Can using CHG Mode be more efficient?

In different thread @KYBlue reported a case in which use of CHG mode and then EV mode from the SoC gained on his trip made the overall efficiency measured by MPG better than just driving the same route on HV mode without using CHG mode.

See his comment in this thread:
Question about battery charging while on the go | Page 2 | PriusChat

I was very intrigued by his finding and wanted to confirm his observation. Unfortunately, I do not do often enough repeated longer distance drives right now, so that my chance to do the testing was very limited. Testing on a shorter trips less than 30 miles is likely to cause the comparison difficult due to more pronounced variations in numbers. I just made a 65 miles/one way drive yesterday and had a chance to make initial comparison.

This is what I did. On my way to the destination, entire 65 miles was driven on HV mode. The EV range was already depleted at start, so that gave me a chance to test driving this distance all on HV mode. The route I took was rural highway with one lane on each side with top speed limit of 55mph. Very light traffic but had a few in town sections with slower speed limit and traffic lights. Overall, rolling hills up and down with slight elevation hike of ~400' to the destination. The entire distance was driven uninterrupted at average speed of 45mph with EV ratio of 65%. This trip resulted in average efficiency of 65.5mpg all from the dash.

Then, on my way home, I started my trip with CHG mode until it stopped at 80% EV SoC. This took 29 miles at average speed of ~45mph and displayed efficiency was 37.5mpg. Then using the 80% SoC, I switched to EV mode and drove next 28miles all on EV until it depleted the EV range. I needed to finish the return trip on HV mode for the last 8 miles. This return trip was also uninterrupted at average speed of 45mph, but EV ratio displayed was 49% and the average efficiency shown on the dash was 72.3mpg.

The summary of the result is shown in the table below:


Now, it is comparing the same distance, but not in the same direction, so it can not be taken as a direct comparison, especially because there is a slight decline in elevation on the return compared to the incline that had to be driven on HV mode on the way to the destination. It was very calm day so head and tail wind are not likely the contributing factors. But I think there is a situation in which the use of CHG mode and using the SoC acquired by the CHG mode to drive on EV mode later on the same trip may increase the overall efficiency.

I would love to try the same drive again with using CHG/EV mode on the way to the destination, and use HV mode on the return for the same trip, but I don't have plan for another visit to the destination again anytime soon. I have to wait for next repeated longer trips for more controlled experiment.

 

Yeah, I have not tried using CHG mode in any of my usual ~35miles round trip commuting drives. I do think longer distance but not overly long trips are needed to distinguish the real difference from daily variation. The ideal distance seems to 60-80 miles trip. Either round trips, or do one way experiment on both legs of the trips twice. But it maybe not. I may have to try a few drives with CHG mode and without to see if it makes noticeable differences for the same ~35 miles trips I routinely make.

My main question is to answer this:

My initial thought was that it would be impossible to use CHG mode to make the car run more efficiently than just using the HV mode, but now I think I was wrong.

 

From my initial observation, CHG mode on a steady 45mph drive took 29miles of driving to gain 0-80% SoC for 28miles of EV drive. If you want to test the full EV range gained by CHG mode, then you need at least 60 miles trip.

But I am thinking that I don't have to gain 80% SoC to test the efficiency comparison. If the SoC gain is completely linear, then 40% SoC gain would need ~15miles of steady 45mph drive to gain ~14 miles of EV drive. That's only a 29miles trip. So, in theory, I can test and compare my routine ~35 miles commute (round trips) with and without using CHG mode. I may try that. But on my routine commute, I will not get an average of 45mph speed. It is more like 30mph since I don't drive a long stretch of non-stop rural highway. It is, after all, only 18 miles one way.

 

I agree 100%. For reference, @KYBlue stated that he was gaining 1mile EV for every 2miles he drove on CHG mode at 74mph on CC. This is vastly different from what I observed on my trip. I was gaining roughly 1mile of EV for every 1 mile of CHG mode drive at steady but not CC controlled average 45mph highway driving. I am sure this number will change if I use the CHG mode in 30mph commuting with some stop and go.

That is exactly my problem. I am not doing much driving now except routine commuting 1-2 times a week and occasional longer out-of-town trips and my wife using PP for around-town errands. It is hard to make more than two longer trips under very similar conditions. Now going into the colder season, the weather factor can be drastically different from one day to the next.

 

As I commented, one trial I did was not in the same direction so it can not be directly compared. But other than #1 on your list # 2-4 were all satisfied. If I make frequent enough trips of the same route of ~65miles, I can easily satisfy #1, but unfortunately, I don't make trips of that distance often enough. Also as I commented, the initial observation made by @KYBlue in another thread was on the round trips.

I plan to do the shorter trips (~35miles) with or without CHG mode to see if that is sufficiently long enough to show a significant difference. I will post the results when I do.