Mercedes Power Unit Hardware & Software

[DR30]

Hmm I would think that the engine seeing the lowest possible backpressure would be an advantage when turbine driven electrically, and likely drive losses at the turbine due to spinning faster than the exhaust stream. I would imagine the turbine spinning in isolation of the exhaust would reduce power required to motor the shaft similar to throttling a compressor.

Even when the turbine is being driven electrically you're still extracting work from the exhaust blowdown, diverting that would only ever be a net loss of power.

Yes you are extracting work from blow down, which in turn creates back pressure / disrupts exhaust scavenging and subtracts from net engine power, providing MGU-H has enough power to do so on its own.
If the MGU-H was not powerful enough to drive the compressor for short periods independently then obviously a full divert wouldn't work but I suspect they do have enough power to do so, at least briefly.
Audibly I would argue it backs up my theory.

[PhillipM]

You can scavenge blowdown pressure with barely any difference in the backpressure. What's more, flowing exhaust gas through both the turbine and the wastegates is always going to flow more air to atmosphere than just the wastegates alone, so that would drop the pressure more than using a diverter setup.

The MGU-H would never be driving the turbine 'faster than the exhaust stream' because by definition spinning the turbo up would be pushing a shedload of mass flow through the compressor and the PU.

[Tommy Cookers]

.... extracting work from blow down, which in turn creates back pressure / disrupts exhaust scavenging and subtracts from net engine power ....

maybe some newer members haven't read the Wright paper 'TC Facts' available as a pdf on enginehistory.org ?
wherein Wright says 'cylinders are relatively unaware of the existence of the turbines'

or page 15 etc of the TERS thread ?
wherein I attempted to highlight the useful potential of blowdown being a choked process

or the rain of posts in the 2014 engines thread in those years c.2014 ?

[saviour stivala]

And some seems to skip page 9 of said TC FACTS on pdf on enginehistory/.org.

[DR30]

.... extracting work from blow down, which in turn creates back pressure / disrupts exhaust scavenging and subtracts from net engine power ....

maybe some haven't read the Wright paper 'TC Facts' available as a pdf on enginehistory.org ?
wherein Wright says 'cylinders are relatively unaware of the existence of the turbines'

or page 15 etc of the TERS thread ?
wherein I attempted to highlight the useful potential of blowdown being a choked process

Key statement being "relatively" and not "totally". Think you have bolstered my argument.

[Tommy Cookers]

.... extracting work from blow down, which in turn creates back pressure / disrupts exhaust scavenging and subtracts from net engine power ....

maybe some haven't read the Wright paper 'TC Facts' available as a pdf on enginehistory.org ?
wherein Wright says 'cylinders are relatively unaware of the existence of the turbines'

Key statement being "relatively" and not "totally". Think you have bolstered my argument.

isn't your argument now saying that Wright were wrong ?

they obtained c.10% power gain (exhausting at 1 bar ambient pressure) - free of any cost in fuel consumption
and sold c.15000 engines on that basis

[saviour stivala]

Yes. they obtained c.10%power gain exhausting at 1bar ambient pressure. they did so by using a 'blow-down' turbine instead of a pressure turbine. please refer to page 9 of their technical findings.

[DR30]

maybe some haven't read the Wright paper 'TC Facts' available as a pdf on enginehistory.org ?
wherein Wright says 'cylinders are relatively unaware of the existence of the turbines'

Key statement being "relatively" and not "totally". Think you have bolstered my argument.

isn't your argument now saying that Wright were wrong ?

they obtained c.10% power gain (exhausting at 1 bar ambient pressure) - free of any cost in fuel consumption
and sold c.15000 engines on that basis

In a formula one engine application yes I guess. If they were right regarding a formula one engine incorporating an MGU-H, there would be no need to run anything other than a log style header.
You are assuming the valve timing requirements of a stationary engine match those of a dynamic formula one engine equipped with an MGU-H which is flawed.

[Hoffman900]

The Wright paper also claims the exhaust gases reach sonic conditions, which we have learned via pressure transducers paired to crank rotation is hard to impossible (in most multi cylinder engines) to accomplish.

In common performance applications, according to one of the best cam lobe designers, it’s only been in the last fifteen years or so that pressure ratios between the intake and exhaust are such that they mimic a NA engines and required cammed accordingly.

[J.A.W.]

The Wright paper also claims the exhaust gases reach sonic conditions, which we have learned via pressure transducers paired to crank rotation is hard to impossible (in most multi cylinder engines) to accomplish.

In common performance applications, according to one of the best cam lobe designers, it’s only been in the last fifteen years or so that pressure ratios between the intake and exhaust are such that they mimic a NA engines and required cammed accordingly.

If you scroll across to ~3:30 into this vid, & run it in slo-mo, you'll see the sonic-shock waves
visibly forming in the exhaust efflux of this Wright radial engine's individual header pipes:

[LaplacesDemon]

The pressure waves always travel at local speed of sound. The actual flow velocity may or may not be sonic depending on the restriction.

[saviour stivala]

Formula one turbocharger. The MG-H does not spin the turbocharger shaft any higher than the exhaust gases/turbine does. Yet. It is only possible to extract maximum power output possible when compressor is in electric supercharger mode with waste-gate/s fully open and exhaust gases are bypassing turbine housing. This means that exhaust gases going through and out of turbine housing (spinning the turbo) still robs some possible engine power output.

[Tommy Cookers]

The Wright paper also claims the exhaust gases reach sonic conditions, which we have learned via pressure transducers paired to crank rotation is hard to impossible (in most multi cylinder engines) to accomplish.
In common performance applications, according to one of the best cam lobe designers, it’s only been in the last fifteen years or so that pressure ratios between the intake and exhaust are such that they mimic a NA engines and required cammed accordingly.

I don't understand what is being said here (or elsewhere apparently)

afaik ...
later NA F1 engines (huge bore:stroke ratio/huge valves) had lower mean gas speeds than eg DFVs (or olders)
but these later engines still made some compromise of EVO timing vs potential losses from imperfect scavenge
(ok maybe less when freeze rules pulled existing huge b:s engines down to 18000 max 17000 general rpm)

nominal EVO timing doesn't pass all gas at EVO - effective EVO depends on actual valve (port) size and actual rpm
ie real blowdown behaviour is rather similar even with supposedly very different engines
and current F1 engines b:s ratio will make their gas speeds somewhere between the DFVs and the late NA F1s

in his book J.C.Morrison BSc PhD MIMechE says he recorded in 1962 a peak exhaust gas velocity of 1968 ft/sec
a 'shock wave' isn't a sound wave - but it is quite likely to be moving at the speed of a sound wave

I have never said that very high exhaust velocities are ideal - quite the reverse
I have never said that low mean exhaust pressures are ideal - quite the reverse
but it's the non-ideal nature of blowdown that allows a turbine recovery that's without cost in crankshaft power ...

the matter in hand is the electric supercharge mode with fixed valve timing .....
how won't the port exhaust speed try to be high when the pressure drops from eg 4 bar (WGC) to 1 bar (WGO) ?

[gruntguru]

The Wright paper also claims the exhaust gases reach sonic conditions, which we have learned via pressure transducers paired to crank rotation is hard to impossible (in most multi cylinder engines) to accomplish.

All engines see sonic conditions at the exhaust valve seat during blowdown. Only requires cylinder pressure >= 2x port pressure. During that period, changing the exhaust backpressure has no effect on flow from the cylinder.

[Hoffman900]

The Wright paper also claims the exhaust gases reach sonic conditions, which we have learned via pressure transducers paired to crank rotation is hard to impossible (in most multi cylinder engines) to accomplish.

All engines see sonic conditions at the exhaust valve seat during blowdown. Only requires cylinder pressure >= 2x port pressure. During that period, changing the exhaust backpressure has no effect on flow from the cylinder.

At the seat, but from the throat and downstream it doesn't, or should I say, it's very hard to do.

If you look at the ports on any of the older Cosworth stuff, the exhaust ports are monstrously overly big.