"EV mode" switch - that's the little white button I added to the car.
It's a simple momentary switch connected to the ECU to give this North American market gen 2 Prius a feature that was unavailable here, but available on the JDM/European cars.
Limited efficiency usefulness... or so I thought at first!
When I put it in, I mostly did it because I thought it would be neat to use EV mode for short distances like moving the car in the driveway, or as a party trick to show passengers.
Ooh, neat! And how far will it go in EV mode?
Everybody wants to know, myself included. So for kicks, I drove it as far as I could from 7/8 bars on the battery SOC gauge until the battery got low enough that the engine automatically fired up.
In mild, dry conditions it went about
1.3 km / 0.8 mi., feather-footing it through suburbia, including 2 stop signs, mixed gradually rising/falling elevation (with maybe a net 5 foot drop from the start), and a max speed of probably 45 km/h.
Of course, using EV mode like that is a terrible efficiency strategy, because once you deplete the battery charge, the ICE automatically fires up regardless of EV setting and then it gets particularly terrible mileage while the engine force-charges the battery back to a minimum level, even when stationary. Mileage in that scenario will be worse than if you'd just driven the car normally the same distance (ending at the same state of charge).
A few scenarios where EV mode can boost efficiency...
- Increasing EV usage before an upcoming significant regen event (eg. descending a long hill).
- Delaying a cold start until the engine is
really needed. Eg. I typically go down hill less than a block from my driveway to the first stop, and now I always do this in EV mode. Which means I wait to fire up the ICE until it can make some useful contribution to forward progress, rather than going along for the ride at high idle, getting terrible MPG. It's like taxiing efficiently to the runway before firing up the big jets.
The best use of the EV switch is as an engine kill switch
Wait a minute, you're surely saying. Doesn't the Prius normally stop the engine automatically when you let go of the accelerator?
Sure it does. But it only does this when the coolant has gotten hot enough AND after you've made a complete stop.
But if you're doing a fairly short trip where you know the car isn't going to get up to temp anyway, or if it's up to temperature but you haven't yet had to make a stop, you can use the EV switch like an engine kill switch in a conventional car: ICE-on for accelerating & cruising, ICE-off for coasting up to stops & turns.
My in-town mileage has gotten much better since I started using the EV switch like this.
The main downside? I would have put the switch it in a different location if I'd known it was going to be much more useful than as a party trick. I put it where it is mainly because that trim panel was easy to remove, and I intentionally put the switch lower down so it would be less obvious when I removed & patched it when I go to sell the car next year. It would be much ergonomically better closer to the steering wheel, like on one of the stalks (signal/wiper/cruise).