Ask the Expert: Turbochargers

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Question: I'm interested in understanding the effect of incoming flow into the compressor wheel on noise and performance. Literature seems to contain Incident angle analysis techniques to minimize flow separation. It would be great if I can get support in terms of figuring out how much I need to swirl the flow in order to minimize noise without effecting compressor performance.


Changing the flow angle coming into the impeller with inlet guide vanes (IGV’s) is a common method of influencing compressor performance.  The effect is usually quite strong.  The Euler turbomachinery equation shows us why:

DH = U2*Cq 2 – U1* Cq 1

DH is the change in total enthalpy across the impeller, U is the local wheel speed, and Cq is the absolute tangential velocity.  The subscripts 1 and 2 are the inlet and exit of the impeller respectively.

Changing the swirl will change the Cq term and directly affect the enthalpy addition and therefore the pressure rise.  The effect on range is also quite significant but it’s not possible to quantify it in a simple equation.  The range is highly dependent on the incidence angle which can be directly controlled through the IGV’s.

The effect on the noise level is much more complicated to estimate.  One would reasonably expect that the intense turbulence and unsteady flows associated with stall would be reduced by the range control that IGV’s allow.  What this effect would be at the normal operating range would require a test program to quantify with any accuracy.

Hope this was helpful.

Question: Why isn’t turbocharging used as much in gasoline and diesel engines?

Answer: Gasoline engines typically run over a wider speed range, which in turn requires a wider compressor range that is difficult to achieve. Boost a low engine speed is always a problem with a turbocharger. In a diesel, this can be compensated by increasing the fuelling rate at low speed, but a gasoline engine can only run over a small range of fuel/air ratios. At high boost pressures, gasoline is more likely to self-ignite and possibly damage the engine. These problems can all be mitigated, if not overcome, and we are seeing more and more turbocharging of gasoline engines.

Question: Is it possible to gain anywhere near a 10-point increase in turbo efficiency? What is possible given higher boost levels needed?

Answer: It is a trade-off. It is possible under limited circumstances, but you will need to accept some severe limits on range. You will also need to be tolerant of a large size, weight and inertia. Transient response will suffer and you may need exotic materials and manufacturing process which will drive up your costs.

Question: What effect do you think Additive Manufacturing will have on turbochargers?

Answer: AM is already having a massive effect. We’re seeing it as impacting engineering in general by increasing the scope. We can manufacture at lower costs, contemplate new materials that weren’t viable before, etc. We are just at beginning of this revolution, but it will have a significant impact. 

Question: How is a turbocharger for gasoline different than one for diesel?

Answer: Any trouble you have with turbocharging a diesel engine is amplified to the nth power in a gas engine. With gas, you can only vary the gas mixture ratio over very small range. You can’t increase the fueling rate to boost the torque.

By nature of combustion, exhaust gas at a hotter temperature means more energy for the turbine, but it also means the turbine is operating in a hotter environment. This means we must use higher temperature tolerant material for the turbine rotor – increasing the cost.

In addition, the speed range of a gas engine from idle to red line is typically twice the speed of a diesel so you have a much wider range of speeds and flowrates which puts more pressure on compressor range. This should then be reflected in the turbocharger technology.

The lower temp diesel engine means less energy for the turbine. This puts more emphasis on turbocharger efficiency. When you optimize the design, you need to go harder for component efficiencies, making compromises that much more difficult. 

Question: How is a turbocharger for low pressure EGR different? What about putting the EGR before the compressor?

Answer: To large degree, low pressure EGR isolates the turbo from the EGR system. This means no matter what percentage of exhaust gas you’re recirculating, the net effect is that you’ve still got the same amount of gas going through the compressor, and you’ve still got the same amount of gas going through the turbine for a given engine condition. In fact, low pressure EGR has far fewer implications for the turbocharger than high pressure EGR does. Generally, it’s not something that causes us a great deal of concern.