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.
