Turbochargers for higher engine efficiency

Posted on January 28, 2021 by Lucie Maluck, Images by Robert Hack

Turbochargers are used to make internal combustion engines more efficient – but their benefits are not restricted to those engines alone. Fuel cells as well need the compressed air that a turbocharger provides.
mtu engineers have been developing turbochargers for sixty years now, and still have plenty of new ideas in the pipeline. The use of turbochargers for fuel cells and hydrogen engines are just two of them.  

We often talk about 'igniting the turbo'   – but did you know that that's not at all accurate?   It actually happens the other way round: it's the job of the turbocharger to make sure that the engine develops plenty of power when it is ignited, so in that respect you could say that it's the lung of the engine. It is the turbocharger that ensures that the engine is supplied with the right amount of air and enhances the power output in relation to the given cylinder capacity. The job of the turbocharger is to pump air into the combustion chamber. The more effectively it does so, the more oxygen is available. And the more oxygen there is available, the more fuel will be combusted. The more fuel combusted, the higher the engine output. The core task of the turbocharger is – in short – to intake as much air as possible, compress it, and deliver it to where it is needed. The great thing about a turbocharger however is that it harnesses what is basically waste. 30% of the potential energy that fuel carries would – without a turbocharger – be simply blown back into the atmosphere as exhaust gas after combustion has taken place. That's why it makes a lot of sense to use this gas for turbocharging.  

The main components of a turbocharger are the turbine and the impeller. They are mounted on opposite ends of a common shaft. The turbine is driven by the thermal and kinetic energy of the exhaust from the engine. On the opposite side of the turbocharger, the aerodynamically shaped blades of the impeller draw in air and force it under pressure through the intercooler and into the cylinders.

Two-stage turbocharging reduces contaminant emissions  

It's important however not just to see the turbocharger as a 'powerpacket'. Turbocharging also helps reduce fuel consumption and contaminant emissions. A special turbocharging technique, called two-stage turbocharging, has been installed on many mtu engines in series production over the last ten years. Here, the air is not just compressed by a single turbocharger before being delivered into the combustion chamber – two turbocharger stages are used. This is particularly important on engines on which other technologies are being used to reduce the formation and emission of pollutants: e.g. the Miller process, exhaust gas recirculation systems or a diesel particulate filter. This is because these systems produce back-pressures that require compensation. With two-stage turbocharging it can still be ensured that enough air is fed to the engine.  

mtu experts at Rolls-Royce are currently developing a highly compact two-stage turbocharging sytem.   Here, air flows onto the turbines in axial direction, which allows a more compact turbocharger design. “This is especially important in the case of marine engines,” explained Dr Johannes Kech, head of mtu turbocharger development at Rolls-Royce.

Electrical turbocharging makes engines more agile

Electrical turbocharging can also be used to make engines more agile as well as to save fuel and enhance their eco-friendliness. With electrical turbocharging, the design weakness which is inherent to the turbocharger – at low speeds, the amount of exhaust gas arriving at the turbine is not enough to drive the compressor sufficiently – is compensated for. This insufficiency is known as the turbo lag and is made up for using electrical power. It works like this:   The classic mtu turbocharger is connected to an electric drive. By using an electric drive, the operating point of the turbocharger can be made almost completely independent of the speed of the diesel engine. Significant delays in the build-up of power i.e. the turbo lag, become a thing of the past and turbocharging can take place optimally in almost any operating mode. “We've already put this technology through its paces in tests and we're expecting the first orders soon,” said Kech. Electrically powered turbochargers are primarily intended for applications where the engine is expected to deliver full power very promptly: e.g. military applications, yachts, and quick-starting gas engines used for standby power.

mtu engineers have developed a new family of classical turbo-chargers for stationary gas engines where the emphasis has solely been placed on efficiency. Unlike diesel engines used in mobile applications, power density is not all-important on the stationary gas engine. This meant that compact design was not an objective during development. “Gas engines clock up a huge number of operating hours. If we manage to raise the efficiency of these engines through highly effective turbo-charging, our customers could save a lot of fuel,” explained Kech. The new turbocharger family is to be tested in stationary mtu gas engines very shortly to demonstrate that the turbochargers can indeed raise efficiency significantly.  

98,000 times per minute, the turbine and turbine and compressor wheels of some mtu turbochargers rotate on their own axis.

Turbocharging on fuel cells

Not yet developed but very much taking shape in the mind's eye of our mtu engineers is turbocharging on fuel cells. Turbochargers on fuel cells will increase their power density and lower hydrogen consumption. The exhaust air from a fuel cell, however, is neither as hot as that coming from an internal combustion engine nor does it flow as quickly. That means that the energy that can be harnessed from it will not be enough alone to drive the turbocharger. So here as well, it makes sense to implement electrically supported turbocharging. However, unlike electrically supported turbocharging on the diesel engine, where the electric motor is used more or less as a booster at the lower end of the speed range, it will always be in service on the fuel cell in order to sustain output.  

The up-and-coming mtu hydrogen engines will likewise need turbocharging. “But we're still in the conceptual phase,” pointed out Kech. What he can tell us: Hydrogen, compared to diesel, is a very expensive fuel. That means that in development, the primary focus will be on efficiency. So it is likely that two-stage turbocharging will be used in this application as well for top efficiency.

“The ultimate aim in turbocharging development is always to make our engines more efficient,” said Kech. Alongside that there are, depending on the application, other aims, such as compact design or the ability to ramp up to full power within a short space of time.

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