How a VNT Turbocharger Works

As a former VNT turbocharger development engineer, please allow me to explain the effect of the angle of the VNT vanes on engine performance.

Take the situation where the engine is running at a steady speed and torque, and the vanes are then quickly rotated to a more closed position. The following steps describe the immediate transient effects:

1. The minimum flow area of the vane channels decreases by a large amount (say, 1/2 the original area) as the vane angle is made more shallow.

2. Immediately after changing the vane angle, the exhaust gas mass flow rate through the reduced area VNT vane channels does not change significantly, due to increase of the exhaust gas density and velocity. Exhaust gas density is increased due to increased exhaust manifold pressure, and gas velocity is increased by gas acceleration in the vane channels.

3. Increased exhaust gas acceleration in the vane channels is due to the increase of pressure gradient caused by increased exhaust manifold pressure.

4. The pressure in the exhaust manifold increases because the cylinders continue to empty into the exhaust manifold at nearly the same mass flow rate.  Initially the same amount of air is still going into the engine and this same amount still goes out; and this does NOT change much when the vanes are closed, because of the reciprocating engine cycle separates the intake and exhaust parts of the engine cycle.  The exhaust mass flow rate out of the cylinders is reduced slightly due to the increased density of the small amount of residual gas in the cylinder @TDC displacing some air on the intake stroke.

5. The exhaust manifold gas pressure, temperature, and energy density are increased due to the increase of piston pumping work during the exhaust stroke. Piston pumping work significantly affects the engine power output, and is also affected by pressure during the intake stroke.

6. The exhaust manifold takes time to accumulate the additional mass of exhaust gas that increases the exhaust manifold pressure. Exhaust manifold pressure is typically higher than the intake manifold boost pressure at most conditions.  Engine fueling may be increased to maintain the engine speed and power output against the rising exhaust manifold pressure.

7. The exhaust gas accelerates through the VNT vane channels due to the VNT vane channel geometry (decreasing in area like a nozzle) and due to the increased energy density in the exhaust flow provided from the exhaust manifold (being at at higher pressure and temperature). However – the process of the gas flowing into the turbine wheel at possibly over Mach 1 (1600 mph or 2300 ft/s, @ 1600F – that’s as fast as a bullet!) is not a significant contributor to the generation of torque on the turbine wheel, as the wheel blade tip speed is typically similar to the gas speed.

8. The pressure of the gas exiting the VNT vanes is typically not far above atmospheric pressure. As the gas flows into the turbine wheel channels, the turbine wheel blades turn the flow to be at an angle to the rotational axis opposite to the entry angle and also act as nozzles.  The impingement of flow on the turbine wheel blade near the turbine outlet causes higher pressure on one side of the blade than the other, this process results in most of the torque generated by the turbine wheel.

9. The increased turbine wheel torque causes the turbocharger rotor group to accelerate, and the compressor wheel speed increases on the intake side. Increased compressor speed results in increased intake manifold boost pressure. Engine intake airflow increases due to the increased manifold boost pressure. Fuel is increased to maintain A/F ratio and the engine power output begins to increases.

10. On the exhaust side, increased exhaust flow into the exhaust manifold results in a further increase of exhaust manifold pressure and temperature and therefore energy available to the turbine. Energy feedback is obtained causing further increase of the rotor speed until the power developed by the turbine balances the power absorbed by the compressor and bearings. The turbocharger speed can be over 200,000 rpm (3300Hz) for a small displacement automotive engine.

11. When the rotor power is balanced, the engine operates at a higher state of intake and exhaust manifold pressure and engine airflow and consequently power output, depending on how the throttle is operated.

Cymbal Vibrations

These are the different shapes that a cymbal makes when it vibrates after you hit it, looking at the top of it.  The white areas are where the cymbal doesn't move, is stationary, but it vibrates all around these areas up & down.  For each picture all the vibrating areas have the same frequency.  The pictures shown range in frequency from 40 Hz to 1135 Hz.

Each picture shows a pattern of motion that occurs at a specific frequency.  Engineers call the shape patterns "modes".  After the cymbal is struck, many of these different mode shapes happen at the same time, at their individual frequencies, so when you look at a vibrating cymbal it appears almost random in its motion, but the motion is really a sum of all of the different mode shapes vibrating simultaneously - see the animations at the end of this blog.

If you vibrate the cymbal with something like a speaker, and slowly increase the frequency, you will find that the mode frequencies are also the resonance frequencies of the cymbal.  You will see and hear the amount vibration increase a lot as you reach each mode (resonance) frequency.  This video shows a membrane being excited by a nearby speaker playing tones at some of the resonance frequencies, the membrane and speaker motion is made more visible due to the use of a strobe light which highlights the motion at longer intervals than you normally see (~20fps):
Circular Membrane (drum head) Vibration


Every physical object has this same thing happen.  Every physical object has resonance frequencies, and a specific shape of vibration associated with each frequency.  When a cymbal is struck, many of the resonances may be excited at once because of the impact.  But most of the energy goes into the lower frequencies of vibration, and each higher frequency typically has less energy than the previous one.  Also, the frequencies that are excited depend on where and how the object is impacted, this is why things sound different when you hit them differently.  If you excite the object at a spot where the mode shape doesn't have a lot of natural motion, you will not excite that frequency very much, and vice-versa.

Interestingly, physics has been able to predict the mode frequencies of complicated objects and their shapes somewhat accurately.  You actually only need to know 3 things:  the material stiffness, material density, and the shape of the object, and it can be solved!  Meaning you can calculate the mode frequencies and the mode shapes.  Here are some good examples I found on YouTube of vibration prediction /analysis:

1. This animation shows the motion of one of the modes (a mode shape) of a circular membrane clamped around the edge:
Vibrating Circular Membrane Mode

2. This animation shows the first 4 modes of a circular membrane clamped around the edge.  Notice as the mode frequency increases the vibration shape gets more complicated.  With the assumptions of continuum mechanics (ignoring atomic-scale effects) there is no limit to how high the theoretical mode frequency can go, but you would eventually need to consider heat, electrodynamics, and quantum mechanical physics.  For instance, visible light is the result of atomic vibrations near 500 TeraHz (500,000,000,000,000 Hz).
First 4 Vibration Modes of a Membrane

3. This animation is an representation of what might happen when the membrane is excited by an impact force, which distributes energy into many modes and so many mode shapes and frequencies occur simultaneously after.  If you look carefully, you can recognize some of the individual modes.  It appears complicated but physics says the motion is represented by the sum of many single modes:
Superposition of 25 Modes of Vibration on a Circular Membrane.

This motion would be similar to the actual cymbal motion as shown is this video by Meytal Cohen:
Meytal Cohen Cymbal Crash Slow-Mo

4. This type of prediction analysis is done routinely on every type of engineering structure where vibration is important!  For instance, here is a vibration mode predicted for a spacecraft antenna:
Vibration Mode of a Spacecraft Antenna


Hope you learned from this!

steve

This is something I've felt strongly about for a long time and I'm happy for the opportunity to say some things about it.

I think it's very important for non-science/technical people to understand and be informed of what the real goals of science are/should be, what it really looks like like in terms of process and data and actual knowledge in terms of data and information and statistics...