Employing newly developed electrically assisted turbocharging, MTU solves turbo lag, improving engine agility and environmental performance
Some of the most complex questions facing engine designers include: how can turbo lag be eliminated? How can increasingly strict emissions regulations be met without compromising an engine’s agility and fuel consumption? And how can turbochargers deliver maximum performance across the engine’s entire operating range?
Engineers at MTU’s Friedrichshafen, Germany, facility believe they have struck upon a solution to address all three questions. The solution that will make ‘turbo lag’ a problem of the past is a new technology that combines conventional turbochargers with an electric motor.
One of the world’s largest manufacturers of high-speed and medium-speed diesel and gas engines, MTU in 2017 acquired from G+L Innotec, Laupheim, Germany, the exclusive rights to use a new technology for the electrically assisted charging of off-highway combustion engines in the power range above 450 kW.
Discussing the technology, MTU director of development turbocharging and fluid systems Dr Johannes Kech said: “Electrically assisted turbocharging is a significant milestone on the road to hybridisation. This technology will allow us to develop engines that deliver increased agility and lower consumption at the same time as enhancing ecological performance.” Added Dr Kech: “All this is made possible by using an electric motor to smooth out weaknesses in turbocharging systems.”
The electrically assisted charging system comprises an electric drive combined with a traditional turbocharger developed and manufactured by MTU. Using the electric drive, the turbocharger can be accelerated and the charge pressure built up earlier. The technology can be employed when the energy required for a faster charge pressure of the turbine would normally not be sufficient.
Using the technology developed by G+L Innotec, MTU can increase the acceleration capability of marine engines, for example, and the load response capabilities of generator drives significantly. Additionally, it can also reduce the engine’s fuel consumption and emissions in a variety of different applications.
Due to the increased load response capability, emergency standby gensets will be able to deliver their full output even faster than was previously the case. This technology is well suited for both diesel and gas engines, says MTU, and has already passed field tests, with the expectations that the first areas of application will be for marine applications and power generation.
Increased demands on turbochargers
Turbochargers are intended to increase performance and efficiency. They utilise the energy contained in engine exhaust gases to drive a turbine coupled with a compressor and deliver more oxygen to the combustion chambers in the diesel engine. However, the physical constraints of the technology mean that the additional performance only initiates at higher engine speeds. At lower speeds, there is not enough exhaust gas to drive the turbine fast enough to support the process. Consequently, there is always a delay – ‘turbo lag’ – until the necessary speed is reached.
In the past, designers tried to solve the problem with ever more complex (and therefore increasingly expensive) constructions, including sequential and alternating switching concepts and adjustable turbine blades.
Today, these technologies have just about reached their limits because the demands made on turbochargers are constantly increasing. Customers operating in more and more applications are demanding even greater acceleration across the operating spectrum. At the same time, turbochargers are increasingly expected to achieve reductions in emissions by helping to prevent the generation of diesel particulates and NOx emissions during the combustion process. Maximum performance across wider speed ranges with simultaneous suppression of emissions – the expectations have now almost surpassed the possibilities open to conventional technology.
“Electrically assisted turbocharging is a significant milestone on the road to hybridisation”
MTU has turned to electrically assisted turbocharging – marrying a conventional MTU turbocharger with an electric drive motor – as the solution.
“The electric motor makes it possible to virtually decouple the operating point of the turbocharger from the speed of the diesel engine,” explained Rudi Rappsilber, who is in charge of testing the new system at MTU.
The result is that significant delays in performance ramp-up are now history, and optimum turbocharging can be achieved in almost every operating state. For development engineers and users alike, that represents the fulfillment of a dream. An additional advantage is that the technology can be implemented with existing turbochargers without excessive complications. The additional installation space required is limited.
The new technology is expected to be available to owners and operators of ships, gensets and land-based vehicles from 2021.
Up until now, MTU manufactured turbochargers for high-load cycle numbers, large performance maps and two-stage turbocharging.
Adding to its development of electrically assisted turbocharging to produce high agility, it has focused on higher efficiency.
Some 45 years ago, an MTU series 396 diesel became the first engine equipped with an in-house ZR exhaust turbocharger. Ever since then MTU has been developing turbocharger systems that are particularly matched to the respective engine’s requirements.
In 2016, for example, when it presented its 16-cylinder Series 8000 diesel engine for navy application for the first time, MTU used a quadruple configuration of ZR turbochargers for aspiration. The 20-cylinder version of the engine was already a top seller in naval applications. As is the case with the 20-cylinder version, the 16V 8000 engine features low overall operating costs, high power density and environmental compatibility. Common rail fuel injection combined with the electronic engine control system make it possible to achieve fuel consumption levels of less than 200 g/kWh and very low exhaust emissions.
Nevertheless, in the past, synergies were exploited, and turbomachines were used across engine platforms.
However, current trends in engine development demand more and more specific turbocharging systems that need individual solutions. They impose various requirements such as efficiency, compressor pressure ratio and map width, power-to-weight ratio and acceleration capability.
To face these challenges in shortening design cycles, MTU is using a two-prong strategy: first, it is making use of a modular design kit to minimise development efforts; second, it is applying modern design methods to cope with the broad set of requirements imposed by the engine application: fully automated, multidisciplinary optimisation methods for wheels, flow guiding systems and bends play a major role in this regard.