Reasons for varying performance at high altitude

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Steven
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Reasons for varying performance at high altitude

Post by Steven » Mon Oct 30, 2017 9:55 pm

Last weekend's Mexican GP has thrown some surprises in terms of performance, not only for fans, but also for the engine manufacturers themselves.

A few things of note:
- Renault were strong but unreliable (ERS and MGU-H issues).
- Renault said they were too aggressive in relation to the climate
- Renault believe Mercedes were very conservative for the altitude
- Honda were stronger than they expected

So, I'm curious how this can actually happen. How can power unit suppliers still be surprised of themselves and their competition when running on a known circuit?

Does anybody have any idea on what is actually being adjusted? I'm aware there are certain measurements to be taken due to the lower cooling capacity, but do they need to work the Turbo harder with the MGU-H? Or slow it down even?

NL_Fer
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Re: Reasons for varying performance at high altitude

Post by NL_Fer » Mon Oct 30, 2017 11:25 pm

Turboshaft rpm, to compensate for the thinner air, the turbo’s have to spin faster and the bearings are overheating. Renault has pushed the turbo’s to far and they blew, bearing/seals failed.

I can think of 2 causes.

1. The split turbo because of it’s mounting positions, it’s bearings rely on watercooling and somehow they could increase the cooling capacity. (bigger radiators, faster waterpumps?) The Renault is mounted behind the engine and relies more on air cooling, through the top air intake and this is harder to increase.

2. Because of the better mounting position, the split turbo has a bigger compressor and can spin much slower and will have less bearing stress at all. Where the Renault turbo is limited in size, because of the position above the gearbox and the it interferes with the aero.

It is either 1 or 2, or a little of both.

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Re: Reasons for varying performance at high altitude

Post by gruntguru » Tue Oct 31, 2017 3:38 am

The air density is about 75% of sea level density so a PU optimised for a PR of 3 at sea level would need to run a PR of 4 at Mexico City to see the same MAP (and therefore the same AFR). Assuming the compressor and turbine were selected for best efficiency at PR = 3, the efficiency at PR = 4 would be somewhat less. Shaft speed would have to increase by 15%.
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Re: Reasons for varying performance at high altitude

Post by henry » Tue Oct 31, 2017 9:33 am

If the turbine runs at lower efficiency it will extract less lower from the exhaust stream, and similarly the compressor will take more power to drive it. This leaves less power for the MGU-H to harvest and so less need for cooling, unless it too is running less efficiently.
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Re: Reasons for varying performance at high altitude

Post by sosic2121 » Tue Oct 31, 2017 12:09 pm

I noticed from ERS telemetry(blue bars) that Hamilton deployed significantly less than Sainz while they battled.

Also FI deployed less than Ferrari.

Am I missing something here? What exactly that blue bar represents?
ES > MGU-K
ES + MGU-H > MGU-K
?

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Re: Reasons for varying performance at high altitude

Post by MrPotatoHead » Tue Oct 31, 2017 1:31 pm

At it's most basic level on a turbocharged vehicle running at higher altitude (thinner air) you are able to recover the power lost due to the thinner air by running the turbo faster to achieve the same air pressure that you would normally run at sea level and thus almost the same performance.
But - along with running faster you will have higher mechanical loading - and we have already seen how on the edge some of the power units are in the MGU-H reliability.

So if you can make the choice to run higher turbo shaft speeds to get the boost back you then have to worry about the fact that with the thinner air you have far less cooling ability from the air when moving at speed. And these cars are already on the edge cooling wise. This was the biggest problem to manage this weekend.

Basically Renault were too aggressive this weekend - and it showed. It's no coincidence that the Renault powered car of Verstappen that spent most of the race in clean air was also one of the only Renaults to survive.

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Re: Reasons for varying performance at high altitude

Post by notsofast » Tue Oct 31, 2017 8:25 pm

Not to hijack the thread, but what changes could be made to the track to lessen the impact of high altitude?

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Re: Reasons for varying performance at high altitude

Post by strad » Tue Oct 31, 2017 9:56 pm

I just keep thinking of how lean they must be running.
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Re: Reasons for varying performance at high altitude

Post by Mudflap » Wed Nov 01, 2017 12:03 am

gruntguru wrote:
Tue Oct 31, 2017 3:38 am
The air density is about 75% of sea level density so a PU optimised for a PR of 3 at sea level would need to run a PR of 4 at Mexico City to see the same MAP (and therefore the same AFR). Assuming the compressor and turbine were selected for best efficiency at PR = 3, the efficiency at PR = 4 would be somewhat less. Shaft speed would have to increase by 15%.
It's probably worth mentioning that a drop in compressor efficiency translates into an increase in compressor outlet temperature for the same PR. Combined with the reduced charge air cooler performance this probably forces teams to derate ?
How much TQ does it make though?

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Re: Reasons for varying performance at high altitude

Post by dren » Wed Nov 01, 2017 3:28 pm

Mudflap wrote:
Wed Nov 01, 2017 12:03 am
gruntguru wrote:
Tue Oct 31, 2017 3:38 am
The air density is about 75% of sea level density so a PU optimised for a PR of 3 at sea level would need to run a PR of 4 at Mexico City to see the same MAP (and therefore the same AFR). Assuming the compressor and turbine were selected for best efficiency at PR = 3, the efficiency at PR = 4 would be somewhat less. Shaft speed would have to increase by 15%.
It's probably worth mentioning that a drop in compressor efficiency translates into an increase in compressor outlet temperature for the same PR. Combined with the reduced charge air cooler performance this probably forces teams to derate ?
Yes, I'd say cooling was the major reason for derating. Running the PU to obtain the same sea level power creates more heat and the thin air decreases your overall cooling efficiency. Everyone was likely derated, just some willing to accept more risk than others.
Honda!

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Re: Reasons for varying performance at high altitude

Post by MrPotatoHead » Wed Nov 01, 2017 3:29 pm

notsofast wrote:
Tue Oct 31, 2017 8:25 pm
Not to hijack the thread, but what changes could be made to the track to lessen the impact of high altitude?
Pick it up and move it to a lower altitude. :D

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Re: Reasons for varying performance at high altitude

Post by notsofast » Wed Nov 01, 2017 6:36 pm

Or build a dome over it and increase the air pressure.

But seriously. If I understand the situation correctly, there was a lack of grip, and the tires lasted quite long. So, how about paving the track with a more abrasive substance? Would that allow for better racing? Just wondering...

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Re: Reasons for varying performance at high altitude

Post by NL_Fer » Wed Nov 01, 2017 8:40 pm

notsofast wrote:
Tue Oct 31, 2017 8:25 pm
Not to hijack the thread, but what changes could be made to the track to lessen the impact of high altitude?
Make it a street circuit wih more slow corners and shorter straights.

OT: It looks like Renault is really on edge with turbochargers rpm. And the problem with Honda’s split turbo is something else. Or we just saw the superior cooling setup a split turbo offers, because of less IC piping clutter.

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Re: Reasons for varying performance at high altitude

Post by roon » Wed Nov 01, 2017 9:19 pm

Do the engine test stands adjust their plenum pressure, or drop the pressure in the entire room? To simulate elevation.

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Re: Reasons for varying performance at high altitude

Post by MrPotatoHead » Thu Nov 02, 2017 1:23 am

roon wrote:
Wed Nov 01, 2017 9:19 pm
Do the engine test stands adjust their plenum pressure, or drop the pressure in the entire room? To simulate elevation.
Adjusting the plenum pressure would not simulate the different air density.
The only way to do it would be to lower the air pressure being fed to the turbo inlet. But that is troublesome because you have to lower the pressure but still maintain enough volume of air for the engine.