There is a growing interest in electric and hybrid-electric vehicles propulsion system due to environmental concerns. Efforts are directed towards developing an improved propulsion system for electric and hybrid-electric vehicles (HEVs) for various applications in the automotive industry. The government authorities consider electric vehicles one of several current drive technologies that can be used to achieve the long-term sustainability goals of reducing emissions. Therefore, it is no longer a question of whether vehicles with electric technologies will prevail, but when will they become a part of everyday life on our streets. Electric vehicles (EVs) fall into two main categories: vehicles where an electric motor replaces an internal combustion engine (full-electric) and vehicles which feature an internal combustion engine (ICE) assisted by an electric motor (hybrid-electric or HEVs). All electric vehicles contain large, complex, rechargeable batteries, sometimes called traction batteries, to provide all or a portion of the vehicle’s propelling power.
EVs propulsion system offers several advantages compared to the conventional propulsion systems (petrol or diesel engines). EVs not only help reduce the environmental emissions but also help reduce the external noise, vibration, operating cost, fuel consumption while increasing safety levels, performance and efficiency of the overall propulsion system. However, there are many reasons why EVs and HEVs currently represent such a low share of today’s automotive market. For EVs, the most important factor is their shorter driving range, the lack of recharging infrastructure and recharging time, limited battery life, and a higher initial cost. Though HEVs feature a growing driving range, performance and comfort equivalent or better than internal combustion engine vehicles, their initial cost is higher and the lack of recharging infrastructure is a great barrier for their diffusion. Therefore, industry, government, and academia must strive to overcome the huge barriers that block EVs widespread use: battery energy and power density, battery weight and price, and battery recharging infrastructure. All major manufacturers in the automotive industry are working to overcome all these limitations in the near future.
Common Types of Electric VehiclesA more universal EVs classifications is carried out based on either the energy converter types used to propel the vehicles or the vehicles power and function . When referring to the energy converter types, by far the most used EVs classification, two big classes are distinguished, as shown in Figure 1, namely: battery electric vehicles (BEVs), also named pure or full-electric vehicle, and hybrid-electric vehicles (HEVs). BEVs use batteries to store the energy that will be transformed into mechanical power by electric motors only, i.e., ICE is not present. In HEVs, propulsion is the result of the combined actions of electric motor and ICE. The different manners in which the hybridization can occur give rise to different architectures such as: series hybrid, parallel hybrid, and series-parallel hybrid. All these different EVs architectures are shown in Figure 2.
Full-electric vehicles or BEV are wholly driven by an electric motor, powered by a battery. There is no combustion engine. BEV may adopt two (or four) in-wheel motors in their powertrains, as shown in Figure 2(a) above. In this case, every motor is driven by a dedicated power converter which control a wheel’s speed and torque. Moreover, a central electronic controller must coordinate speed differences (in steering wheels), whenever needed or as a result of wheel slippage, as long as a differential power splitting device is not present. As expected, the simplification of the mechanical design is attained at the expense of increased complexity of the power electronics and controllers.
To date, BEVs are not as attractive as HEVs because of limited driving range, performance and comfort . Nevertheless, as BEVs are the only zero-emission vehicles, they must be viewed as an effective tool to combat greenhouse gas emissions, air pollution and petrol dependency . Examples of currently available BEVs are shown in Figure 3. They are Tesla model S, BMW i3, Nissan LEAF, Volkswagen e-Golf, Ford focus electric, Mitsubishi i-MiEV, and Fiat 500e. As shown in Figure 3, Tesla model S has a range of 200+ miles while all other models have a range of 100 miles or less.
Hybrid Electric Vehicles
While BEVs are propelled by electric motors only, HEVs employ both ICE and electric motor in their powertrains. The way these two energy converters are combined to propel the vehicle determines the three basic powertrain architectures called hybrid, parallel hybrid, and series-parallel hybrid as shown in Figure 2(b), 2(c) and 2(d) respectively.
As shown in Figure 2(b), in series HEVs the wheels are only driven by the electric motor which also operate as generators during breaking and coasting, augmenting the overall energy efficiency. This topology simplifies the powertrain design, since clutch and reduction gear are not necessary. Speed and torque control are carried out by controlling the electric motor only, via a very efficient power converter. The ICE’s role is charging (or recharging) the battery and supplying energy to the electric motor, always being operated at maximum efficiency. This is another strategy that helps increasing the overall energy efficiency. Series HEVs are said to be ICE-assisted electric vehicles, for obvious reasons. An ICE, one generator and one motor are one of the main disadvantages of series HEV. Moreover, as the vehicles must be capable of cruising with maximum load against a graded road, all the machines, i.e., the ICE, the generator and, of course, the electric motor, must be powerful enough, which will result in relatively over-dimensioned machines. This leads to cost increase . Commercial examples of series HEVs are shown in Figure 4 b, and include GM-EV-1 (General Motors- EV-1), Fisker Karma, BMW-i3 (with range extender), etc.HEVs of all architectures can be recharged in two very distinct ways, as shown in Figure 5: the so-called plug-in hybrid electric vehicles (PHEVs) and the conventional HEVs. While PHEVs can have their batteries recharged directly from the power grid, which is an enormous advantage, the conventional HEVs have their batteries recharged by means of the ICE. In this case, the advantage is the omnipresence of gas stations. The conventional HEVs are potentially less eco-friendly than PHEVs. While the latter can take advantage of the ubiquitous power grid, the impact they can cause to the grid is far from being negligible and depends on the way charging and discharging (as PHEVs can return stored energy to the grid) are done [7,8]. Some of the commercial examples of PHEVs are Hyundai sonata, Fiat 500e, BMW i8, Audi A3 E-Tron, Kia optima, Honda accord, Toyota prius, etc. Toyota prius and Honda accord plug-in HEVs, Figure 6. In parallel HEVs, propulsion can be the result of torque generated simultaneously by ICE and the electric motor. As shown in Figure 2(c) above, this technology provides for independent use of the ICE and the electric motor, thanks to the use of two clutches. One of the key features of parallel HEVs is that, for a given vehicle performance, the electric motor and ICE, can be significantly smaller than what is needed for series architecture, which allows for a relatively less expensive vehicle. On the other hand, wheel propulsion by the ICE leads to superior dynamic performance of this topology. A complex powertrain controller may enable up to six different operation modes:
- Electric motor on and ICE off;
- ICE on and electric motor off;
- Electric motor on and ICE on, with both of them cooperating to propel the vehicle;
- ICE on supplying power to drive the vehicle, and to drive the electric machine that, in this case, runs as generator to recharge the batteries with energy coming from the fuel tank (maximum overall energy savings can be achieved by running the ICE at maximum efficiency speed, while pumping the excess energy to the batteries);
- ICE on and dedicated to recharge the batteries through the electric machine (i.e., the vehicle is stopped); and
- Regenerative braking, with energy being stored in the batteries (or in a supercapacitor), via the electric machine.
This profusion of operation modes can be conveniently handled by the controller to optimize the driving performance or fuel savings, for example. Parallel HEVs are said to be electric motor-assisted ICE vehicles and their architecture are most appropriate for vehicles of the high-class segment and full hybrid . Honda insight, Toyota prius, Chevy volt, Honda accord, Ford fusion Energi SE, etc. are some of the commercial examples of parallel HEVs, shown in Figure 6.At the expense of one more electric generator and a planetary gear, a quite interesting architecture for the powertrain is obtained (Figure. 2(d) above), which blends features of both series and hybrid topologies, and is conveniently named series-parallel architecture. Though more expensive than any of the parent architectures, series-parallel is one of the preferred topologies for HEVs, especially when automakers target excellence in dynamic performance and high cruising speeds for their models . One of the commercial examples of series-parallel HEVs is ford escape which is shown in Figure 7.
Subclassification of HEVs
Additionally, HEVs are classified into three different categories, according to the electric motor power under the hood , as shown in Figure 8. This classification is a measure of the hybridization degree of the HEVs . In other words, it indicates how much of a role the electric motor plays in the vehicle propulsion.
Micro hybrids use an electric motor (EM) of about 2.5 kW at 12 V . The EM is only a helping hand to the ICE, for the start and stop operations of city driving. In this driving mode, energy savings is only about 5% to 10%. This is a very poor economy, obviously with a negligible impact on fossil fuel dependence, metropolitan area air pollution and greenhouse gas emissions, the challenging triad. C3 Citroen is a commercial example.
Mild hybrids use an EM of 10-20 kW at 100-200 V . The EM is only used as assistance when starting and for greater power delivery when overtaking, a concept known as “boosting”. As expected, energy savings is greater reaching about 20%-30%. Commercial models are Honda Civic and Honda Insight.
Full hybrids use an EM of circa 50 kW at 200-300 V. In city driving, this yields energy saving of 30%-50% . Thanks to complex control algorithms which manage to operate the ICE, when needed, the engine is always at maximum efficient region and directing the excess energy to batteries. Energy is also recovered and saved into the battery and/or supercapacitor, during coasting and regenerative braking. Toyota Prius is a genuine member of this family.
Thermal Management in Electric Propulsion
BEVs and HEVs propulsion systems require the development of high-power electric machines with integrated power electronics. Here, cooling remains an issue because of the dense packaging and close integration. The high-power density associated with these propulsion systems requires an advanced thermal management system (TMS). These thermal management systems control the temperature of the EVs or HEVs by technology relying on thermodynamics and heat transfer. The importance of thermal management in an EVs cannot be overstated. The performance of the EVs can fluctuate greatly depending on how warm or cool it is. A case of serious overheating can be a sever safety concern. Efficient TMS in electric vehicles propulsion significantly improve the efficiency, health, and extend the overall lifespan of electric components such as the motor, generator, battery etc. These electric components generate large amount of heat during operations, which should be managed by TMS, making them a crucial part of electric vehicles propulsion system. Keeping an EVs battery at an optimum temperature can help to preserve its capacity, optimise its length of charge and retain the health of its cells. Driving range of EVs is also a real point of interest for both industry and consumers, the greater the EVs range on a single charge, the more likely the EVs is to sell. TMS is vital to ensuring new vehicles meet those expectations. However, it is not just the battery pack that needs to be considered. Other electric components including motors, generator, etc, must also be considered.
In EVs batteries, current flow, both charge and discharge, generates heat inside the cells and in their interconnection systems. This heat is proportional to the square of the flowing current multiplied by the internal resistance of the cells and the interconnect systems. The higher the current flow the more the heating will be produced. Heat can be generated from multiple sources including internal losses of joule heating and local electrode overpotentials, the entropy of the cell reaction, heat of mixing, and side reactions . This heat should be removed from the battery pack when battery temperature reaches the optimum temperature or even in advance to avoid thermal issues. Thus, proper thermal management of EV batteries is required to maintain adequate and consistent performance of the battery and the vehicle.
Types of Cooling used in EV
There are various cooling methods are in use today as part of thermal management of EV batteries. Among these are air cooling, liquid cooling, direct refrigerant cooling, phase change material cooling, and thermoelectric cooling. Liquid cooling is shown in Figure 9.A liquid cooling system uses water glycol coolant as the thermal medium. This cooling system consists of a cooling pipe that winds through the battery pack and carries a flow of water-glycol coolant, thermal contact with the cells is through their sides by thermal transfer material. This removes heat from the side of the cells. Due to heat transfer, the cells at the end of the cooling pipe will have significantly higher temperature than cells at the beginning of the cooling passage.
In EVs propulsion systems, an electric motor also generates a relatively large amount of heat due to the operating and driving cycles. The main challenge of the TMS in electric motor is to efficiently remove this heat and maintain the motors temperature within the prescribed range while minimizing energy consumption. Thus, an efficient cooling system is required for the TMS of motors.
Similarly, like batteries, there are several cooling system strategies used in TMS of electric motors such as air-cooling, liquid cooling, heat pipes cooling, hybrid cooling with heat pipes and liquid, etc. An example of hybrid cooling with heat pipes and liquid cooling is shown in Figure 10 below.Figure 10 shows a hybrid cooling system, which features two parallel heat transfer pathways – heat pipes and liquid cooling systems. In liquid cooling system, a coolant enters from the inlet of the coolant jacket and the motor heat is removed by the flowing coolant before exiting through the outlet. In a heat pipes cooling system, as shown on the right side of Figure 10, the presented heat pipe has two sections: an evaporator (hot end) and a condenser (cold end). A heat pipe evaporator is inserted into the motor housing to absorb heat and the heat supplied to the evaporator end. The working liquid in the heat pipe vaporizes as a result of the input heat. The vapor is then pushed towards the condenser and condenses back into liquid by rejecting heat. Finally, the liquid from the condenser end returns to the evaporator via capillary forces. Fins and a centrifugal fan are added to enhance the heat dissipation rate. By adding heat pipes cooling with liquid cooling system, the workload of the liquid heat transfer pathway is reduced, and therefore minimizes the cooling system’s energy consumption.
Electric motor cooling methods should be selected based on the machine’s classification, power level and working environment. To learn more about other cooling system strategies used in the TMS of electric motors, click here to return to the previous blog on TM in electric propulsion series.
Such types of cooling systems, whether in a battery or electric motor, make it possible to remove the heat generated during various operations and help to control the thermal behaviors of each. This makes an efficient thermal management system in both batteries and electric motors. Such types of cooling flow systems can be modelled and analyzed using AxSTREAM NET™ by creating a cooling flow passage and analyzing it. Click here to see the example of how to model a cooling through convection and conduction between a hot and a cold flow in AxSTREAM NET™.
In AxSTREAM NET™, by simulating convective heat transfer between fluid flow and solid structure, we can estimate cooling or heating system effectiveness, based on values of heat flow rates, temperatures of fluid flow, and solid bodies.
To design or analyze centrifugal fans, like the one coupled to the electric motor of Figure 10, SoftInWay offers a complete turbomachinery design and analysis tool called AxSTREAM® which can generate optimized designs with less time and effort starting from the specifications. Using this tool, designers can generate thousands of designs from scratch with minimal data available. The flexibility of this software allows a person, with basic knowledge of fan design and use of design tools, to perform complex fan design and analysis tasks. An example of a centrifugal fan designed using AxSTREAM® is shown in Figure 11.
However, there is no universal architecture for EVs or HEVs that can be considered superior in all practical aspects of energy efficiency, vehicle performance and range, driver comfort, manufacturing complexity, or production cost. Therefore, in practice, manufacturers may choose different EVs or HEVs architectures to achieve different goals and meet distinct transport segment requirements.
To completely replace the world noisy ICE-based fleet by a silent EV-based fleet in the coming decades, the following barriers need to be overcome for widespread use of EVs. First, the price of EVs, mainly due to the battery cost, has to be lowered- which can be the result of present and future investigations on battery technology. Secondly, the driving range of EVs has to be significantly extended, at reasonable battery price. Finally, huge investments in infrastructure for EVs have to be carried out. It is well known that there is still a lot of space to completely replace the ICEs by a full EVs, and this is what industry, government and academics are focusing on.
For anyone who is looking to develop a new concept in turbomachinery, SoftInWay can be a strong partner. With many years of experience in automotive industry and with the aid of our AxSTREAM® platform, we can ensure the optimized propulsion system for EVs or HEVs.
Are you interested in learning about how SoftInWay can help you to develop a new concept in turbomachinery design? or/ to design or analyse a cooling flow path and fan using AxSTREAM NET™ and AxSTREAM® platform? Reach out to us at Info@Softinway.com to schedule a demo!
 Clement-Nyns, K.; Haesen, E. & Driesen, J. (2011). The Impact of Vehicle-to-Grid on the Distribution Grid. Electric Power System Research, Vol.81, (2011), pp. 185-192.ology Magazine, (June 2009), pp. 80-89.