Electric-Turbocharger: The Future of Turbocharger Technology
Electric-Turbocharger: The Future of Turbocharger Technology
The turbochargers has always been the time between pressing your foot to the gas pedal and waiting for the engine to respond with the desired power. This lapse in engine response, generally named turbo lag, is what has hindered turbochargers from delivering optimal performance. The objective of a turbocharger is to give additional power, better efficiency and lesser lag in power delivery. Engine efficiency is getting more important than ever previously, leading to the development of smaller engines. still, the power conditions aren't reducing which means the loss in engine displacement from small designs must be picked up with alternate technologies, such as turbochargers, which can help enhance power delivery and fuel economy.
Figure 1: Garrett Motion electric turbocharger due for production in 2021.
Electric turbochargers(e-turbos) give a solution to eliminating turbo lag while adding further performance benefits. This allows for larger turbocharger designs which can give larger power and efficiency gains, stay cooler over longer days of use, and drastically enhance engine responsiveness. Garrett Motion are developinge-turbos for mass demand passenger vehicles set for launch in 2021, with a claimed fuel efficiency enhancement of up to 10%. When used on diesel engines, this e-turbo could be over to a 20% reduction in NOx emissions. In utmost cases, energy efficiency will be enhanced by about 2 – 4%. Other manufacturers such as Mitsubishi and BorgWarner are previously developing their own electric turbos and are anticipated to have announcements in the near future matching the trend in electric-turbo development.
What is a turbocharger and how do they work?
First, let’s understand how conventional turbochargers works. Naturally aspirated engines draw air into the combustion chamber by way of a partial vacuum created by the movement of pistons and opened intake valves. Turbochargers help to enhance the fuel-air proportion in the combustion chamber and to regulate more precisely the quantity of air entering by pushing air under force into the chamber. Exhaust gases from the engine rotate a turbine which then powers a compressor by means of a rotating shaft. The compressor pulls in air, compresses it, and pushes air into the cylinders. The downside of turbos is the lag in getting the engine up to speed, from low pressure and low RPM to high pressure and high RPM, needed to spin the turbocharger. Larger turbo compressors cause longer lag, but if the turbo is too small, it'll not operate efficiently at high RPM which leads to a shut down to cool. Remember that the turbine is rotated from high temperature exhaust gases, causing temperature management issues for the whole turbocharger.
Figure 2: BorgWarner e-Turbo slated for production in 2022.
E-turbos practically eliminate turbo lag by exercising a large electric motor to turn a larger compressor at low speeds. When the revolutions have increased enough, the turbine can take over either partially or completely to turn the compressor. This allows the turbocharger to be larger, resulting in further power and improved efficiency. What’s more, the electric motor can work in reverse. Residual exhaust gases and the inertia of the turbo’s spinning when the vehicle is decelerating down or coasting can be harnessed by transferring the power to the vehicle’s battery. This enables acquiring energy that would else be lost to the exhaust, operating in improved efficiency as well as providing energy that can latterly be used to spin up the compressor once again. This basically turns the electric turbocharger into an integrated generator.
Figure 3: Mercedes F1 PU106A hybrid power unit with the MGU-H visible on the left hand side.
Electric turbocharging technology has been around for a while, although the move to incorporate them into commercial vehicles is a recent development. The technology was introduced in 2014 as part of the complex hybrid- electric power units of Formula One cars, and is still used now. Under the name MGU- H, these-turbos would serve alongside regenerative braking technology for improved performance, making them the most effective combustion machines to date. Like numerous of the innovations developed in Formula One, these technologies are now making their way into commercial vehicles.
Need for Electric Turbocharger
The low-end power is missing but there is abundance of high-end power available this is mainly because of the exhaust threshold and the turbo lag but when the revs climb, the high-end boost comes into play as the turbine is now spinning at a very high rpm and so is the impeller which in turn is going to suck in more air and send it to the intake manifold.
Turbo-lag: There is a time difference when we press the throttle and when we get the boost from the turbocharger which is known as the turbo-lag or boost-lag. It is because when we rev the engine it takes some time for the exhaust gases to spin the turbine and the compressor to deliver the boost. Sometimes turbo-lag is often mistaken with the threshold exhaust or boost but they both are totally different things. Nowadays manufacturers are trying to reduce the turbo-lag as much as they can by using and implementing different technologies like modifying the aspect ratio of turbine, reducing the losses due to friction at bearings, using an anti-lag system, using multiple turbos be it parallel or sequential, using a spool valve to increase the speed of the exhaust gases, increasing the compressor discharge, etc.
So to overcome this cons of turbocharger, Electric Turbocharger plays the role.
Fig. 4. Dyno graph of power (in hp) vs engine rpm of a naturally aspirated engine against a turbocharged engine
What are the challenges faced in e-turbo design?
The development of an e-turbo is a challenging design task. A balance must be reached between optimal compressor, turbine and electric motor size as well as keeping the overall weight as low as possible. One benefit of electric turbochargers is that the turbine can be designed with a narrower operating range than conventional turbochargers since the electric motor will spin the compressor at low RPM and low pressure. Turbomachinery design tools similar as AxSTREAM ® allow the design, analysis and optimization of turbocharger factors similar as the compressor and turbine. These tools help to design the machines at the optimal design points while the creation of turbine and compressor charts using the integrated tool AxMAP allows an engineer to assess at what pressure and rotational speed ranges the electric motor is needed.
Due to the high temperature of exhaust gases used to spin the turbine, heat management is another important issue in turbocharger design. Special care must be taken in the choice of bearings due to the tough working conditions. Current research is being undertaken in the development of spiral groove bearings and ball bearings that can handle high heat transients and varying loads, typical of turbocharger working conditions. Complex rotor dynamics issues also pose a problem in the design and operation of turbochargers. Coupled rotor dynamics and bearing analysis can be accepted using AxSTREAM Rotor Dynamics and Bearings tool. These programs allow an engineer to gain information about the rotor- bearing- support system including static deflection forms; critical speed maps; Campbell diagrams for lateral and torsional analysis; and much further.
CONCLUSION
Electric turbochargers are the future of turbocharger technology and propose a novel solution in the move to smaller engines without reducing power delivery while improving overall efficiency. Turbo lag can be all but eliminated, improving drivability and performance of vehicles. Many challenges are faced in the design of such systems, but thanks to modern design, optimization and analysis tools such as the AxSTREAM software suite, these challenges can be much more easily overcome, resulting in exciting new turbocharger designs which are sure to help power vehicles for many years to come.
References:
https://newatlas.com/garrett-turbo-promises-more-power-and-efficiency-less-lag/
AUTHORS:
HARSH SHET
VAISHNAVI JADHAV
TEJAS JOSHI
ROHAN KACHAVE
PRAJWAL KAKADE
SHARDUL KANDARKAR
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