Turbocharger Question: 2x Less Rotating Mass Benefits?

[yellowf40]

Hello guys I am new to this. I have a question about turbochargers that is really giving me a headache maybe someone can help. If there are 2 turbochargers they are the same in every way. The only difference is that one turbocharger has 2x less rotating mass than the other turbocharger. Will both turbochargers require the same amount of energy to compress a specific volume of air to a given pressure?

I know this may sound dumb, but try to understand what I am saying. By having 2x less inertia, is faster response to get to a certain rpm the only benefit? or does the lighter turbo require less power to compress the air than the heavier turbo since the heavier turbo needs more power to get to a certain rpm?

If we consider the adabatic compression process which turbochargers follow, nearly all the turbine shaft work is used by the compressor to raise the temperature of the air and the pressure. If one turbo has 2x less inertia than the heavier turbo wouldn't the lighter turbo have less available shaft power than the heavy turbo, because less power is needed to overcome the inertia of the lighter turbo? Would this affect the overall adabatic temperature rise?

 

[Mech_Engineer]

If there are 2 turbochargers they are the same in every way. The only difference is that one turbocharger has 2x less rotating mass than the other turbocharger. Will both turbochargers require the same amount of energy to compress a specific volume of air to a given pressure?

The advantage of a lighter turbo is it can spin up faster when the car is first accelerating (one turbo might begine supplying full pressure at 3000 rpms, while the lighter version might spool at 2300 rpm). After either turbo has spooled up, they will both compress the same amount of air to the same state given a certain amount of energy and a specific engine rpm. So, if you look at any steady-state point after the turbos have spooled up, both are capable of doing the same amount of work at that instant.

It gets a little fuzzier if you integrate over the entire RPM range though. Since the lighter turbo has a faster spool up time, it is able to supply boost to the engine for more of the time during an acceleration run. Therefore, when looking at an entire RPM range, the lighter turbo is able to supply more compressed air, and therefore more work, since it has been compressing the full amount of air it can for a longer period of time.

Make sense?

 

[yellowf40]

Ok so the key here is steady-state. I see what you mean and it does makes sense, but now I am seeing something else. For example let's say the required kinetic energy to get a turbo to 50,000 rpms is around 2kw. Will this energy go to compressing the air? You see someone put it like this and it caused too much confusion on my side. They said once you supply the required power to overcome the inertia of the turbo to a certain rpm, then at a steady state the compressor takes power from the turbine and not the momentum of the compressor and turbine at that speed.

12 people agreed with him, but I am having trouble seeing this. You cannot just convert heat to compressed air something must be exchanged I said, and that exchange is the decrease in turbocharger momentum for air momentum, but they said that it is not so.

To further explain this we talked about a turbocharger with x amount of wieght and they determined it can take a few kilowatts to get the turbo to 80,000 rpms. After this they said that the energy from the momentum of the turbocharger at 80,000 is not used to make compressed air, but that the compressor takes power from the turbine at a steady-state of 80,000 rpms. If you look at what the compressor is flowing and the pressure rise the energy needed by the compressor is more than the total kinetic energy of the rotating assembly, so how can the compressor take more power and keep at a steady state and not increase in rpm?
I don't understand such explanations they gave maybe you can answer this

 

[Mech_Engineer]

The kinetic energy used to spin up the turbo goes to just that- spinning the turbine/compressor. The turbo hits a balance when accelerating from dynamic to steady-state, where the energy being imparted on it from the turbine is transferred through the turbine/compressor interface and then used to compress the air on the compressor side. Once the turbo has hit steady state, all of the energy developed by the turbine is transferred into the intake stream to compress the air.

 

[yellowf40]

Ok I understand now. I remember talking to a physics about turbochargers, and he told me the lower the temperature into the turbine the more ideal the turbomachine. Now concerning exhaust driven turbochargers, from what I am understanding power from the exhaust is used to spin up the turbine and then compressor the air. Turbos are considered to undergo adabatic compression. So if we go back to the original power source, which is used to spin up the turbine and compress the air, that is the total turbine power. No wonder why the compressed air is getting so hot. It is because the turbine side is hot, and this rule apply's for all exhaust driven turbos.

What I am seeing is that the heat of the turbine is allowing us to compress air but at the same time it is also self defeating because compressor power is turbine power which is exhaust gas power.

If we look at the opposite extreme to exhaust gas turbocharger theory, which is highly improbable since the market is saturated using exhaust gas theory, I am starting to think we can expect a better turbocharger that has less outlet temperatures of the compressor for less power and like pressure of exhaust driven turbochargers.

For example highly improbable but just to get a point across. Super light wieght and strong thermoplastic turbine and compressor, being driven by a super cold tornado, would be a whole different level of theory than exhaust driven turbo, and most likely a more efficient machine