How To Increase Driving Range
The most compelling reason to use an IEdrives transmission on your electric vehicle, delivery truck or city bus is to gain a 30 to 40% increase in vehicle driving range. How is this possible? This is the first of three white papers that will explore this question and hopefully provide understanding as to why an electric vehicle may want to add the burdensome cost of a transmission.
Standard drive train design practice used today for electric vehicles and for series hybrid electric vehicles is to select a fixed, drive reduction ratio to send power through to the drive wheels. This gear selection represents a compromise between a high ratio acceleration gear, used to start the vehicle moving from rest, and a much lower ratio gear used to match a high efficient motor speed and torque to the power requirements of the vehicle’s top speed.
At the most functional level, a multi-speed transmission allows the operational time spent by the vehicle’s traction motor in low to very low efficiency regions to be greatly minimized. A typical city bus only averages 13 to 15 mph as it follows its route. Delivery trucks operating in the same downtown city centers and surrounding metro areas would have similar speeds. All of these vehicles experience frequent stops for traffic lights and slow moving traffic. For these types of electric vehicles, the majority of their operation is very much concentrated in the zero to 10 mph speed range. This speed range coincides with one of the low to very low efficiency regions of all traction motors.
Before proceeding with this discussion, it is important for the reader to understand that an electric motor does not have a single efficiency value.
Though a peak efficiency value is sometimes presented as a standalone number, that is number is somehow representative of the motor’s efficiency at all speeds and loads, this is most certainly not the case.
A traction motor efficiency map will help to convey where these regions of low efficiency, higher efficiency and peak efficiency are located. In addition, traction motor efficiency maps are a critical and very effective tool that can greatly aid electric vehicle manufactures in their selection and systems level analysis and integration of an electric traction motor into their vehicle. The efficiency map from UQM, www.uqm.com , is shown above in Figure 1. This particular map includes the speed controller inefficiencies, and as such, offers a complete representation of this systems efficiency. More generally, this mapping can clearly illustrate the general shape of all electric motor/controller performance mappings. The highest efficiency regions are in the central region, located adjacent to the maximum speed/torque line for that motor. Moving along any vector from this peak efficiency point, results in entering a lower efficiency region. The lowest efficiency regions lay adjacent to the vertical and horizontal axis and have been highlighted in Figure 2 (below).
The region close to the vertical axis represents motor operation every time that the vehicle is accelerated from stop. For delivery trucks and buses, this is a very frequent area of operation, resulting in a majority of vehicle operation in this very low efficiency region.
How aggressive the motor efficiencies decline (increased electrical consumption) in these regions is very different from traction motor/controller to traction motor/controller. While some traction motor and controllers maintain higher efficiencies over a much broader region than others, all traction motors have zero percent energy efficiency at zero rpm. This zero efficiency point occurs every time the vehicle is stopped (zero rpm) and the driver is attempting to move it. How quickly, or slowly, that the electric motor can move through this inefficient region will determine little or how much excess electrical power is consumed.
A further consideration for the use of an electric vehicle transmission is for the other end of the speed spectrum; specifically steady driving at the upper speed ranges of the vehicle. The region close to the horizontal axis is often entered when the vehicle is under steady state driving conditions. This begins from medium speeds (30-40 mph) to top vehicle speeds (60-80 mph for trucks and buses), see Figure 2.
Figure 3 and figure 4 illustrate a vehicle/traction motor/drive train analysis performed by IEdrives. This analysis was performed on a production Class 4-5 electric delivery truck. This vehicle uses a fixed ratio drive train. Figure 3 plots two different curves for the stock vehicle. The first curve is a constant power acceleration line from “near rest” to 15 mph. The second curve joins the first at 15 mph and characterizes the steady state power requirements for this vehicle for the various increasing speeds, up to the maximum vehicle speed of 52 mph.
In order to reduce electrical power consumption, the maximum current allowed to the motor under acceleration is restricted. In addition, drive train limitations impose a maximum motor speed of 6000 rpm or approximately 52 mph on the delivery truck. Both of these attributes are reflected in this analysis, along with vehicle mass, aerodynamic drag coefficient, rolling resistance, tire diameter, differential ratio and a secondary motor rpm reduction box. Headwind and road inclination were both considered to be zero at all times. A constant inefficiency factor for drive train losses is included and kept the same for both configurations. Because there were no efficiency maps for the particular motor or controller that is used in this vehicle, IEdrives arbitrarily assigned 100% efficiency to both of these devices over the entire speed range. When such information does become available, IEdrives will update its analysis appropriately. The blue efficiency lines on both Figure 3 and Figure 4 were placed there by IEdrive and are only intended to be representative and as an aid to the reader in understanding how a transmission can be used to “move” these vehicle speed curves from low efficiency to higher efficiency regions.
What becomes clear from Figure 3 is that the many constraints imposed on this well received vehicle results in the traction motor being forced to operate extensively in its low to very low efficiency regions. Other than those infrequent times, in which there is hard to very hard acceleration from 20 to 30 or 30 to 40mph, the peak efficiency region of this motor is completely irrelevant to the typical operation of this vehicle. The single most severe constraint that drives the electric motor into these low efficiency regions is the fixed gear ratio of the drive train. Once the gear ratio is selected, then the top speed of the vehicle is immediately defined. If that top speed is outside of the motor’s rpm capability or its maximum torque capability, then that top speed is not possible. For that gear selection, an achievable, lower top speed must be electronically imposed. If the top speed is not acceptable to the end user, then a lower ratio gear is required. This final lower ratio gear choice will further degrade the quality of vehicle’s acceleration.
The presence of a transmission allows for a different set of final drive train gearing choices to be suddenly available. Choices that can dramatically improve vehicle acceleration. Choices that can dramatically improve the energy use efficiency of the vehicle. Choices that can significantly increase the amount of operational time of the traction motor into its higher to much higher efficiency regions. All of these positive drive train variations resulting from the availability of a transmission are additive. Each contributes to an improvement in the vehicles efficient use of electrical propulsion energy.
Figure 4 depicts one possible drive train configuration that is possible with an IEdrives 3 speed electric vehicle transmission. The first significant improvement is depicted in the constant power acceleration area from zero rpm to 15 mph. An IEdrives 3:1 first gear reduction allows for a 3 to 1 reduction in current flow to the electric motor and still maintain the same acceleration rate. Why is this relationship reasonable? In reality, there could be a greater than 3 to 1 reduction in current due first to fact that the motor/controller efficiencies are very low in this speed region. Remember, all motors have a zero efficiency at zero rpm when current is being fed to them to start a vehicle moving. The second reason that current could be reduced beyond the 3 to 1 is that the motor/controller efficiencies are very rapidly improving, literally with every increase in each rpm. This rapid rate of change in this region is depicted in Figure 1. The dramatic torque reduction offer by an acceleration gear allows that motor/controller to much more quickly move through its low efficiency region. For simplicity, it is reasonable to assign the same current reduction ratio value to same as the torque reduction ratio. At zero to low rpm, there is no consequential rotational kinetic energy in the motor shaft. All motor power in this low rpm range is then strictly driven by current flow to the motor coils, which is used to create the torque. As such, all torque is a consequence of current to the motor. No current, no torque. Small current equals small torque. Big current equals a quickly depleting battery.
Torque amplification, by means of an acceleration gear, allows for a smaller current to generate a larger torque and produce faster vehicle acceleration. Since torque generation is very proportional to current at low speeds, the area difference “under the curve” between a 300 N-m and 100 N-m is representative of the reduction in current flow into the motor to accomplish the same end goal…..15 mph in the same time period. In reality, because the motor/controller efficiency is improving so quickly by every rpm from the zero efficiency starting point, the 1/3 lower current acceleration profile will be faster. When an acceleration gear is available, an electric vehicle could improve its acceleration and reduce current flow by 60%. If only a 30% reduction in current flow is sufficient, then vehicle acceleration can be aggressively improved.
The second significant benefit that a multi-speed transmission brings to the table is to opportunity to re-specify the final drive ratio. A multi speed transmission allows a very high rpm motor too much better fit into the drive train component world which has been refined for over a hundred years to best fit the needs of low rpm engines. Figure 4 illustrates a very different steady state efficiency that is possible when a multi-speed transmission can provide a dedicated acceleration gear. Based upon OEM available differential ratios, an IEdrives transmission can be used to dramatically force the operation of the traction motor into a much higher efficiency region. This repositioning of steady state operation brings other benefits beyond the obvious reduction in current consumption. One of the more customer valued benefits is that top speed for the vehicle can now be increased nearly 15 mph to 65 mph. Another benefit would be driver perception. Simply by repositioning steady state operation, this vehicle’s responsiveness to acceleration commands is greatly improved, independent of the use of a transmission. Moving from the stock 52 mph operating point to the new “3rd “ gear 52 mph point, the maximum available motor “acceleration” torque available at that speed more than doubles from approximately 150 N-m to over 300 N-m. How valuable this is to the overall vehicle energy use efficiency is directly related to how often the vehicle is accelerating from zero, driving at a steady state, or accelerating from one speed to another.
Improving the vehicle’s efficient use of electrical energy will and does result in a direct, one for one increase in the vehicle’s driving range. For delivery trucks and buses running daily inner city routes, this increase in range is the one most important considerations made by the buyers of these vehicles. If the electric delivery truck cannot complete its daily route, or has to run a reduced route, the fuel and emissions savings it offers is of minimal value to the fleet operator. A fleet operator or owner cannot accept either undelivered packages or a vehicle with a dead battery. This problem leaves the operator with an unhappy customer. The second problem leaves the operator with a vehicle that now needs a service truck to recharge or tow their stranded electric vehicle back to the service yard. Neither problem is acceptable.
Find out how multi-speeds can improve your vehicle.
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