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Increasing the range of EV with the same battery size
– Part I – The efficiency
Electric Vehicles (EV) are the future of mobility, but the biggest roadblocks to mass adoption among consumers is the range anxiety and price. While using larger batteries would be an obvious solution to increase the range, it would drastically increase the cost of the vehicle. As we discuss in this blog, it is indeed possible to increase the range of an EV with the same battery size.
Just like a gasoline car has a gas consumption measured in MPG or l/100km, an EV has an energy consumption rate measured in kWh/km or miles per KWh. The more efficient the electric powertrain is, the less energy an EV consumes to run and the further it will go until the battery is depleted. Increasing the range of an EV while keeping the same battery size is all about improving its efficiency.
An electric powertrain (Figure 1) includes usually four main components: a battery, an inverter which converts the direct current (DC) into multi-phase currents (AC) to control the electric motor, the electric motor which uses the electric energy to generate magnetic fields to make it turn and, in most of the case, a DC-DC converter which adapts the voltage of the battery to match the voltage of the electric motor and the power demand.
Figure 1 – Electric Powertrain Efficiency
The efficiency of the electric powertrain is the ratio between the energy output of the battery and the energy output of the motor. A ratio of 100% would mean a perfect conversion of the electric energy into mechanical energy… but it is never the case. Many losses happen at different stages in the energy conversion process. The efficiency is not even a constant value. For example, the combined efficiency of eMotor and Inverter is ranging from 60% to 96%, depending on the drive profile, the speed and the torque of the motor and its position on the drivetrain.
The efficiency is usually represented in a graph (efficiency map) on which the y-axis and x-axis are respectively the torque and the speed, providing for any speed/torque combination a given efficiency. As shown in Figure 2, the optimal operating points where the efficiency is at its peak are located in a restricted area in the Speed / Torque space. The usage of the electric motor in this area guarantees an energy efficient system. But in real driving conditions (as we discussed in our previous blog note), the Inverter/eMotor are used in wider operating points and keeping the system in its optimal range is not possible. New techniques need to be employed to expand the optimal operating range, without any compromises on the overall performance while keeping the costs low.
Figure 2 – Combined Efficiency Map of an Electric Motor and Inverter [1]
Possible solutions to increase the efficiency – A difficult compromise
There are 2 solutions which are currently being utilized, alone or combined, to increase the efficiency. Some car makers have decided to widen the electric motor optimal operating range by increasing the size of the motor, or adding a second motor. A recent example is Tesla’s usage of dual motors, one in the front and one in the rear– one motor is optimized for power and another motor optimized for range. Another approach is the usage of multi-speed gear box in the drive train, which has the effect to reduce and center speed and torque in the optimal area of the efficiency map. Recently ZF introduced a 2-speed drive for the electric cars of their customers.
Figure 3 – Dual motor configuration for Model 3 (©Telsa) |
Figure 4 – 2-speed e-drive (©ZF) |
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But in both cases, the efficiency gain provided by these approaches generates an ineluctable and significant increase of cost and weight. More cost is not helping to drag the price of EV down and more weight is not helping to push the range up. . Moreover, these solutions are sub-optimal: they do not fix the root causes of efficiency drops but patch the consequences of low efficiency by adding mechanics and material.
A 3rd way is possible, which will not request a bigger eMotor or the add-on of a gear box, which will cost nothing and add no weight, and which will surpass in range improvements mechanics and material: A better software!
A better software is a software where the electric motor, inverter and DC-DC converter are controlled in such a way that it increases the amount of optimal operating points, reduces the losses in every component, and improves the overall efficiency.
An algorithmic approach to achieving high efficiency
This is the path followed by Silicon Mobility with OLEA technology. A novel approach to achieving high efficiency without adding complexity and increasing the costs. A unique hardware platform and advanced software where the control uses the motor electrical angle position to permute between several control strategies so it can adapt the requested power (Torque x Speed) to the best possible efficiency. On the electric motor and inverter components, the efficiency is increased by 20% in comparison to conventional control when measured on a real driving cycle.
The table below shows a comparison of the various approaches discussed above.
In our next blog note, we will discuss the challenges of getting wider operating range and higher power from an electric motor and learn which appropriate control algorithm can mitigate or compensate the undesirable collateral effects.
Check out www.silicon-mobility.com for more details.
[1] S. Faid. – A HIGHLY EFFICIENT TWO SPEED TRANSMISSION FOR ELECTRIC VEHICLES, EVS28, KINTEX, Korea, May 3-6, 2015 http://www.evs28.org/event_file/event_file/1/pfile/EVS28_Saphir_Faid.pdf
