Brettus

iTrader: (2)

I think we can resolve this if we think about the flow through a pipe of air at different temperature but the same mass flow a

Seeing as you are the physics guy how about doing the calculation for me .

50lbs/min of air through a 1" diameter open ended pipe at 1400f
vs 50lbs/min of air through the same pipe at 1700f

What is the kinetic energy in each case ?

If the 1700f example has more kinetic energy then i think you will have proved your point (in my mind anyway) .

TeamRX8

iTrader: (25)

why do you think Pr and efficiency varies for a given mass flow (vertical line on the map)?

Harlan

iTrader: (3)

Ok, how about a simple kinetic energy between different volumes flowing at the same mass flow rate.
First we have to convert to an absolute temperature:
1400F is 1033K for T1
1700F is 1200K for T2

Then we use the one of the ideal gas laws to find the difference in volume with respect to temperature (mass is constant).
V2/V1=T2/T1

So V2/V1=1.16 meaning the difference in volume is about 16% or V1 times 1.16 is V2.

If you have a pipe of 1000ft long and this pipe can hold V1 then you need a pipe of length 1160ft to hold V2 assuming the same diameter. Or if you pump V1 and V2 at the same mass flow rate down the same example pipe so that V1 will travel 100ft/sec then V2 will travel 116ft/sec.

Now to do a ratio of kinetic energy. Where we use V for Velocity instead of volume.
Where E=1/2mv^2
Since the ½ is constant, and m is constant. The ratio of kinetic energy is E1/E2=V1^2/V2^2
Or E1/E2=1.16^2=1.34
So there is 34% more kinetic energy in the hotter exhaust.
Again this is an oversimplification using classical mechanics instead of fluid flow, but it gets the job done.
Yeah I said 20% and not 16% because it's a ball park based on temperature assumptions. It is significant enough though to be important for sizing a turbo.

Brettus

iTrader: (2)

So are you saying 16% less mass flow required to spin the turbine ....................or 34% ?

Harlan

iTrader: (3)

That's a great question... I wish it was as simple as it seems. Let me see if I can find a better answer. I'll also see if I can figure out the change in maximum power available to the compressor comparing exhaust temperatures. There is a reason why I used a ratio based off the temperatures instead of going this deep into the problem, things get complicated and a lot of assumptions need to be made which can skew results.

Brettus

iTrader: (2)

This also begs the question ......... if it requires less mass flow and same volume flow to power up a rotary turbo . Why do people always insist that rotaries require larger turbines/AR ?

Harlan

iTrader: (3)

Keeps back pressure down and allows you to run turbos large enough to eat small children. Not necessary but why not?

logalinipoo

If it requires less flow, less back pressure. So a smaller then normal turbine should be usable with no loss and allowing a faster spool. Any extra volume would be sent to the waste gate, right?

Harlan

iTrader: (3)

Exhaust gas out the WG is just wasted energy. you can save that energy if you run a larger turbine with lower back pressure, but it will take longer to spool.

To produce the same power to the compressor you can run a turbo that requires a low backpressure but high flow, or a high backpressure with a lower flow.

Just because you can create all the power you need from a tiny turbine, doesn't mean it's ideal, any more than using a giant turbine just because you can spool it at high RPM. It is a matter of preference.

Brettus

iTrader: (2)

Perhaps it's more to do with the rotary having a lower tolerance to the effects of back-pressure than a piston engine .

TeamRX8

iTrader: (25)

No, you can't see the forest because the trees are in the way.

In addition to having a 30% lower VE than a reciprocating engine, a rotary engine also has a much higher volumetric flow rate relative to displacement.

TeamRX8

iTrader: (25)

http://www.full-race.com/articles/efrturbotechbrief.pdf

Brettus

iTrader: (2)

Yes . But I wasn't talking about making the same power as a piston . Then of course you need a bigger turbo /turbine housing to achieve the same backpressure . I was talking about where massflow is the same through either engine with the same turbo .

Brettus

iTrader: (2)

Still confused

If you now do this calculation again assuming the energy to turn the turbine is the same either way .
E1=E2

How can volume flow be the same ? You only have one remaining variable ... mass .

say E = 1 and E=1/2mv/\2

if mass was....................... 0.5 then velocity =2

also................ if mass was 0.4 then velocity =2.23

For this example
mass is................................ 0.4/0.5 = 20% less
velocity (and therefore volume) is 2.23/2 = 11.5% more

Not sure how to do the maths for our piston vs rotary example ?

TeamRX8

iTrader: (25)

Not sure what you mean? At 50 lb/hr the piston engine is in theory making 500 bhp and a rotary engine is making 350 bhp. The mass rate is the same, the rotary has 30% less VE. Also due to the volumetric flow difference the rotary is operating at a lower Pr. The boost is lower in the rotary for a given exhaust manifold pressure. They both work against us. Maybe that's what you're missing?

Bottom line, for a given mass flow level the rotary needs a more exhaust flow capability, whether turbine, wastegate, turbine efficiency, etc. as compared to compressor than a reviprocating engine.

Howard Coleman focuses on the aspect for evaluating turbos on RX7Club. His method is not always accurate though.


.

Brettus

iTrader: (2)

Yes that example it exactly what i'm talking about . Whether the rotary needs a bigger turbine housing than the piston in that scenario .

What Harlan has been saying is that the volume flow to power the turbo is the same but mass flow is less .I'm now suggesting that mass flow is less but volume flow is more .... hence the need for a bigger housing . Sure there are other things that come into it like turbine efficiency at lower rpm but it would be good to put an approximate figure on it .

TeamRX8

iTrader: (25)

Hopefully you *both* will figure it out some day ...

Brettus

iTrader: (2)

Yeah ... and you will learn something too ..............obviously .

TeamRX8

iTrader: (25)

I believe you misunderstood me.

I don't care because it's an irrelevant discussion IMO.

Brettus

iTrader: (2)

If I don't care about something ...........I ignore it

TeamRX8

iTrader: (25)

No you don't

Harlan

iTrader: (3)

Mass flow rate will be lower to turn the turbo, that's what I said in the beginning. I was just showing the difference in energy in the exhaust stream, which is what you asked for. Yes volume would be slightly different for the same energy using this calc, and yes I didn't do it this way the first time. Enthalpy went up with the hotter exhaust gas too and that wasn’t fully accounted for either.

I made an assumption to make the math easy. The actual problem is much more complex than I think you realize.
At the same mass flow the THERMAL EFFICIENCY is around 30% less, this corresponds to the higher BSFC. The VE is the difference between the swept volume of the engine and the actual amount of air consumed is higher than most piston engines which is a good thing.

The same turbocharger compressor operating on a piston engine vs a rotary engine at the same horsepower levels will have a higher flow and lower PR because the mass flow will be higher as mentioned above. But operating at the same mass flow and same PR the turbo compressor won’t care. All you need to know is how much air your engine can take, and what that pressure it takes it.

Agreed.

Power Cycle Analysis Online Calculator ...
I played around with a very useful Brayton cycle calculator. The difference in power outputs (yes a few assumptions were made) between 1085K exhaust and 1280k exhaust was 26%. Meaning you would need either 26% less mass flow to produce the same power, or you would need to drop the pressure ratio from 2.0 to 1.7 for the same mass flow. Either way it’s about a 30% difference in sizing.
Again this all hinges on what Garrett assumes for exhaust temperature. If they assume hotter than 1400F our numbers will be closer to theirs. Also I assumed 1700F for us, so if you run hotter or colder it will change the ratio. Just don’t assume things are the same!
In the end you can run about 20-30% bigger than what Garrett says for the same mass flow and run a slightly larger gate, but maintain the gate/turbo split. Or you can go to either side of it for your preference.

TeamRX8

iTrader: (25)

Thanks for the the clarifications.

The Renesis side-port exhaust typically runs several hundred degrees less than the 13B peri-port exhaust.

Harlan

iTrader: (3)

I've seen several side by side comparisons of cruise temps, but no side by of comparable output turbocharged engines. I suspect it has more to do with compression than anything else and with higher outputs the exhaust temps will be closer. If you know any better please share the info.

Brettus

iTrader: (2)

So we are all agreed that there would be less mass flow and more volume flow to power the turbine for the same shaft speed . Excellent .

If we focused on the volume flow and worked that back to what the pressure difference would be ...............that would tell us how much bigger the turbine should be to maintain the same backpressure.
And that number would be less than 20-30% .... surely ?

BTW : Some good stuff in your post #72.