How to Assess "Electric" Performance
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H. W. Heyman, B.Sc. (hens.) J. H. Cansdale, M.I.E.E. ASSESSMENT of performance is more difficult in the case of a
. battery electric than in that of a petrol or oil-engined vehicle. In a petrol vehicle, fuel consumption, expressed in terms of gross ton-miles per gallon; is generally accepted as a satisfactory basis for comparison. The exact equivalent for the battery-driven vehicle would be gross ton-miles per . kw.-hour, which, however, leaves out of consideration speed and acceleration.
These are important, not only for . their own sakes—although there is no doubt that drivers prefer alively vehicle—but because high acceleration and, often, high average speed serve as an indirect indication of a vehicle's hill, climbing capacity.
Low electricity consumption by itself, : and not related to other performance details, is not an aim in electric-vehicle design, particularly as the cost of charging current is a relatively small item. The main expenditure on " fuel" may be said to be the cost of the battery, and its effective use in service . should be the basis of an efficiency test on electric vehicles.
Battery Size
The size of the battery will naturally affect test figures of range, speed, etc.; in other words; a larger battery may give an apparently better performance, but not increased efficiency. On the contrary, practical experience has shown that every vehicle has its optimum size of battery beyond which it would be uneconomic to go. Incidentally, it is both curious and gratifying that the optimum battery for the various sizes of vehicle happens to be just right for the requirements of an average collection or delivery round.
High efficiency in a well-designed vehicle may reduce costs in one of several ways. It may enable a smaller battery to be fitted, it may prolong the life of the battery thnaugh working it always well within its capacity, or it may even permit the user to arrange his rounds so as to reduce the number of vehicles required. —
It is obviously difficult, with 'so many factors to consider to arrive at a fair comparison of the efficiency and suitability of the different vehicles offered to a prospective buyer. A rough calculation, which could, in most cases, be undertaken with a simple road test, would yield a performance factor arrived at by the following formula:— speed x Mileage x PANIoad
Battery kw.-bra.
Speed would be the average nonstop spied over the first, say, 10 mites with a fully charged battery. Mileage would be the continuous mileage covered until the vehicle would not go any farther, i.e., came to the first definite -standstill. No attempt ,should be made to obtain additional mileage through recuperation of the battery. Payload should be the permissible payload of the vehicle, the figure for which is obtained by deducting the weight of the unladen vehicle, cornplete with driver and all necessary equipment, from the gross weight allowance guaranteed by the manufacturer. The term 'battery kilowatthours" denotes the stated, nominal capacity of the battery.
For instance, in a 1-ton van, the battery might consist of 36 cells (72 volts), 210 arop.-hours at the five-hour rate, which is equivalent to a nominal
capacity. of 72 x 210 = 15.12 kw. hrs. It is unnecessary in this calculation to take account of reduction of effective capacitythrough a higher rate of discharge than the five-hour rate. The cost of the battery is related to the nominal capacity and this is the figure which must be used in assessing performance. '
Optimum Capacity
The resulting factor relates performance to battery capacity, assuming that range, speed and load are of equal importance. Increases obtained in any of these figures by installing larger batteries would not improve the performance factor if the battery capacity were increased beyond the optimum size for the particular vehicle.
On the other hand, real efficiency in effective exploitation of the battery. is not, perhaps, shown satisfactorily, as the formula, so far, has taken no .aecount of consumption and acceleration where there are frequent stops and Starts, as on a normal delivery or collection round. These are bound to have a decisive influence on the average speed and range to be obtained in service.
If calculations were made as described, two vehicles of similar payload might have an identical performance factor, i.e., the same speed and range, yet vary greatly. in technical efficiency, and the range and average , speed of the one might be much inferior. Many design features which greatly help technical efficiency do not come into operation unless the vehicle is engaged on a frequent stop-start service, as, of course, is normal for an electric tiehicle.
Parallel-series battery control iS an obvious example, as is the use of a powerful motor which gives high acceleration without an appreciable drop in efficiency. Such a motor would. incidentally, also ensure ability to climb steep inclines.
To obtain a more telling comparison it is, therefore, advisable to establish a further performance factor, leaving load and battery capacity'as before, but obtaining the average speed and range for a given number of stops per mile. The average speed is the total distance in miles divided by the actual running time in hours; times for stops would be deducted from the total time. The range, as before, is the mileage covered until the vehicle will go no farther. The formula should now be written: P.F.2 Average Speed x Stop-Start Range X Payload
flattery kw.-hrs.
Different methods of driving may affect the results obtained in the stopstart test. Coasting between stops, with little or no use of a brake, would reduce the average speed, but increase the stop-start range. The effect of this practice tends, however, to cancel out. and the resulting performance factor would still be a good indication of average efficiency.
By deducting P.F. 2 from P.F. 1, a figure is obtained which shows, so to speajc, how far stops and starts affect the overall performance of the vehicle. Clearly, a small percentage difference indicates that the vehicle is well designed for the duty it is expected to perform.
Frequent Stops
To enable the performance of different vehicles to be compared, it is suggested that there should be standard tests giving the performance factor for a continuous run, another with four stops per mile, and a third with eight stops per mile. These tests would, of course, have to be taken over the same route and under identical conditions.
, The test rotate could be established in some main town, and should include at least one stiff gradient, in order that the average speed will be comparable with that obtained in normal service.
Again, the stops, in the case of the second and third tests, should be of a specified length, so that the average speed could be determined accurately, unless an integrating stop-watch 'were used.
If standard tests on these lines could be carried out, possibly under independent auspices, such as those of "The Commercial Motor," less experienced users would be given valuable guidance. Meanwhile, it should be possible for fleet operators to carry out tests of this nature, or even to ask manufacturers to demonstrate on this scientific basis.