Can using CHG Mode be more efficient?

I happen to have some gas I need to burn. Before the winter I like to refresh the gas can for the generator and snow blowers. It is over 6 mo old of gas in the can. I just hate to buy new gas at the current high price. At the current rate of driving, I am doing, a full tank in PP should last just about 6 months provided I don't do the long-distance out-of-town drive. But come winter, I would have to burn gas to get the heater running anyway.

 

I also did a few tests today. Very similar to what you did, but I did not do the resetting Trip meter at the instant SoC flipped to the next %. So my numbers can have a bigger margin of error +/-0.99%? But at least the result is quite similar to what you observed. That is, the extrapolated amount of gallons needed to fill 100% SoC was ~0.9 gal (average 0.890gal, with range 0.826 to 0.943, n=3). However, I observed that for three trials I did, each time CHG mode got more efficient. It could be within the margin of error, so may not be significant, but the trend seems to be as the engine got warmer, the more efficient CHG mode generator got. Three trials were done 2-3 hours apart each after parked for that duration, so, the engine temperature difference is likely caused by the ambient temperature as it got warmer during a day.



Still, if we believe the number 0.9 gal gas for CHG mode in PARK to generate 100% SoC is true, then it is not likely that using CHG mode as a generator and using the SoC for EV mode later is going to beat the efficiency of the HV mode only for the same distance traveled that is short EV range trip of 25-35miles. But I still believe there is room for a debate that CHG mode used during DRIVE can under certain circumstances be more efficient than straight HV mode.

I have a weekly chore to run tomorrow, so I can test to see if using CHG mode will make the overall mpg better than HV mode only on this route.

 

I agree. For a very short trip, HV mode mpg suffers a lot. I can never get above 50mpg if I am only going down the road for 5 miles. For those short trips, yes, CHG mode used to generate SoC to be used on EV mode later would make perfect sense.

 

Yesterday, I drove for my weekly chore. I drove the first half (16.8miles) in CHG mode which resulted in exactly 50% SoC gain and returned the second half (16.8miles) in EV mode using the SoC. I started CHG mode with 2% SoC left on the display to make sure I am not charging any over usage of SoC below EV range - - miles. I returned home with 3% of SoC left, meaning it actually used 49% SoC on the EV mode. The mpg displayed in the first half was 33.0 and at the end of the return trip was 66.1mpg.

Comparing this to the same trip I did a week earlier all on HV. The difference in mpg is negligible, CHG/EV 66.1mpg vs HV 66.9mpg. The road conditions and environmental factors were very similar. The average speed was slightly faster for the CHG/EV mode trip.



Now, this is only an n=1 data, but I now think neither CHG mode nor HV mode has an edge when compared under identical conditions. This may be explained by the fact that either using CHG mode to feed the SoC and use it on EV mode or under HV mode the engine comes on as needed to feed the SoC and allows the car to drive in EV, there is no fundamental difference here. If the driving conditions are identical, then either on HV mode or CHG mode, the generator efficiency is the same. If I can, I will try more experiments for both HV mode and CHG/EV mode, but the gas price got so high now, and cold weather is starting, I may not have any chances.

However, there is one thing that seems to be fairly clear from the result. The generator efficiency of CHG mode is clearly better under DRIVE compared to when the car is in PARK. This is because if we believe the earlier results that CHG mode under PARK generates 100% SoC using ~0.9gallon of gasoline. If I apply this number for the CHG mode in the DRIVE result I just got, it means to generate 50% SoC the car must have used ~0.45gallon of gasoline. If the first leg of the trip was driven the same way, then at 67mpg, 18.6miles first half of the trip was using (16.8/67) 0.278gallons of gas. Add them together, the total amount of gas used under CHG mode would be (0.45+0.278) 0.728gallons of gasoline. Now since the second half of the trip 18.6miles was on EV mode, the overall mpg for the 33.6 miles trip would be (33.6/0.728) 46.2mpg. The empirical mpg from the dash was 66.1mpg. Even with the dash mpg display being optimistic, the difference is too big to explain if the generator efficiency of CHG mode under DRIVE is equal to or less than CHG mode generator efficiency while in PARK. Thus, the only explanation left is CHG mode generator efficiency is better while in DRIVE compared to in PARK.

Interestingly, when I was using the CHG mode during the DRIVE, I observed the following. The first 10% SoC gain to 12% took 9 min. This is 2 min shorter than the 11 min it took to gain 10% SoC while in PARK. Then the next 10% SoC gain to 22% took 5 min, and thereafter, every 10% SoC gain took 4-5min until I stopped the CHG mode at 52% SoC. This may be explained by the fact that as the engine gains more loads under DRIVE, the efficiency of the generator was increasing.

 

Yeah, but now I read back and think it through, to confirm faster charging speed during CHG mode in Drive, we have to check the speed is constant when the car is in PARK. I and you did only 10% SoC gain while at PARK. It is possible that maybe even at PARK, the speed increases after the first 10% SoC.

Also, I did not CHG all the way to 80% during my experiment, the speed may slow down as the SoC get near full.

 

Even if my single data point holds to be true, and CHG/EV mode has no advantage nor disadvantage in terms of efficiency compared to using HV mode, there still are some applications of CHG mode that may prove to be useful. One was already mentioned which is to use CHG mode before reaching the destination when there is no plug-in and know there will be a need for EV range at the destination.

Another I now think maybe useful is using CHG mode on a very cold morning below 14F. I use to drive HV mode in my morning commute and saved my EV range from the wall for my afternoon commute back home. This was because, during winter I needed an engine to heat the cabin in the morning, and the EV range was reduced to almost half so that I could only drive one way on EV mode. If I knew CHG/EV mode is as efficient as HV mode, then I could have used CHG mode in the morning commute and used the SoC for the EV mode on my way back. Two years ago, gas was cheaper than electricity, if I did that, it would have been more economical. But now, with the higher price of gas, it makes no sense to do that.

 

Doing some search, I found someone has already tried what we have been experimenting with and came to basically the same conclusion.

This proves 11 min/10% stays constant all the way to 80% (88min).

And @OptimalPrime had guessed it way before us that the CHG mode in PARK is less efficient than the CHG mode in DRIVE. As for the amount of gas used by CHG mode in PARK in his experiment is consistent with our findings. That is ~0.7gal for 80% or ~0.9gal for 100%.

 

Yes, I agree. I don't have an opportunity to test higher speed testing for I rarely get on an open highway to attain that speed. And a few opportunities I get on a long trip with higher speed, I don't do it repeatedly to be able to do the comparison. But, sure at higher speed, the CHG mode generator efficiency may get even better. And there may be a special circumstance in which CHG/EV would beat the HV efficiency.

As I commented in #85, the sweet spot (the highest generator efficiency) is likely to be at ~2400RPM and a Torque of ~100N.m.

But this sweet spot is the same for HV mode and CHG mode. I still think if you can do the comparison under identical conditions, CHG mode efficiency would be identical to HV mode. That is my working hypothesis now.

 

Not sure what you are getting. Having read the comments and results in another thread, it seems clear to me that CHG mode generator speed in P is constant for the duration of entier 80% SoC charging. If I did not read that comment, it would have been interesting to check if the CHG mode generator speed would increase after the 10% SoC mark, like it did while in D. But, for me, reading the results from the test done by others is good enough to satisfy my curiosity.

Of course, you are welcome to repeat the experiment. It is probably easily reproducible unlike many real scientific experiments of today. I am not interested in wasting gas on my car just to repeat what has already been tested.