My first real test of Charge Mode efficiency

Well you'll just have to test it out and see.

 

That's great. I have 90 and 200 mile road trips this week so I'll get to play with this a bit. I'm very impressed with the vehicle over the 3rd gen and it was no slouch.

Very well said.

 

OP's result is too good to be true m'thinks.
If he is reading the trip mpg off the car meter I am even more skeptical

I'd like to see the results determined by actual gasoline consumption as measured by starting with a full tank and refilling to full at the end of the trip as a x-check

 

It would be interesting to see how this system would work on a longer trip. Start with a full battery, use it to zero, then use charge mode to refill it. Repeat.

 

Certainly. Then compare it to the same route without the charge mode application. I do not own a Prime or I would be doing it myself.

 

I'm with @Lee Jay on Charge Mode. In fact, I would like to propose that in some fairly common scenarios, Charge Mode should be used for optimal fuel economy. Hear me out. This is counter to my initial expectations/understanding when I first got my Prime. Here is a test (the proof is in the numbers), followed by my reasoning of why it is more efficient for this (and similar) scenarios. For anyone curious, I removed my roof accessories before these tests, so it's basically stock.

Test 1:
I drove on steady flat ground (engine already warm) with climate control off for approximately 50 miles at 70 mph using cruise control. The outside temperature was about 50F. The car was in HV mode with the EV range depleted. A trip meter set at the beginning of this test read 52 mpg for this test, which steadied out much earlier than 50 miles. This mpg value is in close agreement with an estimation I've seen of mpg in the range between 40 and 80 mph equalling 120-X (where X is your speed in mph).

Test 2:
Under the same conditions (warm engine, flat ground, no climate control, 70 mph using cruise control), starting from 0 EV range, I engaged Charge Mode for 18.3 miles (before construction began and I had to reduce my speed). During that time, I gained 10.9 miles of EV range. A trip meter set specifically for this test read 40 mpg. A separate test in Charge Mode gave 38 mpg, again at 70 mph. Therefore, 39 mpg under these conditions seems about right. For these tests the miles_charged/miles_driven ratio is ~60%. Therefore, if you drive 10 miles in charge mode, you'll gain 6 EV miles. A reminder: these values apply to this situation and are probably non-linear, so don't go using them for other scenarios.

Now, consider this scenario: you have to drive 100 miles on flat ground and you have no EV range to start off with.

Scenario 1:
You drive in HV mode and the engine is on the whole time. You average 52 mpg and use 1.92 gallons of gasoline.

Scenario 2:
You immediately turn on Charge Mode and drive 31.5 miles. You gain 18.5 miles of EV range (I chose 31.5 miles so that adding the EV range results in 50 miles, to make this example simple). Turn on EV Mode and use up the 18.5 miles. Do this one more time and you'll hit 100 miles. You average 39 mpg in Charge Mode for 31.5*2=63 miles. The rest of the miles are EV and use no gasoline. Therefore, you use 63 miles/39 mpg = 1.615 gallons of gasoline. The other 37 miles used no gasoline (directly). The overall mpg is therefore 100 miles/1.615 gallons = 61.9 mpg.

You get 10 better mpg by utilizing Charge Mode this way in this example. [An aside: you could argue that the MPG gauge on the Prime is unreliable; I do think it might be skewed high by 3-4 mpg, but I think it's relatively reliable and it should at least comparable to itself]. Here's why I think using Charge Mode is more efficient in this situation: At 70 mph on flat ground in HV mode, the engine just stays on the whole time, purring at a low RPM. Sometimes the battery is being charged, sometimes it helps propel. Nevertheless, the engine is always on. For the second scenario, the engine is off 37% of the drive. All of the ICE inefficiencies are gone while the engine is off. When the engine is on, it's revving higher, yes, but in a more efficient part of its operating curve. This is somewhat like the concept of a regular hybrid in city driving taken to the extreme (shut the engine off whenever you can). I will also mention that I'm at half a tank and this ~62 mpg figure seems accurate (rough estimation: I've driven 284 miles on pure HV and I'm at at half a tank (halfway before indicator light comes on), so I should have used about 9.5/2=4.25 gallons, which gives 283/4.25=66.8 mpg).

I think there are many such circumstances where Charge Mode can be used. In fact, I'd suggest that any moderate-speed driving on relatively flat terrain should use CHG/EV cycling. Some situations you should not do this:

• If you have city driving, I doubt you should do this because that'll be less efficient (engine mainly as generator) and you should have saved EV from earlier.
• If you have nice big hills, you'd have to use Charge Mode in a more advanced fashion. Charge Mode steals your power and it'll rev high on a steep incline. Also, the hills act sort of like a battery, storing your potential energy and allowing the engine to shut off on the downhills, helping out your overall mpg (flat ground doesn't allow this). Also, you might want to save EV for power on an uphill, etc.
• If you need lots of power (passing or small steep hills), you won't want to be in CHG mode during those high-demand situations.
• If you need heat, you might think that you'd just opt for full-time HV to utilize "free" engine heat. However, I noticed that with this CHG/EV usage, the vehicle was still able to maintain 72F inside using residual heat from the engine (while it was off) and 38F outside until it was time to engage CHG again.

 

For test #2, did you actually drive the 10.9 miles on EV and demonstrate that was the actual mileage gained or was that just what the car estimated?

I ask because we know the estimator isn't always that accurate so, for instance, the car was actually only able to go another 8 miles @ 70mph (even thought it initially said 10.9), then you would have only achieved 56mpg using charge mode. Yes, that's still better than just HV mode, your results do look positive.

 

Switching the display to report PERCENT rather than estimated distance would be more informative.

 

I do not understand the 52 MPG in scenario 1. This summer I drove a round trip of 600 miles in my Prime, averaging 65 MPG. I started out with a full battery but did not charge on the trip. I set cruise control at 65 MPH the entire trip, except when going through perhaps six towns. Starting elevation was 1500 feet, with destination at 8500 feet. 58 MPG the first 300 miles, and 72 MPG the last 300 miles. Used AC the entire trip. Is there that much difference between 65 and 70 MPH?

 

Good test. I'm anxious to try this on our new Honda Clarity PHEV which strangely allows recharge up to 50.7%. Perhaps Honda (and Toyota) determined the potential pay-back isn't worth it above their set limits.

Your test makes sense to me in that engines have inefficiencies just to run themselves. When you're driving at highway speeds those inefficiencies are in effect already so adding the charge load can use up any extra energy.

One of our regular trips is down highway five in California's central valley which is straight and mostly flat. We could easily do some similar testing.

 

5 mph would give you about 5 mpg better. Starting charge (30 miles?) about 3.5 mpg better. Driving in town, who knows, but definitely an increase. The AC doesn't use too much energy once at temperature.

 

I did just what you're saying on a 317 mile road trip. Going same direction in both attempts (to account for the net elevation difference of about 150 feet). My best mpg I could get in HV only with no starting battery charge was 77.3 mpg. I then did the trip starting with no battery charge, using Charge Mode to charge from 0-80%, and then EV Auto to deplete to 0% (because I wasn't fully aware of the difference between EV and EV Auto at the time, embarrassing I know, but I still managed to not have the ICE come on once), then repeated until the end of the trip where I tried to use Charge Mode just long enough to give me enough EV range to get me to my ending distance and end with zero battery. I got 63.2 mpg doing this (but I also had different tire pressures since the air compressor was out of order in my second attempt, and in one run there was a 16-18 mph wind blowing south).

Both videos are here if you want to see test methods (not the most controlled test as I'm sure some of you will pick apart but far better than any other Prius Prime mpg tests I've seen on YouTube and the tape was more to get a distinguishable thumbnail than drag but I'm sure it had a minor effect):

 

Hey @Charged Up, thanks for the videos. I'd seen your first one at some point. I like your tests and I do not mean to pick anything apart. I want to try to determine what's different between our tests from a technical perspective:

• Your second test averaged 65 mph versus 58 mph. This is a big difference. According to data in several places (several here on PriusChat), with the 120-X (X being mph) estimate for mpg (aka 1 mpg per 1 mph), that's about a 7 mpg differential I would expect. That could be 7 of the 14 mpg differential between your tests in favor of CHG/EV.
• You mentioned you had no tape in the second video. I don't think that really had much effect (the Prime is already very slippery!), but I appreciate your dedication in the first attempt!
• I think utilizing CHG in the city is inefficient. At highway speeds, some of that power is always going to the wheels and then extra to the battery. In the city, there are times where it's all to the battery and operating the engine as a pure generator is not as efficient as simultaneous propelling + generating, I believe.
• I saw at one point you were in the red "power" zone on the MID. The engine is revving fairly high in this zone. At 70 mph I was able to stay out of this zone except for a portion of one or two small hills. I believe allowing the engine to rev into this region hurt your mpg. I'd turn EV back on if you need power for a short time.
• Because you had a route with traffic and were in control of the accelerator/brake yourself, there are many variables that were not controlled (I'm not trying to criticize--it's very difficult to deal with traffic and have a test with the fewest changing variables as possible). My tests were both at a constant speed on flat ground with no traffic. Of course, my tests have their own issues. I did not document the wind speed. Wind speed can be a big factor, so somehow that needs to be accounted for accurately. Although the ground was flat, there were very minor slopes and my tests were not on the same portion of road.
• I hope that the mpg readings are fairly accurate in all of our tests. I have to assume they are (at least comparable in the Prime family).

If you have another opportunity (it seems you might), please repeat this test noting the above potential issues: lower average speed, no red zone, CHG/EV only between maybe 55 and 75 mph, etc. I recognize these speeds are not optimal for mpg overall, but my proposal is that this is the region where the method makes sense versus HV. Below 55 mph (or higher in hilly areas), it seems even with minor hills the engine is able to shut off in HV mode--meaning that it's being able to operate a little more efficiently (like a real hybrid) by cycling off and on. I agree that CHG/EV would not be beneficial in those circumstances. Basically I'm advocating that CHG/HV is appropriate compared to HV in a condition where the engine is not cycling off/on (in HV mode) and is humming at a low power state (and you don't need lots of power). Although this seems like a very specific situation, there are many flat interstates and highways with 65-75 mph speed limits--that's a lot of potential Prime driving that might be best done with CHG/EV (if indeed this is the more efficient thing to do).

 

Yeah you're right, some of the variables could have been more controlled. Now I have winter tires and the lake effect snow can be brutal - I want to be safe for myself and all others on the road, so I'm going to wait until spring. During the video when it was in red, that was on a long hill, using Charge Mode. I was trying to raise my speed before the hill so that my momentum could help me not be in the red zone at all on the hill, but the hill was too long and I had slowed down to about 45 mph just before that. I'm not sure if the gaining speed before the hill and then easing on the accelerator method is best way to approach hills, haven't done much research into it.

My channel originally wasn't intended to be documentation of engineering tests, it was intended to try showing people considering purchasing a Prime some of the features and what to expect. Its also much easier to film/edit showing features because I can redo the clips however many times I want. When I do tests, a lot of them are a one-shot deal, and I'm still quite the amateur behind the camera, which makes it more difficult to do those and do a decent job. Hopefully in the spring I can reattempt and make a better video.

 

This is a matter of personal preference. The mile estimate seems to simply be the percent remaining multiplied by usable EV kWh and then multiplied by your average miles/kWh. If I see 5% left, that means little to me, but if I see that I have 1.5 miles left and I can drive more efficiently than my average, I know I can drive that distance before running out of EV. (Also if you're moving and have 1 mile left, you'll gain some distance from your current kinetic energy, but if you're stopped, you won't make it a full mile). The estimation is pretty reliable when you account for these things. My current max EV range is 33 miles so 10.9 miles remaining on my car is 33%.

This is a great point. I'd intended to, but construction lowered my speed so I gave up on monitoring the trip meter while using up the EV and just documented my remaining distance. I'd have gotten less because my average miles/kWh at 70 mph is around 4 miles/kWh but my overall average is 4.7. This would bring that ratio I mentioned to 51% and the CHG/EV estimated mpg to 59, closing in on the HV mode's 52. I decided to look up wind data and it seems like during my HV test there was a side wind but during the CHG/EV test there may have been a 5 mph wind at my back (effectively reducing my speed 5 mph, giving me 5 mpg better?). Sigh. Variables. Another test...

I had to drive more on the interstate more today and tried this again at steady speeds of 70 mph. The HV test was 57 mpg for 16 miles. The CHG/EV test took 37.5 miles to generate 19.7 miles of "quoted" EV range, which resulted in 16 miles of actual range. This ratio of 16/19.7 is nearly equal to the ratio of 4.0/4.7 supporting my belief that the distance estimated is based on average miles/kWh from the Drive Monitor. During Charge Mode it averaged 40.2 mpg and the stated mpg when running out of the charged EV was 57 mpg. If you perform the same calculation I did before (this time the miles_charged/miles_driven ratio is 42.7%), the estimate is 57 mpg under these conditions. It was 57 all around, so in this pair of tests the two driving modes tied.