Overcoming the Use of ICEs in Hybrid Electric Vehicles with Turbomachinery – Micro-Turbine Range Extenders – Part 2

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As introduced in the last blog regarding Micro-Turbine Range Extenders, we will continue the discussion of turbine engine applications in the automotive sector in this blog.

Looking to solve the problem of range anxiety in electric vehicles, many companies have started exploring the business model of recharging electric batteries in automotive vehicles with a parallel turbine engine driving a generator – coined under the term ‘micro-turbine range extender’ (or MTRE).  As seen in the turbine-powered car programs initiated in the 50s and 60s, issues with low efficiencies, slow throttle response, and capital cost of the powertrain rendered all of these programs futile shortly after their inception.  However, the revolution of electric vehicles and hybrid technologies has allowed this technology to resurface from a different direction.  With battery-driven electric motors designated as the main driver, these cars are equipped with a technology that has both energy efficient low-end torque as well as groundbreaking throttle response and many of the former drawbacks during its initial iterations are solved using an electric drivetrain.  The turbine-engine, instead of operating as the main driver, will now only operate at its most efficient power output mode and work to simply drive electricity through the generator, recharging the vehicle’s battery packs.  Acting as an isolated thermo-mechanical system, a micro-turbine range extender can be designed and optimized without having to worry about the varying duty cycles and idling that is inherent in the vehicle’s drivetrain. The thermodynamic model of a typical micro-turbine range extender can be seen below in Figure 1.

Figure 1 – Thermodynamic Formulation of a Micro-Turbine Range Extender Model in AxCYCLE™

One application within commercial vehicles that has benefitted from this technology utilizes a MTRE system developed by Wrightspeed.  The specific application lies within retrofitting refuse trucks with this electric powertrain in order to help them save an estimated $35,000 a year on fuel and maintenance costs.  In such heavy-duty applications, it is obvious that the potential for fuel cost and maintenance savings is much higher due to the large fuel burning needed for these vehicles as well as the harsh drive cycle a refuse truck goes through.  The question in the expansion of this technology generally comes in two forms: What makes the micro-gas turbine range extender a better alternative than a normal ICE hybrid option? – and – What is the viability of scaling this for consumer vehicles given the capital cost of the drivetrain?

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Overcoming the Use of ICEs in Hybrid Electric Vehicles with Turbomachinery – Micro-Turbine Range Extenders

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The concept of turbine-powered automotive vehicles is not necessarily an unfamiliar idea or a technology that has yet to be explored.  In fact, several prominent automakers explored this concept as early as the 1950s and 60s – with real, functional prototypes.  Notably, Rover-BRM in the UK as well as Chrysler and General Motors in the US employed turbine engine programs to test the viability of such engines in the commercial market.  The Chrysler turbine engine program began its research back in the late 1930s and eventually ran a public user program from September 1964 to January 1966 where a total of 55 cars were built.   General Motors had tested gas turbine-powered cars with its many iterations of the Firebird in the 50s and 60s.  Rover and British Racing Motors developed several prototypes of their Rover-BRM concept that actually participated in the Le Mans race three years in a row, from 1963 through 1965.  However, even Chrysler, which was considered the leader of gas turbine research in automobiles, had to eventually abandon their program in 1979 after seven iterations of the turbine engine.  Many of the initial issues with heat control and acceleration-lag were improved during the program’s lifetime, but the program had never paid off in the retail automotive sector, and its continued development was deemed too risky for Chrysler at the time.

Chrysler Turbine Car
Figure 1- Chrysler Turbine Car – Now at Display in the Walter P. Chrysler Museum

Several decades later, we are seeing a resurgence of turbine motors in automobiles, but now serving as a range extender generator for electric vehicles instead.  As with many upcoming technologies, learning from past research and failed historical attempts can bring light to the most elegant and innovative solutions for today’s modern challenges.  This revolution of an old concept shares many of the qualities that made turbine engines attractive back in its initial development phase.  Such advantages include the ability to run on any flammable liquid and the high power density that results in a significantly lower weight and size contribution than its piston engine counterpart.

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Leveraging AxSLICE for Centrifugal Pump Upgrades and Retrofits

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Often, service companies are faced with the challenge of redesigning existing pumps that have failed in the field with extremely quick turnaround times. While there are quick-fix methods to return these pumps into operation, other more complex problems may require taking a step back and analyzing how this particular pump could be redesigned based on its current operation.  These engineering upgrades could solve recurring issues with failure modes of a certain machine, and they could also solve new capacity demands that are imposed by a customer based on their system’s upstream or downstream changes. While efficiency increases could be beneficial to the overall system, many times it is more important to solve capacity requirements and increase the life of the pump by decreasing the Net Positive Suction Head Required (NPSHr).

In this blog post, we will investigate how to move an existing centrifugal pump through the AxSTREAM platform in order to solve engineering challenges seen on common OEM pump upgrades.  With the use of AxSTREAM’s integrated platform and reverse engineering module, many of the CAE tasks that are common in an analysis such as this one can be realized in record speed. The first step of the reverse engineering process occurs in obtaining the necessary geometrical information for the desired pump. Through AxSLICE, the user can take an STL, IGES, CURVE file, or a generated cloud of points and properly transform this 3D profile into a workable geometry inside the AxSTREAM platform. In a matter of minutes, the user can outline the hub and shroud and transform a blank 3D profile into a profile defined by a series of segments.  Seen in Figure 1, the centrifugal pump is now defined by a hub, shroud, and intermediate section.

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SuperTruck II Program and Waste Heat Recovery Systems

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Familiar to many, the 2011 SuperTruck program was a five-year challenge set by the U.S. Department of Energy to create a Class-8 truck that improves fuel efficiency by 50 percent.  Hoping for even more groundbreaking achievements this time around, the Department of Energy has initiated a second five-year program to bring further fuel-efficiency advancements and near closer to eventual commercialization.  Cummins, Peterbilt, Daimler Trucks North America, Navistar, and Volvo Group remain the five teams involved in this R&D endeavor.  Michael Berube, head of the Energy Department’s vehicle technology office mentioned “SuperTruck II has set goals beyond where the companies think they can be.”  SuperTruck II is looking for a 100 percent increase in freight-hauling efficiency and a new engine efficiency standard of 55 percent.  With such lofty goals, the SuperTruck II development teams will need to tackle improvements in freight efficiencies from all sides.

Figure 1 - Daimler SuperTruck
Figure 1 – Daimler SuperTruck

Material considerations, body aerodynamics, low-resistance tires, predictive torque management using GPS and terrain information, combustion efficiency, and several other improvements methods on the first iteration have demonstrated how the SuperTruck II will require a multi-phase and integrated systems approach to achieve equally successful numbers. However, with an engine efficiency target that is 31 percent above the SuperTruck’s first go around, special attention will be required on engine advancement to achieve an efficiency standard of 55 percent.

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Foil Air Bearings for High-Temperature Turbocharger Applications

Within the realm of turbocharging, there are a number of different design challenges that influence the design process on both large-scale marine applications and smaller-scale commercial automobile applications.  From aerodynamic loads to dynamic control systems to rotor dynamics and bearing challenges, turbochargers represent a special subset of turbomachinery that requires complex and integrated solutions.  Turbocharger rotors specifically, have unique characteristics due to the dynamics of having a heavy turbine and compressor wheel located at the overhang ends of the rotor. The majority of turbocharger rotors are supported within a couple floating-ring oil film bearings.  In general, these bearings provide the damping necessary to support the high gyroscopic moments of the impeller wheels.  However, there are several disadvantages of working with these oil systems that have allowed different technologies to start to surface for these turbomachines.  With the floating-ring oil models, varying ring speed ratios and oil viscosity changes significantly influence the performance of the rotor dynamic model.

Dan blog bearing for turbochargers
Figure 1 – Floating-Ring Bearing Model for a Turbocharger

The application of oil-free bearings have started to emanate due to the overall consistency of their performance and the minimized heat loss associated with air as the damping fluid. Studies on these bearing types for turbomachinery applications are neither trivial nor unique, as they have seen plenty of exposure within the commercial and military aircraft industries within turbo compressors and turboexpanders. However, the success of these specific applications are due to the fact that these turbomachines operate with light loads and relatively low temperatures. The main design challenges with foil air bearings are a result of poor rotor dynamic performance, material capabilities, and inadequate load capacities at high temperature/high load applications.

Foil Air Bearing
Figure 2 – Foil Air Bearing

Foil air bearings operate based on a self-acting hydrodynamic air film layer during normal operation, but they exhibit serious wear on start up and shut down if not properly attended to. Prior to developing a gas film on start up, these bearings must handle the sliding that occurs between the rotor and the inner surface of the bearings. For this reason, solid lubricants like polymer foil coatings were considered for these bearings. Polymer coatings have a serious temperature restriction which do not allow them to be considered for high-temperature applications above 300 °C. Different chrome oxide based coatings have shown greater performance at higher temperatures. Initial testing of these coatings showed significantly poor performance at lower temperatures of 25 °C and difficulties with adhesion through repeated thermal cycles. However, NASA has developed a new high temperature PS400 formulation of this coating that performs well under different load conditions and between the temperature range of 25 °C and 650 °C. Essentially, the viability of these bearings within the automotive market has become a reality with individualized bearing designs. The question now becomes whether the foil gas bearing manufacturers can penetrate the market from a larger-scale and create a standard for these turbocharger setups to run free of oil altogether. To learn more about the simulation of both floating-ring oil film bearings and foil air bearings using the SoftInWay platform, please visit: http://www.softinway.com/software-applications/bearing-design/

References:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000004303.pdf

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090033769.pdf

 

Simultaneous Design for Turbocharger Compressors and Turbine Wheels

AxSTREAM Blade Profiling
Figure 1- AxSTREAM 3D Blade Profiler for Radial Designs

Increasing regulation for reducing emissions has forced the automotive industry to accept different technologies over the years in order to stay ahead of the market. In an industry that is so accustomed to internal combustion engines, new solutions such as electric motors and turbocharger systems have allowed experts in other industries to cultivate an influence in the automotive market. Specifically in the realm of turbomachinery, increased development has gone into designing turbochargers in order to minimize the effect and size of internal combustion engines. Design challenges are inherent in the fact that an engine is a positive displacement device whereas the turbocharger falls under aerodynamic turbomachinery. The two separate machine types have distinctly different flow characteristics, and the proper sizing of a turbocharger for its parent engine requires proper modeling of the engineering system as a whole.

In general, initial turbocharger sizing becomes a matter of obtaining the necessary boundary conditions required for a preliminary design. A thermodynamic cycle analysis of an ICE-Turbocharger system will allow the designer to obtain an initial idea of the bounds

Axmap for turbocharger
Figure 2 – Simultaneous Turbine (color) and Compressor (dotted) Maps – Power vs. MFR (left) & Pressure Ratio vs. MFR (right)

necessary for the compressor and turbine design. Given the engine information, necessary inlet conditions of the compressor such as temperature and pressure, efficiencies required, and heat transfer of the system, the user can then obtain the boundary conditions for the turbocharger compressor and turbine wheels.

From this point, the process becomes an exercise in turbomachinery design and analysis. With SoftInWay’s turbomachinery design and analysis platform, a boundary condition realization of the system eventually manifests into a full 3D design of the turbine/compressor wheel. Once the engineer designs both the turbine and compressor wheels, they will be left with two discrete physical systems. However, these two designs must eventually coincide into a harmonious system that accurately represents the “turbocharger”. In order to facilitate this representation, the user can overlay the different compressor and turbine maps based on a number of varying parameters. Given the Power and Pressure Ratio curves for a number of varying shaft speeds and temperatures, an off-design performance of the turbocharger system can be analyzed via AxSTREAM’s matching module (Figure 2). Another simultaneous analysis of the turbine and compressor wheels must be made on the component that connects them, the rotor. Rotor design, rotor dynamics, and bearings analysis are crucial to a legitimate turbocharger design and will be a topic of a next week’s blog post. If you would like to learn more about turbocharger design and analysis methods, please follow this link

References:
http://www.automotive-iq.com/engine/articles/high-boost-and-two-stage-turbo-power-systems

Axial Compressor Challenges in Hyperloop Designs

Back when the California high-speed rail project was announced, Elon Musk (CEO of SpaceX and Tesla Inc. and perhaps the most admired tech leader of present day) was not only disappointed with this project, but also introduced an alternative to this system called the Hyperloop in 2012.  Since the abstract of this project was introduced, many engineers around the world have started to evaluate the feasibility of this “5th Mode of Transportation”.

Hyperloop Alpha Conceptual Design Sketch
Hyperloop Alpha Conceptual Design Sketch

The general idea for the Hyperloop consists of a passenger pod operating within a low-pressure environment suspended by air bearings.  At the realistic speeds estimated by NASA of 620 mph, the pod will be operating in the transonic region.  While Japan’s mag-lev bullet train has succeeded at achieving speeds of up to 374 mph, the scale and complexity of a ground transportation system rising above 600 mph bring to surface an unusual number of engineering challenges. As well, brand new designs such as the one proposed by Musk have a certain amount of risk involved due to this technology inherently having no previous run history on a large scale.

Of the many concerns with his original design, perhaps the largest resides on how to design and operate the axial compressor in front of these pods. The supposed function of the compressor is two-fold. The first function would be to overcome the Kantrowitz limit. Musk uses an analogy between the pod and tube and a syringe:

“Whenever you have a capsule or pod (I am using the words interchangeably) moving at high speed through a tube containing air, there is a minimum tube to pod area ratio below which you will choke the flow. What this means is that if the walls of the tube and the capsule are too close together, the capsule will behave like a syringe and eventually be forced to push the entire column of air in the system. Not good.”

Aero Booster
Figure 2 – Safran Aero Boosters Low-Pressure Compressor – Assembly View

An onboard compressor in front of the pod will allow the collected column of air traveling in front of the pod to flow through the system without compromising the increasing velocities of the pod. A second function of the compressor would be to supply air to the air bearings that support the weight of the capsule throughout the passage.

Traditionally, axial compressors are coupled with a complimentary turbine at the exhaust that provides mechanical power to the compressor. In the hyperloop, the proposed compressor arrangement will be driven by electric motors instead of turbines. This is a relatively new design that has only been tested on a handful of electric powered jet aircrafts for research purposes. Furthermore, Musk proposed a compression ratio of about 20:1, which would require several compression stages for an axial compressor arrangement and an intercooler system. The temperature increases resulting from this high order compression require a complex cooling method or a traditional steam pressure vessel for the proper dumping of hot air. A final challenge on the compressor end would be the fact that it will be operating at a very low pressure. Only a handful of companies like Safran Aero Boosters have the necessary experience with low-pressure compression.

In general, while this new proposed mode of transportation is very exciting and innovative from an engineering standpoint, the following challenges specific to the on-board compressor will require serious collaborations amongst the leaders in the compressor design industry:

  • Electric Motor Driven Compressor
  • High Compression Ratio – 20:1
  • Complex intercooler system
  • Low-Pressure Compression Environment

If you would like to learn more about SoftInWay’s integrated platform for axial compressors, please visit our axial compressor page

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