On the virtues of the EV switch ....

I installed an EV switch, as described here, a little over a year ago. At the time, I had researched it fairly well and concluded that the opportunities for EV-induced improvements in fuel economy were rather limited. Nonetheless, I knew I had a few select opportunities to use it on my regular commute. Furthermore, the modification was inexpensive and, as it turns out, fairly easy to install. I knew there had been at least one instance of Toyota initially denying a warranty claim for battery replacement because of the user’s EV switch installation. But I knew battery problems were rare anyway, and the burden would be on Toyota to demonstrate that the same switch they include from the factory on overseas Prii somehow harmed the identical battery on my U.S. model. (I also acknowledged to myself that that wouldn’t stop them from trying and that I might be confronted with a legal fight if it came down to it.) Finally, I had a good sense of how it could actually hurt fuel economy if misused. So I concluded that though the potential benefits might be limited, the risks were small and mostly manageable.

So I embarked on my EV voyage using the switch for short, low power-demand situations where ICE operation likely would be inefficient: moving around the driveway, through parking lots, between closely-spaced red lights, and extending low-speed glides a bit. I also used it as an “Engine Veto” switch (to borrow a term coined by PC member Dan) in S2 or S3 operation, forcing ICE shutdown during sub-34 MPH glides.

Using it for the latter at the time was somewhat limited. I have an engine block heater (EBH), which allows the ICE to reach S3/S4 temps within just a couple of minutes in warm weather, minimizing the need for Engine Veto. A year ago I was generally able to plug the EBH in every day for the morning commute and, from my part time job site, for the evening commute. I can’t plug in at the full time job, but I couldn’t use EV much on the evening commute last year – the route had moderately heavy traffic on mostly two-lane roads with speed limits of 35 or 45 MPH.

I moved last October. Good thing about the move is that the part time commute distance (which I drive 6-8 times a month) was cut in half, and the full time commute, though a tad longer and with more red lights, can be done mostly at lower speeds. Bad thing is that I have nowhere to plug in the EBH at home. (Actually, I might be able to stretch an extension cord across the backyards of two neighbors between me and the parking lot. But I need to get to know them a little more before I broach that issue; it was only a few weeks ago that I even met the second one.) So I’ve really been poking the EV button, especially on the morning commute.

Despite the assertion of many of the PC elder statesmen that the EV switch’s effect on total fuel economy is marginal, I have become convinced over the last few months that it makes a significant difference for me. It seems especially helpful in the morning on my part time commute. That route through the middle of Richmond, VA has 43 stoplights on 14 miles of mostly urban four- and six-lane streets with 25 and 35 MPH limits.

A trip a few weeks ago is a case in point. Unusually light traffic, even for a Saturday morning, allowed me to use low speeds and time red lights without fear of disrupting traffic or inciting road rage. It was several minutes and 15 lights into the ride before I even had to stop completely. With the exception of two rather steep downhills, I kept it below 34 MPH without fear of running the ICE during S2 glides – I had my EV button! In fact, my switch-mediated low-speed glides were long enough and frequent enough that the ICE never reached S3 temperature until about 2-3 miles from the destination. The result: 80 MPG for the trip, with an outside air temp of 56F.

Certainly I otherwise could have gotten into ICE-off conditions early in the drive without the switch, but that would have required continuous ICE operation for the first several minutes, forcing ICE-on glides and eventually a complete and unnecessary stop to provoke S4.

Given the prevailing thoughts around these parts on the usefulness (or lack thereof) of the EV switch as a fuel-saving tool, I’ve been reluctant to report this without hard data. Well, more recently I got my CAN-View data capture working. And I now have data. These are from my full time morning commute the past two days.






The main argument against the switch’s use is that depleted battery energy has to be replenished, with conversion losses suffered through discharge and recharge. But again, my main use is not for propulsion, but instead to force ICE shutdown during its warmup when propulsion is not needed.

Nonetheless, I sought to control and account for SOC. Though the goal is not to propel the car on battery power, the ability of the ICE to recharge the battery is lost with EV mode. Therefore, I attempted to begin and end each run with identical SOC. The non-EV run was on Monday, and I simply began and ended it with what the car gave me: 54% to begin and 59% to end, both as displayed by CAN-View. Initial SOC Tuesday was 56%, so on first startup (with EV activated) I ran the AC until the display showed 54%, at which point I began data capture and the drive. SOC was displayed at 60% as I approached work, so I ran with a short final EV mode segment to bring it down to 59%. Turns out, the recorded data showed the EV run beginning at 54.5% and the non-EV run at 53.5%. (More on that in a minute.) The data showed ending SOC for both runs to be 59%.

I determined from the data the "SOC equalization" point where SOC from the EV run had climbed to match SOC from the corresponding point on the non-EV run. It is at that point that I consider the MPG difference to be most meaningful -- whatever battery energy was used for EV mode has been replenished. I adjusted for the 1% startup difference in defining the equalization point: it was where 60% for the EV run matched or exceeded 59% for the non-EV run.

I noted cumulative trip MPG at each of 5 different benchmark locations, including destination. As luck would have it, the SOC equalization point fell very near benchmark #3 for the EV run, with the same benchmark for the non-EV run lagging by about a half minute. So I use that benchmark's numbers for my results comparison: 73 MPG with the EV switch versus 69 MPG without.

Given that the EV run was on the cooler of the two days, the MPG difference likely is even more significant. The Prius MPG Simulator predicts a 2.3% improvement in fuel economy at 22 MPH with an increase in OAT from 60F to 65F.

The benefit almost certainly is greater in cold weather when the car’s ICE starts out colder, takes longer to warm, and cools more quickly during ICE-off periods or when running cabin heat.

22 MPH, BTW, was the combined average speed of the two runs up to the equalization point. Other average speeds:

• EV run, up to SOC equalization: 23.1 MPH
• EV run, total: 22.0 MPH
• Non-EV run, up to SOC equalization: 20.3 MPH
• Non-EV run, total: 20.7 MPH

On both days, skies were clear and winds calm or nearly so.

In conclusion, I am convinced of the fuel-saving benefit on an EV switch for situations similar to mine. Others without an EBH – or with no place to plug one in – and who make frequent low speed (<34 MPH) drives with a cold ICE may benefit as well. The potential warranty issue still must be considered, and the switch installed at one’s own risk.

 

After the original post, something in the charts jumped out that bothered me a bit. The non-EV run had three complete stops at red lights before the second benchmark. To find them all red is typical. But on the EV run I got lucky, in a sense, catching two of them green. So not having to completely stop could have biased the results in favor of the EV run.

So I repeated the EV run this morning. OAT was closer to that of the non-EV run: 64F. Winds were calm and skies clear. Two of the three early lights were red this time. Significantly, the SOC equalization point was considerably earlier, very near the second benchmark. Starting SOC was 54% (after another brief period of AC), so I adjusted for the 0.5% difference in starting SOC in defining the equalization point. Ending SOC again was 59%.

Given the equalization point's location this time, I consider fuel economy results at the second benchmark most significant for this comparison: 70 MPG vs. 60 MPG. Not needing to stop for the third red light could account for a small part of the difference, but nowhere near 10 MPG.

Here's the chart for the second EV run, along with a repeat of the non-EV chart. Pointers for the early stops are included; that and adjustment of the equalization point are the only changes to the non-EV chart.

 

Yes, it's a short but rather steep segment approaching a stop sign. See this for a detailed description of the commute. It's the segment between miles 5.2 and 4.2. (The mile markers work backwards in that post for easy comparison to the PM commute on the same route, documented first in the previous post of that thread.)