321apex wrote:chip engineer wrote:321apex wrote:...
The point I was making is that the Mach index determines at which point an engine will start making less and less power at the peak. To make this argument understandable you must assume constant manifold pressure. At such condition, the airflow can not increase with increasing RPM, which adds internal friction (pumping + mechanical) as it rises.
I still don't see how this makes the point that a wide power band is so difficult even in a 2014 F1 turbo engine.
Maybe oversimplifying: Just design the valves and cam timing for decent performance at 15000 rpm (no choked flow problem), then use whatever boost you need for full fuel flow power down to 10500 rpm (or even lower).
Of course, my statement says nothing about efficiency over the band, but the Cosworth simulations show very good efficiency from less than 9000 to about 14000 rpm. So going all the way to 15000 was not their choice, possibly for some of your reasons, but their power band is still very wide.
BMEP=IMEP-FMEP
BMEP -> torque at the crank
IMEP -> mechanical potential from combustion
FMEP -> all frictional losses, pumping, mechanical
The rules allow the following fuel flow:
9500 - 91 kg/h
10000 - 96
10500 - 100 = <<<-------GOLDEN POINT
The higher the engine speed with limited fuel, the higher are the friction losses (FMEP)... and LESS brake POWER. So ideal power peak is at 10500 RPM, however you must factor in the RPM spread for the gearing, which will force you to rev the engine in a zone of DIMINISHING FUEL efficiency [BSFC].
Engine speed of 15k RPM is the worst possible option. IMHO
Think about this:
- Will an engine make more power at 11k or at 10k?
- How much less power will you make at 15k vs 11k? What will be your BSFC at 15k?
When those questions are answered with precision it is then possible to establish a workable power band spread for gear ratios and decide how high in the rev band it must be pushed.
But, Apex, the original point you made was:
"What that means is that the power peak and torque become "cast in stone" as an event in rev range. By adding or subtracting boost you may move these peaks up or down in value but not much in RPM."
What I am arguing is that for a 2014 F1 turbo engine, that is definitely not true. In fact the opposite: fuel flow limits the power; you can't change the power at all (except minor efficiency changes). But you can operate almost at any rpm you want from 10500 to 15000.
Of course, efficiency (and therefore power) may be a little better at some rpm values than others, but according to the Cosworth simulations, the self-sustaining mode power is pretty flat from 11000 to 13000. If you had the Cosworth engine, this is the range you would like to use in qualifying.
For best efficiency (which is what matters in the race during times you are saving fuel), there could be a reason to go below 11000 rpm if:
1. the ICE has higher efficiency there (probably the case)
and
2. you do not need much MGU-H energy because you almost fully recharge the energy store under braking (probably not).
Otherwise, you want to run where the total efficiency (including MGU-H) is highest: 11000 to 13000 rpm (in the Cosworth case).
I think you are totally overlooking the importance of MGU-H power.
