Essay: Transportation technologies that lower carbon emissions

Over many years transport has been a source of economic growth by moving goods and workers around the country but has also been a major source of greenhouse gas emissions [1]. To tackle these emissions including other harmful emissions from transport, it has been at the motivation of researchers and manufacturers to develop and improve transportation technologies that lower carbon emissions. This has led to the increased use of electric vehicles and plug‐in hybrid electric vehicles, therefore making motor drives, energy storage systems, and power electronics important related issues [1].
Optimization and sustainable use of energy sources is the main goal of many electrical machine manufacturers around the world. One of the ideas was to apply an electric powertrain to a motorcycle dating as far back as 1895. The first efforts in creating an electric powered single-track vehicle can be found in the patent index from 19 September 1895 in Ohio, USA, where Ogden Bolton Jr. patented an “electrical bicycle” [2][3].
In the following years, new constructions suitable and relevant to the state of technology of the time were created for example in 1919, the Ransomes, Sims & Jefferies company had created a prototype of an electric motorbike powered from a battery located in a sidecar and named it Orwell [3][4]. As a result of technological advances in construction and materials technology, new innovations were created and presently there is a wide choice of models to choose from. This report will discuss the present state of technology and the design criteria incorporated in a racing electric motorbike which has gone beyond proving performance is possible with an electric motorcycle in comparison to traditional internal combustion engines.
Several specialist electric motorcycles and traditional motorcycle manufacturers have electric bikes in the market which are used as daily transportation as well as for racing or have concepts awaiting to be showcased [6]. Today’s racing motorbikes have mixed performance requirements due to different consumer targets and racing promotes innovation as it gives a chance to push the technology with limited liability to its limit prior to mass manufacture as well as publicising the technology. Examples of electric motorbikes in the market are; KTM Freeride E, Alta Motors Redshift MX, Zero SR, Victory Empulse TT, Brutus V9, Tacita Diabolika, Energica Ego, Harley-Davidson Project Livewire etc which are mainly designed around transportation.
Figure 1 [5] below shows the University of Nottingham electric superbike drivetrain system and components. It has three main parts; battery pack (including the battery management system), power converter and motor as shown in figure 1. It is a typical arrangement with typical main parts of most of the electric motorbikes either intended for racing or transportation in the market and the focus of this report will be based around the main component parts and how they function individually and/or together. They must be optimized as a closed loop system to meet the required user performance requirements for the motorbike’s intended purpose [7].
Figure 1: UoN Electric Superbike – an “Electrical Energy Conditioning and Control” system [5]
A racing electric motorbike design can be demanding as the chassis, batteries, motor and power converter can have a significant impact on the overall weight of the design hence optimisation is key when doing the design. Several batteries are required to achieve the required voltage. For higher voltages, several batteries must be placed in series as higher voltages result in a lower current hence lower losses and higher overall efficiency. To handle higher voltages, bigger coils are required in a motor which inherently increases the size. The motor and drive efficiency are also an important factor in the optimization of the system [7] and there is a need for improved technology, for example, exploring current technology options and improving it or coming up with something new to make the batteries & motor smaller and reliable with a longer range. Optimisation makes the racing platform agile and nimble, with the ability to accelerate quickly out of turns and lightweight as a heavier bike requires more acceleration energy.
Researchers are constantly looking into ways of making the motorbike lightweight and fast so as it becomes appealing to racing corporations interested in racing electric motorbikes through optimisation.
 
 
 
Electrical System Design and Technology
a. Power Converters
Power converters can be of different variations, for example, DC/DC, AC/DC or DC/AC depending on application or preference. When choosing a power converter several factors must be considered such as its input voltage with the benefits of a higher voltage outlined in paragraph 1 page 3, its ability to drive the chosen motor as this limitation determines the current and voltage ratings of the motor [7], cooling system as the converter is prone to getting hot during operation, longevity if used in conventional purposes but not so much of an issue on a racing bike, weight and volume of the converter are also critical in the overall bike design. The chosen power converter for this design is the DC/DC converter Hybrid pack ™ 2 from Infineon [12].
The device is robust and programmable which makes it versatile when being implemented in different applications. It is designed to be used as a Motor and/or generator inverter for Hybrid- and Electric Vehicles and Range Extenders providing 100kW of continuous power. Accommodates a 3-phase Six-Pack configuration of Trench-Field-Stop IGBT3 and matching emitter-controlled diodes as well as designed for a 150°C junction operation temperature. The chips are rated at a maximum of 800A/650V and includes a Pin-Fin baseplate for direct liquid cooling which significantly improves the thermal cycles capability extending the lifetime of the power module and enabling a very high-power density [12]. It is a popular choice amongst electric and hybrid vehicle manufacturers. Figure 3 below shows the layout of the module including its component parts [12] showing the electrical energy transformation from dc to ac interfaced with high-speed motor, coupled to the rear wheel.
Figure 3: Application Block Diagram of HybridPACK™ 2 [12]
b. Electrical Power System
The electrical power system comprises of the battery management system (BMS) which is an electronic control system for the lithium battery cells and the battery pack which is a key enabling system component in electric motorcycles as shown in figure 1 (page 2). Low-cost lithium batteries cells with higher power densities and improved power factors have changed the way power is used and stored onboard an electric motorbike making them suited for mobile applications [8]. When choosing lithium battery cells, the amount of energy storage required, the rating of the motor and power densities needs to be considered. An appropriate number of series and parallel lithium battery cells matching the motor rating with respect to the maximum voltage allowed by the relevant electric motorbike racing competitions are chosen. For example, MotoE regulations have battery voltage limitations of +/- 300V.
Lithium battery cells have a very non-linear charge/discharge curve making them relatively difficult to manage [8]. For a lead acid battery, measuring its voltage is the easiest way to tell how charged it is and from that data, the state of charge can be worked out. With a lithium-ion battery, it is slightly difficult to work out making it very hard to tell the state of charge of a cell as the voltage relative to the state of charge is very flat around 60% of the charge/discharge curve with sharp inflections at both ends [8]. This makes it relatively easy to cause permanent damage by both over and undercharging the cell and damaged cells can overheat and get into a thermal runaway condition [8]. This is where the BMS plays a crucial role in managing risk and ensuring safety.
The BMS accurately reports the state of charge of the battery pack and monitors the cells detecting any problems to ensure that the pack can be safely shut down when needed. The BMS consists of several separate circuit boards typically such as module control units or MCUs which will monitor the voltage of a small group of cells or individual cells very accurately in the battery. The MCU will also include some temperature measurements to monitor inside of the battery pack and possibly the individual cells linking up to a master BMS module which will combine data from all the slave MCUs measuring overall current flow in and out of the battery pack. With this information, it can work out what the battery state of charge is and reported to the driver via a display or warning light(s) [8].
Ability to charge each cell up to its maximum safe voltage and discharge it down to its minimum safe voltage limits the battery pack capacity. Cell tolerance in the manufacturing processes gives them slight variations in how quickly they will charge and discharge. The BMS also balances the cells making sure that the battery pack is not limited to the capacity of the cell that gets emptied and filled quicker by using charge pumps or balancing resistors. They switch on and off to discharge off small amounts of power from small groups or individual cells and make sure that the maximum pack capacity can be exploited and not limited to the weakest cell capacity [8]. The BMS also protects the battery for example, by making sure it restricts the rate at which it is possible to charge the battery if it detects that the battery temperature is getting too high [8]. It is currently mandatory for all-electric racing motorbikes to be fitted with a BMS.
c. Electrical Machines
A typical AC motor consists of two parts; stator (with coils) and rotor. The stationary stator coils are supplied with AC current producing a rotating magnetic field and the rotor connected to the motor shaft given a torque by the rotating field as shown in figure 2 below. The stator of an induction motor is normally made from a laminated magnetic steel structure and has slots to hold the windings with each slot containing one or more copper coil conductors. These must be insulated between adjacent turns, the core and from other phases with each coil then connected to one of the three phases. Coils of the same phase are normally connected in series and depending on the direction of connection current can be reversed. For a 3-phase motor, the winding may be star or delta connected with the ends of the three windings terminated in the terminal box where 3-phase A.C. supply will be connected. The rotor construction can either be squirrel cage or wound rotor, but wound rotors can be expensive to manufacture hence they are hardly used in industrial applications.
Figure 4: 3 Phase Induction Motor
For a given power rating, the volume and therefore weight of an electric motor are proportionately reduced. This combined with the desire to reduce unsprung mass on the vehicle suspension dictates using a high-speed motor, coupled to the rear wheel via some mechanical reduction gearing [7]. The UoN racing electric motorbike uses a 3 phase GVM Parker Hannafin permanent magnet motor which can reach speeds of up to 10,500Rpm as shown in figure 1 [9]. Other motor models could have been chosen but the GVM motors from Parker are specifically dedicated for Mobile applications and specially designed for rugged atmospheres and harsh environments for traction, pump, auxiliaries, and steering. These motors are brushless synchronous servomotors, with permanent magnets which are made of rare earth magnets allowing high-temperature operation, water or oil cooling system and a resolver/encoder speed sensor. The water cooling increases the torque density allowing a silent operation [9].
This motor is preferred due to features such as high efficiency due to no current in the rotor hence losses in the rotor are very low due to the absence of brushes, commuters or slip rings, high dynamic performances, high power density, wide speed range as speed control is easy and efficient, high precision, high motion quality, Low inertia, compact dimensions & robustness, large set of options & customization possibilities such as a wide variety of applications of high speed/power to low speed/power, can be used either as motor or generator and operating voltages available from 24 to 800 VDC [9].
Conclusion
There are many electric motorcycle manufacturers currently producing racing motorcycles with promising characteristics to rival their gasoline counterparts. Riding performances regarding acceleration, speed, and handling are already as good as any other motorcycle even in some cases, the electric motorcycles outperforming. The automotive industry is undergoing a radical transformation with 2025 as the next target date cited by manufacturers as a tipping point when everything from fuel, materials, and cost are set to change dramatically [13]. Certain technological issues are still present such as the range, battery, and motor sizes but as the technology rapidly changes through intensive research electric motorbikes might be in the market before 2025.
Electric racing superbikes have demonstrated that they can be raced at high speeds on demanding tracks to the point of even attracting racing corporations interested in investing in the technology. This a positive sign that electric motorcycles of the future will be exciting and fun to drive as well as environmentally friendly [7] with gasoline motorcycles being a technology of the past.