Supervisory Control Methods for Electric Hybrid Vehicles – A Review
In modern day era where increasing population poses a difficult challenge, the crave of every individual or at least every family to own a personal vehicle boosts the sales and manufacturing of modern day transportation vehicles. This leads to a relationship between the increase in population and the consumption of natural energy resources, which is ever increasing, as well as between the increase in population and increasing environmental pollution (Mi et al.
, 2011). The solution to the postulated problem, up to a certain extent, can be realized by hybrid electric vehicles. Such vehicles utilizes conventional internal combustion engines as the primary source of energy conversion (chemical to mechanical) and an electric component (electric motor/ electric generator) as secondary source of energy for vehicle propulsion.
Electric vehicles is not a new concept, actually the concept of electric propulsion system for the passenger vehicle was realized much before the discovery of gasoline based combustion engines.
The concept of hybrid electric vehicle was first realized by Dr. Ferdinand Porsche a few years after the discovery of combustion engine. He invented and developed the first Hybrid vehicle using combustion engine as the primary source and electric generator and electric motor to transmit the required energy to the wheels for the vehicle propulsion. Electric vehicles were quite popular in that era as well and contributed to a fair share of the automobile market, but due to high technical developments in the field of combustion engines the concept subdued.
In addition to this, electric propulsion system, may be due to lack in technological advancements, was not able to produce competitive power to the vehicle and investments for such vehicles were comparatively high. The focus shifted again to the concept of electric and hybrid electric vehicles in 1970’s when diesel prices hiked up and the low availability of the natural energy sources was realized. (Mi et al., 2011)
The trend in the technological development and researches in the field of hybrid vehicles have picked up its pace in last few decades. Newer technologies or refurbished concepts are being realized and automotive manufacturing giants are trying to prove their metal in this upcoming and competitive field. With the realization of hybrid concept, in which more than one energy sources are utilized in vehicle propulsion, it becomes essential to understand the power distribution (i.e. which source is fulfilling the power requirement of the vehicle propulsion system).
As defined earlier hybrid electric vehicle refers to a vehicle that utilizes the energy from two energy sources available in the propulsion system of the vehicle, conventional fuel (i.e. the chemical energy) and electric storage units/electric components (i.e. electrical energy). Hybrid electric vehicles have a certain upper-hand on the conventional vehicle, because realization of different operation modes is possible (G?rges, 2019).
· Engine on/off functionality: Engine can be turned off when not required, which results into significant reduction in idling losses.
· Regeneration of kinetic energy while the vehicle is being decelerated: A certain fraction of kinetic energy can be restored using electric motor, while applying brakes. This can be helpful, as in conventional braking system (i.e. friction braking) kinetic energy is dissipated in form of heat.
· Load point shifting: As shown in figure 1, the efficiency of the IC-engine can be realized using two factors, angular velocity and the torque of the engine. Thus the load of the engine can be shifted by operating the electric component in either motor mode (load point decrease) or in generator mode (load point increase), to optimize the efficiency of the engine. Decrease in load point implies that the motor is assisting the engine to drive the vehicle and increase in load point implies that the extra torque is utilized by the generator to charge the battery.
· Electric driving: When the electric storage unit (usually electric batteries) consists of sufficient charge, than the vehicle can be driven in pure electric mode.
· Phlegmatization of the combustion engine: this can result into lower pollutant emission.
Figure 1. Engine fuel consumption map and significance of LPS: Load Point Shifting
These operation modes helps significantly in consumption of gasoline fuel (as the efficiency of the combustion engine is optimized) and environmental pollution, because of the possibility of lower emission of the pollutants (G?rges, 2019). Along with this, noise pollution can also be reduced due to electric driving and high degree of freedom in energy sources can be realized, especially with the variants of hybrid electric vehicle such as plug-in hybrid electric vehicle (in such vehicles batteries can be charged from the external electric grid sources) (Mi et al., 2011). Hybrid electric vehicles can be differentiated on the basis of the architecture of the propulsion system. In general three architectures of the hybrid electric vehicles can be realized: ‘Series hybrid electric’, ‘Parallel hybrid electric’ and ‘Combined hybrid electric’.
Series architecture for the hybrid electric is easy to understand. In this architecture three components plays a major role: Internal combustion engine, electric generator and electric traction motor. The generator in mounted in front of the engine, which implies that the mechanical output of the IC-engine is fed as the mechanical input for the generator, which than converts the mechanical energy into electrical energy. This electrical energy can be used, via the electric motor, directly to drive the vehicle or can be used to increase the state of charge (available charge in the on-board battery) of the battery. Figure 2 explains this type of configuration, where thick links indicates the mechanical connection between the components and thin link implies electrical connection. Different modes of operation can be
realized in this architecture depending on the available battery power, but downsizing of engine is not possible. (Guzzella & Sciarretta, 2013)
Figure 2. Series Hybrid Electric Architecture. E: Engine, G: Generator, B:Battery, P: Power link, M:Motor, T: Transmission link, V: Vehicle, Thick links: Mechanical Coupling, Thin Links: Electrical Coupling
In any propulsion system, mass of the body that is desired to be propelled plays an important role. In series hybrid electric configuration engine has no direct connection to vehicle transmission drive, thus there is no requirement of clutch, but still the architecture requires three different components, which adds significantly to the vehicle mass and affects the performance of the propulsion system. The performance of vehicle with such configurations have similar performance to vehicles based on highly efficient IC-engines, which was attained after years of research. (Guzzella & Sciarretta, 2013)
Examples: Faun Rotopress Dualpower, Mercedes-Benz Citaro G BlueTec Hybrid, MAN Lion’s City Hybrid, Solaris Urbino 18 Hybrid Vossloh-Kiepe, Opel Ampera (G?rges, 2019).
As the name suggests, two different drive trains can function in parallel within a vehicle with parallel hybrid electric architecture. Both mechanical and electrical components are mechanically connected with the vehicle transmission links and operates on different axles of the vehicle. One of the disadvantages of such architecture is the requirement of the clutch. As IC engine is mechanically coupled with the vehicle transmission, clutch becomes a necessity to realize the engagement and disengagement of the engine with the mechanical drive. But due to the parallel operation, all the driving modes are possible, which increases the overall fuel consumption efficiency of the vehicle and thus increasing the performance of the vehicle. Along with this both the components can be sized properly to function at the maximum efficiency, which helps in the energy management of the vehicle. Moreover, only two components are required in the vehicle architecture to successfully propel the vehicle: IC engine and electric traction motor. (Guzzella & Sciarretta, 2013)
Figure 3. Parallel Hybrid Electric Architecture. E: Engine, G: Generator, B:Battery, P: Power link, M:Motor, T: Transmission link, V: Vehicle, Thick links: Mechanical Coupling, Thin Links: Electrical Coupling
Examples: John Deere 7530 E Premium, Volvo 7900 Hybrid, Porsche Panamera S E-Hybrid, Citro?n DS5 Hybrid4 (G?rges, 2019).
Benefits of both the aforementioned architectures can be realized in the vehicle with the combined architecture. The architecture provides parallel functioning of the engine and the electric motor along with an extra electrical component (i.e. a generator), which is mechanically coupled with the engine and can be used to charge the battery, using the mechanical output of the IC-engine. In such architecture power split devices (devices used to split the power between mechanical and electrical sources) plays an important role in control of the propulsion system. Different complex transmission arrangements can be considered for such a task, but a popular choice includes a gear box containing planetary set (i.e. planetary gear set) and has been implemented previously in market vehicles (Guzzella & Sciarretta, 2013).
Figure 4. Parallel Hybrid Electric Architecture. E: Engine, G: Generator, B:Battery, P: Power link, M:Motor, T: Transmission link, V: Vehicle, PGS: Planetary Gear Set, Thick links: Mechanical Coupling, Thin Links: Electrical Coupling
Examples: Toyota Prius II (G?rges, 2019).
Functionality of the hybrid electric vehicle can be classified on the basis of degree of hybridization (i.e. how many operation modes are possible and with what effectiveness). Vehicles can be divided in micro-hybrid, mild-hybrid or full-hybrid based on the power delivered by the motor. Degree of hybridization can be understood as the ratio between the power rating of the motor and the power rating of the IC-engine (Guzzella & Sciarretta, 2013). Thus size of the motor plays an important role in deciding the hybridization of the vehicle. Along with this, there are few more variants available in hybrid vehicles, where electric driving dominates than other operating modes, such as plug-in hybrid electric vehicle and range extender hybrid electric vehicle.
Architecture of hybrid vehicles paves the way for control strategies, which structures the functioning of propulsion system of the vehicle and decides the mode in which vehicle should be operated, based on the power requirement of vehicle. This control strategies can be understood as algorithms that are used to control the pollutant emission and minimise the fuel consumption of the vehicle, by splitting the overall power requirement of the vehicle between conventional thermal sources and electrical components (Malikopoulos, 2014). Based on the design complexity, parallel hybrid electric architecture can be considered as ideal foundation to develop an algorithm, which splits the power requirement as both the energy sources (conventional and electrical) are each connected to the transmission drive mechanically. Such an algorithm can then be manipulated for simpler vehicle configurations like series hybrid and for complex architectures like combined hybrid (Malikopoulos, 2014).
A reliable control algorithm can be developed on the basis of three input factors: power-split ratio, status of the clutch and status of the engine. Power-split ratio (u) can be understood as the ratio of power delivered by the electric motor and the overall power requirement, status of the clutch indicates whether the engine is engaged or disengaged with the transmission drive and status of the engine indicates whether the engine in on or off. These control parameters are of Boolean type (i.e. they can either acquire value 1, which implies on or 0, which implies off) and can be used in different combinations to realise different vehicle modes. (Guzzella & Sciarretta, 2013)
Operating modes Power-split ratio Clutch status Engine status
Table 1. Driving modes and control factors.
As discussed earlier different control algorithms can be realized on the basis of these control factors and these algorithms can be classified on the basis of different control strategies. Figure 5 lists down different control strategies, which can mathematically be further explored to derive an algorithm for the supervisory control. Such control algorithms aims at delivering the required power by functioning the engine and the electrical components on the optimum efficiency, thus assuring the best feasible fuel efficiency, lower pollutant emission and charge sustainment in the battery (if the vehicle in discussion is not plug-in hybrid) (Salmasi, 2007). Pros and cons of most common control strategies have been summarised in table 2, which can provide a starting point for the comparison between the available control strategies (G?rges, 2019).
Table 2. A summary of control strategies.
The study aims at presenting an overview of the control strategies, which can be realized in modern day field of hybrid electric vehicles. Hybrid electric vehicles have enticed a research trend and energy management of vehicles has always been a major topic of discussion. Energy management implies the most efficient utilization of the fuel (chemical energy), which can be attained by decreasing the losses in energy conversion during the vehicle propulsion. Comparing the pace with which the efficiency of conventional engines and electrical components is being improved, refinements in the control algorithms to improve overall efficiency of the propulsion system proves to be a better strategy. Additionally with the support of research being conducted in the field of vehicle communication, it becomes more convenient to develop an algorithm that can easily predict the future driving profile of the vehicle and an optimum (global) solution can be attained along with the prospect of real time implementation. A thorough research in the field can thus be helpful to answer many difficulties in enhancement of supervisory control which implies the optimum performance of the hybrid electric vehicle.
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