Do FSAE engines actually reach choked flow?

[RacesWithWolves]

You seem to confuse quite a bit and i was not specific enough.

The decrease in density at the restrictor throat increases the volume flow rate at the restrictor throat. Thus the decrease in density leads to a chocking at lower mass flows compared to a calculation with atmospheric pressure like you did.

How do you come up with this 0.6 mach? The restrictor chokes at mach 1 and a good designed restrictor difusor can regain most of the pressure loss that occurs before the restrictor actually chokes the mass flow. -> A good restrictor only affects your engine performance when your mass flow reaches the choke massflow of the restrictor.
And this does happen at 10.500 to 11.000 with a VE of 1 for a 600cc engine. Recheck your calculations! At this point the flow reaches a velocity of mach 1 at the restrictor throat and you reached the critical pressure difference. A further increase in overall volume flow is not possible! Please (re)read some literature about nozzle flow and if its just the Wikipedia article on "choked flow" which includes the correct formula to calculate the mass flow through a nozzle.

As to the VE of the engine. The VE is always based on atmospheric pressure. So of course the VE is a variable dependent on the charge density, which is dependent on the gas exchange of the engine. Pressure losses decrease your VE and charge effects from your intake runners or a compressor increase your VE.

Thanks for the clarification, I am starting to understand now. I will do some more reading and then redo my calculations.

[Jan_Dressler]

What section of this page are you referring me to? The minimum pressure ratio section? If I understand it correctly, choked flow should occur IF the pressure through the restrictor reaches about 50 kPa.

Sorry that my first answer was a bit short, but it seems you are on the right track now

By the way, if you are interested, there are some actual dyno measurements of different cars on the same dyno available here: http://www.formulastudent.de/fileadm...no_Results.xls

Jan

[mech5496]

Slightly offtopic, but I have a (possibly stupid) question. Please forgive my ignorance as I am by no means an engine guy, I just like to learn stuff.


Assuming a turbocharged engine while its' restrictor is chocked. The fact that is chocked, should mean (according to Wiki) a pressure ratio of 0.528 across the restrictor, thus meaning that the turbocharger inlet pressure is only 7.76psi. Now, assuming a (relatively high for a GT12) turbo pressure ratio of 2.5, you end up with 19.4psi on your intake (or 4.7psi of boost...


Possibility is that you regain some of the pressure before the turbocharger inlet (like you would in an NA engine plenum) but is this the case?

[Jay Lawrence]

mech5496, not sure if that pressure ratio is correct, especially for the entire choked RPM range. Our results showed a maximum manifold pressure of about 220kPa abs. (about 17psi boost pressure) at something like 7000RPM using a GT15. The compressor inducer is exposed to ~atmospheric pressure a little bit prior to choking (assuming a decent intake path and restrictor), and as you choke and as engine speed increases you reduce that pressure thus reducing your pressure ratio but maintaining ~0.07kg/s mass flow. Thus the turbo helps you to choke for the largest RPM range possible (greatest torque magnitudes and spread). Towards the end of our RPM range, we were seeing ~atmospheric pressure in the plenum, indicating that by that point the choked restrictor is having the effect you describe.

[mech5496]

Jay, thanks for your clarification. I understand that mass flow rate in chocked flow remains constant at about 71gr/sec, but I would like to see what happens and if teams get past the surge line in their turbochargers. The original thought-question in my head was how I could use an electric motor coupled to a compressor side of a FSAE sized turbocharger and how the massflow/boost map would be with respect to motor RPM.

[Jay Lawrence]

If you are getting into the surge area of the compressor then you are not at choked flow (or you have chosen the wrong compressor). If you were to drive the compressor with an electric motor you would have to heavily gear the motor or you wouldn't reach sufficient speed. The best bet for that application would probably be to make it a fixed speed compressor (rather than using some kind of VFD on the electric motor) attached to a fixed speed engine driving a CVT. If you were to use it as a generator you would probably just create too much "backpressure" for it to be useful.

[Dan Radulescu]

Hi

I did my calculations on the choke flow.
I did it by 2 methods, from different sources.

I the theoretical engine airflow demand using the equation
ThFlow= Displacement x RPM x VE( assumed =1 ) / Engine Stroke ( 2 ) and converting it to m^3/sec


Method 1
by Fundamentals of Fluid Mechanics 5th - Bruce R. Munson, Donald F. Young, Theodore H. Okiishi

Initial conditions ISA
p0=101325 Pa
t0=15 deg Celsius
rho0= 1.1839 kg/m^3

I ended up with a Mass Flow Rate of 0.075 Kg/s in the following critical conditions
cP= 53528.152 Pa
cT=240.125 deg Kelvin
cRho= 0.777 Kg/m^3

If I calculate Volume flow rate as VFL=MFL/density
using ambiental air density I get 0.062 m^3/s
using critical air density I get 0.098 m^3/s


Method 2
using an 2 euqations found in an article entitled
Improvement of Intake Restrictor Performance for a Formula SAE Race Car through 1D & Coupled 1D/3D Analysis Methods
by Mark Claywell and Donald Horkheimer from Univeristy of Minnesota

Using an equation for volume flowrate at choking point I get 0.063 m^3/s


In the case I get 0.063 m^3/sec I will reach choked flow at around 12600 RPM
In the case I get 0.099 m^3/sec I get it nowhere near the operable RPM

Where am I doing wrong in here? Should I take the ambiental air density when calculating the Volume flow rate ? If that's so I will reach choke flow at around 12600 RPM not 10500.

I attached a download link with my calculus ( Excel & Mathcad ) for both ways, available for 7 days starting 11 December 2013.
http://we.tl/R405Tf0r5i

Thank you in advance!

[Simon Dingle]

Hi Dan,

Try converting your "theoretical engine airflow demand" into mass flow rate and comparing that to Method 1. I'm afraid I'm not familiar with Method 2, but I've used Method 1 and got an almost identical answer to you.

A couple of things to be aware of when doing this sort of analysis:
1. The volumetric efficiency of an engine can exceed 1 if the intake manifold is designed correctly. (I'm assuming that you're using an R6/CBR600 or similar)
2. The calculation you describe in Method 1 is for finding the isentropic mass flow rate. In reality any restrictor that you make will not actually achieve this flow rate. One of our team members measured the isentropic efficiency of two throttle/restrictor/diffuser assemblies and found them both to be between 80 and 90%.

I hope that helps