Efficiency comparison Normal vs HV mode

[Michael L.]

I have been keeping detailed stats since I purchased the car in Dec 2017. I had an '15 Accord I was leasing that I was using on trips and just used the Clarity around town until I turned in the Accord in Mar 2018. Now that I've been taking the Clarity on trips and actually using the gas engine now, I felt it was time to do a comparison between driving the vehicle in Normal mode (i.e. using up the battery and then auto HV) vs strategic use of HV mode (using only on sustained highway drives that are beyond my electric range).

So far my results indicate that it is more efficient to strategically use HV mode vs driving the car in Normal mode.

In April 2018, I went on 2 trips the first half of the month and just let the car select the propulsion (Normal mode). During the second half of the month I went on another 2 trips that were to different places but the mileage and driving conditions were nearly identical and good enough to do a comparison. This time I strategically used HV mode on the highway.

From the picture, for the first half of the month, the vehicle attained a 57.9 mpg-e rating using Normal mode. For the second half of the month, the vehicle attained a 56.4 mpg-e rating strategically using HV mode. Yes this is lower, but to compare apples to apples we have to mix the same amount of EV driving to the second half of the month that I did during the first half. Once you hypothetically equalize the EV driving (i.e. same amount of Electricity used) to make an apples to apples comparison, the vehicle attains a 63.7 mpg-e rating. So far, the stats support using the propulsion sources advantageously.

 

[Michael L.]

By the way, my overall cost per mile, the best "where the rubber meets the road" figure to compare against a gasoline car is $0.045 per mile. The Accord hybrid was $0.057 per mile.

 

[Ken7]

Interesting, but I think you'd have to do more comparison tests before any fair conclusion can be drawn. You'll also have to make note of the ambient temperatures encountered during these tests in different modes.

 

[Michael L.]

Interesting, but I think you'd have to do more comparison tests before any fair conclusion can be drawn. You'll also have to make note of the ambient temperatures encountered during these tests in different modes.

Fair enough. I will continue to post my results in the coming months.

 

[Tangible]

Thanks for posting this, Michael. Can you say a bit more about what "strategic" use of HV entails?

 

[Michael L.]

Thanks for posting this, Michael. Can you say a bit more about what "strategic" use of HV entails?

Tangible, sure no problem. What I mean by strategic is basically this:

1) Determine whether your trip is going to take you beyond your current available electric range.
2) If answer to 1 is yes, then push the HV button when you get on the highway.
3) If/when you encounter slower, stop and go driving thru towns/cities, push the HV button again and go back to EV mode.
4) Push HV button again when you are back to sustained highway speeds.
5) Repeat 3 and 4 until you have determined that your current available electric range is sufficient to get you to a place to charge.
6) If you are within the available electric range to get to your charging location (home, for example), turn off HV and finish the drive in EV mode.

I would only use HV charge if you don't have enough charge in the battery to take advantage of step 3.

 

[Demian Johnston]

Has anyone determined if there is a crossover point (highway speed and/or incline) where the electric motor is less efficient than the ICE?

Practically speaking I'm wondering if there exists a possible situation where on a $/mile basis there is an advantage to drive in HV mode even if you have electric range to spare.

 

[Tangible]

Thanks, Michael. My first trip > EV range isn't for a few weeks, so I have time to practice.

 

[Michael L.]

Has anyone determined if there is a crossover point (highway speed and/or incline) where the electric motor is less efficient than the ICE?

Practically speaking I'm wondering if there exists a possible situation where on a $/mile basis there is an advantage to drive in HV mode even if you have electric range to spare.

I'm not sure there is one Demian. Electric is more efficient than gas any way you slice it. In Mar 2018 when I drove nearly 100% EV, including some highway speeds around town within my electric range, my cost per mile dropped down to $0.036 from my overall average of $0.045. I can still get 100 mpg-e at highway speeds around town.

Sounds like an experiment you could conduct and report the result.

 

[dstrauss]

The stats are great to see, but for me its a case of comfort and convenience. I like to use HV mode and save the battery for the in town portions of my drives where the cabin quietness is more appreciable. It's also more convenient, and I can ALWAYS stretch more EV miles in town than on the freeway (the old Prius driver in me - when on a 40-45 mph street in town I glide a lot further than 0.1 mile distance for each 0.1 mile of battery range)...

 

[Michael L.]

The stats are great to see, but for me its a case of comfort and convenience. I like to use HV mode and save the battery for the in town portions of my drives where the cabin quietness is more appreciable. It's also more convenient, and I can ALWAYS stretch more EV miles in town than on the freeway (the old Prius driver in me - when on a 40-45 mph street in town I glide a lot further than 0.1 mile distance for each 0.1 mile of battery range)...

That's the great thing about this car dstrauss, it gives you the options. You could just get in this car and drive, no button pushes required, for convenience. Or you can strategically use the gas and electric power sources to maximize efficiency and comfort. Best car I've owned.

 

[Viking79]

Electric is more efficient than gas any way you slice it.

Exactly, even in the '12 Volt where I checked efficiency in the dead of winter with sub 0F highs and I was getting 1.5 mi/kWh I was still using less energy than a gas car getting 50 mpg.

 

[Kendalf]

Has anyone determined if there is a crossover point (highway speed and/or incline) where the electric motor is less efficient than the ICE?

Practically speaking I'm wondering if there exists a possible situation where on a $/mile basis there is an advantage to drive in HV mode even if you have electric range to spare.

If you're talking purely costs then it would greatly depend on the average $/gallon for gas and the cost for charging in your particular location. The break point would be different for regions with high gas costs and low electricity costs vs high electricity/low gas, etc.

Energy efficiency would be a different matter. Like @Michael L. said earlier, I think that driving on electricity is always more efficient, but at higher speeds you lose battery charge at an increasingly faster rate. In my experience, at speeds above 65mph battery charge seems to drop quite a bit faster, thus making it worthwhile to switch to HV mode until you know you have sufficient charge to complete the trip.

It's a bit of a game for our family on longer trips, as we try to figure out when we should turn off HV mode to just make it home with as little charge as possible. It's complicated by a pretty long hill that we have to climb about 7 miles from home that really eats up battery charge, so we always have to plan on additional battery charge beyond just the miles left before we reach home.

 

[Michael L.]

Using the navigation system certainly helps with that HV -> EV switch over on the last leg, but as @Kendalf pointed out, it's probably best to give yourself a little mileage buffer before the switch is made. For me, it's mostly highway driving to home, including a short stint at turnpike speeds (75+mph) before I get there.

Electricity is more efficient in any driving, the issue with our cars is the energy storage. Since it takes more energy to drive at sustained high speeds (I have had the same observation as @Kendalf on the 65 mph threshold), and the bulk of this energy is in the gas tank, not the battery (7 gal of gas x 32.78 kWh/gal) vs (14.3 usable kWh in battery), it makes sense that in these situations the gas tank should be used. This is also where the gas engine is most efficient. Better to save that limited electric energy to capitalize on the vehicle's supreme efficiency in the slower/stop and go town/city driving.

 

[Demian Johnston]

I guess that if I am doing a 20 mile round trip, with even 15 at highway speeds, its still better to do all Electric when possible

 

[Michael L.]

I guess that if I am doing a 20 mile round trip, with even 15 at highway speeds, its still better to do all Electric when possible

@Demian Johnston do you have a level 2 charger at home? If so, do your 20 mile round trip then charge it and log the kWh to recharge it back to full. Then, on your second round trip, use HV mode instead. After you get back, charge it up (shouldn't be much at all since you used HV mode), and fill the gas tank up and report the gallons used. Convert the kWh used to gasoline gallons (32.78 kWh/gal of E10), and then compute your mpg-e for both trips and compare. My guess is that EV would win in efficiency. The second part of the calculation (and the real truth of the matter) is the cost. Attain your cost per kWh from your electric bill (mine is $0.119/kWh), and the cost to fill up the gas tank. Then compare costs. If you can do this experiment, report the result to us. Thanks.

 

[Demian Johnston]

@Demian Johnston do you have a level 2 charger at home?

If and when I get a L2 charger (with good stats), I will conduct this. Currently I'm still using my included charger. Really, I'm not putting mileage on very fast.

 

[Michael L.]

You can do a rough calculation using the HondaLink app. You have to make note of the % charge in the Battery tile when you first start charging. Then multiply the % difference by 17 kWh to approximate the amount of kWh used on your 20 mile round trip.

 

[Kendalf]

@Michael L. Shouldn't the percentage be multiplied by ~14.3 kWh rather than 17? I believe that going from 100% - 0% only drains the usable 14+ kWh, not the full capacity of the battery.

Also, when it comes to the car's actual efficiency while on EV, we may need to factor the ~10% loss of energy while charging on Level 2. However, if we're doing a comparison of costs between electric vs gas usage then we don't factor that in as the 10% loss is a necessary part of charging the car.

 

[M.M.]

Has anyone determined if there is a crossover point (highway speed and/or incline) where the electric motor is less efficient than the ICE?

The answer is "never" from a thermodynamic standpoint, but in a less rigorous hybrid-mode sense it is probably "when the ICE can be directly connected with the transmission" (which appears to start at around 50mph for me). Honda determined that mechanically coupling the ICE to the wheels with a transmission is more efficient than electric-only ICE-generator-motor coupling at some speeds (backed up by the fact that they went to all the trouble of building a mechanical transmission into the car for that purpose). So it would stand to reason that at the speed it makes more sense to use the mechanical energy of the ICE directly, you could sort of consider that the point where it is being run at its most efficient.

Practically speaking I'm wondering if there exists a possible situation where on a $/mile basis there is an advantage to drive in HV mode even if you have electric range to spare.

This will depend almost 100% on the cost of electricity and gasoline in your area. You can use the MPGe figures to get a kWh/mile approximation (for high and low speeds, if you prefer; it's very generally in the 3-4miles/kWh range, so maybe 0.3kWh/mile). You can use the MPG figures to get a gallons/mile approximation for the same speed ranges (very roughly 0.022/mile). Look at what electricity costs per kWh and multiply it by about 1.1 to account for charge inefficiency. You now have a $/mile value for electricity. Do the same for the cost of gasoline at local prices. You now have a $/mile value for gas. There's your answer. The numbers will differ slightly depending on speed due to the non-linier efficiency curve of each mode.

For my gas and energy prices, gasoline becomes cheaper at 20-25 cents per kWh.

(Actually, because I recently installed a PV system big enough to supply all my energy, even if gas were free it's still cheaper to use electricity, since my marginal cost of electricity is zero and running the ICE reduces oil life.)