Physics of Q3 engine modes


Points Scorer
In recent years there has been much talk of the "magic" Q3 qualifying mode that Mercedes (and now Ferrari) have. In particular Mercedes always seem to bring something extra when entering Q3.

I find the existence of such a mode puzzling in the current engine formula. In the past, the notion of "turning up" the engine for qualifying was very natural. The maximal power output was limited by how much fuel you could burn in the engine. The effective limit was the result of mechanical limitations of the engine, and was certainly possible to exceed the limitations for a short time (by increasing the max revs or boost pressure) at the cost of reducing the mechanical life of the engine.

However, the current engine formula is fundamentally different. The maximal power output of the engine is (due to fuel flow limit) limited by how efficiently the engine burns its constant supply of fuel. A temporary increase of power therefore has to come from a temporary increase of the thermal efficiency of the engine. I have a hard time imagining ways in which one could do this that could not be sustained for longer periods of time, or would limit the life of the engine.

Of course, the alternative is that the engine is drawing from some other limited supply of potential. Clearly, this is where the speculations of engines burning lubricant oil for a power boost come from. I guess the engines could also be consuming itself (either for fuel or some substance acting as a catalyst leading to a more efficient burn).

Any ideas on the physics behind the special Q3 modes?
The engine could be run a little leaner, which over the course of a race could damage the spark plugs or even the turbo, but over one or two laps it could survive. Or the Mercedes power boost has to do with the energy recovery system and it could be harvesting more aggressively, which would cause overheating of batteries or brakes over the course of a race but over 2 laps, it could be fine. I keep thinking it has to do with something allowed to run hotter over a short period vs a race distance that would overheat.
Unless I am mistaken, the amount of harvested (MGU-K) energy that can be deployed during a lap is limited to 4MJ. Hence I do not think more aggressive MGU-K harvesting can explain the powerboost.

On the other hand, there is nolimit on the energy transferred from MGU-H to MGU-K. I suppose it is possible to let the engine run hotter than normal in the setup lap, effectively storing thermal energy in the engine coolant. During the qualy (hot!) lap, on could conceivably harvest more energy using the MGU-H.
I guess the "free flow system" mentioned in that post may be part of the answer. However, it is unclear to me why one wouldn't run the FFS for all of qualifying (vs. just Q3).
I suppose it would depend on whether you needed to or not.

With the engines needing to last significant periods of time, over-stressing unnecessarily would be a potential issue, so you would use the highest mode only when needed. There would be some teams where this would be Q1, Merc and Ferrari would be unlikely to need full beans until Q3.
The top drivers do not even go their fastest in Q2 in order to conserve their tyres; it always amuses me when the commentators get frantic about who is 2 milliseconds faster than his teammate, they're not going flat out stupid.
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