Vehicle electrification and in particular mild hybridization of a conventional powertrain at 48V is a rapid growth area for the automotive industry. As a reduced cost concept, it aims to make more intelligent use of electrical energy to achieve the desired reduction in fuel consumption and CO2 emissions. The low voltage approach compared with full hybrid and EV nominal voltages, which are typically in the range of 300 to 600V, avoids the need for high cost safety features and large battery packs.

Sixty volts is defined by the United Nations Economic Commission for Europe (UNECE) regulation R100 as the boundary between low and high voltage in a direct current circuit, and so is a critical point where a step change in system cost can be identified. Further standards such as LV-148 as defined by the Verband der Automobilindustrie (VDA) are providing further structure to the 48V nominal voltage approach, which allows high power energy recuperation without exceeding 60V.

—Nick Pascoe, CPT’s chief executive

SpeedStart and TIGERS technologies are aimed not only at the C-segment car platform represented by the Ford Focus, but also are relevant from the B-segment to the largest passenger car and rapidly growing SUV platforms. In the UK and Europe, B to D segment vehicles are responsible for almost 80% of passenger car sales, and there are further significant opportunities to apply the technology in the US, Japan and China. The technology can also be applied to medium and heavy duty commercial vehicle platforms.

Potentially this low-voltage technology combination could be applied to something approaching 50 million vehicles per year, CPT said. The successful application of the technologies would allow a global vehicle manufacturer to reduce its in-use carbon footprint of a typical vehicle by 50g/km—a 30% reduction on today’s baseline.

SpeedStart. CPT’s SpeedStart paper (2014-01-1890) describes the specification of a 48V belt integrated starter generator (B-ISG) developed to meet the growing market demand for low voltage mild hybrid passenger vehicle applications. The main areas discussed are the vehicle and application variations, considered both in terms of electrical and mechanical architecture and how these transfer into the motor design. The paper focuses in particular on the challenges of balancing the need to customize the motor for different applications against the need to maintain common variants to minimise cost, reduce risk and accelerate development cycles.

An SR based B-ISG is a relatively simple mechanical design construction consisting of a wire wound stator and central laminated rotor with no windings. The key difference feature of an SR electrical machine compared to other motor technologies is that it utilizes no permanent magnets, relying on current passing through the stator coils to generate the electromagnetic forces to drive the rotor with either positive or negative torque.

Within the SpeedStart design water-cooled power and control electronics are housed within the rear of the machine to reduce cost, improve control response times and minimise losses in the power electronics. SpeedStart is capable of delivering peak recuperation power levels in excess of 12kW which requires a greater degree of thermal management of the stator and power electronics.

Retaining a common stator and rotor design across multiple voltage levels enables the package space and mechanical/electrical interfaces to remain identical, thereby offering the OEM a modular design solution. The stator windings configuration, power electronics layout and control system strategy are the variables that enable the system design to be effectively tuned to meet increasingly widening, and sometimes conflicting, OEM requirements.

The number of architectures under consideration for 48V systems is still growing, particularly across different vehicle classes. This results in a range of system level requirements from different manufacturers, dictated by vehicle targets and other issues such as battery state-of-charge and energy storage capability.

By maintaining a common core of rotor and stator laminations and a common coolant system, it is possible to accelerate the development of each application. This common core also provides greater confidence in the robustness of prototypes, while minimizing the cost of variants further supports the development of 48V systems. SR machines are ideally suited to this concept of low cost development, while reducing the cost of production components. Moreover, compared with other electrical machines they have a high efficiency across a large speed range, good controllability, and reliability.

—Peter Scanes, senior manager for SpeedStart

TIGERS. CPT’s technical paper (2014-01-1873) describing its turbo-generator integrated gas energy recovery system known as TIGERS is similarly focused on the simulation, machine design, control system development and validation of the technology. The unit can be applied to naturally-aspirated or forced induction engines—though most likely will be applied to downsized turbocharged engines as an additional unit to the existing turbocharger.

The electrical machine design and control system for this application is particularly challenging for a number of reasons, the authors note. The turbine is capable of rotating the shaft at speeds greater than its critical rotating limit which is 60,000 rpm. Rolling element grease filled bearings are used to allow application flexibility; these have an operating temperature limit of 200°C. The exhaust gas can reach temperatures greater than 900 °C, whereas the turbine upper functional limit is 850 °C. The power electronics integrally mounted in the machine have a maximum thermal operating limit of 120 °C.

Considering that TIGERS is expected to harvest energy from the exhaust gas it is essential that it not only lives in this harsh environment it must also produce work with no impact on vehicle performance or fuel efficiency. The TIGERS design and control system have been designed to do exactly this through a number of thorough modeling exercises.

To ensure that the bearing system, generator core and integrated electronics are all kept within their working ranges, the coolant system taps into the engine coolant to operate between 80-105 °C.

Waste heat energy recovery concepts gaining interest for automotive applications include thermoelectric generation (TEG) and organic Rankine Cycle systems, whilst heavy duty markets already offer mechanical turbo-compounding systems as an efficient solution where space is less restricted.

The concept of direct turbo generation, using a turbine coupled to an electrical generator to extract the potential and kinetic energy of the fast flowing exhaust gas, differs significantly from these waste-heat recovery devices, which rely solely on heat transfer to extract energy. With a correctly specified turbine, there is the potential to harvest significant electrical energy over a large proportion of the engine operating range as the turbo-generator can take advantage of both temperature and mass flow rate of the exhaust gas provided by the engine.

—Dr. Rick Quinn, senior manager for advanced engineering

As with the SpeedStart device, the liquid cooled switched reluctance generator at the core of the TIGERS technology provides many benefits over conventional permanent magnet generators. Due to the absence of rotor windings, SR machines possess low rotating inertias thus minimizing rotor losses. In addition, due to the exclusion of permanent magnets there is no risk of torque loss due to de-magnetization or uncontrolled generating modes at high-speed.

The SR type generator also facilitates cost savings across the design, including minimal tooling investment due to the straightforward motor construction and a reduction in the rating of the power electronics due to low switching frequencies. The 65,000 rpm high speed capability of the Tigers electrical generator also facilitates a direct coupling to the turbine rather than a reduction gear set. This reduces the number of components in the device, making the unit design inherently lighter, cheaper to manufacture and simpler to assemble in both its prototype and volume production form.

The TIGERS system has already benefited from two research projects supported by the UK’s innovation agency, the Technology Strategy Board (TSB). The HyBoost project consortium partners included CPT, the European Advanced Lead Acid Battery Consortium, Ford, Imperial College London, Ricardo and Valeo; and the Vehicle Integrated Powertrain Energy Recovery project known as VIPER consortium members comprised BP, CPT, Ford, IAV, Imperial College London, Jaguar Land Rover, and the University of Nottingham.

There is a continual drive within the auto industry for low carbon vehicle technologies. The new 48V DC standard provides the opportunity for a cost effective approach based on switched reluctance motor generator technology with integrated power electronics and control software. SR technology not only eliminates the need for permanent magnets made from increasingly difficult to obtain rare earth materials, but also is uniquely well suited for electrical boosting and energy recovery in downsized petrol and diesel engines. Our various applications of low voltage switched reluctance motor-generator technology are now approaching the level of readiness required by carmakers to meet near-term European CO₂ emission levels—reducing further to an indicative 68-78 g/km by 2025.

—Nick Pascoe

Resources

• John Kelly, Peter Scanes, Paul Bloore (2014) “Specification and Design of a Switched Reluctance 48V Belt Integrated Starter Generator (B-ISG) for Mild Hybrid Passenger Car Applications” Technical Paper 2014-04-01

• Andrew Haughton, Andy Dickinson (2014) “Development of an Exhaust Driven Turbine-Generator Integrated Gas Energy Recovery System (TIGERS)” Technical Paper 2014-01-1873