Members of the Otago Electric Vehicle Incorporated Society conducted an experiment on 25 November as part of an outing for a strategy meeting and group BBQ. Twenty EVs participated (19 Leafs and a Zoe) to (i) test the impact of load on efficiency; (iii) measure whether the ‘Eco’ and ‘Drive’ settings affect outcomes and (iii) measure comparative efficiency (km/kWh) of driving from Dunedin to Waitati (a small town north of the city) and back via the Motorway, or via the “back route” (North East Valley).
As usual in science, some of the results came out as expected and others were intriguing different. Here is a brief summary of the methods and some preliminary results:
What we did:
- EVs assembled at Woodhaugh Gardens in Dunedin for recording odometer, and zeroing the average speed (km/h) and efficiency (km/kWh) meters, estimating the weight of drivers and passengers, and measuring tyre pressure (more on that later). We also recorded whether or not the drivers had selected ‘Eco’ or ‘Drive’ mode for the journey, which was 18 km each way.
- Participants were assigned randomly into two groups: (a) ones who added load by placing 4-8 sandbags (each weighing around 18kg, so roughly the equivalent of 1-2 additional adult passengers) for the outward journey, or (b) ones with no added load on the outward journey.
- Half of group ‘a’, and half of group ‘b’ travelled to Waitati via the motorway, and half via the “back route” up North East Valley. These are both hilly routes, but the back route does not climb quite as high as the motorway.
- People drove at their normal comfortable and safe speed.
- Once at Waitati the efficiency and average speed were recorded and the instruments reset. The bags were then swapped between the two groups for the homeward journey. This is called a “switch-back” experiment – it helps remove the variation between cars and drivers so we can get a better signal of the size of the experimental effect (load).
- EVs that went out on the motorway came back via the back route, and vice versa. The same driver was used for outward and inward legs, and ‘Eco’ or Drive’ were on the same setting throughout to minimise any effect of different driving styles and settings on efficiency.
- Everyone returned to Woodhaugh Gardens to unload sandbags and measure the efficiency and average speed in the return journey.
A few photos from the event are posted below.
What we found:
- The dominant effect on efficiency was route (there is less than a 1 in a 1000 chance that this difference was just a chance sampling ‘fluke’). Efficiency across the back route (at average weight and speed) was 6.6 km/kWh, compared to 5.6 km/kWh on the motorway. This is roughly 18% increased efficiency to go via the back route.
- EVs travelled faster on average down the motorway (57 km/h) compared to the winding and somewhat narrow back route (36 km/h).
- There was no statistically significant evidence that speed itself reduced efficiency to explain the difference between the two routes. We would need a lot more EVs and to ask some of them to drive relatively fast and others slowly if we wanted to get a better idea of the influence of speed on efficiency.
- A somewhat surprising pattern emerged between the effects of load and route. There was a statistically significant “interaction effect” between load and route i.e. increasing load reduced efficiency considerably on the back route, but it had no detectable effect on efficiency for EVs travelling down the motorway! See the graph of the efficiency predicted for EVs with 100, 150, 200, 250 and 300 kg on board. It is unlikely that this interaction effect would have arisen by chance (a 2.7% chance remains that it is a statistical “fluke”). Roughly, the effect of adding 3 adult passengers alongside the driver resulted in a 13% drop in efficiency.
- But wait, there’s more! The ‘Eco’ setting led to lower overall efficiency than the ‘Drive’ setting over the experimental route (compare the upper and lower graphs). There is only a 1% probability that this was a chance effect). Go figure – this is the opposite of what most people would have predicted. What do you reckon causes this effect? Would it be the same on a relatively flat route? Drivers may have driven differently when they were in ‘Eco’ mode compared to ‘Drive’ mode (a formal “blind” experimental test would have kept each of us ignorant about which experimental group we were in, but … we figured it was better that we could see the dash and where we were going for this one!!).
It would be great if you shared your ideas about what all this means and how it might be interpreted. We’ll write it up your comments in our joint report.
Over 30 of the Flip the Fleet members from throughout Otago are also participating in a wider month-long experiment in November and December in which they are switching between ‘Eco’ and ‘Drive’. Those results will perhaps help us confirm and interpret the result we got yesterday.
Implications for Dunedin drivers:
- If you are pushing your range when going north or coming home, take the back route between Dunedin and Waitati.
- Selecting ‘Eco’ mode won’t give you much help, at least going up and down hilly terrain (you could probably gain the best of both worlds by climbing up in ‘Drive’ and switching to ‘Eco’ for a cruise down the hill (but we need to test this hypothesis)
- Don’t get too fretful about having a big load on if you are using the motorway, but be aware that it could compromise your efficiency along the back route compared to prior experience with a light load. However, even when you are carrying 300 kg, the efficiency along the back route will be similar to going via the motorway.
Potential implications for all EV drivers
We need to do more experiments to test our ideas, but if our interpretation is correct,
- The ‘Eco’ setting may be counterproductive for efficiency in hilly terrain
- Load may have comparatively little impact on efficiency and range in very steady motoring conditions
- Local routes and steepness of terrain have the main impact on the variation in average efficiency we see in the Flip the Fleet results from around the country
- Foote Haulage lent us the sand for the experiment, and Farmlands gifted us the bags which are now stored ready for more fun and games in the sun for another EV driving experiment next year.
- This experiment and Flip the Fleet’s growth this year has been partly funded by the Otago Museum’s ‘Participatory Science Programme’. Thanks!
27 November 2017.