Pumping Up the Boost: Everything Depends on Everything Else and Nothing Is Certain
The pumping and spooling capability of a centrifugal supercharger driven by an exhaust turbine is so complex that you'll need a compressor map to understand it--and then some.
Pumping ability changes with compressor speed (and in a nonlinear way!). Below about 40,000 RPMs, boost pressure is off-the-map low because centrifugal compressor pumping is an exponential function (at first, almost nothing, eventually tons). The compressor has to accelerate from essentially zero to 40,000 RPMS before you see any boost on the gauge at all.
And everything depends on everything else: Compressor air flow depends on compressor size and trim, speed and discharge pressure. Compressor speed depends on compressor discharge pressure and air flow, and available turbine energy. Air flow depends on instantaneous compressor thermal efficiency, engine displacement, engine volumetric efficiency, engine speed, intercooler efficiency, ambient air temperature, and throttle angle, as well as turbine speed and torque. Turbine speed depends on compressor load, exhaust energy, turbine size and trim, turbine nozzle size and leverage, the reactants-to-products ratio of combustion, exhaust pulse speed, exhaust volume, exhaust gas temperature, and exhaust manifold pressure--all of which depend on engine displacement, engine speed, boost pressure, air-fuel ratio, and all of which are changing constantly as the engine speed and rate of acceleration go up and down. And that's not even considering the effect of the wastegate strategy (how and when you bypass exhaust gases around the turbine to prevent overboost).
Got that? Take a deep breath and try again: The speed of a compressor depends on how much energy it takes to turn the compressor (a function of manifold pressure, throttle angle, engine volumetric efficiency at RPM, engine RPM and rate of increase in engine RPM, current compressor speed, and rate of increase in compressor speed), available exhaust energy to turn the turbine that's powering the compressor, dynamically increasing exhaust energy from increasing boost pressure (or the reverse), residual heat in the turbine and housing, turbocharger inertia of the complete rotating assembly, friction in the turbocharger (ball bearing or not, oil viscosity, etc), and so forth.
Of course, it's not just pumping (CFM) that determines air mass moving into an engine, it's also intake charge temperature: For a given boost pressure, hotter air is less dense, meaning there's less of it. As you drive, compressor speed and boost pressure move all over the map (literally!), and the thermal efficiency of the turbocharger will vary, as it heats the air more or less. In addition, induction systems, intercoolers, and other components can heat soak. Which tends to diminish air density. The difference between compressor air flow at the first tickle of low-RPM boost and maximum air flow at high compressor speed and high engine VE is tremendous and nonlinear. But as a centrifugal compressor approaches the outer limits of maximum design air flow, it will begin to "choke," meaning the compressor can pump no more air at a given level of boost no matter how fast it turns, but will simply be heating the air with increasing ferocity.
On the low end of air flow, if there is too much boost pressure at too low total mass air flow, the pumping performance can become unstable and the compressor will begin to surge. At this point, the compressor looses its ability to further squeeze the air, which is then free to expand and flow backward as well as forward through the compressor. This "compressor stall" can sound like a sudden "backfire" or series of backfires or "chuffs" of air, with high-pressure air abruptly reversing direction and exploding backward through the compressor wheel in a shock wave that instantly lowers compressor discharge pressure and can damage bearings and damage or destroy the compressor wheel and shaft (and scare the hell out of you from the noise).
Since a turbocharger's freewheeling rotating assembly is able to seek its own speed as compressor and turbine pressures and other factors permit, the turbo is free to out-accelerate the engine (if it can!) and surge ahead to make boost while the engine hunkers down to make more power without necessarily changing RPM a bit. Similarly, just because the engine increases in speed does NOT mean the compressor will accelerate--though it might if engine VE or air flow increase with RPM. Conversely, above peak power, with VE falling, the engine will pump LESS air with RPM increases, which can cause the turbo to slow down and pump less, beginning a negative feedback loop that results in a power-loss death-spiral. Meanwhile, on the high end, if the compressor is small for the engine but the turbine is not yet a bottleneck, if engine VE is increasing with RPM, the turbo might turn so fast it spools up into dangerous above-the-redline overspeed line at the upper right of the compressor map. (Actually you can over-speed a turbo by going too far to the right (in mass flow) OR too far up (in pressure ratio).
By Corky Bell
Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!