Can using CHG Mode be more efficient?

I have read completely opposite of your statement. The larger the engine, the higher the efficiency. And maybe you can't compare the generator in Prius against a portable small engine generator.

Here is one quote I found by Google search. Not saying this is the definitive proof or anything. But there are many search results saying a similar thing. And if you extend your search to any generator, you will find comments and articles with over 90% efficiency of generators.
https://www.plantengineering.com/articles/large-engine-generator-sets-keep-getting-better/

 

I don't know about this. If the larger gas engine generator is more efficient under load but not when low load. It seems to be related to gas being wasted when idling (low load). I can see a larger engine being more wasteful than a smaller engine if it is constantly idling. I think using CHG mode in PARK is similar to using a large gas engine generator under a very low load. The gas is being wasted for idling. I have no data or proof to support this but I have a feeling CHG mode is more efficient while driving than being stationary.

 

I have no training in this area to fully understand this paper. (full citation is posted below) But if my interpretation of the graph below is correct, then the highest efficiency of the Prius generator is 93% and it can be mapped at speed of ~250rad/sec and a Torque of ~100N.m. The lowest efficiency is 40% and it is at idling (speed=0).
EDIT: I checked it online. 250 rad/sec is ~2400RPM



CITATION: Alzuwayer, B., Abdelhamid, M., Pisu, P., Giovenco, P. et al., "Modeling and Simulation of a Series Hybrid CNG Vehicle," SAE Int. J. Alt. Power. 3(1):2014, doi:10.4271/2014-01-1802.

 

I still think engine running while driving will make it more efficient in generating the same amount of energy. Partially because I saw the graph I posted above. It is not about CHG mode on Prius Prime, but regular Prius has the same generator as far as I know, just not a big enough battery to put extra energy generated. The generator efficiency is the same whether it is for a small battery or for a large one.

But that's enough arguments without any data. If you want to prove your hypothethis, now you would have to do two experiments, measure the use of gas and amount of kWh generated while PARK and while Driving. Please post when you have some data to share. I will go the empirical drive testing route and will post when the gas price becomes more reasonable or if I have to drive without EV range left in the battery.

 

Well, you will also have to do the measurement of CHG mode generator efficiency while driving. @MTN already thinks the result will be in favor of HV only under your testing setting.

Whatever the result, it only tells how efficient the CHG mode generator is while in PARK, and tells us nothing about how it behaves while driving. Nonetheless, it will be interesting to see what kind of results you get. If you have a stretch that long, then it would be easy to modify it so that you drive half the distance on CHG mode and rest on EV mode from the charge generated by CHG mode (plus HV mode if EV range is not sufficient to cover the half of the test course), and compare the overall MPG against all HV. You don't need to have a full 80% SoC to test the CHG mode efficiency. This is essentially what I am proposing to do on my regular commuting route of ~36 miles. Still, you need to perform the test more than once and average the results. More the batter.

For the testing scheme that you describe, make sure to switch to EV mode immediately after CHG mode reaches the max EV SoC of 80%. I noticed this last time I did my experiment, the CHG mode did automatically stop at SoC80%, but the car stayed in HV mode. It may be because the car was in HV mode before starting the CHG mode. But I had to actively switch to EV mode to start driving on the traction battery.

I also wonder what really happens to the mpg display during CHG mode at PARK and after you start driving on EV mode from the charge generated. I have never done it, so I don't know what happens, but while the car is in PARK and in CHG mode, the mpg will stay at 0 mpg. But in reality, it is actually using gasoline so the amount of gas on the denominator of the mpg calculator continues to be increasing. After you start driving on EV, the mpg display should start showing a positive number and keep increasing. Is that really what happens? If it is, and the mpg number does get up to mpg obtained by the HV mode before the end of the run, then CHG mode is more efficient, if it never gets to that mpg, then HV wins.

Yes, I gathered that much. They are using Prius as a reference model for the simulation of the natural gas GENSET engine serial hybrid concept car because the Prius generator efficiency map (the figure I posted) was already known to the authors presumably from the previous testing done on Prius. I just don't understand all the formulas and equations in that paper.

 

If you know that the mpg indicator does keep track of the used gas despite the fact the car has not moved, then for the purpose of calculating the amount of gas used by CHG mode at PARK, you can simply drive the car 1 mile on EV mode after CHG mode is completed and record the MPG displayed number. If the number shown is, say 10.0 mpg when you drove only 1 mile on EV mode, then you know ~0.1 gal of gas was used by CHG mode. Again, this does not tell you anything about the amount of gas used during CHG mode in drive, which seems to change quite a bit depending on the driving condition.

I just did very short (5 miles) drives on CHG mode. Although not identical 5 miles, the driving conditions and weather were similar. The first 5 miles produced 10% SoC and overall MPG 32.6mpg. The second 5 miles produced 20% SoC and overall MPG 34.6mpg. I think the low SoC produced in the first 5 miles is because there was no SoC left in the traction battery at the start. That is the EV range was - - miles. At that point, the SoC can be a negative number as low as 5% off. 10% SoC gain may be in reality a 15% SoC gain. But still, that is 25% less than 20% SoC generated in the second 5 miles of CHG mode.

 

If I get a chance, I will try CHG mode at PARK to gain SoC 10% (but should not be done when the EV range is - -miles) before driving off my driveway. Then I will check the mpg at the 1-mile mark. I will try repeating it several times at, least 5 times but more the better, under similar temperatures and other environmental conditions. An 1 mile of drive from my driveway would be as constant as I can manage on any other road, so there should be very little variable for that 1 mile other than weather-related changes. It will be interesting to see if the number (mpg) I get is more or less constant. If this number varies widely from one testing to the next testing, then it will be very difficult to make a meaningful conclusion from the rest of the experiments.

 

Not sure what you mean by pressing the HV button. I am only interested in seeing the mpg display at the 1mile marker, not after the trip. Changing to HV mode at the 1-mile marker would slow down the climb of mpg numbers, but still, keep changing afterward. Also, if I do this test only once a day, then miles/kWh statistic can be looked at by checking the daily Eco log, but if I do it more than once on the same day, then there isn't an easy way to keep the miles/kWh statistic separated, that is unless I "reset" the EV efficiency for each trial. But I would like to keep my EV efficiency un-reset for the life of the car to keep the number going corresponding to ODO reading.

One possible problem with this experimental procedure is that the frequency of the reading of the mpg monitor may be too low for a 1-mile trial. I know the mpg reading does not update every second. IIRC, it is more like every 30 sec or even longer. With a longer interval of update, it is likely that mpg reading right before and right after the update may show big differences.

 

Well, the experiment is to test to confirm that there is No variability in mpg. Meaning, it is a test to confirm that gasoline used under CHG mode at PARK is constant (or at least very close each time). But if this number varies a lot, then the rest of the experiments are not very meaningful. And I will not be looking at the gauge next to the speedometer. That gauge up-and-down so rapidly, there is no way to pinpoint what the number (and there is no numerical display) the bar is indicating. Rather, I will be looking at the AVG MPG below the mile/kWh (or MPG) gauge.