Turbo Diesel Compressor Surge

Ive noticed that without having a blow off valve there doesnt seem to be a way to have a simulated compressor surge. Even though compressor surge isnt from BOVs, rather the lack thereof. I feel like it would be a nice touch to have compressor surge for diesels when they are pumping large amounts of boost. For example, my I6 Cummins (coming soon soon dont worry) can make up to 30-50 psi of boost on the single turbo variant, and it just feels weird letting off and hearing nothing.

 

Aren't those the sound of blow off valves you can hear ? Cause if not then wouldn't you just hear the turbo spin
go off gradually on release of throttle ? Like SSSSOOOOOOWWW ?

 

The infamous stutututut sound is from the lack of a bov, or a bov that isnt large enough.
The stutututut sound is the pressure from the turbo bouncing back and forth in the intake when you let off, as there is no where for the pressure to go since the intake is closed.
Since diesels dont have bovs, when they run high amounts of boost, the stutututut is very common.
When you have a bov, its more of a *PSHHHHHH* when you let off, as all of the built up boost is just escaping from the valve.
Compressor surge (no bov):

BOV:

 

Diesels don't have throttle butterfly, there is nothing that would close intake etc.

 

Could it be that as throttle pedal is lifted, no fuel is injected and turbo is spinning fast, suddenly exhaust turbine is sucking faster than possible as there is no expansion of hot gases without fuel burning and expanding gases?

Idk really why such happens, but would think it being cavitation of some sort. Hydrodynamics, aerodynamics, what is really the difference, with other things just happen at slower velocities

 

Seems like that could be it. I dont know the ins and outs of diesels, but i would assume something happens when you let off that causes a variation of pressures through the system that causes this flutter.
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What do diesels have to regulate airflow into the cylinder? I'm not too familiar with the inner workings of diesels.

 

air is always allowed in. fuel injection is what varies the speed/load of the engine.

 

Like it was mentioned, nothing, diesels run lean just happily because fuel/air mixture is ignited by a knocking, not by a timed spark.

Also reason why diesels get really loud without intake sound restrictions as it make similar amount of noise to full throttle with gasoline engine, but there is more to it of course.

So you just adjust amount of fuel, that is why you can tune old diesels to have more power easily by turning one screw, there is a limit for it though and some drawbacks, but more fuel is more power with diesels.

Ships had turbos way before cars:
https://www.marinesite.info/2013/09/7-causes-of-turbocharger-surging.html
Rapid variation (reduction) in load.Reduction in load means less air intake leads back flow of savage air leads surging.


While diesels don't have throttle to close, amount of gas changes a lot, think how much temperature rises and how much gas must expand at full throttle of such tuned vehicles, when you cut throttle turbo will still be spinning fast while there is not enough expansion of gas so my guess is turbine side stalls.

Older big rigs had surge sound with every shift, 4 bars of boost with a LOT of added fuel and when you lift loud pedal amount of gas is cut short, big heavy spindle in turbo keeps going though, much faster than air moves.

So while turbo compressor pushes lot of air to cylinder, it is nothing compared to amount of gas running trough exhaust at full throttle as heat is not expanding gases, that means blades are stalling, spindle is rocked around it's bearings and turbo gets little damage each time sound is made.

Pressure compensation of fueling is also worth noting, it can do odd things with sudden throttle changes.

How much change turbo can then take before making sound, with stock vehicles such does not happen much, but with a lot of boost it starts to happen, if change in exhaust volume larger than X then make sound, simple Yeah, right, never so simple

 

I hope this is added some time in the future, so I can do this:

 

This picture is gasoline engine turbocharged. The BOV is there because of throttle valve, the BOV only works when the throttle is lifted-off rapidly, and does nothing with gradual throttle movement. Because if the throttle is closed quickly there's little room for air to go and the air will choked between the compressor and the valve, damaging the compressor. One way is to put the throttle upstream of the compressor, but leads to massive turbo lag. Superchargerd engines put the throttle before the superchargers because they don't have turbo lag.

In a diesel engine, there's no throttle valve and BOV because it's not required, the pressure before the compression stroke is constant regardless of the gas pedal position at a given turbo RPM.
No throttle means less air resistance, the turbo boost may reaches full boost (and the wastegate starts to open) at 50% ges pedal, this also improves efficiency because of more complete combustion. If the gas padal is 0% since there's no throttle valve, all the heat(and pressure) from the compression stroke, returns back to mechanical energy during the expansion stroke. This means the turbine inlet pressure(exhaust manifold pressure) is about the same as the intake manifold pressure at 0% gas.
When 100% gas, the turbine inlet pressure(exhaust manifold pressure) is much higher than the intake pressure, when you lift off the gas, the exhaust pressure is dropped to the intake pressure, however the turbo is still at high RPM, at this very moment, the work of the turbine is reversed, sucking air from the exhaust manifold and push it to the catalytic converter. The turbine is never designed to reverse, thus the air is disturbed and the flutter sound is heard. BOV does nothing, instead, a "blow-in valve" should be installed between the exhaust manifold and the turbine to eliminate the flutter.

 

Diesel compressor surge is quite the mystery. Either way I hope it can be added into the game. Maybe you can have it like the turbo sound settings where you can choose a coefficient to determine at what psi the surge sound plays?

 

So I figured it out, when the turbo is at WOT and then you suddenly let off, the rapid deceleration of the turbo causes there to be no more force pushing air into the engine, so some of the built of pressure escapes back out of the turbo. Even though the engine is still taking in the boost, the lack of turbo force when you let off let’s a large amount of the boost pressure to escape back out through the turbo. Which seems to be why it’s a deeper and less rapid flutter. When you go from WOT to let off, the decrease of fuel injected means less exhaust gas being pushed out, so there’s a large, sudden drop of exhaust pressure, which suddenly stalls the turbo, meaning that it is no longer building pressure in the intake, which then allows that previously built up intake pressure to escape back out through the suddenly slowed turbo. This seems to be what I can gather from reading different forums and stuff.