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Part of Elica's Wellness Outside Walls (WOW) initiative is the Health on Wheels (HOW) mobile medicine program, featuring state-of-the-art mobile health clinics with full exam rooms that provide services to two primary groups: hard to reach populations with barriers to accessing care and underserved students at select schools within Elica's service area.

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HOW mobile clinics visit area schools to provide primary care and related services, including vaccinations, sports physicals, and more. We do this by invitation from the schools or through formal agreement, with the San Juan Unified School District, where we provide services four days a week at select campuses during the school year.

Already enacted carbon-dioxide (CO2) limiting legislations for passenger cars and heavy duty vehicles, drive motivations to consider electrification also in the sector of non-road mobile machinery. Up to now, only the emissions of the vehicles themselves have been restricted. However, to capture the overall situation, a more global assessment approach is necessary. The study described in this article applies a tank-to-wheel and an extended well-to-wheel approach based on simulations to compare three different powertrains: a battery electric drive, a parallel electric hybrid drive, and a series electric hybrid drive. The results show that electrification is not per se the better solution in terms of keeping CO2 emissions at a minimum, as battery electric powertrains are accountable for the lowest as well as the highest possible CO2 emissions of all powertrains compared. A battery electric machine is not economically competitive if its battery has to last a whole working day. Parallel hybrid systems do not achieve much of an advantage in terms of CO2 emissions. In this global assessment approach, the most promising propulsion system for wheel-driven-mobile-machinery appears to be the series hybrid system, which shows to offer up to 20% of CO2 saving potential compared to the current machine.

It is most likely that ongoing discussions about air quality in European cities will lead to a further lowering of allowed emissions by law. Possible scenarios include emission-free protection zones in and around urban areas. These measures will eventually also affect the applicability of mobile machinery.

Another fact motivating the present study is the recent development towards an additional CO2-limiting legislation. Since 2019, the certification process of heavy duty vehicle includes the assessment of emission values according to EU VI as well as a vehicle-individual evaluation by a longitudinal vehicle dynamics simulation software (VECTO = Vehicle Energy Consumption Calculation Tool) [2]. By 2025, this should result in CO2 reductions of 15% compared to the reference year 2019, and to minus 30% by 2030. This in turn limits the energy consumption of heavy duty vehicles, because the fuel consumption directly correlates with CO2-emissions. Thus, technology that is accounted emission free is necessary to achieve a high number of CO2 reduction. Typically, the legislation for mobile machinery lags behind the emissions legislation of heavy duty vehicles. By not taking into account the energy carrier production, however, current legislation follows a simplified approach, called tank-to-wheel, in the assessment of emissions. The consequence of this simplification is that electric- and hydrogen-powered drives account for zero emissions, even though they both in fact cause emissions during a complete life cycle. Depending on the production process of the energy carrier, the according emissions vary considerably. Therefore, this study argues for a consideration of the total emissions, which requires at least a well-to-wheel or better a cradle-to-grave analysis to reveal the powertrain with the lowest emissions.

First insights into fuel saving potential of electrified mobile machinery powertrains can be gained by looking at already published studies. According to current literature, fuel savings between 5 and 40% could be achieved, depending on the powertrain structure and the machine usage [3,4,5,6]. Independently of the powertrain structure, a more cyclic working process (frequent successive acceleration and braking or load lifting and lowering) of the machine leads to a higher energy recovery potential and, consequently, higher fuel saving potential. Of course, this potential still needs to be exploited by a well-adapted operating strategy and layout of the vehicle. A prerequisite for recuperation, however, is the use of a rechargeable energy storage system.

In the present study, focus is laid on battery electric and partially electrified powertrains for wheel-driven construction machinery, since the potential of recovering energy is considered particularly high with such machines. For example, potential energy can be recovered when lowering a load and kinetic energy during braking. In this way, the total energy demand can be reduced. The investigation focuses on machines with a dominate share of traction-drive use during operation including wheel loaders, industrial trucks, mobile excavators, dump trucks, telescopic handlers, and dumpers (Fig. 1).

The method applied in this study is based on the idea of a backward calculation using stationary efficiency maps of each energy converter to gain the overall energy consumption, but was further developed to suit the study of mobile machinery. The most important difference to be considered compared to other vehicles like cars is that beyond wheel propulsion, the propulsion of the work hydraulic system also needs to be considered. For mobile machinery, an analysis of the drive train itself through a typical simulation of longitudinal vehicle dynamics based on velocity and gradient over time would only account for isolated driving operations. Mobile machinery, however, often uses the work hydraulic system and the drive train at the same time; as an example, one can think of the filling of the bucket of a wheel loader. At the moment the bucket touches the ground and enters the pile of bulk material, there is additional friction between the bucket and the ground, which is not covered by a simulation of longitudinal vehicle dynamics. The present study covers phenomena like this by carrying out simulations based on measurements gained from a real machine operating in different typical transient working situations.

The material handling machine studied has a broad field of application. The most representative working task, a Y-cycle, was chosen for the comparison of the different electrified powertrains, because it requires wheel propulsion and additionally addresses the usage of the work hydraulic system. It is called Y-cycle, because when loading material from a pile into a transport vehicle by a material handling machine, normally, a Y-shaped track is driven. In a Y-cycle, dynamic measurements were performed by Liebherr Werk Telfs GmbH. In this study, the data gained are used to simulate electrified drive train topologies and to compute the power at the red points in Fig. 2. This set of measured/computed power is the basis for the MATLAB-based simulations carried out in this study.

The current hydrostatic powertrain propels the wheels with a series arrangement of a hydraulic variable axial piston pump and a variable axial piston motor (Fig. 2). The hydraulic power \(P_\text {Hyd}\) between these two energy converters is calculated based on volume flow \(\dot{V}\) and pressure difference \(\Delta p\) for each time step:

In a first step, the effective power at the cardan shaft \(P_\text {HM,eff}\) is computed incorporating efficiency maps of the hydraulic motor (arrow nr. 1 in Fig. 3). The effective power at the cardan shaft \(P_\text {HM,eff}\) represents the reference power for all considered electrified powertrains and is therefore of special interest for the following investigations. All powertrains considered must produce the same effective power at the cardan shaft. The wheels and axles are assumed to remain unchanged. The calculation is done based on an efficiency map and is performed for every measured data point by interpolating the value of the efficiency \(\eta _\text {HM}\) depending on the pressure difference \(\Delta p\), the swivelling angle, and the rotational speed of hydraulic motor. It has to be noted that the following equations only represent the propulsion case:

\(P_\text {DT-sH}\) is obtained by dividing the effective power at the output shaft of the hydraulic motor \(P_\text {HM,eff}\) (see Eq. 2) with the total efficiency of the electric drive train \(\eta _\text {el,tot}\). Using the power \(P_\text {HM,eff}\) secures that the same effective power is used to propel the cardan shaft and thus the wheels:

\(\eta _\text {el,tot}\) is the calculated efficiency of the two electric motors arranged in series: one working in most of the time as generator, called working motor (WM), and the second as propulsion motor for the wheels, called traction motor (TM). In the calculations, the power electronics is taken into account, as well:

The recuperation power \(P_\text {Recup}\) in Eq. (25) is obtained in a similar way as for the parallel hybrid (see Eq. 14). However, two important values are changing, the maximum recuperation power \(P_\text {EM,max}\) and the mean efficiency of the hydraulic motor \(\bar{\eta }_\text {HM,mean}\) which is assumed to be 74%. Additionally, instead of multiplying with the efficiency \(\bar{\eta }_\text {HP,mean}\), as in case of the parallel hybrid, the measured hydraulic power is divided by the mean efficiency of the hydraulic motor \(\bar{\eta }_\text {HM,mean}\). \(\bar{\eta }_\text {HM,mean}\) is used, because the point of recuperation moves closer to the wheels and with that the efficiency chain shortens. More recuperable power than with a parallel hybrid is possible, because the traction motor is sized for its maximum propulsion power (here 70 kW). For short periods, an overload factor of 2 is assumed to be possible, resulting in the value of \(P_\text {EM,max}\) = 140 kW: e24fc04721