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High aspiration

11/30/2012 | Words: Lucie Maluck | Pictures: Robert Hack

Turbo charger

Aesthetic lines. Sculpted forms and contours, finely crafted details, glittering surfaces– it looks so good you could almost hang it around your neck on a chain. No, we are not talking about an expensive piece of jewelry, it’s the impeller from a turbocharger. When engine developers get onto the subject of turbochargers, their eyes glaze over as they effuse adoringly about what they call one of the engine’s most important parts. Are they right?

It isn’t easy for a diesel engine. To start, it first has to get its flywheel moving, a large steel disc that takes a while to set in motion. Only when that is going, do things start to happen. But then all the more so, because that is when the turbochargers come into play. Inside a few milliseconds, they suck in air, compress it and blast it into the combustion chamber.

“The turbochargers are what give our engines their power,” says Ronald Hegner, who leads the MTU turbocharger design team. And he quickly adds that, much more than that, “They give the engines their unmistakable character, and affect their economy, dynamic response and emissions.” A lot for a single engine component to be responsible for. And reason enough to take a closer look at this key contributor.

What exactly makes the turbocharger what it is? Put simply, the turbocharger is the engine’s lungs. It gives it its athletic power and makes sure that performance can continually be improved without increasing cylinder capacity. That is because the turbocharger pumps the air into the engine’s combustion chambers. The better it does so, the more oxygen there is available for combustion. More oxygen means more of the fuel can be burned. And more complete fuel combustion means higher power output. So, in short, the actual purpose of the turbocharger is to draw in as much air as possible, compress it and deliver it to the engine. And the best part is, to do so, the turbocharger effectively uses “waste energy”. Thirty percent of the energy contained in the fuel is simply expelled into the atmosphere as exhaust. So it makes perfect sense to utilize that energy for turbocharging. It is the flow of exhaust that drives the turbocharger’s turbine. On the opposite side of the turbocharger and rigidly attached to the turbine by a common shaft is the impeller with its aerodynamically shaped blades that draw in air and force it under pressure through the intercooler and into the cylinders. 

So is the familiar expression “to fire up the turbo” the right way of describing what you do when you need to put your foot on the gas? No, you don’t fire up the turbo, it is actually the other way around. The turbocharger “fires up” the engine by giving it a boost of extra power. “The turbochargers are among the engine’s key components. They are so decisive that whenever higher specific requirements and more power are demanded we always develop and produce them ourselves,” Hegner recounts.

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98.000 revolutions per minute is the speed at which the impeller of an MTU Series 2000 turbocharger spins around its own axis. Blindingly fast compared to the 2,450 rpm of the crankshaft. But even though that figure is impressive, the speed of rotation is not the decisive factor. Much more important is the peripheral velocity. If the impeller were rolling along the ground, it would travel virtually 600 meters in one second. That is more than twice as fast as a commercial airliner. By that reckoning, an impeller could travel the distance from the Earth to the moon in only one week. Of course, such speeds place enormous stresses on the materials. So the MTU developers use threedimensional computer modeling to simulate the airflow and mechanical structure loadings on the turbocharger from an early stage in the design process. 

?490 different turbocharger models are developed and manufactured at the MTU lead facility in Friedrichshafen. The different engine sizes alone make variations in turbocharger design necessary. But even within a design series, the developers adapt the turbochargers to the specific requirements of the individual applications. Thus electricity generator engines, which are run constantly at the same speed, need a different turbocharger setup than vehicle engines. Because, unlike a generator engine, a vehicle engine is not run constantly at one speed but rather over a wide range of speeds. It has to deliver high performance from idling speed right through to maximum revs. The challenge in that is to dimension the turbocharger exactly for the type of use. Therefore, for engines for mobile applications, MTU has designed the turbochargers to deliver sufficient boost pressure while covering as broad a range of engine speeds as possible. The variable parameters in design development are features such as the pitch of the turbine and impeller blades or the size of the housings.

5 turbocharger groups are used on the 20-cylinder version of the Series 1163 marine engine. Each group consists of a high-pressure turbocharger and a low-pressure turbocharger. When the vessel is only moving slowly and the engine speeds are correspondingly low, the boost pressure is generated by only one of the turbocharger groups. As speeds gradually increase, the other turbochargers are brought successively into action. They then provide sufficient mass air flow and pressure for higher engine speeds and power outputs. Using the principle of sequential turbocharging, the various turbochargers can be matched precisely to their specific operating range. Especially in the case of highly dynamic applications such as yachts – which demand fast acceleration – that is a decisive aspect. Ships can accelerate extremely quickly with this engine as there are large amounts of intake air available right from low power levels.

850 degrees Celsius (1562° F) is the surface temperature of the turbine housing on a military vehicle engine when the turbo is spooled up to maximum speed. To stop it overheating at such temperatures, it is made of ultra heat-resistant material. “For turbos that are subject to especially high thermal stresses, we even make the impellers out of titanium,” explains production manager Wilfried Kempter. On some turbocharger versions, the impeller housing is water cooled to prevent the surface temperatures getting too hot. On turbochargers used in marine applications, the turbine is also housed in a water-cooled connecting block.

50 is the number of years MTU has been developing and producing its own turbochargers. The Maybach Type MD 650 diesel was the first of the company’s engines to have a turbocharger developed in-house. Though the first turbocharged fast-running large-scale diesel engine, the Maybach GO6, was developed nearly 80 years ago by Karl Maybach, founder of Maybach-Motorenbau, the company from which MTU originated. In those days, however, the turbochargers were not made by Maybach but were supplied by the Swiss manufacturer Alfred Büchi.

5 bar is the air pressure inside the turbocharger. It doesn’t sound a lot when you think that a tire on a racing bicycle is inflated to as much as 11 bar. “Although higher boost pressures are entirely conceivable, it doesn’t make sense because the components would then be overstressed,” explains MTU developer Ronald Hegner. And, of course, the boost pressure is not the same for every engine. “It is one of the variables we can adjust to fine-tune the turbochargers to suit their applications,” Hegner adds.

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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.

270 degrees (518° F) is how hot the intake air gets when it is compressed by the turbocharger. But hot air takes up more space than cold air. So to deliver more air, and therefore more oxygen, to the engine, an intercooler lowers the air temperature to about 50 degrees Celsius (122° F). In a two-stage intercooling system there are even two intercoolers. The first is positioned between the low-pressure and the high-pressure turbocharger. That means the high-pressure turbo is supplied with cooler air and can compress it further. Intercooling

34 is the number of years since the first car with a turbocharged diesel engine appeared on the market in 1978 – and so a much shorter time than in the case of commercial vehicles. However, the Mercedes 300 SD was sold mainly in America. In Europe, the “turbo-diesel” didn’t make a breakthrough until the mid-1990s. But once established, the turbocharger and direct fuel injection gave the diesel engine an entirely new image. No longer was it the dependable but sluggish and noisy commercial vehicle engine – it was transformed into a fuel-efficient and punchy performer.

2-stage regulated turbocharging is MTU’s answer to the constantly lowering limits for the emission of soot particulates and nitrogen oxides. Instead of the intake air being compressed and delivered to the combustion chamber in a single stage by a turbocharger as before, it now passes through two turbocharger stages. First of all, the air is precompressed by a low-pressure turbocharger, then cooled by an intercooler before being further compressed by a high-pressure turbocharger. Controlled by an engine management system developed by MTU itself, this regulated turbocharging system ensures that the engine is always supplied with the same amount of air even when there is a lot of backpressure. Because higher backpressure is one of the side-effects of using emission-control technologies such as the Miller process, exhaust recirculation and diesel particulate filters. To put it simply, the turbocharger then has to force more air into the combustion chamber to provide the same amount of oxygen for combustion. In many cases, singlestage turbocharging is no longer capable of doing so. The first MTU engine to feature regulated twostage turbocharging is the new Series 4000 rail engine unveiled in 2010. With a cooled exhaust recirculation system and a diesel particulate filter, it meets the EU Stage 3B emission requirements in force from 2012. Regulated two-stage turbocharging is also definitely planned for future versions of the Series 1600, 2000 and 4000 engines in other mobile applications such as construction and mining vehicles. For static applications such as electricity generators in which the demands on turbocharger dynamic response are not so high, the more economical single-stage turbocharging will continue to be used.

30.000 hours is how long the turbocharger on an MTU rail engine can be run before it needs servicing. That is nearly three years even if the engine runs 24 hours a day. On vehicles that have a very dynamically variable load profile, however, the turbochargers have to be serviced more frequently because the frequent load changes subject the materials to extreme stresses. Durability is a key consideration from the early stages of turbocharger development. They are analytically optimized using efficient computer modeling and simulation tools long before they are first tried out on the test bench. To simulate the structural and mechanical stresses on the turbo, for instance, the developers use methods such as three-dimensional computer modeling.

80 employees produced 7,600 turbochargers at MTU in 2011. “In recent years we have completely replaced the production machinery,” Wilfried Kempter relates, going on to explain that the special expertise was in the geometry of the turbine and impeller blades. The minutest changes can result in significantly less air being delivered to the combustion chamber. On some types of turbocharger, the geometry is even variable. That enables the power delivery and response characteristics of the turbocharger to adapt better to the engine operating conditions. The exhaust passes over adjustable guide vanes to the turbine blades so that the turbine spools up quickly at low engine speeds and subsequently allows high exhaust through-flow rates. Another important consideration is that the turbine and impeller have to fit precisely inside the housing. “We are talking about manufacturing tolerances measured in microns,” Kempter emphasizes. The gaps between the turbine housing and the turbine wheel must be absolutely exact if the air is to be used efficiently. 

The content of the stories reflects the status as of the respective date of publication. They are not updated. Further developments are therefore not taken into account.

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