Cold weather hill climb induces engine ON

Time to investigate:

@Lee Jay a couple of questions:

• tires - snow tires or OEM?
• tire pressure and temperature when last set?
• do you have a ScanGauge II with the Prius Prime fix and tire pressure Xgauge?
• velocity profile?

This is my local test hill:


To avoid the known engine ON cycle, I'll drive in manual mode until SOC is 80%. Then I'll set cruise control to complete the route to the top of the hill.

UPDATE: cold soak 28F, at 9.8 mi indicated, 30% SOC, 65 mph climb using cruise control, the engine ON by itself with a significant growl and excursion into power band.

Here is where the engine ON occurred:

• ~25 mph - 0.0-0.5 miles
• ~55 mph - 0.5-5.5 miles
• ~65 mph - 5.5-10.0 miles and engine ON

Due to the distance, the battery was exhausted on the trip home and I had to flip to hybrid mode to reach home. It will be at least 5 hours before the car is fully charged and I can run another test.

It is predicted to be 35F at ~4PM when I try it again. The dash cam has built-in GPS and ephemeris data.

We have another predicted freezing night early Tuesday that looks to be nearly identical temperature to the first test.

Bob Wilson

 

Thanks,

I can safely handle 55 mph on the test route. I'll also use CHARGE mode to return to shorten how long it takes to get fully charged for the Monday-Tuesday test. This will increase the cold-soak time.

Since this appears to be easily reproduced in freezing weather, let's speculate about mitigations:

• early engine operation - the sudden engine on meant a cold engine forced to high power which is the worst for wear and efficiency. It may make more sense to enter HV mode during lower speed sections to warm it up or even preserve charge during the climb. This may also be a case where "Auto EV" makes sense.
• reduced climb speed - using the power bar, reduce climb speed so it does not peg the maximum EV rate. This may require using the hazard flashers for following traffic.

Does pre-conditioning have an effect?

Bob Wilson

 

Perhaps I'm confused:

The hill climb, engine ON, happened just once?

Bob Wilson

 

Do I understand correctly that both happened without pre-conditioning?

Bob Wilson

 

We're learning winter car operations for predictable and efficient driving scenarios. So we have options:

• preconditioning good - with an exception, it appears that preconditioning is key to EV-only operation. Secondary alternatives under investigation: (1) minimum duration preconditioning, and (2) double-READY. These assume the commute distance fits within battery range.
• engine on strategies - a gentle warm-up helps both efficiency and wear. So turning on the engine in low load, moderate speed conditions BEFORE a known heavy load makes sense. When high power is needed, the engine is at efficient operating temperatures and won't suck down the gas. It may be that "Auto EV" makes sense in cold temperatures on commutes longer than EV range.

In about 2-3 months, Dixie will be warmer than freezing temperatures so my window for investigation will come to a close. Hopefully we'll have lessons learned to optimize winter driving.

Bob Wilson

 

Captured the event:

• 34.51416 -86.59512
• 57 mph
• use "," to backup one frame and "." to advance one

Doing a detailed examination shows a stretch of ~100 ft with a significantly higher grade. This triggers the subsequent engine ON operation.

Bob Wilson

 

If I fat-fingered a decimal longitude or latitude, my apologies. The YouTube segment has the dash cam reported values. Trying to correlate to Goggle Earth is equally likely to have GPS ‘drift’ issues. That 28% grade is off the scale BUT it is a relative steeper part based on my butt-o-meter and the same spot as the previous day.

I can use a 400 Hz, 3-axis accelerometer and get a more precise grade map but it would not change the engine ON event significantly. It is the local increase in grade that appears to trigger the engine ON. There may be a delay but it would be in the seconds range.

My next test will be to confirm a minimum duration, preconditioning inhibits this engine ON event.

Bob Wilson

 

The first documented case:

Prius Prime hit piece on electrek.co | PriusChat

Prius Prime hit piece on electrek.co | Page 2 | PriusChat

I contacted the reviewer and let him know of our progress.

Bob Wilson

 

In this we agree:

I was looking at this primarily because @Lee Jay reported a hill climb induced, engine-ON event. Also, @VTBIGDOG on a bridge approach. So I was surprised at how easy it was to reproduce after a cold soak under 40F. But @Lee Jay pointed out "my hill-climb engine on was after a cold soak outside at 16F with charge starting at 100% after an all-night charge and preconditioning." There was a second report where a family member might not have seated the J1772. Yet using key fob initiated, preconditioning did not inhibit it which puzzled me. Fortunately this behavior is documented:

My experiments show this can happen at 30% and 33% SOC. Furthermore, key fob initiated preconditioning had no effect. So review mitigations for the cold-soak, hill climb problem:

• If possible, switch to HV mode 2-3 minutes under low load conditions before the hill climb and back to EV at the base of the hill. - this avoids slamming a cold engine ON when it is cold if the load requires the engine ON. This requires planning for a known route.
• If an unexpected engine ON occurs, as quickly as safe, pull over and restart the car. - This will put the car in EV mode. While stopped, put the car in HV/CHARGE mode to warm up the engine. Then in HV mode, resume the climb and switch back to EV mode after the crest of the hill. This minimizes the fuel burn and allows the catalytic converter to properly warm up. Note there is a similar problem with the BMW i3-REx when driving at low, 6.5%, SOC. It slams the engine ON but the BMW control laws protects the engine during the warm up and this can involuntarily reduce the speed. There is (was?) a law suit about this documented behavior.
• Keep the car fully charged, preconditioned, in a +50F garage. - @Lee Jay has shown that to be the case.
• Use a battery heater in the empty space. - For example, Expertly Designed Battery Heaters | Phillips & Temro Industries . A pair of them with fiber-fill on the bottom could be laid under the battery and fixed with the radiant surface up. Better still, somehow raise one to touch the center, bottom of the pack for maximum heat transfer. Although 80W seems small, it should be enough to bring the battery pack to a better, operating temperature. Yes there is a built-in battery heater but we have no metrics on when it is used and how much power is required. It may be scheduled preconditioning is needed to give it enough time to warm the battery since the key fob preconditioning appears to be just cabin.

There is probably an SOC vs battery temperature graph with gradient lines that could show the EV limits climbing a hill. However, we don't have the battery temperature and power thresholds that trigger the engine ON. That was a long gulp to say hill climb, engine ON could be predicted.

I have mapped another, loop that includes two grades greater than 10% that I can drive at 35 and 45 mph. Driven at 5AM to avoid other traffic, I could could use this route to map some points on the curves. A simple physics problem, the rest of the curves at one temperature could be plotted. Only, who besides a few of us would care?

It is 44F outside so I'm thinking about recording a practice run.

Bob Wilson