E-Turbos: The Future of Turbocharger Technology

The Achilles heel of 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, commonly termed turbo lag, is what has hindered turbochargers from delivering optimal performance. The aim of a turbocharger is to provide more power, better efficiency and less lag in power delivery. Engine efficiency is becoming more important than ever before, leading to the development of smaller engines. However, the power requirements are not decreasing which means the loss in engine displacement from small designs must be picked up with alternative technologies, such as turbochargers, which can help improve power delivery and fuel economy.

Figure 1: Garrett Motion electric turbocharger due for production in 2021. Source

Electric turbochargers (e-turbos) provide a solution to eliminating turbo lag while adding additional performance benefits. This allows for larger turbocharger designs which can provide larger power and efficiency gains, stay cooler over longer periods of use, and drastically improve engine responsiveness. Garrett Motion are developing e-turbos for mass market passenger vehicles set for launch in 2021, with a claimed fuel efficiency improvement of up to 10%. When used on diesel engines, this e-turbo could be up to a 20% reduction in NOx emissions. In most cases, fuel efficiency will be improved by about 2 – 4%. Other manufacturers such as Mitsubishi and BorgWarner are already developing their own electric turbos and are expected to have announcements in the near future matching the trend in e-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 improve the fuel:air ratio in the combustion chamber and to regulate more precisely the amount of air entering by pushing air under force into the chamber. Exhaust gases from the engine spin 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 will not operate efficiently at high RPM which leads to a shut down to cool. Remember that the turbine is spun up from high temperature exhaust gases, causing temperature management issues for the whole turbocharger.

Figure 2: BorgWarner eTurbo slated for production in 2022. Source

E-turbos practically eliminate turbo lag by utilizing a large electric motor to turn a larger compressor at low speeds. When the revs have increased enough, the turbine can take over either partially or fully to turn the compressor. This allows the turbocharger to be larger, resulting in more power and higher 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 slowing down or coasting can be harnessed by sending the power to the vehicle’s battery. This enables capturing energy that would otherwise be lost to the exhaust, resulting in improved efficiency as well as providing energy that can later be used to spin up the compressor once again. This essentially 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. Source

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 today. Under the name MGU-H, these e-turbos would function alongside regenerative braking technology for improved performance, making them the most efficient combustion engines to date. Like many of the innovations developed in Formula One, these technologies are now making their way into commercial vehicles.

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 such as AxSTREAM® allow the design, analysis and optimization of turbocharger components such as the compressor and turbine. These tools help to design the machines at the optimal design points while the creation of turbine and compressor maps using the integrated tool AxMAP allows an engineer to assess at what pressure and rotational speed ranges the electric motor is needed.

Figure 4: Compressor model after Preliminary Design stage in AxSTREAM

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 undertaken using AxSTREAM Rotor Dynamics and Bearings tool. These programs allow an engineer to obtain information about the rotor-bearing-support system including static deflection forms; critical speed maps; Campbell diagrams for lateral and torsional analysis; and much mor

Figure 5: Example of bearing calculations using AxSTREAM Bearing

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.