**Abstract**

Recent regulations on pollutant emissions have pushed working machines manufacturers towards research and development efforts to meet the strict limits imposed. For a long time, the use of gas aftertreatment systems have been the most widely accepted solution to reduce the amount of pollutants produced per unit of work done. However, lower emissions limits lead to larger systems and consequently higher difficulties in vehicle integration. Thus, alternative solutions have been studied in the last years to solve the emissions problem using wisely the on-board space. Hybrid electric technologies represent a valuable alternative in this direction. In this work, a review of the current state of the art in the adoption of hybrid and electric technologies on working vehicles is proposed. Due to the high amount of application fields and concepts for special applications, the analysis focused on the three major fields which however includes most of the working machines: Construction, Handling and Agriculture. This work highlights how the requirements of each specific field, strongly affects the design of an optimal hybrid electric architectures.

**Keywords:** Construction machinery, Handling machinery, Agriculture machinery, Hybrid electric systems, Energy saving

### **1. Introduction**

Worldwide, air quality is now recognized to be affected at different levels by each human activity field [1–4]. The transportation field is generally addressed as one of the major contributors to air pollution. However, residential and commercial heating as well as industrial processes [5, 6], play an important role when it comes to CO2, NOx and particulate matter production.

Transportation covers a wide range of vehicles categories, from light/heavy duty road transport up to railway, maritime or aviation transport of people or goods. Each of them affects in a different way the total production of some pollutant elements. This is the reason why regulations have been imposed in the last years to force vehicle manufacturers to satisfy certain quality standards in terms of pollutants production. Passenger cars and light duty vehicles have now emissions levels way lower than two decades ago [7] but there is still room for further improvements. New technologies have been developed to properly treat exhaust gas and to

increase the overall vehicle efficiency. In this direction, hybrid and electric vehicles have demonstrated to be a realistic alternative solution for the near future. Lower footprints on CO2 emissions have been measured in Real Driving Scenarios with Portable Emissions Measurement Systems (PEMSs) [8] but still open is the discussion on the production of other pollutants like NOx and Particulate Matter (PM) [9]. However, proper control strategies of a hybrid power unit can reduce the overall emissions with respect to traditional thermal engine powered architectures [10].

If on one hand the electrification process is a well-established trend in automotive, there are also other fields of application where this technology is demonstrating its capabilities as alternative propulsion system for traditional powertrains. This is the case of Non-Road Mobile Machineries (NRMM), vehicles which can be used both for transportation and for heavy industrial works. According to the definition given by the EU regulations [11–15], machines can be classified as NRMMs if they are used in construction, handling, agriculture and farming, forestry and gardening. However, also railcars, locomotives and inland waterway vessels fall within the given definition, although they represent a totally different segment of vehicles. Several studies have shown that due to the high level of resources invested to improve road transports' emissions, NRMM are becoming a not negligible source of pollutants [16–18]. Historically, these machines have been equipped with high power Diesel engines known for their high efficiency, durability and reliability. However, despite of the high performance over total cost of ownership (TCO) ratio for this type of propulsion technology, diesel engines have been addressed by many researchers and non-academic authorities as one of the greatest contributors to air pollution [19, 20]. Exhaust gas aftertreatment systems have been widely used by OEM engine manufacturers to meet international emissions regulations and adopted by NRMMs' companies which use these systems to power their machines. However, the stricter regulations have become, the higher the volume required by aftertreatment systems to properly filter the exhaust gas stream from dangerous pollutant elements [21, 22]. The on-board volume required to install these filters is space dedicated only to exhaust gas elaboration. This space can be considered as a dead volume from the productivity point of view, in terms of space used to add functionalities to the machine. This is one of the reasons why several NRMM manufacturers are looking for proper alternatives to standard Diesel-based propulsion systems. Hybrid electric architectures represent a viable solution to increase the overall efficiency of the machine [23]. The hybridization level of the architecture [24, 25] optimized to the specific working cycle helps in reducing the amount of pollutant produced per Unit of Work performed [26, 27]. At the same time, the more sophisticated technology involved in these architectures allows to add extra functionalities to the machine, opening new working scenarios to the same machines.

This work aims to give an overview about the electrification process that is involving the field of working off road machines. Starting from the definition of the basic architectural topologies and their comparison within the scope of off-road heavy-duty applications, an analysis of the proposed concepts and products during the last two decades is shown. Since the NRMM classification cover a wide range of working fields, this review focused the attention on the three major working fields: Construction, Handling and Agriculture. All the considerations developed for these fields of application can be extended to other specific projects.

#### **2. Basic hybrid and electric architectures**

Vehicle electrification involves the adoption of electric and electronic components within a mechanical system to provide power as a primary source or together *A Review of Hybrid Electric Architectures in Construction, Handling and Agriculture Machines DOI: http://dx.doi.org/10.5772/intechopen.99132*

with other power units [28–32]. Combining properly these components, many powertrain topologies can be obtained allowing for a high level of performance optimization. This characteristic is crucial when it comes to NRMMs, where each field of application requires special custom solutions. Thus, the knowledge of the load characteristics and of the working cycle of each type of machine is mandatory to design an optimal architectural solution [33, 34]. Looking closer at the structure of all the possible electric topologies, three basic functional schemes can be identified: full electric, series hybrid and parallel hybrid [35–39].

#### **2.1 Battery electric vehicles**

From a system point of view, a full electric architecture is the simplest solution when it comes to powertrain electrification. As shown in **Figure 1**, a full electric architecture consists of a single power source used to drive all the possible mechanical loads applied to the vehicle: the driveline, the hydraulic system and all the PTOs (Power -Take-Off s) are electrically driven. By means of an electronic converter, the electric energy previously stored in a battery pack (DC voltage and current) is regulated to provide alternate voltage and current (AC) to the installed electric motor (EM) [40, 41]. One or more electric machines can be used to optimize performance of the specific machine [42, 43].

The lower number of moving parts involved in electric machines increases the powertrain mechanical reliability when compared to Internal Combustion Engines (ICE) [44, 45]. Nowadays, power converters represent a well consolidated technology. If temperatures are well managed with proper cooling solutions, aging mechanisms related to thermal cycles can be mitigated leading to good reliability over the entire life of the vehicle [46–48]. Full electric architectures would probably replace all modern powertrains solutions if the available energy storage systems (ESSs) would perform better under different aspects [49, 50]. The most developed and promising ESSs for vehicle applications are based on Lithium-Ion Batteries (LiB) [51–54]. In terms of energy density, the ratio between 1 liter of Diesel fuel (≈10.9 kWh) and 1 liter of LiB (≈0.25 kWh) is currently 44. Considering an average conversion efficiency of an ICE (≈30%) and of an electric system composed by an electric motor and its power converter (≈85%), the gap reduces to 15 times, still too shifted in favor of thermal engines. If a proper battery pack design can satisfy the energy needs of a machine typical working cycle, still precautions are required to safely use the stored electric energy. LiB manufacturers prescribe a Safe operating Area (SoA) [55–58] in terms of temperature ranges and power limits where the chemical stability of each single cell is usually guaranteed. To operate in the SoA, a proper cooling system must be considered to avoid undesired thermal runaway phenomena which could damage permanently the entire battery pack [59–64]. Moreover, a proper Battery Management System must be designed to continuously monitor cells behavior, to avoid them to work outside their voltage limits both during charging and discharging [65–67]. This approach is necessary both for short and

**Figure 1.** *Full electric architecture for a working vehicle.*

long term performance analysis based on the State of Charge (SoC) [68–72] and State of Health [73–77] estimation using different modeling technique and specific testing activities [78–81]. If proper care of the battery pack working conditions is guaranteed, chemical aging mechanisms can be stemmed achieving a total life of thousands of cycles depending on the specific battery chemistry [82–84].

Battery electric vehicles represent today a promising alternative to traditional thermal powertrains. The actual state of the art LiB technology suggests that with proper design strategies this solution could be a suitable choice to propel also NRMMs. However, the actual cost [85–88] of the commercially available LiB solutions prevent from the widespread adoption of this type of architecture. New battery chemistries promise to increase the average LiB energy and power densities which could place this architectural solution closer to traditional powertrains.

### **2.2 Parallel hybrid electric vehicles**

In a parallel hybrid electric vehicle, the power coming from an ICE and from an EM is mechanically combined to satisfy the power demand from all the different mechanical loads. This architectural solution allows to satisfy the same peak power demand of a traditional powertrain with a smaller ICE. This is called engine downsizing [89, 90] and is particularly useful when the average power demand is consistently lower than the peak power capability of the thermal engine. This is a very common problem especially in NRMMs where their multipurpose nature prescribes high power engines to satisfy all the possible loading scenario the machine might face during its operating life. Thus, the oversized engines usually work far from their nominal working conditions leading to higher fuel consumption. Using an electric machine coupled to a smaller engine, it is possible to cover the average power demand with the thermal unit and the peak power with the boost given by the electric system (**Figure 2**).

The parallel hybrid topology increases the overall efficiency of the vehicle requiring less amount of fuel per unit of work performed [91–93]. Moreover, the use of an EM and the fast response of the electronic units allow to quickly accommodate rapid variations in the external mechanical load. On the other hand, the topology intrinsically reduces the level of optimization on the ICE operating point. The mechanical connection between the engine and the external load does not allow it to work in the most efficient conditions. The rotational speed required by the application is intrinsically related to the actual engine speed so if the load requires a specific operating speed due to the limited amount of gear ratios of the transmission, the engine will rotate at a speed different from the optimal one. Thus, on this type of architecture, at least one clutch and a gearbox are required. The presence of these components increases the efforts required to integrate an external electric system on an existing vehicle layout.

**Figure 2***. Parallel hybrid architecture for a working vehicle.*

*A Review of Hybrid Electric Architectures in Construction, Handling and Agriculture Machines DOI: http://dx.doi.org/10.5772/intechopen.99132*
