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# Compressor maps airflow vs psi and turbine speeds

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Hi, I am trying to understand compressor maps, the attached image is a compressor map for a centrifugal supercharger, 0.5kg/s is about 65 pounds per minute.

I understand it as psi is the restriction of the engine, and airflow is the power made. So to me the greenline and red line in my attached image are the same airflow so soame psi boost.

Am I right in thinking that at a pressure ratio of 1.5 and 59k turbine rpm that works out to the psi will be the same at a pressure ratio of 2.0 and 64k turbine rpm. so having the turbine spin faster for higher ratio will be more efficient temperature wise, but not over boost the engine pressure wise?

Or have I got it all wrong and it will be higher psi boost on the engine?

Pressure ratio is the absolute compressor outlet pressure divided by the absolute comp inlet pressure.

So as an example for 100kpa (14.5psi boost), assuming atmosphere on the inlet (100kpa) and 200kpa outlet (100kpa boost + 100kpa atmosphere), Pressure ratio would be 2.0.

For a pressure ratio of 1.50, you would need 150kpa outlet pressure or 50kpa (7.25psi) boost.

Thanks Adam, but that is the answer to a different question :-) Let me try to rephrase my question as looking at it again I don't think I explained it very well the first time

If the compressor was not hooked up to any tubes, that is pumping onto the atmosphere the what would the PSI be then? I think it would be same as atmosphere. So the only way to have a psi reading higher than atmospheric pressure is to have the output restricted, for example pushing it into an engine intake manifold.

Looking at the image attached to my original post, on the 0.5kg/s line, where it intersects with the 2.0 and 1.5 pressure ratios, is it pumping the same mass of air? I think it is pushing 0.5kg's per second at all pressure ratios intersecting that vertical line

If that is right (and I don't know hence my question) wouldn't the PSI in the intake system be the same for both points on the map given it is the same mass of air being pushed into it? And if that is the case then I should use pulley sizes to have it reach the 0.5kg's at pressure ratio of 2.0 when my engine RPM's are at or about the peak power rpm shouldn't I?

All the compressor map shows is the compressor efficiency for a given mass flow and pressure ratio. It has nothing to do with how much air the engine will consume.

However, the mass flow the engine will consume is proportional to manifold pressure as you are changing the density.

So if you think your engine is consuming 0.5kg/s at a 1.5PR (50kpa boost) you will have a compressor efficiency of about 62%.

If you increase boost to a PR to 2.0 (100kpa boost) then your mass flow will increase to 0.67kg/s and your comp efficiency will be about 63%.

Really the compressor is not well suited for either scenario.

Why would it be pushing 0.67kgs @ 63% efficiency if the compressors is only spinning at, lets say 65,000 rpm? It's belt driven supercharger, so max speed is control by the pulley sizes and engine rpm. So at about the 64-65k rpm I think its at 72-73% efficiency on that map, what am I not understanding about that relationship?

As far as the compressor not being suitable, that is what I am trying to understand a swell. It's hard to find compressor maps, it seems some manufacturers don't like making them available which to me is a big red flag

I have highlighted the 60% efficiency island/line in green and the 65% efficiency island in red. If we plot the two operating conditions that you mentioned above they would be the intersection of pink lines. You will see both conditions intersect roughly halfway between the 60 & 65% efficiency lines which is where my quoted efficiency numbers came from.

Ideally you want to be operating in that 72% island in the middle of the map.

thanks I understand that, but the turbine would not be spinning fast enough at 65,000 rpm to push that much would it? Maybe I misread that but to me it looks like the RPM line is a curve on the map, so it would need to spin to 72 or 73 thousand rpm to reach that point. I am looking at the thick black lines thinking they represent RPM plots

Why do you think it will be doing 65K? If you need to flow 0.67kg at 2.0PR, then your compressor speed will need to be 72/73K.

Because the gearing on the pulleys would be set for that speed at around peak engine RPM, so that it is sitting at 0.5kg's per second and in the 72% island

See in this image (if I inserted it properly) the blue line is the 65k rpm line

EDIT: I attached the image, I need to work out how to insert images on this forum

Correct, that would be the case if your engine consumes 0.5kg/s at 2.0PR. But it is not going to consume the same mass flow at any PR like you originally were suggesting, the flow will vary proportionally to manifold pressure.

So usually you would start by calculating the boosted mass flow you need to achieve your power goal, then divide that by the mass flow your engine would make naturally aspirated to find the PR you need to achieve the boosted mass flow. Then with the required mass flow and PR as known variables you can plot them on the compressor map to see if the compressor is suitable.

Roberto is talking about superchargers and it is a bit different with them. First you calculate how much boost a particular combination of pulleys will make, then you find out the total mass air flow and then you plot it on compressor map. That is talking about positive-displacement superchargers. Centrifugal superchargers are also a bit different.

The good book to read about them is the one in the attachment

yea, it's getting late and my brain hurts, I have been reading this page and doing some maths, and I think I am getting an understanding of it even thought it is a turbo page adn a not a SC page

https://www.garrettmotion.com/news/newsroom/article/how-to-select-a-turbo-part-2-understanding-calculations-to-turbo-any-engine/

Tomorrow I will have a search for that book

Replying to shota; I think the original post has been edited so some of the context to my replies has been lost. It was originally written like Roberto was trying to pick the correct size supercharger for his engine, so in that case final drive ratio and pressure ratio is unknown, you have to approximate what those will need to be from the desired goals. Regardless of positive displacement or centrifugal pump the same laws apply.

yes, I did poor job of asking the question so re-phrased it

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