Plug-In Prius: real winter driving

With all the feedback about moisture on the inside of the windshield due to some owners not wanting to run the engine for warm defrosting, today's opportunity was a real treat.

I would drive to work with the heater running the entire time. The blower direction was set to window & foot, the fan on 2 bars, and the temperature set to "65". We got about 1.5 inches of new snow and the temperature was 25°F. That made for a messy morning commute on the along-the-river route. In other words, plenty of moisture.

After driving the usual 3 blocks in EV to get out into traffic, I fired up the heater. Consequently, the engine started. It ran to warm-up completion (a coolant temp of 130°F) then shut off. EV automatically engaged and I drove that way for a few miles. When the coolant dropped to 96°F, the engine fired up again and then shut back off shortly afterward.

Back in EV, the system was now entirely warmed. To my surprise, the coolant temperature dropped all the way to 87°F before the engine started again. That was great!

My drive continued, moisture free the entire time. Upon arriving at the parking ramp, with the electricity all used up, the final distance tally was 17.3 miles and the average 120 MPG.

Set it, then forget it. No need to mess with the heater. Just drive it.

 

Heater Threshold

I had noticed enough of a difference with coolant temperatures and heater threshold to want to research further. I hoped to detect a pattern. Could I uncover & confirm reactions to specific settings and reliably repeat those results? The answer is, yes!

The catch to performing all that testing was to not allow the engine to heat up the coolant too much. That's what has been preventing us from noticing detail in the past. Everything really needed to be measured within a small window of time, not spread out over weeks like we've been doing.

We already knew with the heater set at 65, the coolant temperature would drop well below 100 before starting the engine to warm it back up again. What I wanted to know what happened when it was set higher. Check this out:

68 = 118
72 = 118
74 = 118
76 = 123
78 = 127

The heater setting on the left corresponded to the coolant temperature on the right, several times at each level. I was even able to confirm results while driving and while stopped.

In other words, if you set the heater at 74, the engine will remain off until the coolant temperature drops to 118. At 76, it's 123. At 78, it's 127. No longer is there a single cutoff point like the regular Prius. The plug-in model takes a tapered approach.

 

Outside Temperature

It clearly is a parameter taken into account for the heater, beside the coolant threshold... which explains why observations have varied so much.

My recent evidence of this was watching the engine shut off when the coolant temperature warmed up to 112°F with the heater set at 70. What the heck? That didn't fit in the pattern I had measured a few days earlier. The obvious difference was it now being a little above freezing outside, rather than well below.

The next observation of not-fitting-the-pattern was when it got even colder outside. Not using the heater at all, the engine shut off and 105°F and I was able to drive for 4 miles in EV. In the past, initial warm-up was always to 130°F anytime the engine started. That didn't seem to matter. Maybe time is a parameter too.

Yesterday, my cold morning errand running with heater had an engine shut-off at 123°F instead. That was yet another temperature. Hmm?

Today, with it 12°F outside and the need to get to work shortly before noon, I set the heater at 70 and jumped on the 70 mph highway with a full battery and a cold engine. Upon reaching the 55 mph section, there was plenty of reserve heat built up for EV driving... or so I thought. When the coolant dropped to 136°F, the engine started back up. Huh?

Of course, in the end, does it really matter? The point is to deliver a significant MPG boost from plugging in. That is indeed what I've been getting.

The same advice we gave in the past applies now too... Just Drive It.