Hybrid Charge Mode Question

The chief engineer said it would take "between 1/4 and 1/3 of a gallon" to bring the battery from 0 Ev miles to 80%.

 

Can you please elaborate on the 25% and 6.5 miles? Which is a reading from car report and which calculated by you?
Also, your trip report attached indicates average speed of 61 mph and not 65.

 

Thanks for paying attention to detail. I set cruise speed to 65mph. To safely take a picture, I exit highway and go down the ramp while waiting at the first traffic light right off highway I took the picture. 61 mph because it account for the slowing down till stop process.
6.5miles and 25% are from the screen.

I was expecting big drop on MPG but surprised it still get 37mpg with 6 miles EV range energy stored. There must be efficiency loss in the process of kinetic to electricity then back to kinetic.

It's possible that toyota computer calculation is wrong although less likely.

I am more intrigued now. We should be a design simple experiment to verify if those dream number are real. Some engineer deep inside toyota probably know the answer we are pursuing.

 

You want to use the car as a generator. As you know, any generator can not achieve 100% efficiency. Using the car generator vs. the power grid is ill-advised.

 

I was skeptic in the past but:
@bowang reports 25% in 11 miles @ 65 mph and 37.2 mpg. That's 35 miles to 80% and 0.94 gal at that speed.
35 miles @ 56 mpg gives 0.63 gal leaving about 0.31 gal for the charge.

Could be that the 9 kW added load makes a substantial improvement in engine efficiency for quite wide (constant) speeds band, an improvement equal or greater than the losses in the electrical pass.

We need more data points.

 

24 minutes highway driving round trip.
Running a loop i.e. starting point is end point
Adaptive cruise speed set to 65mph
Average speed 61 to 62 mph. Depend on traffic.
62.0 MPG reading from dashboard when 100% hybrid mode
Same condition except no hybrid mode just CHG mode mix with EV mode resulting 62.6 MPG! (battery level is the same 76% at beginning and the end)

Do you believe this?

Top is CHG mix EV resulting 62.6 MPG
Bottom is hybrid mode 62.0 MPG


However, gas meter decrease about 0.6 gallon indicate 42MPG not 62.6MPG. I think longer trip is needed to avoide inaccuracy caused by measuring error.

 

I keep getting the impression from comments in other threads that while in HV mode it will not charge beyond the "HV" level of the traction battery, i.e. other than regen it will act as if you have a regular Prius traction battery and not charge more than that level. Maybe I have misread things but that is what it sounds like. Is that correct?

If so then my next question is why is that, why wouldn't HV fully charge the traction battery if it can do it efficiently, i.e. I would expect that after 200 miles of highway cruising at 65mph I would have a fully charged traction battery, with only a relatively small impact on mpg during the 200 mile drive. Instead it sounds like you have to switch to charge mode, and be savvy enough to do so only when you know it is an efficient time to charge. Unless I am misunderstanding how HV works in the Prime. Or maybe the math would prove my theory wrong about the 200 mile trip. I try and avoid math during my non-working hours, and thus appreciate those of you who do all of these calculations.

 

I believe it.
In HV mode trip ICE was on 62% of distance, so "ICE on" mpg was 62*0.62=38.44mpg
In the other trip ICE was on 50% of distance, so "ICE on with charge mode" mpg was 62.6*0.5=31.3mpg
In the "charge mode + EV mode" trip you have used 25.3*0.5/31.3=0.404 gal.
For the same "ICE on" distance in HV mode you would have used 25.3*0.5/38.44=0.329 gal

One may conclude that 0.075 gal were used for charging, but it may be a bit more:
If, at this constant highway speed, charge mode caused the ICE to work at a higher thermal efficiency then the gallons used for propelling the car would have been a bit lower than in the HV mode trip and as a result the gallons for charging - a bit higher than 0.075 gal. How much is that bit? (if any) we will probably never know with the tools available to us.

 

Because it's not designed to do that.

Hv is designed to target roughly holding the traction battery at the same level of charge, CHG is designed to target 80% full on the traction battery. Different targets = different results.

The only way Hv will fully-charge your battery is if you descend a very long hill.

 

As others have speculated (I am not an engineer) but 65mph the engine is likely already outside the peak efficiency curve so asking it to charge is putting on more strain. Basing this on the gen3 displacement graphs that have been posted here that claim maybe charge mode is best at a constant speed of 75km/h or less.

That said if your numbers are accurate sounds like you burned ~0.2 gallons to get 12 miles of range which isn't a terrible tradeoff especially, as others have pointed out, it might allow you skip some engine short cycling when you hit the city.

 

Charging half in ~15 min is full charge (80%) in ~30 min which confirms reports by others. That is 9+kW rate, almost 3 times the L2 wall charge rate.
There is more and more evidence that charge mode at constant speed of 60-65 mph and using the charge later, you are not loosing efficiency as compared to HV drive at these speeds.

 

I realize that HV has a different purpose than charge mode and will thus have different criteria for when it charges. I will try asking the question a different way. The regular Prius has a 1.3 kWh battery. Until now I assumed that the only reason it does not have a larger battery (say 2.0 kWh) is because a 2.0 kWh battery will weigh more and cost more, and the designers felt that the additional cost and weight was outside of the price targets for the Prius.

But your answer seems to indicate that even if additional battery capacity is available for "free" i.e. the additional kWh is already bought and paid for and taking up space in your plug-in, that HV mode refuses to use the additional capacity because other than descending a long hill it will never be efficient at any time to charge beyond the HV target. Is that the reason? And about the long downhill, is it using ICE to charge past the HV target, or just regen?

 

No.

It will use the extra energy capacity in Hv mode, it's just that there isn't much more energy available in most driving situations.

For example, you could argue the primary purpose for the battery in a regular Prius is to recapture acceleration energy - store the energy from coming off the highway for later use.

But let's see how much that is:

E = 1/2 * m * V^2 = 1/2 * 1800kg * 30m/s^2 = 810,000J = 225W-h = 0.225kWh. The regular liftback can store all of that easily. The only way you're going to need more room to store energy is a big descent.

The biggest decent around here (from the continental divide down into Denver) is from 11,300 feet to about 5,300 feet.

E = m * g * h = 1800kg * 9.8m/s^2 * 1,830m = 32,281,000J = almost 9kWh. Now we're talking Prime, not lift-back sorts of energy storage.

So, yeah, the Prime will use the bigger battery in Hv mode, but it will only need it on a big descent.

 

In regular Prius the ICE will automatically charge the battery only when charge is below target and will get more aggressive help from the battery when SOC is above target (like after regen). I assume you know that driving a Prius.
Size of battery is IMO an optimization between investment and possible returns.
Prime and PiP work exactly the same in HV mode, the difference is when you have a regen opportunity long enough to exceed the max capacity of the HV portion of the battery (a case where hydraulic brakes take over in regular Prius) this regen is captured into the EV portion of battery.
Prime will charge battery beyond max capacity of the HV portion in two ways:
Long regeneration - no ICE.
Charge mode forced by the driver.

 

That's a case where engine braking takes over, unless you aren't driving the car properly on a long decent.

I generally pride myself on being able to get over the continental divide and back without ever touching the brakes. I just did it two weeks ago.

 

Agree, hydraulic or engine brake.

 

I guess I was just never aware that the efficiency of HV is so limited. It sounds like you are all saying that from a purely cost/emissions standpoint, HV really should only generate electricity by regeneration. Because apparently every drop of gasoline used to charge the battery is wasteful, except for very rare exceptions. If so, then apparently the only reason that HSD uses ICE for charging is because otherwise people would grumble in situations where they have used up all the regen charge and only have ICE for power.

If this is correct. then I must have been misled in the past by a misconception that there are times during most trips when ICE is underutilized, allowing HSD to generate electricity relatively cheaply, then use it later during acceleration, providing a net gain in efficiency. Apparently that is rarely if ever the case? If so then I guess I understand now that the only real advantage of additional battery capacity is as a place to store grid power which is usually more efficient than gasoline. And as a side benefit the additional capacity gives you more room for storing regen although most of the time you don't need it. But using the additional capacity for HV would be totally inefficient.

 

FWIW. I have been able to charge my battery to 80% on HV on the fairly even roads on CHARGE MODE, but when I hit the downhill I quickly change to EV mode to milk the rest of 20% to come in to Valencia at 100%. Guessing you just have to be actively monitoring to make the best out it. I would recomment it would mainly make sense to take advantage of CHG mode when going downhill or like others have mentioned when you will NOT have opportunity to charge and you do not mind paying extra on gas for that REX effect.

 

A thought about charge mode, it would seem to run counter-intuitive to Toyota's advice for prolonging battery life. Isn't dumping in at 9kwh likely to have a larger negative impact on li-ion life compared to charging at, say, 1.4khw @ 120V?