Florian Hermann, key account manager, KBB Turbochargers discusses how changing regulations and engine demands are impacting the development of turbochargers
When we look at the combustion engine we see it has undergone a process of continuous improvement. One of the ramifications of this is that air-flow capacity has increased (often due to downsizing) which makes additional demands of the turbocharger.
For single-stage turbocharging, at present we require a ratio of about 5.5-1; seven years’ ago the mean value of our matched single-stage turbocharging was about 4.7-1 – so the progression is clear. Two-stage turbocharging is currently above 10-1.
The challenges for turbocharger manufacturers in matching engine developments are many: there is an effect on the compressor wheel, which is responsible for determining the pressure ratio and the flow rate and a further impact on the turbine. The compressor housing and the turbine housing both have a major impact on the flow rate and must be engineered accordingly.
Furthermore, higher-pressure ratios mean more heat in the turbocharger which must be managed; the increased temperature and higher-pressure ratio also mean changes to the turbine management.
In addition, the shaft sealing is the only sealing between the exhaust gases, the pressure gases and the engine oil. A failure here would result in contamination.
Last but not least, if you have a high-pressure ratio and a high flow rate, it may increase the torque being introduced in the shaft which can impact the bearing system.
So how are turbocharger manufacturers meeting these challenges? An efficient turbocharger will maximise the benefits of new engines and new fuels and help minimise fuel consumption. We have achieved efficiencies by, for example, modifying the rotor – a key component of each turbocharger – on our new ST27-EP-Series.
We have also increased the compressor flow capacity – which is particularly relevant for dual-fuel engines – and turbine flow capacity and efficiency have also been adapted. This latter point is very important in terms of engine downsizing.
“An efficient turbocharger will maximise the benefits of new engines and minimise fuel consumption”
Comparing the map on our ST27 units against our newer EP units, the pressure ratio increases from 5.5 to 6. We have the option of extending the maximum efficiency out further and increasing the pressure ratio, which can improve fuel consumption.
Changes to the rotor help reduce weight and the inertia, which is crucial to reducing emissions and black smoke.
We must also factor in the use of scrubbers and new fuels to meet current and future regulatory demands. Scrubbers increase exhaust back pressure and we have worked with engine makers, comparing systems with scrubbers fitted against those without. The main impact is that turbocharging speed falls, which we can counteract by choosing a smaller nozzle ring.
We are currently developing next-generation two-stage turbo charging and while three-stage turbocharging has been discussed, I see its application as being very limited for the time being. Three-stage turbocharging also introduces far greater complexity and associated maintenance costs.
New fuels, like LNG and biofuels, introduce additional demands; for example, we supply a lot of turbochargers to stationary engines running on biofuels and we have detected a kind of glazing on the turbine housing. If the fuel quality is very bad this can also affect the shaft sealing.
LNG creates an exhaust temperature of around 600°C, which is still lower than we see on biofuel applications. Despite this, we expect less contamination – for example turbine washing and oil leakage because of carbonisation – than compared to heavy fuel oil.
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