22. May 2017 | Engineering Service

High flexibility, reduced operating costs

Range Extender Systems for E-Drives for Medium-Heavy Transport Vehicles

As part of its search for an optimized drive configuration for medium-heavy duty transport vehicles, FEV investigated the potential of serial hybrid systems compared to conventional and pure electric drives. The results show that, even using today’s realistic cost scenarios, the operating costs for serial hybrid solutions can be reduced compared to conventional drives. In order to achieve this, the energy strorage and the drive sources must be dimensioned to suite the requirements and optimized  with a view toward the typical driving profile. Such computational investigations are also a tried and tested means of determining the most suitable combination of drive solutions for a specific vehicle fleet based on requirement profiles and fleet data.  The acquisition costs of such systems will always have to be assessed against the background of the current political discussions and the dynamically changing market prices.

The growth of the global population, along with a disproportionate increase in transportation capacity, represents one of the biggest challenges with regard to sustainable mobility concepts. In particular, increasing urbanization and the simultaneous intensification of the debate regarding the reduction of pollutant emissions in metropolitan areas is leading to the electrification of commercial vehicle powertrains. For lightweight transport vehicles, such as courier service and mail distribution vehicles, there are already large numbers of corresponding, electric-only solutions, in large numbers, on the market. It is to be expected that this technology will also find its way into the heavier vehicle classes.

The combination of electric motors with an additional range extender represents an obvious solution for the typical medium-duty commercial vehicle segment, which also considers changing daily requirements with regard to the delivery routes to be served. A serial arrangement of the drive sources already offers significant potential, and can be implemented fairly easily.

Computational Simulation

As part of a simulation study based on a purely electric vehicle, FEV has examined various drives and has assessed them with regard to consumption potential and operating costs. The base vehicle was defined as a typical, two-axle distribution vehicle with an authorized total weight of 12 tons.
To dimension the purely electric drive, fleet data from a shipping company, located in the Cologne-Aachen area were used, which indicated a daily delivery route of about 100 km, corresponding to about 50% of all journeys over a period of 10 days.  The battery capacity for all hybrid drives was then calculated, resulting in a total capacity of 100 kWh in the selected driving cycle, considering 50% of the maximum payload and a usable energy content of 75% of the battery. A larger battery for a range of 200 km in pure electric operation was also investigated. Due to the torque characteristics of electric motors, an electric power of 150 kW was selected.


 

>> EVEN WITH CURRENTLY REALISTIC COST SCENARIOS, THE OPERATING COSTS FOR SERIAL HYBRID SOLUTIONS CAN BE REDUCED COMPARED TO CONVENTIONAL DRIVES

Range Extender Concepts

In addition to a 7.5-liter 6-cylinder diesel engine with a rated output of 180 kW, representing a typical conventional drive unit in this vehicle class, low-cost passenger car engines were also chosen as possible range extenders, including a 2-liter diesel engine and a 1.8-liter ottomotor with 130 kW nominal power. A fuel cell system with 100 kW electric power was also considered. Different exhaust gas aftertreatment systems and tank sizes were applied to the engines investigated, and the corresponding weights were taken into account. All systems were investigated from a design perspective with regard to packaging feasibility by means of existing data from a typical vehicle. The batteries were placed in the frame for safety reasons.  As an electric drive, two motors on the rear axle with the above mentioned total power of 150 kW were considered.

Graphic - Range Extender

Serial drive concept for medium-duty transport vehicles

Power and Fuel Consumption

Using a simulation tool that is based on a model library, the power and fuel consumption of the vehicle for the different drive variants are calculated into the Worldwide Harmonized Vehicle Cycle (WHVC), as well as the Urban Delivery Cycle (UDC). The UDC was established in various studies and also takes height profiles into account. The operation strategy foresees the initial provision of the power required by the battery until the defined minimum charge status is achieved – in this case, 20% of the full charge. From that point on, the charge status for recharging the battery is kept more or less constant by the starting of the combustion engine. The control of engine operation prevents high specific consumptions and uses the engine operating points that provide the best operating area for a specific performance. Compared to a layout with only one operating point, this operation with a changing rotational speed is acoustically much more pleasant for the driver.
In WHVC, the Diesel range extenders showed a consumption reduction of 2%, respectively 7% in charge sustaining mode, compared to a strictly conventional drive. Due to the higher specific consumption, consumption increases by 18% when a gasoline engine is used as an additional drive source. In this context, it should be noted that the consumption of the conventional drive with an additional weight equivalent to the electrical components increases by around 7% in WHVC. If this is used as a basis, the consumption of the hybrid systems is, accordingly, more cost-effective. The comparison with the results in the Urban Delivery Cycle, which are characterized by a lower share of high speeds, more frequent acceleration and braking processes, as well as a height profile, shows a higher basic consumption, but also offers more significant savings potential in hybrid operation. Overall, the power to be used for the cycle is less than what electric-only operation shows.


 

>> IN WHVC, THE DIESEL RANGE EXTENDERS SHOWED A CONSUMPTION REDUCTION OF 2% RESPECTIVELY 7% IN CHARGE SUSTAINING MODE

Calculation of Costs for Daily Operation

Generally speaking, the concepts must be assessed in real operation, using the daily driven distance, with regard to the running costs and the power requirements of the vehicle. As part of the FEV analysis, the pure power costs are used, but initial costs, depreciation and wear costs for the systems are not taken into account. In order to take the currently prevailing uncertainty with regard to future energy costs into consideration, the costs for fuel and power are reflected in two scenarios. In the first scenario, moderate fuel prices based on average prices between 2011 and 2016 are used, while the power price is based on the household price. In the second scenario, the maximum fuel price from the period considered is used, while a reduced power price for industrial clients is applied as an alternating classification of fleet operators can be predicted with an increasing number of electrified vehicles.

Results

The costs for the conventional, or electric-only, drive increase linearly with the distance driven, as per the respective cost scenarios. This changes upon switching to serial hybrid operation: starting at a daily distance driven of 100 km, the cost gradient decreases in accordance with the calculated fuel consumption and power costs.
Within the delivery distance considered (that of up to 200 km per day), the operation costs for serial hybrid operation with Diesel engines are always cheaper than those of a conventional engine. With gasoline engines, however, due to the higher power prices and the higher specific consumption, there is also a price disadvantage in scenario 1. The operation costs for a fuel cell, due to high hydrogen prices – which were kept constant at EUR 7.70/kg in both scenarios – are higher than basic operation, starting at 125 km, respectively 150 km.

 

Graphics - Range Extender

Fuel and power consumption of powertrain variants


Table - Range Extender

Fuel and power consumption of powertrain variants

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