Ensuring electric commercial vehicles are cost-effective
Mike Savage, chief engineer, Drive System Design (DSD) discusses the measures that can be taken to implement the electric powertrains in Indian commercial vehicle segment.
Indian commercial vehicle manufacturers are under increasing pressure to switch to electric power in order to reduce the harmful emissions which contribute to poor urban air quality. The authorities are using a ‘carrot and stick’ approach. The carrot is exemplified by the recent FAME India announcement which will incentivise EV purchases and initiate the setting up of the necessary charging infrastructure. The stick comes in the form of restrictions such as the recent ban in Delhi on diesel engines over 10 years old, by the National Green Tribunal.
The biggest obstacles to sales of commercial electric vehicles (CEVs) are concerns over the high initial cost and subsequent vehicle range limitations.The battery pack remains the dominant cost driver, for example a 207kWh lithium-cobalt oxide battery to provide a 200km+ range for a 13 tonne GVW medium duty truck currently costs approximately $58,000 (Rs 40 lakh) and weighs 1.25 tonnes. By comparison, a typical Indian diesel truck of a similar GVW costs around $29,000 (Rs 20 lakh).
The solution to these concerns is likely to come from a systems approach to the design of future CEVs, in which the electrified powertrain is optimised for cost and range as a complete system, rather than focusing on the individual elements. In addition to this, by matching the powertrain characteristics to the particular requirements of the Indian market, significant cost savings could be possible without compromising the required operating range.
Typical duty cycles for a commercial vehicle in India involve much lower speeds and range than in other markets and, class for class, significantly smaller engines are used, which implies the acceptance of lower outright performance levels. These parameters can be used to good effect in the optimisation process to tailor solutions specifically for local requirements, for example, the rating of the motor and inverter, simplifying the transmission and reducing the battery capacity.
Although initial purchase price remains the key factor for the Indian CV market, the purchase of a commercial vehicle is usually based on the total cost of ownership (TCO). When comparing the TCO of a CEV to a conventional diesel truck or bus, it requires higher initial investment but this is offset by lower running costs. In order to bring the numbers to a point which favours the CEV, the initial cost must be driven down without sacrificing the benefits in operating costs.
Optimisation techniques, processes and tools, such as those developed at Drive System Design, quantify the impact of the various subsystems on overall electric powertrain performance using a systems approach. This provides insight into the interactions between different subsystems within the powertrain, and highlights what design considerations are most influential.In a recent study, DSD simulated over 4000 different powertrain permutations to find the optimal configuration for oneparticular application, a 13-tonne medium duty truck.
Specifications of the powertrain were derived by computing the vehicle resistive power during a range of operational use cases, calculated using models based in MATLAB/Simulink. The vehicle model used road load equations to account for the vehicle inertia, rolling resistance, aerodynamic drag and gradient to calculate the power and torque required at the wheels, verified against measured data.
Based on the vehicle’s functionality, hypothetical targetsprovided a basis from which the power and torque required were calculated, for example using top speed targets, minimum gradeability and minimum range required.Actual range was thendetermined by calculating the energy consumption during the drive cycle, in which the vehicle is subjected to a range of accelerations and decelerations, based on the battery capacity and opportunities for energy recuperation through regeneration.
The correct choice of duty cycle and load cases is essential when optimising the cost of the powertrain, if the performance requirements are to be met. In this case, adding the effect of typical gradient changes to the base drive cycle increased the peak power requirement by 15%.
The systems approach can lead to unexpected or counter-intuitive solutions which might otherwise be missed. In this example, changing to a more expensive, more efficient motor permitted a significant reduction in battery pack size while maintaining the desired range. This was because, across the drive cycle, the more efficient motor drew less energy from the battery pack, which could therefore be reduced in capacity. A similar effect is sometimes observed when introducing multi-speed transmissions in order to downsize the motor. In each case, the overall result is a lower cost for the total powertrain and a reduction in weight, which liberates the capacity for additional payload, making the vehicle more profitable.
Apart from identifying optimal solutions that may otherwise not be apparent, the strength of a systems approach is its broad range of potential applications. The process is equally valuable when applied to light, medium or heavy duty vehicles using drive cycles in which urban, rural or highway conditions predominate.
India, as well as other markets around the world, require next-generation electric commercial vehicles. By analysing the duty cycle of a specific market, vehicles can be optimised for that region, significantly reducing cost and accelerating adoption.
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