1.2 Overview of different electric propulsion systems

Propulsion systems for vehicles can generally be classified into conventional systems powered solely by an internal combustion engine⁶ and electric propulsion systems. In the following, all propulsion concepts that can cover at least a portion of the distance propelled purely by electricity are classified into this second class, no matter whether they receive the energy from a battery or from a fuel cell system (Spath et al., 2011).

The variety of electric propulsion systems is big and can either be differentiated by the range of hybridization (micro-⁷, mild, full-, or plug-in hybrid) or the architecture of the drivetrain. For the architectural view, the configuration of the powertrain components, that is, the combustion engine, transmission, electrical motor (including power electronics), generator, or battery is important (Hofmann, 2010). Based on the configuration, the three types—serial, parallel, or power-split hybrid—can be differentiated. Figure 1.2 shows the range of electric propulsion systems.

In the following, we look at the different drivetrain architectures (Braess 2013; Hofmann, 2010; Spath et al., 2011; Spath et al., 2012; Wallentowitz, 2010).

1.2.5 Fuel cell vehicle

The propulsion concept of the fuel cell vehicle (FCEV) in comparison to the previously discussed electric drive systems is that hydrogen is used as the energy source stored in a special hydrogen tank. The fuel cell system works as an energy converter supplying the electric motor. The traction motor in existing fuel cell vehicles is in the range between 30 and 100 kW. This propulsion concept has a smaller dimensioned battery (1–2 kWh) for the function of recuperation.

As evidenced by the large variety of different propulsion systems, there is not just one right application. In terms of vehicle concepts, there is a difference between vehicles of personal transport such as passenger vehicles or two wheelers, public transport vehicles including buses, or off-road vehicles such as different types of mobile working machines. For each type of application, different propulsion systems have to be analyzed for their suitability. Figure 1.3 demonstrates one alternative way to determine the fit between propulsion system and application. Therefore the two dimensions of “driving profile” and “load profile” have been chosen.

As we have already seen, there is not just one right solution for tomorrow’s mobility. The existence of the different propulsion systems and their use in various applications require extensive technological knowledge, which has to be further developed in many ways.

1.3 Advantages and disadvantages of electric vehicles

Electric mobility can be regarded as a cross-industry development. Although the combustion engine technology is fully industrialized, electric propulsion systems for the use of the automotive and off-road industry are still in a very early stage of their industrialization. In addition to the changes in the key components of a vehicle, several more aspects have to be considered when talking about the advantages and disadvantages of electric vehicles. In that context new ways of energy consumption and supply have to be discussed. The current strong dependence on fossil fuels is one of the most important issues when talking about future mobility. To increase the amount of renewable fuels, electricity based on renewable sources and hydrogen can be considered to be suitable alternatives (Thiesen, 2011). Additionally there are new research fields and activities regarding information technologies and the connectivity of vehicles and their surroundings which can benefit from the latest developments within electric propulsion systems. Together they could provide interesting mobility solutions to commonly known and conventionally driven vehicles.

Considering electric vehicles in general, there are some quite obvious advantages as well as disadvantages when comparing them to the original solutions based on conventional combustion engines. The functional principle of the electric propulsion system provides some clear advantages to this solution when it comes to the efficiency of the system. Compared to systems based on combustion engines, electric propulsion systems work far more efficiently. Some electric motors can reach an efficiency of up to 95% or more, which is about three times higher than the efficiency of a combustion machine (Spath et al., 2011). On the other hand, one of the big disadvantages is the relatively heavy weight of the electric propulsion system, especially in the case of the hybrids. The electric machine (and, in some cases, the electric generator), the power electronics, and the heavy battery system combine for more weight than the combustion engine. As a compensation for the heavy battery system, however, is another advantage to the storage device—its rechargeability. This can be achieved either through the recuperation function or through the charging option via the grid. An advantage when evaluating the comfort or the drive characteristics is the outstanding acceleration and power transfer of the electric system over the conventional system. The relatively low energy density of today’s batteries, which results from the complex development in battery science, is one main disadvantage. Improvements have been achieved in the last couple of years in the fields of material, cell design, and processability.

The costs of electric vehicles remain a disadvantage in the current development state owing to the early level of industrialization, limited product range, and overall low market sales. The key components of electric vehicles, especially the battery systems, are very expensive. Much work remains to be done in the assurance of quality aspects in existing production sites. However, the acquisition cost of the electric vehicle is not the only criteria when it comes to evaluating the economic feasibility of the vehicle. Life-cycle costs also have to be taken into account. Lower maintenance costs and reduced efforts for energy are big advantages when considering all costs of the vehicle over its lifetime.

Another benefit is the local-free emissions of electric vehicles. Especially in urban areas, this is one of the major advantages; they reduce pollution in cities and lead to an environmentally friendly image. But to achieve the full potential of electric propulsion systems, the energy mix in today’s power generation has to be changed to a more sustainable system by increasing the amount of regenerative energy sources. Additionally the carbon footprint of vehicles over the entire product life cycle has to be monitored and optimized regarding the CO₂ emissions in the process chain, only than electric vehicle concepts can use their potentials (Sammer et al., 2011; Spath et al., 2011).

Table 1.1 summarizes the advantages and disadvantages of electric vehicles. This list does not encompass only the advantages or disadvantages of a purely technical nature but also includes ecological and sustainability characteristics.

In addition to the more general advantages, there are clear benefits for electric road and off-road vehicles when regarding different applications in their typical use. One example is in the use of electric vehicles in the delivery business in urban areas. Because the maximum range of the electric vehicle isn’t reached when only local deliveries are made, there can be some quite good fuel savings. In some cases, they can reach a cost savings of approximately 20% as compared to a diesel equivalent. Other benefits are the fast acceleration in urban traffic and the potential for lower maintenance costs because of fewer mechanical parts in the propulsion system (Department of Transport, 2010).

1.4 Applications in the field of electric road and off-road vehicles

As there is a large variety of electric road and off-road vehicles, the following chapter gives an overview of both categories in terms of characteristics and known applications. In the European Community there is an enormous set of vehicle categories defined on the primary basis of their technical requirements. The overall aim, therefore, is to support activities in terms of registration, sale, and entry into service of vehicles within the European Community (EG, 2007).

(Electric) road vehicles can be defined as any type of vehicle designed to drive in public transport. The most important categories are passenger cars for individual transport, motorcycles, commercial vehicles, and buses. For each category numerous electric alternatives are in development or even on the market. We have already described existing vehicles in the passenger car market as types of hybrid-, pure electric-, or fuel cell propulsion systems.

Currently, commercial vehicles are widely used in numerous fields of action. Clearly some of them would benefit from the new developments in electric drives. Especially in refuse collection or other municipal activities, hybrid or even pure electric drives are alternatives for commonly used trucks based on the internal combustion engine.

In the case of the bus there are also various forms of hybrid propulsion systems. In addition to the classic drivetrain architecture with a central electric motor, there are alternatives with electric hub motors. One product example in public transport is the Mercedes-Benz Citaro G BlueTec Hybrid (Mercedes Benz, 2013; Figure 1.4).

In the field of electric road vehicles there are previously described concepts, some more or less new applications, which cannot be put easily in one of the defined categories. In former times the European Commission subdivided the market of passenger cars, for example, into various segments starting from segment A (so-called mini-cars) ending at segment J (sport utility cars) (Commission of the European Communities, 1999). New electric vehicle concepts such as the Renault Twizy⁸ do not really fit into the categories for motorcycles or into the passenger car category and initiated a discussion about the need of a new vehicle segment. Another example is the vehicle concept of the so-called pedelecs.⁹

Compared to the category of road vehicles, the majority of the regarded information sources define the term off-road vehicles as any type of vehicle that is designed to be capable of driving off of public roads, especially on rough terrains such as unpaved roads, trails, and beaches. Off-road vehicles do not include only quads or buggies, but also forklifts, cranes, or wheel loaders and several more (also called mobile working machines) (Beck and Grünbeck, 2012).

These mobile working machines are essential to many industries such as forestry and agriculture and also include specialized urban vehicles (Nijs, 2013). Especially in the agriculture sector, the developments for several applications are strongly influenced by the upcoming exhaust-emission standard Tier 4. To fulfill the necessary requirements new solutions to reduce CO₂ emissions are being developed. Agriculture machines with increased electric power (up to 50 kW) are being discussed. One efficient solution for an electric system is a power take-off shaft generator that can bridge the gap between the mechanical tractor and the electric device (Herlitzius, 2013).

Compared to the development of electric road vehicles including passenger cars or trucks, some of the aforementioned off-road vehicles such as forklifts already have a long history of using electric propulsion systems. This current state is a result of the need to avoid exhaust gases inside buildings, such as factories. The trend of integrating electric propulsion systems can also be seen in off-road vehicle applications other than just forklifts.

The combination of a diesel engine and an electric engine inside a wheel loader, for example, leads to a reduced fuel consumption and exhaust emissions. Also, the combustion engine can be downsized and the fuel savings estimated at up to 30%. Generally hybrid drives are ideal for applications with low average power but high peak requirements for a short time. For that reason, a mild hybrid drive can also be used for other off-road vehicles such as excavators, cranes, or airport tow trucks. All of the mentioned vehicles are distinguished by frequent acceleration, short travel distances, and high load peaks. Through these characteristics hybrid systems are able to support the combustion engine by engaging the electric motor as soon as power peaks occur (Figure 1.5a). On the other hand, this sort of off-road vehicle also tends to break down frequently. As a result, excess energy, induced by the braking operation, can be fed back into Li-ion batteries (Geimer and Synek, 2013; Brun, 2008). The concept of using a mild hybrid for an off-road vehicle is generally familiar ever since the first electric hybrid wheel-loader entered the market (Equipment World, 2013).

Hybrid drive systems that can be used in wheel loaders or tractors, however, will probably not be a 100% suitable alternative propulsion technology for machines under a full load, such as drilling vehicles, agricultural tractors, or harvesters, because they operate mostly under a full load. In hybrid drive systems for these types of off-road vehicles the electric engine could provide energy to side aggregates, which would originally have been supported with energy by the combustion engine. As a consequence, off-road vehicles under constant and full loads could almost run at their best engine operating point (Figure 1.5b) (Geimer and Synek, 2013; Albers et al., 2008).

Inasmuch as the movement of cargo is increasing permanently between countries and continents, ports will have to be expanded and developed. At the same time, limited exhaust emission standards are being set. In this regard, a wide range of vehicles used at ports (e.g., straddle carriers, reach strackers, terminal tractors, forklifts, mobile cranes container handlers, warehouse stackers, or automated guided vehicles [AGVs]) can also be equipped with hybrid drive systems. In some cases, these vehicles are already available. For example, the Los Angeles Port and the Hamburger Hafen und Logistik AG are investing in emission-reducing and sustainable technologies. In addition to the benefit of fuel savings of up to 30% or more compared to the origin solutions with a combustion engine, these organizations will also benefit from the hybrid system in the forms of lower noise emission, higher lifetime of the components, lower maintenance costs, and a higher availability. Compared to the number of hybrid off-road vehicles inside the forestry and agriculture industries, the range of hybrid mobile working machines for ports is quite comprehensive. Other industries may have the opportunity to learn from the development of hybrid mobile working machines at harbors (Geimer and Synek, 2011).

Although there is already a number of applications, the development of integrating hybrid propulsion technologies inside off-road vehicles has just begun. Until lithium-ion batteries are able to fulfill the required demands for performance, short-time energy storage solutions such as “Supercaps” and hydraulic hybrid systems will be used in hybrid solution systems for off-road vehicles (Geimer and Synek, 2011).

1.5 Conclusion

A wide variety of electric propulsion systems are available as alternatives to commonly known combustion engine-based systems. In the field of electric road and off-road vehicles, numerous applications can benefit from this kind of propulsion system. These applications are not only for the use of passenger transport but also for commercial vehicles and off-road vehicles. In addition to the well-known advantages in the use of an electric concept, there are several deficits in current technologies. For the battery system to be a key component of the electric drive train, it has to fulfill several requirements and be adapted to the different applications. After this short overview to the range of electric drives for different applications, the following chapters will give a closer look at the key research fields of battery technologies for electric road and off-road vehicles.