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Question: My project’s budget can’t afford Inlet Guide Vanes, are there other possible “simple” solutions for improving operating range of my turbocharger?
Answer: You could use a Recirculating Casing Treatments on the compressor side, which is sometimes referred to as a “portable shroud”. This feature is frequently used in automotive turbocharger compressor design to extend its operating range. In the low flow region of the map, surge margin can be enhanced by bleeding a fraction of the main flow from the impeller casing back to the stage inlet. In the high flow region, choke margin can be improved by bleeding flow from the stage inlet to a location downstream of the impeller throat. The benefits of recirculating casing treatment are greater at higher rotational speeds. It is important to remember that the benefits are usually accompanied by an associated penalty in efficiency, which may or may not be acceptable, based on your needs.
The challenge for the designer is to create a recirculating casing cavity configuration which enlarges the map width, while minimizing its impact on efficiency.
Figure 1 - Comparison in COMPAL of the same design, with and without RCT
In our Centrifugal Compressor meanline program, COMPAL®, it is possible to model the RCT and evaluate its impact on the performance early in the design process.
Question: What is a typical design workflow when designing a turbocharger for a specific engine?
Answer: Based on the Fundamentals of Turbocharging (Baines, 2005), the general approach follows the general steps:
An integrated approach is necessary to evaluate not only the turbomachinery components but the matching between them, the effects on the engine performance and the engine network (EGR, intercooler, waste gate, etc.).
Question: Generally speaking, what are the common technical features for those turbochargers on trucks? Do we see more vaned or even variable geometry designs on them compared with passenger vehicles?
Answer: Like everything, it comes down to cost/benefit. Generally speaking, the larger the system, the more cost can be justified since the net benefit is larger. Turbochargers on large systems like ships and locomotives often make use of active control devices like variable IGV’s, turbine nozzles, and even diffuser vanes. These are less common with trucks but not unknown especially variable turbine nozzles.
Question: Is 'high-cycle fatigue' a common form of failure in practice? Is there a 'rule of thumb' to avoid it instead of a harmonic response analysis?
Answer: High cycle fatigue is indeed a common risk factor. The critical thing to avoid is any resonance frequency matching any source of excitation. Some common source of excitations: blade passing frequency, rotational frequency of the turbocharger, engine firing rate, etc.
Question: Are then any good sources of turbine/compressor matching procedures with HPL EGR? I would expect the turbine/compressor matched so at to allow for sufficient EGR in the peak torque region and not choking at rated condition.
Answer: This is an issue everyone is facing using EGR systems. If the EGR is designed to give the right amount EGR at peak torque at for instance 1000 rpm the boost level has to be high enough so the right in cylinder lambda is achieved. Then at peak power let say 1800 rpm the boost level will be far too high, so therefore waste gating or variable geometry is needed on the turbine side. When we are designing turbochargers here at Concepts NREC we use our software code TurboMatch to find the right balance on turbine sizing and waste gating sizing. So as usual, turbo engineering is often times about finding the right characteristics and sizing on the different components involved in the system.
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.
Answer: 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.