near redline if you’re doing it right. Both will measure slightly different geometric compression ratios (aluminum being lower) but the same running compression ratio.

Binotto as an engine guy should know this. People building engines in their garages who struggled through high school even know this. This sounds more like sour grapes than anything.

Thought you might find this interesting...

https://youtu.be/hLzto55W3RU?si=Bu9_J7zOVmZDJMLX

It largely said what I have said over a few posts, just with a lot more words.

Interesting he brings up “dynamic compression” which basically calculates compression ratio starting at the crank angle at intake valve closing (IVC). This thought comes from the 2 stroke world where they don’t have valves and the engine can’t begin compressing the mixture until the piston closes off the ports. On a 4 stroke it’s a little bit misleading as once the engine is “on song”, cylinder pressure is above 1 atmosphere before intake valve closing (IVC) as the inertia of air against a rapidly closing intake valve raises port pressure, so compression is occuring to a small amount prior to IVC.

A certain and controversial performance authorhas coined “effective compression ratio” as “dynamic” and has tried to correlate to octane needs for a given combination. This is wrong as octane needs are driven by lots of things like peak firing cylinder pressure, turbulent kinetic energy (TKE), plug position, atomization (and its effect on the mixture temp in the cylinder), etc.

There is nothing dynamic about “dynamic compression ratio”.

Further complicating things is on a Miller Cycle engine, which F1 engines and some of the diesel LeMans engines before 2014 were, closes the intake valve before BDC on the intake stroke. This means the engine has a higher expansion ratio than compression ratio, which in turn lowers cylinder temperatures. This was used by Mazda and Subaru in the early 90s, still used in locomotive engines. It requires big boost / high air flow from the turbo or supercharger to make up for lack of open time of the intake valve. Pat does a great job describing it here starting at 13:00 . As he points out, by closing the intake valve so early you do lose some of the effect of a longer intake period adding in cylinder motion in the cylinder (typically tumble on a 4 valve head), or when you factor in a few more things, TKE. So that’s why thints like turbulent jet ignition (TJI) are sought out. Honda has a good white paper published this past September about developing TJI systems for large bore sport bike engines, why you want it (the jets increase TKE and create a larger flame surface area, thus reducing knock, and allowing for a higher geometric compression ratio for a given octane), but they also point out it struggles at part throttle operation and is part why you haven’t seen it widely adopted on the street, but I digress…


The Wikipedia and subsequently the AI searches that have trained on the wiki part are wrong about the Miller Cycle. Overall, as he brings it up in the video, the dynamic compression ratio isn’t entirely well explained and it just felt like he was dropping it in there to sound smarter, and also has no idea what it means.

Yep - dynamic compression ratio is a big nothing in the context of this conversation. Miller cycle (early IVC) is simply a means of controlling DCR - it reduces DCR in fact.

The geometric CR limit of 16:1 has nothing to do with limiting how much the engines are permitted to compress the charge - the Miller timing is already being used by the teams to deliberately limit this.

The GCR limit is actually about limiting the "Expansion Ratio" which limits thermal efficiency. We all know the Otto cycle efficiency is limited by compression ratio but this is misleading because the Otto cycle assumes compression ratio = expansion ratio and it is the expansion ratio that is critical.

[Hoffman900]

Thought you might find this interesting...

https://youtu.be/hLzto55W3RU?si=Bu9_J7zOVmZDJMLX

It largely said what I have said over a few posts, just with a lot more words.

Interesting he brings up “dynamic compression” which basically calculates compression ratio starting at the crank angle at intake valve closing (IVC). This thought comes from the 2 stroke world where they don’t have valves and the engine can’t begin compressing the mixture until the piston closes off the ports. On a 4 stroke it’s a little bit misleading as once the engine is “on song”, cylinder pressure is above 1 atmosphere before intake valve closing (IVC) as the inertia of air against a rapidly closing intake valve raises port pressure, so compression is occuring to a small amount prior to IVC.

A certain and controversial performance authorhas coined “effective compression ratio” as “dynamic” and has tried to correlate to octane needs for a given combination. This is wrong as octane needs are driven by lots of things like peak firing cylinder pressure, turbulent kinetic energy (TKE), plug position, atomization (and its effect on the mixture temp in the cylinder), etc.

There is nothing dynamic about “dynamic compression ratio”.

Further complicating things is on a Miller Cycle engine, which F1 engines and some of the diesel LeMans engines before 2014 were, closes the intake valve before BDC on the intake stroke. This means the engine has a higher expansion ratio than compression ratio, which in turn lowers cylinder temperatures. This was used by Mazda and Subaru in the early 90s, still used in locomotive engines. It requires big boost / high air flow from the turbo or supercharger to make up for lack of open time of the intake valve. Pat does a great job describing it here starting at 13:00 . As he points out, by closing the intake valve so early you do lose some of the effect of a longer intake period adding in cylinder motion in the cylinder (typically tumble on a 4 valve head), or when you factor in a few more things, TKE. So that’s why thints like turbulent jet ignition (TJI) are sought out. Honda has a good white paper published this past September about developing TJI systems for large bore sport bike engines, why you want it (the jets increase TKE and create a larger flame surface area, thus reducing knock, and allowing for a higher geometric compression ratio for a given octane), but they also point out it struggles at part throttle operation and is part why you haven’t seen it widely adopted on the street, but I digress…


The Wikipedia and subsequently the AI searches that have trained on the wiki part are wrong about the Miller Cycle. Overall, as he brings it up in the video, the dynamic compression ratio isn’t entirely well explained and it just felt like he was dropping it in there to sound smarter, and also has no idea what it means.

Yep - dynamic compression ratio is a big nothing in the context of this conversation. Miller cycle (early IVC) is simply a means of controlling DCR - it reduces DCR in fact.

The geometric CR limit of 16:1 has nothing to do with limiting how much the engines are permitted to compress the charge - the Miller timing is already being used by the teams to deliberately limit this.

The GCR limit is actually about limiting the "Expansion Ratio" which limits thermal efficiency. We all know the Otto cycle efficiency is limited by compression ratio but this is misleading because the Otto cycle assumes compression ratio = expansion ratio and it is the expansion ratio that is critical.

Good post, Grunt. Don’t worry, no one will read ourn posts anyway and keep arguing things that don’t matter

[BassVirolla]

Yep - dynamic compression ratio is a big nothing in the context of this conversation. Miller cycle (early IVC) is simply a means of controlling DCR - it reduces DCR in fact.

The geometric CR limit of 16:1 has nothing to do with limiting how much the engines are permitted to compress the charge - the Miller timing is already being used by the teams to deliberately limit this.

The GCR limit is actually about limiting the "Expansion Ratio" which limits thermal efficiency. We all know the Otto cycle efficiency is limited by compression ratio but this is misleading because the Otto cycle assumes compression ratio = expansion ratio and it is the expansion ratio that is critical.

But having a GCR of 18:1 vs. 16:1 allows you a greater expansion ratio of a given air mass.

If I'm not seriously misguided (which ranges from possible to probable ) this just means that you have to "millerize" even more your valve timing for a given amount of fuel maintaining the same AFR, increasing somewhat the efficiency.

Edit: Now I fully got your point. DCR has nothing to do when you are already at Miller cycle, if I understood you rightly.

[BassVirolla]

Now thinking about reducing the in cylinder air mass but minimizing pumping losses (to achieve a Miller cycle), what would be better:

- Intake valve opening late (reducing intake time, but generating in cylinder vacuum)
- Intake valve closing late (returning air from the cylinder to the plenum)
- Exhaust valve closing late (bypassing air from intake to exhaust via cylinder).

Surely here are some posters that know something about this...

[Tommy Cookers]

inlet valve closing early
the charge then cools to below ambient (remember rules ban cooling the charge below ambient in the induction system)
so you can then compress it a lot without causing detonation
and so expand it a helluva lot

[vorticism]

IVC early would bring you back to Otto or, if really early, relative to Miller-type extended IVO, to some other Miller-like cycle (maybe there's a term for the that)

In-cylinder expansion cooling I doubt would offer much since the charge will be recompressed--where would the heat go? Or is the suggestion that, if fueled during IVO, the fuel vaporizes more in such a mode to provide phase change cooling

Regardless of valve timing, if greater expansion ratios are ever better for these Miller cycle engines, you are still right back at reducing the CC volume at TDC via whatever means i.e. the ongoing discussion across several months

[Gooch]

OK I have a dumb question, is there anything that prohibits the use of pistons were the crown extends above the top deck and into the cylinder head? I have seen it on heavily modded 2 strokes before and remembering that got me thinking maybe the crown could cover a series of very tiny holes to a second chamber drilled on around the edge of the head (think oil drain holes but smaller) when the engine is heated but I have a feeling this might have been debunked already.

edit: Actually, this would probably be horribly inefficient.

[Farnborough]

IVC early would bring you back to Otto or, if really early, relative to Miller-type extended IVO, to some other Miller-like cycle (maybe there's a term for the that)

In-cylinder expansion cooling I doubt would offer much since the charge will be recompressed--where would the heat go? Or is the suggestion that, if fueled during IVO, the fuel vaporizes more in such a mode to provide phase change cooling

Regardless of valve timing, if greater expansion ratios are ever better for these Miller cycle engines, you are still right back at reducing the CC volume at TDC via whatever means i.e. the ongoing discussion across several months

There's some interesting adpect within this topic (I'd put on forum before, from Pat Symonds presentation) which was kicked back as being untenable

The point about early closing of inlet valve, before BDC, interests as it doesn't seem initially to be logical if looking at purely outright power ONLY.
If the intake charge at say 3 x atmosphere from turbine fills competently the cylinder before reaching absolute BDC, then closing it could avoid the "dead" relatively speaking, spot around BDC without much compromise. This ultimately to avoid port flow reversal against turbine derived pressure and losing very little by that closing time.

Switch to regeneration though .... then closing early with waste gate opened can underfill the cylinder, to ultimately reduce pumping loss as it comes up to TDC and compression. This reduction effectively a gain for torque travelling through crankshaft to MGU-K destination. Coupled with late, after TDC fuelling and ignition event (retarded in braking phase of driver demand) possibly firing at 1 in 4 possibilities, could ultimately keep as much transmission torque available for regeneration while crankshaft torque is not called by driver. Downshift "blip" facilitated by ignition & fuel advances to remove peak torque demand from MGU-K from gearbox dogs.
A misfire type pulse is audible (possibly from that late burn timing) out through exhaust/waste gate exit during brake and downshift period on them, accompanied by that characteristic high pitched whine of transmission set into MGU-K architecture.

This would cut fuel use, while achieving maximum torque throughput from wheels to MGU-K in pumping loss/friction mitigation.

[michl420]

There is this rumor about that seperat room with a very small conection to the combustion chamber to "practical" increase the compression ratio.
Would such a small connection let any meaningfull amount of gas through, or to put it another way, can this work?

[Hoffman900]

The Miller Cycle depends on big boost and a ton of mass flow. That’s how it works with IVC before BDC. Honda has even showed this to you all:

https://www-jsae-or-jp.translate.goog/e ... r_pto=wapp (Figure 4)

It’s something like 0.66 kg/s at 4.5 pressure ratio at sea level.

Remember, on an Otto Cycle engine, the cylinder is still filling on the compression stroke. On high rpm racing engines, IVC (seat closure) is 65+ degrees ABDC.

As for recovery; Honda showed you how it’s done here: https://www-jsae-or-jp.translate.goog/e ... r_pto=wapp

And Ferrari here:

https://www.mdpi.com/1996-1073/14/1/171

Not only do they lay out how harvesting happens on straights, but corner exit too.

All this has been shared here multiple times as well…

[vorticism]

IVC early would bring you back to Otto or, if really early, relative to Miller-type extended IVO, to some other Miller-like cycle (maybe there's a term for the that)

In-cylinder expansion cooling I doubt would offer much since the charge will be recompressed--where would the heat go? Or is the suggestion that, if fueled during IVO, the fuel vaporizes more in such a mode to provide phase change cooling

Regardless of valve timing, if greater expansion ratios are ever better for these Miller cycle engines, you are still right back at reducing the CC volume at TDC via whatever means i.e. the ongoing discussion across several months

There's some interesting adpect within this topic (I'd put on forum before, from Pat Symonds presentation) which was kicked back as being untenable

The point about early closing of inlet valve, before BDC, interests as it doesn't seem initially to be logical if looking at purely outright power ONLY.
If the intake charge at say 3 x atmosphere from turbine fills competently the cylinder before reaching absolute BDC, then closing it could avoid the "dead" relatively speaking, spot around BDC without much compromise. This ultimately to avoid port flow reversal against turbine derived pressure and losing very little by that closing time.

Switch to regeneration though .... then closing early with waste gate opened can underfill the cylinder, to ultimately reduce pumping loss as it comes up to TDC and compression. This reduction effectively a gain for torque travelling through crankshaft to MGU-K destination. Coupled with late, after TDC fuelling and ignition event (retarded in braking phase of driver demand) possibly firing at 1 in 4 possibilities, could ultimately keep as much transmission torque available for regeneration while crankshaft torque is not called by driver. Downshift "blip" facilitated by ignition & fuel advances to remove peak torque demand from MGU-K from gearbox dogs.
A misfire type pulse is audible (possibly from that late burn timing) out through exhaust/waste gate exit during brake and downshift period on them, accompanied by that characteristic high pitched whine of transmission set into MGU-K architecture.

This would cut fuel use, while achieving maximum torque throughput from wheels to MGU-K in pumping loss/friction mitigation.

I guess, but the discussion has been in the context of supposed Miller cycle engines which by definition have IVC occurring after BDC. That’s why I said earlier IVC in that context, on a Miller cycle engine, just gets you closer to an Otto cycle. It could be that recent F1 engines were merely Miller-like in the sense of utilizing incomplete cylinder filling regardless of which side of BDC that IVC occurs on.

[Hoffman900]

They weren’t utilizing incomplete cylinder filling, they are ramming the air in with a ton of boost, that’s a design feature of the Miller Cycle and pretty much the only way it can work.

[vorticism]

Utilizing only a portion of a cylinder's displacement (swept volume) to define the intake charge would be incomplete filling in terms of displacement utilized, via valve timing, and regardless of boost pressure or the absence of it (Atkinson). That's where the advantageous asymmetry arises from--the expansion ratio being greater than the compression ratio.