on hunkering, Gen 3 style

I just had the chance to check out, in my Gen 3, the same techniques for winter-storm hunkering as in my Gen 1 on hunkering thread from 2012.

Differences:

• Then a Gen 1; now a Gen 3.
• Then temperature 2 C, winds gusty; tonight, temperature −20 C (brrr!), winds light.

The two techniques I tried five years ago were:

1. Keep heat on AUTO, set for the lowest usable temp setting (that is, 65 F, as the next lower setting is a demand for unconditional cooling).
2. Turn heat OFF, rely on a heated blanket.

My Gen 1 results suggested about 3 days of hunkering possible with (1), or about 9 days with (2), under those conditions, and on a full tank.

Tonight in my Gen 3, I tried three techniques:

1. Keep heat on AUTO, set for lowest usable temp setting.
2. Turn heat OFF, rely on heated blanket.
3. Leave heat ON, set for lowest temp setting, with fan on lowest setting (not AUTO).

Technique (1) had the engine cycling about 2 minutes 25 seconds on, 1 minute 50 seconds off, about a 57% duty cycle. When running, it tended to burn about 0.3 gallons per hour according to the ScanGauge. Assuming 10 usable gallons in the tank, that looks like about 58 hours, or 2.4 days. (Bear in mind tonight is a lot colder than the night I tested the Gen 1.)

Technique (2) did not seem to be a good idea for the Gen 3 in these conditions, which surprised me. While I was testing (1), the engine was happily starting and stopping at coolant temperatures well under nominal (it would start at around 50 C, and stop at around 60 C). With the heat turned OFF, the system did not seem willing to compromise on operating temperature, but would start the engine and try to run it up to 90 C before shutting off. At tonight's temperature, it could barely get past 87 C by idling, leading to absurdly long run times for the technique you'd expect to be efficient.

I don't know if this is a change in control strategy from Gen 1, or just something I would only notice tonight because it is so much colder out. Anyway, I can't recommend technique (2) in a Gen 3, at least not for temps like this.

Technique (3) seemed to be a sweet spot. I just hadn't thought of it five years ago. The heat is still on, and the temperature still can't be usefully set any lower than 65, but the fan can be forced to run at lowest speed, rather than the speed AUTO would pick for it. At lowest speed, it sucks a lot less heat out of the coolant, and the engine-off time increases accordingly.

The engine-off times averaged 2:49, with the on times averaging 1:12, for a 30% duty cycle.

Gallons per hour usually read about 0.3, but maybe one cycle out of four it would burn at around 0.63 GPH to put more charge in the battery. I'll call that about 0.38 on average. (I was just eyeballing the ScanGauge for this; a more accurate average could be obtained by datalogging.)

Again assuming 10 usable gallons in the tank, that looks like about 88 hours of hunkering with technique (3), about 3.7 days. I know that doesn't sound nearly as good as the 9 days I projected for a Gen 1 with technique (2), but again, that night was 2 C, not −20 C like tonight.

Also, technique (3), with the fan running on low, is an improvement for comfort. It may still get cold in the cabin (that's why the heated blanket), but at least some air is moving, and a lot less ice building up on the insides of the windows.

-Chap

Edit: just for completeness, I do have a rubber sheet behind the grille. It probably didn't make much difference tonight (only light winds), but I didn't want to leave it unmentioned.

 

Electrical power was being used also, but (at this ambient temperature) the coolant was losing heat a lot faster than the battery state of charge was dropping, and the engine was restarting and stopping at fairly predictable coolant temperatures (though it clearly has some sort of adaptive algorithm trying to figure out what temperatures it can get away with). Every so many cycles, it would also burn fuel at a higher rate (more like 0.63 GPH rather than the 0.2 to 0.3 at idle) and use the extra juice to bring the battery state of charge back up a few percent.

I can't think of a reason to set the temperature anywhere other than the lowest usable setting (65 for US cars) for this purpose. If you're trying to conserve, you basically don't really want the car to be trying to reach even 65. You'd like it to be more like 40 or 50, while you bundle up inside (ideally with the heated blanket). They don't give you the option to set a lower target temperature, but you can sort of get there by setting the fan speed to minimum, so the car may be trying to reach 65 inside but can't extract enough heat to actually do so. Those end up being the settings where you extract the least heat from the coolant (short of turning the heat completely off).

-Chap

 

Ok, I guess that splits into two cases. In one case, low fan speed simply cannot move enough air to achieve the temperature setpoint you've chosen. That's the 'hunkering' case I've been considering here, just a way to force the car to be maintaining a lower temperature than you can actually set.

You seem to be talking about milder conditions, where it might be possible for the cabin to hit 72 using only low fan speed. I'd predict yes, if the conditions are mild enough to do that, and it meets your other needs, then that will consume less engine heat than a higher fan setting. Why? Because the heater usually draws air from outside, which we've stipulated is below your setpoint, so the more cabin air it is replacing with fresh, the more heat is needed to maintain the same temperature.

Your "other needs" might include things like visibility, so if low air replacement isn't enough to keep the windows clear, it might be the wrong strategy at that time.

You can, of course, get minimum air replacement by changing the intake setting from fresh to recirculate. That will conserve more energy, but serious humidity issues like window fogging, waking up in a soggy sleeping bag, etc., may ensue. (The HV battery holds a veto on recirculate: it can force fresh if it needs more airflow, but that's more likely to be an issue in hot weather.)

Another thought: if the conditions are mild enough to hold your chosen setpoint using low fan speed, low fan speed is probably what AUTO mode will eventually settle on, so you might not really be changing much by overriding it. In fact, auto mode might settle on an even lower fan speed; the controller has about 31 speeds, but only a handful you can pick from the front panel. Sometimes you can't beat what the car does on its own.

Instead of pipe insulation, I use a heavy vinyl sheet I slip behind the grille, less of a fashion statement for passersby. The same one, in fact, that I made for my Gen 1 and described in that post, only now I slip it in with the edge with holes up, the Gen 3 electronics radiator being on top rather than below in Gen 1. (As when I first used it in Gen 1, I think I made the holes in an excess of caution; had I had a ScanGauge at the time, I would probably have tried with no holes and kept an eye on the temps, and probably had no problems.)

In case I didn't mention it, that block was in place for this test. The car was stationary and the wind was light, so it might not have had much effect. It could be interesting to test on a windier night.

-Chap

 

You're still going to get jostled every few minutes ... but it's pretty regular, which seems to be all my brain needs to tune something out and sleep through it. My furnace at home is pretty quiet, but the last one wasn't, and I slept through that all the time. And the A/C condenser is right outside the bedroom window....

I notice my Gen 3 actually applies the brakes before starting the engine when stationary, to help keep the jostling to a minimum. Pretty clever....

-Chap

 

Hrm, starting the ICE is one thing the 12 volt battery does not do. It does provide all the power for all the 12 volt devices in the car, when the car is not in READY.

When the car is in READY, the 12 volt battery is not disconnected from the 12 volt circuits, it's just that the DC/DC converter is also operating then, and its output is a volt or two higher, so whatever power any 12 volt devices are using will be coming from it, and not the 12 volt battery (which, instead, is getting a bit of a charge).

Starting of the ICE, which can only happen in READY mode, is done by MG1, powered by the traction battery.

-Chap

 

Thanks!

There's one extra mode you didn't name: ON but not READY (the mode you're in after two power-button pushes without the brake; ACCessory is one power-button push without the brake). The 12 volt battery is providing all the power in that mode also. The DC/DC converter is only online when the car is READY.

-Chap

 

A little more on-hunkering research reveals there's a workable technique for extra-extended hunkering.

So, a little review from last time:

Reviewing why (2) did not work well for Gen 3: when the heat is off, and the coolant temperature is anywhere near operating temperature (80 to 90 C or so), the car will cycle the engine to try to keep the coolant temperature in that ballpark. That's unnecessarily hot.

Why (3) worked better for me: when the heat is on, and in ECO mode, and the car figures out you're just sitting parked and using the heat, it will use lower limits for the coolant temperature. Over a few cycles, it will gradually reduce the setpoints until it is cycling the engine on around 50 C and off around 60 C. That's still plenty hot enough to make warm air for you, but it slows the rate of other heat losses, and slightly extends the time you can hunker.

Now. here's a thing in the control strategy that you might not guess: if you wait with temp at min and fan at min and ECO on, until the car has settled into the 50 C to 60 C coolant temp cycle, and you wait until the engine has just cycled off, around 60 C, and just then you turn the heat off, the car will not try to bring the coolant to operating temp. (!)

If the engine is stopped and the coolant is under 60 C when the heat is turned off, the car will decide that it is far enough below operating temp that there is no point starting the engine to bring it up. It will then ignore the coolant temp, and not start the engine again until needed for battery charging, when SoC drops below 40 %.

That gave me an engine-off time of 39 to 41 minutes. Today, at −23 C (−9 F), in a polar vortex of generational severity. That was with electrical load that included the seat heater and a 40 watt heated blanket. (And it stayed quite cozy there between the seat and the blanket.)

This is not a set-and-forget technique. Once 40% SoC is reached and the engine starts, because the heat is off, it will want to run up to operating temperature before shutting off.

So here's the trick: as soon as the engine starts, touch the fan switch, so the heat is back on again at low fan. Now the engine will cycle off again at 60 C instead of running all the way to full operating. Once it cycles off, touch the fan off switch again.

Somebody could build a "hunker-extender" that does this. Wait for engine on, simulate one fan-speed button press, Wait for engine off, simulate one fan-off button press.

You end up getting (in polar vortex conditions) around 40 minutes off, 4 minutes on. That's a 10% duty cycle, compared to the 30% I could get with technique (3). However, the engine runs about twice as hard for those four minutes (around 0.6 gallons per hour compared to 0.3) because it is charging the battery up from 40 % SoC. So this is not really multiplying hunker time by 3, but by 3/2, so you get half again the time, or a bit better (I guess my old technique 3 fuel consumption I was calling 0.38 GPH, not just 0.3).

At 0.6 gallons per hour, assuming 10 usable gallons in the tank, 16.7 hours of burn, at 10% duty, 167 hours of hunker, or just about seven days.

Maybe the best thing is the length of the cycle: at 40 minutes off, 4 minutes on, the engine cycling will only jostle you about a dozen times during eight hours of slumber, instead of every three ever-lovin' minutes.

 

A public-service announcement on hunkering

About a week ago, as a way to make sure the car got some run time, I took a laptop out there with some work that needed doing and just sat in front in 'hunker' mode doing it. I did not bother with the fiddly extended-hunker technique, turning the heat on and off; I just used the technique (3) above, temperature to the lowest real setting and fan overridden to low speed.

Sitting in the heated seat with the 40 watt blanket on top, I was entirely comfortable the whole time. Eventually too hot, in fact, and turned the seat off and relied just on the blanket.

I did a couple hours of work in perfect comfort, shut the car off, and brought myself and the laptop back inside. That's when my toes thawed and I realized I had perhaps my most-serious-ever frostbite. It seems I was so comfortable with the seat heater and blanket keeping my core temperature up, and my toes hadn't bothered me because they were numb, it really snuck up on me.

I'm sure I would have been just fine if I'd been Prius-camping, sleeping flat in back all covered up and with the heated blanket. But a seated position can be deceptively comfortable, I guess, and may need some extra provision for the feet.