Plotting compressor map for restricted car
Turbo Mini Forums

Hi Everyone,

Was wondering if anyone could help me out with plotting some operating lines on a compressor map, the car we are dealing with has an inlet restrictor before the compressor which causes things to go a bit funny.
I understand once you reach the choke limit of the engine the air mass flow rate won't increase, and as the mass flow rate goes up the inlet pressure before the compressor will go down, and thus a higher pressure ratio will be needed, I think.

Has anyone got an examples of a compressor map with a line of this type plotted on it for reference?

If it helps, its a 600cc engine revving from about 3-4k up to 11-12k, 20mm or 19mm restrictor (depending on petrol or E85 respectively), looking at using a GT12
(map: www.turbomaster.info/img/aplicaciones/GT1241-756068-0001-compmap.jpg)


Cheers,
Jake

This might be a good forum to ask

http://efi101.com/forum/viewforum.php?f=10

But I think monitoring impeller speed is a better way of optimising performance when restrictors are involved

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

The restrictor reduces the inducer pressure, thus increasing the pressure ratio, and therby reducing the effective mass flow of the compressor. Basically as the mass flow increases towards the maximum mass flow that the particular restrictor can achieve, the pressure drop across it also increases. Since the higher pressure available is atmospheric pressure, the compressor inducer pressure is actualy in a vacuum. The compressor map works pretty much the same way as it does without the restrictor, but the pressure ratio usually noted on the Y axis will increase with the lowering inducer pressure as mass flow increases and sustained exducer pressure.

The key is the restrictor. Work out the mass flow and pressure drop of the restrictor, work out that pressure ratio from that, then you should be able to plot that on any compressor map

But restricting it also causes the impeller to spin much faster, and I think that's where the risks lie. Overspinning the unit.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

The published compressor maps are based on atmospheric inlet pressure and temperature (i.e. density) at the inducer. Hence the pressure loss through the restrictor will reduce the density of the air entering the inducer.

So to adjust the map to take into account of the restrictor you need to correct the inlet conditions. Roughly speaking, if you create a 0.5 Bara condition at the inducer inlet, you will need to halve the X axis of the map.

The Y axis will be unchanged but, of course, the inlet pressure has changed so the outlet pressure will also change, but the pressure ratio will stay the same.

There is a point at which the inlet pressure will no longer supply enough air and the compressor inducer will fail on suction performance.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Sounds like someone is working on a formula student car! how's it going?

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

sounds interesting, going a bit off topic, where do you buy your e85 from?

Team Racing

Good question, we have never run it in the past as it means you need a smaller restrictor and results in less power when NA, but when boosting the knock resistance makes it worth it. At the events its supplied by Shell I believe, so I guess it would be a case of speaking to them. And I'm guessing it will come in drums rather than out of a pump.


As for Paul's scaling of the X axis, would this actually need to be done? As the garrett maps have air mass flow rate, hence density/ temperature/ velocity is already taken into account? I understand that would be the case for the maps that have a volumetric flow rate.
Or is it a mass flow rate calculated from atmospheric conditions?

Cheers,
Jake

Given the drive losses with a supercharger, dont think it would even be worth looking at compared to a turbocharger.

Unless it allows you a larger restrictor or something

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

I understand in power terms its inferior due to more losses, but in terms of drivability it would be better.
The courses are very winding, and you don't get much/any practise before they expect you to drive it, so having good throttle response and drivability are key.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will