Question about fuel usage to heat Gen 4 during a snowpoclase like on I 95

1gal/24h

 

Even my old Subaru with 2.5L engine would burn just 0.25 gal/hour when idling continuously after warmup. My new one is closer to 0.20 gal/hour.

The Pruis has a smaller engine and doesn't idle continuously, but cycles off most of the time, so will average far less fuel burn. My winter cold start tests long ago found its fuel burn starting near 1 gal/hour, then declining as it warmed up, to about 0.20 gal/hour just before its first ICE auto-shut-off. So count the time it runs vs being off, and figure its 'duty cycle', and multiply that fraction by 0.2 gal/hour to get a coarse estimate.

Yesterday (Wednesday), crossing my state's primary mountain pass into a storm forecast that was sure to close it soon, I made sure to stay on the top third of the tank to have plenty of reserve fuel. The road closure didn't happen until a half dozen hours after I crossed, and it will now stay closed until at least Sunday. But travelers there don't usually get stranded without opportunity to turn around and go back to other civilization.

 

I am not sure if it saves the gas, but if it is Prius Prime and the temperature is above 14F, then you may do better to run the engine on CHG mode and charge up the traction battery up to 80%, and then keep the engine off for as long as the traction battery SoC remains and can keep up with heating demand using just heat pump.

I have not compared the difference in gas usage between simply idling the car on gas engine vs on CHG mode, but on CHG mode it takes about 10min to charge up ~10% SoC. Unfortunately, heating with a heat pump at below 32F is not very efficient, so it may not work very well if it is very cold. And below 14F, the heat pump will not work at all, so charging up the traction battery may waste more gas (and heat).

 

Your thread title says Gen 4, but the forum is Prime, so there could be some differences involved.

For a Gen 4 non-Prime, I think you could get pretty close to the answers you want by starting with the Gen 3 hunkering thread. If you have a chance with similar weather conditions, you could do some tests in your driveway and report the results for Gen 4. I would begin with the same 3 techniques tested in the Gen 3 thread, and see if you discover any noticeable changes in Gen 4 behavior.

For Prime, there may be substantial differences, not just because of the larger battery capacity but also because of different programmed rules controlling it, and also because of the heat pump. It might still be productive to start with the three Gen 3 hunkering techniques, see what results you see for those, and explore further from there.

edit: ... or the four Gen 3 hunkering techniques (there's an "extra-extended" one described later in that same thread).

 

FWIW, Technique (3) is exactly what I use for my HVAC setting when I am driving on EV mode during winter when I need heat. That setting uses the least amount of traction battery power (lowest A/C load ratio). If the temp is low enough (below 14F) and kicks on the engine even in EV mode since the heat pump does not work, I usually switch to HV mode so that I can get both traction and heating from the combustion engine.

 

And now come to think of it... the "extra-extended" engine cycle technique you described is very similar to what I do with my HVAC setting during cold morning drive.

I never checked my PP's engine coolant temp, so I don't know what temp it was cycling. But my HVAC control goes something like this.
Temp below 14F... the engine comes on even in EV mode with plenty of SoC. At this low temp, I usually put the car in HV mode immediately after noticing the engine ON. Keep driving on HV mode with HVAC set at ~65F and lowest fan speed setting (i.e. not AUTO). At some point after the engine had sufficient time to warm up and heated to comfortable cabin temp, I switch OFF HVAC and change to switch back to EV mode. Continues to drive on EV mode until it gets too cold in the cabin, then switch back to HV mode again and set the HVAC set at ~65F and lowest fan setting. And repeat. With this method, I can usually drive without losing the EV range to A/C load. I am not sure if that also saves the gas or not, but seems to be more economical than keeping it on HV mode and HVAC on all the time.

 

I've got a hunch it can't be that much better, just "trying" to think like a physicist. A regular Prius does ultimately get all it's energy from gas; the hybrid battery is kind of like a surge pile?

 

My observations happened while defrosting windshield ice, with cold starts, in 'warmer' weather than ChapmanF's hunkering tests. And I didn't go to the continuous cycling mode, but drove off once the windshield was clear. I'm not sure why my ScanGauge reading different than his, other than different ambient temperatures plus maybe his engine was making up for off-mode heat losses while my observations weren't run long enough to stabilize in a regular pattern. I had presumed that my Subaru's steady consumption was mostly for friction and pumping loss, and wouldn't greatly increase while hunkering in serious cold.