Thank You Bart, and PY, very much. Tom V.
The Compressor Maps posted in Post #18 are good as a basic way to identify where conditions change as the Turbocharger is operating.
The First Picture shows a map with the SURGE LINE identified.
The Pressure Ratio numbers are on the left side and listed in a vertical order with the higher pressure ratio numbers at the top.
If you have a Pressure Ratio number of 1 then you have 1 atmosphere of air entering the induction system.
If you have a Pressure Ratio of 2 you have about 15 psi GAGE "BOOST PRESSURE".
If you have a Pressure Ratio of 3 you have about 30 psi GAGE "BOOST PRESSURE".
Same deal for Pressure Ratios of 4 or 5 or 6.
A Pressure Ratio of 4 being about 45 psi GAGE "BOOST PRESSURE".
A Pressure Ratio of 5 being about 60 psi GAGE "BOOST PRESSURE".
Once you get over 5 Pressure Ratio you are dealing with very expensive Turbos like the ones used in Tractor Pulling or the people are using COMPOUND BOOSTING methods.
One Boosting Device pressurizing a second boosting device.
Back on the Surge Line for a minute. The Surge Line drawn on the map is not a ABSOLUTE LINE, the Surge Line can move so that on one engine the line may be a bit more to the left on the map and on a different engine the line may be a bit more to the right. The line also has a variable vertical path that can deviate from the Turbo Suppliers "Mapped" turbo on the Turbo Test Stand.
So what is SURGE? Here is a little Video where a guy is explaining "Turbo Flutter" or SURGE.
https://www.youtube.com/watch?v=FwzUffAuQyc
Turbo Surge is caused by the Turbo trying to pump more Mass Flow thru the engine vs what the engine Throttling System is allowing. The Throttling System could be that the
physical engine size is the wrong size for the Turbocharger used.
Turbo Surge can also occur when the Turbo and engine are happy at a given rpm and load point and the driver suddenly closes the throttling device. Now the turbo is still trying to pump Mass Flow thru the engine but you suddenly stopped that mass flow.
THE TURBO IS NOT HAPPY WHEN IT GOES INTO SURGE. IN SOME CASES THE COMPRESSOR WHEEL TIPS MAY TOUCH THE COMPRESSOR HOUSING. This is very hard on the Turbo "Shaft Bearing System" and the thrust bearing in the Turbocharger CHRA.
CHRA is the Compressor Housing Rotating Assembly. It is made up of the Shaft, the Compressor Wheel, the Turbine Wheel, The Support Housing/Oil housing for the Turbo Bearing Assy, and and on some turbos the Water Cooling System for the Turbo.
So a Turbocharger is a complicated device. The Turbocharger CHRA is also balanced to allow over 100,000 rpm shaft & wheel speeds.
So when the Turbocharger is in SURGE, things are not happy and the Turbo typically makes a unique noise.
One way to eliminate SURGE when the Throttle Blade(s) is closed is to use a Compressor By-Pass Valve. You close the Throttle quickly and vacuum (negative pressure) is restored in the intake manifold. You can use that Vacuum to open the CBV and eliminate some of the Surge. In some cases it may take more than one CBV to quickly remove the potential for Turbo Surge.
Centrifugal Superchargers also use Compressor By-Pass Valves to reduce/eliminate Surge. They are plumbed basically the same way that a Turbo Set-up would be plumbed.
The one application where you would not typically need a Compressor By-pass Valve would be when using a Draw-Thru system.
A Draw Thru system has the Throttling Device in front of the Boosting Device.
A Holley Carb/ Roots Supercharger would be a Draw Thru System, No By-pass Valve Required.
The Pontiac 301 Turbo System was a Draw Thru system that used a Rochester Q-Jet Carb in front of the Turbocharger. No By-pass system required. A picture of the 301 components is shown below.
More on Surge Later
Tom V.