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analysing turbo perfomance

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hi guys

1. question reguarding the pressure ratio on a turbo compressor map I understand you have to divide the barometric pressure vs the compressor out let pressure to see where the turbo is working in the map, now on the pressure ratio axis the numbers just say 1.5 2.0 2.5 are they boost pressure numbers the turbo is capable of? also are those number including barometric pressure?

2. the efficiency islands as we start to get closer to the choke line the efficiency starts to drop off is that due to the fact that as were running higher boost pressures were heating the air and decreasing the air density in the intake?

3. is there a way we can work out at what rpm we will start coming on boost based off the compressor map?

4. andre was saying a turbo cant be 100% efficient whys that? what would it mean for a turbo to be 100% efficient?


The ratio is the absolute pressure after the compressor relative to the pressure at the compressor inlet. Say you are at~900m altitute and have 2kPa pressure drop from atmosphere to the compressor inlet, so you end up at 90kPa absolute at tge compressor inducer. If you are operating at a PR of 2 you have 180kPa absolute after the compressor or roughly 90kPa or boost. You have to remember it is a ratio not a linear offset so filter/pipe loss and charge heating present real problems.

As the charge inlet air approaches the speed of sound you reach the flow limit of the compressor. The ratio of frictional loss to air mass moved increases at this point.

You can get a rough idea when a compressor will start working by how much air the engine will swallow with no boost but you need to consider the turbine and turbo inertia too.

Due to the laws of thermodynamics there is a limit to ideal compression/expansion without heat transfer which results in temp/rise of fall with pressure change, you can look up equations. Due to frictional effects we cannot achieve that so add slightly more energy again to the charge as heat, same goes for the turbine.

Since it's a ratio, the higher the altitude, the more the pressure ratio increases. Also, the more restrictive the piping at the inlet, the higher the pressure ratio. Then you end up with a higher compressor speed and you move to a different part of the map.

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