Start with HV Mode and 100% SOC for longtrips.....

Are you going to recharge upon reaching your destination?
If no, you might want to use HV for the trip portion and EV for putting around at your destination.
There is more than one correct answer to your question.

 

This thread has quite a bit of conversation on driving HV vs. EV on longer trips. It also deals with ECO vs. NORMAL modes, but I'm sure you can skip over those comments and follow your main interest.

My short answer is that the Prime seems to get better mileage with some range left in the EV portion of the battery, but there have been no scientific tests that I've heard of to test the theory. However, it is prudent to use some of the EV range at the start in case you need extra room for charging in mountains or other long descents. Whether you have 10% SOC showing on the display or 90% won't make any difference.

 

i don't see how the premise can be true.

 

I'm still on the fence. Last night, for example, I used up all but about 4 miles of EV on our outing while we were still about 10-12 miles from home. At 4 EV miles left, I switched to HV to see what it would do. Driving mostly at 50 mph or slightly over and using the air conditioner, the car would go for about 2 miles or so at a time in EV on its own once the ICE warmed up and repaid its energy loan from the battery. Then the ICE started and charged pretty aggressively for a while giving about 25-30 mpg, then it tapered off, giving me about 60 or so mpg before it shut off again for a couple more miles. Looking at the bars on the 11.6" screen, it looked like the ICE was on about 50% of the time. But then again, I had to stop every 1 to 5 miles. I don't seem to remember it going that far on EV when the EV range was used up. If it's indeed going farther between startups of the ICE, that means less energy spent starting the ICE. I can see how that would boost economy but have no way to know if it's enough to be significant. That would take a controlled test on a closed loop course.

 

Another thing to consider is the battery charge and discharge limit. I've been watching this for a while on the Hybrid Assistant app. I'm going by memory, so don't quote me on these numbers...

The Charge Current Limit (CCL) starts around 25kW and quickly increases to 40kW by the time you use about 10% of the EV range, and usually stays at 40kW into HV mode at long as the battery temperature is mild.

The Discharge Current Limit (DCL) seems to start in the low 60's fully charged, increases to a maximum of around 68 or 69 kW between 40-90% of the EV range (as indicated on the MID, not SOC indicated by Hybrid Assistant), and starts to decrease as the EV range approaches 0%, with the sharpest decrease happening in the last 5-10%. This one seems to be particularly sensitive to cold, especially if the car sat out overnight without charging. I've seen DCL as low as around 30kW with the EV range nearly used up. When it gets that low (even below 45kW), the engine will sometimes start if you try to accelerate quickly or if you go up a hill on the highway, presumably to make sure you always have enough power available to accelerate if needed. Last time I saw that happen, the engine started on the highway in EV mode (not EV Auto), and it bypassed the engine warm-up cycle and immediately started driving the wheels.

It is surprising how much DCL varies. I would expect most of the variation to be outside the operating range of the car (for example with a warm battery it should be higher, even if it's still limited to 69kW, so that with a cold battery you still get 69kW), but it's not. Maybe it was really a stretch to get 69kW out of this battery. An interesting thing to note is, if you floor the accelerator, the moving bar display (can't remember what it is officially called) still shows the same range (the bar stops just below the PWR area) even if the discharge current is limited. You just accelerate more slowly.

With the EV range fully used up, driving in HV mode, the DCL is probably around 20-30kW (with battery temperature around 50-60F. I never paid attention to this in the summer). But the car seems to limit it more, since the engine will usually come on when accelerator input exceeds the EV driving bar on the MID. I can't remember the limit, but I think it's around 7-10kW.

It's possible that if there is some EV mode range left, the battery DCL could be higher, which could result in higher efficiency if the car takes advantage of it. For example it might let you accelerate from a stop in EV mode, rather than turning on the engine if you try to accelerate faster than a snail (as it usually does in HV mode).

 

even if the car allows more charging and discharging in hv mode, isn't that less efficient unless you're on a route where more regen would make it advantageous?

 

I'm not familiar with the article, but if that's what it says, I'd say it's dead wrong to get home with a full charge in the battery and waste all the gas you could have saved by using the electricity. --- Unless gas is really cheap and electricity is really expensive. But if that was the case, I wouldn't have a plug in vehicle. And as mentioned earlier, the battery doesn't need to be at 100% to help with efficiency. Anything over a couple miles of EV range showing will help just as much as a full charge and could even help more depending on terrain.

As for the 640 mile range, that's assuming getting the EPA rating for fuel economy and using up all the EV range. Anyone can do better than the EPA mpg rating unless they are fighting bad headwinds or deliberately driving wastefully. Last May on a 6,000 mile trip I had a tank that went well over 600 miles and still had well over one gallon left.

The only way to be sure is with a day at the race track. Do about 100 laps in each condition (0 EV %, 50 EV %, and 100 EV %) on a day without temperature or wind variations and see which is better.

(edited a couple minor typos)

 

You're probably right that being able to use EV mode more is not necessarily more efficient. Once the battery is depleted all the energy ultimately comes from the engine. It could theoretically be more efficient to have a more powerful EV mode if you only needed the engine for a short burst.

There regen limit is usually the same even when the EV portion of the battery is used up, but I like to keep a few percent remaining in EV mode so that I can use B mode for regen with less chance of the engine coming on.

We're talking about supposedly only a slight increase in efficiency. It's probably also only a slight change in the programming for when to run the engine and how much, otherwise we probably would have noticed it.

 

My many experiences on starting a long trip (of more than 300 miles) in HV mode from start with 100% charged battery and not use charge depleting EV mode do not show any clear indication of increased HV efficiency in MPG shown compared to starting a long trip (of more than 300 miles) in HV mode from start with 0% charged battery. Of course, if you have 100% SOC and use that SOC during the trip, that will always increase overall MPG due to the fact ~25 miles are driven by pure EV.

On the thread @jerrymildred pointed, I started to do my own testing not on the long trip, but on my short daily 18 miles commuting. Not enough data yet to validate or repute the hypothesis "that the Prime may be getting substantially higher HV mpg with some EV charge (about 10% or more) remaining vs. no EV charge remaining at all (close to 0% EV charge)." I just started taking note of my daily HV drive record. So far only n=1 for each category one with EV SOC or another with 0% EV SOC.

 

I I've decided that if the difference is so small that we can't be sure of it then it's not worth fussing with. I just try to leave a few miles of EV in case I need to do some parking lot driving when I'm out & about. But I always try to use that up before I get home or some other destination where I can charge. If the ICE is warm, I can still easily do the last mile in HV without the engine running.