Turbo limitations - help me with the theory here!

[aceparts]

Am I right in thinking that flow into / though a turbo (or any other orifice!) is limited by the inlet hole? As atmospheric pressure is pushing the air INTO the turbo, and that's limited to 1bar, the total flow cannot exceed the limits of the inlet?

(Assuming no + pressure from moving the aperture through the air etc).

[SuperchargedVR6]

I'm no expert but I thought it was down to a complex relationship between the inducer and exducer? Having said that, if you restrict the turbo intake, then it can only draw so much air in.

[stevieturbo]

aceparts said:

Am I right in thinking that flow into / though a turbo (or any other orifice!) is limited by the inlet hole? As atmospheric pressure is pushing the air INTO the turbo, and that's limited to 1bar, the total flow cannot exceed the limits of the inlet?

(Assuming no + pressure from moving the aperture through the air etc).

Ultimately of course any flow is limited by a hole it has to pass through.

But not really sure what you're asking ?

[SamR380]

Yes, this is how the WRC restrict the power of their cars (they mandate the use of a restrictive orifice).

This thread has a post that explains it well...
http://www.pistonheads.com/gAssing/topic.asp?h=0&a...

[fatjon]

aceparts said:

Am I right in thinking that flow into / though a turbo (or any other orifice!) is limited by the inlet hole? As atmospheric pressure is pushing the air INTO the turbo, and that's limited to 1bar, the total flow cannot exceed the limits of the inlet?

(Assuming no + pressure from moving the aperture through the air etc).

Kind of but not quite, you are not considering both sides of the hole. The flow through the hole is proportional to the pressure difference on each side of the hole. Of course atmospheric pressure is a fixed number but there is a depression inside the air inlet caused by the turbo removing the air and stuffing it into the engine. The more the turbo sucks the greater the pressure difference across the orifice and therefore the greater the flow through the orifice.

[Pumaracing]

aceparts said:

Am I right in thinking that flow into / though a turbo (or any other orifice!) is limited by the inlet hole? As atmospheric pressure is pushing the air INTO the turbo, and that's limited to 1bar, the total flow cannot exceed the limits of the inlet?

(Assuming no + pressure from moving the aperture through the air etc).

Yes you're correct. The turbine blades compress whatever air passes through the inlet and present it to the engine at a higher pressure and density than atmospheric but they can never process more air than can flow through that inlet at a pressure difference of 1 atmosphere, 14.7 psi. 1 bar however is actually a slightly different measure being about 14.5 psi.

In practice it's rarely going to be an issue though. Every square inch of area of a well rounded orifice operating at 1 atmosphere pressure difference can process enough air for around 400 bhp.

[anonymous-user]

Pumaracing said:

aceparts said:

Am I right in thinking that flow into / though a turbo (or any other orifice!) is limited by the inlet hole? As atmospheric pressure is pushing the air INTO the turbo, and that's limited to 1bar, the total flow cannot exceed the limits of the inlet?

(Assuming no + pressure from moving the aperture through the air etc).

Yes you're correct. The turbine blades compress whatever air passes through the inlet and present it to the engine at a higher pressure and density than atmospheric but they can never process more air than can flow through that inlet at a pressure difference of 1 atmosphere, 14.7 psi. 1 bar however is actually a slightly different measure being about 14.5 psi.

In practice it's rarely going to be an issue though. Every square inch of area of a well rounded orifice operating at 1 atmosphere pressure difference can process enough air for around 400 bhp.

The big issue for inlet restricted engines that are also turbocharged is the effect that restrictor has on Compressor Work & Efficiency!. If we take a critically choked inlet restrictor (M1.0 at choke) the total pressure at that point, for a standard atmosphere upstream (1bar, 20degC) is just 52.8kPa(abs). The regulators generally force one to have the inlet restrictor close to the compressor wheel front face (typically a max of 50mm away) for very good reason. It's all about pressure recovery, or the lack off.

Consider a turbo charged engine making 1bar(g) boost pressure in a standard atmosphere without a restrictor. if we ignore the small pressure losses in the precompressor intake system, the compressor pressure ratio will be 2 (1bar boost = 2bar(abs), so 2/1 = 2).

Now we put an inlet restrictor in place, that has very little pressure recovery after it (because it's so close to the compressor wheel, there is no time/space to decelerate the incoming air from M1.0 and rebuild the static pressure before that air hits the compressor wheel). Lets assume we now have say just 60 kPa (abs) upstream of the compressor say.

So, for the same level of boost (1bar) our pressure ratio is now 2/0.6 = 3.3!

In both cases, if we assume an intake massflow rate of 0.2kg/s (735kg/hr about the max available through a choked 34mm restrictor), the compressor work is as follows:


At a fixed 78% compressor efficiency,

Case 1) with a PR of 2.0 is 16.7kW and a Compressor temp rise of 80.9degC

Case 2) with a PR of 3.3 is 31.2kW and a Compressor temp rise of 151.6degC


But of course, at a pressure ratio of 3.3, the compressors efficiency will also be reduced (extra blade spill and leaking due to higher pressure across the blades) typically, you'd lose something like 5% efficiency, so at a 73% efficiency we get:

Case 2) PR of 3.3, 73% CompEff, is 33.4kW & 162degC air out


And it's not all over yet: Because that extra work has to come from somewhere, namely the exhaust gases, there is a corresponding increase in preturbine pressure to furnish that extra compressor work, which reduces the engines pressure ratio for the same 1bar(gauge) intake manifold pressure. Hence you get a fall in manifold volumetric efficiency, and would need to use a higher manifold boost pressure to get the same intake mass flow.

And: Higher preturbine pressure means less expansion of the exhaust gases, which means a higher EGT, which may need more fuel to prevent thermal stress of the exhaust line components.

And: Higher preturbine pressure means a higher proportion of in-cylinder exhaust gas residuals at EVC, which on a heavily boosted relatively low speed engine, generally results in detonation and the requirement to retard ignition angle further away from MBT (meaning less torque, and higher EGT)

And: More compressor work means higher compressor air outlet temps, requiring more intercooling work, or leading to an increase in intake manifold air temp, again reducing density (lower mass flow) and possibly resulting in detonation again.


Basically it's a horrible ever decreasing spiral of effects, and precisely the reason intake restrictors are used for effective power limits in motorsport, and why engine designers/developers have spent so much time, effort and money trying to optimise every last facet of the engine and support systems for the inlet restrictor that is mandated!