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accomplished through several means, including in-cylinder combustion optimization and exhaust gas recirculation, without exhaust after-treatment systems for Tiers 1–3. With the addition of exhaust after-treatment systems for the Tier 4 interim stage, some engines require diesel exhaust fluid to catalyze pollutants in the system (e.g., urea). Some manufacturers claim as much as 5% greater fuel efficiency for their Tier 4 interim engines compared with Tier 3 models [14]; however, these entail increasing complexity, dimensions, and maintenance costs. Although most construction and agricultural vehicles include a driving mode tractor as a primary power unit, most modern models provide power for implementing a power takeoff (PTO) shaft and/or fluid power hydraulics. Moreover, working machine engines can stay idle for a notable amount of time [15]. Advanced engine controls are being introduced to reduce fuel consumption by lowering engine idle speeds and even shutting off the engine during extended idle periods. Examples of these strategies are found in existing patent applications, which indicate intentions of further development of these strategies [16]. Hybrid electric pro- pulsion systems allow the combustion engine to operate at maximum efficiency and ensure both a considerable reduction of pollutant emissions and an appreciable decrease in energy consumption. Over the last few years, many configurations of hybrid propulsion systems have been proposed, some of which are also very complex. The fuel efficiency in this operat- ing mode is greater than in a conventional machine for the following reasons: • the fuel and energy consumption is limited only to the vehicle work time; • the electronic control selects the engine speed to minimize fuel consumption depending on the state of charge of the batteries and the vehicle power demand; • the power transmission from the electric motor to the gearbox ensures greater energy sav- ings compared with hydraulic power transmission; • the electric motor acts as a power unit to charge the batteries, while the vehicle is slowing down/stopping. The automotive field has the largest number of studies, published patents, and proposals for hybrid and electric vehicles. Recently, intensive research has been carried out to find solutions that will enable the gradual replacement of the conventional engine with a highly integrated hybrid system. In the construction and agricultural working machines field, the number of concepts is limited and sporadic, and only recently has the market shown great attention to these studies. Thus, hybrid architectures allow the development of work machines character- ized by high versatility and new features. Such machines can be used both indoors and out- doors because they can operate in both full electric and hybrid modes. The advantages to end users are reduction of running cost due to greater fuel efficiency and use of electric energy, and better work conditions due to low noise emissions. From a system engineering point of view, the different solutions are described by introducing a specific hybridization factor suitable for work vehicles that include two main functionalities: driving and loading. The high-voltage electrification of work vehicles is also currently under development [17, 18]. According to Ponomarev et al. [19], in order to be competitive, manufac- turers should offer energy-efficient and reliable hybrid vehicles to their customers. Compared with automobiles, the introduction of electric drives in work vehicles would allow expanded Trends and Hybridization Factor for Heavy-Duty Working Vehicles http://dx.doi.org/10.5772/intechopen.68296 5