[kishkaru]

Hi all,

I just recently purchased a 2018 Volt, and I'm liking it so far except for one thing: EV efficiency at highways speeds.

I was a long time gen 1 Volt owner (2014), and I was able to get 3 mi/kWh (333 Wh/mi) driving at a constant speed at 80mph. With it's 10.1 kWh usable battery, I was able to get 29-30 miles of realistic range cruising at 80mph. This is with HVAC on Fan Only.

With gen 2, I was expecting that same kind of efficiency, but that isn't my experience so far. In the same route I usually take, cruising at 80mph, I'm only able to get ~32 miles of effective driving range. With 14.2 kWh usable in the battery, this comes out to only 2.28 mi/kWh (439 Wh/mi)! This is at similar ambient temps as with gen 1.

As a comparison, I did the same route again cruising at 75mph and I was able to get 36 miles of EV range. This comes out to 2.55 mi/kWh (392 Wh/mi). A tiny bit better, but still much less efficient than gen 1 Volt. I should also mention that in both range tests, I had the cruise control turn on with a set speed.

Any idea of what could be going on here? Did the switch from 1 electric motor to 2 motors negatively effect the efficiency? Or did GM tune the gen 2 Volt to be more efficient at lower speeds, thus sacrificing efficiency at high speeds? I'm really bummed out about this, as I was hoping to get at least 42 miles of realistic driving range at 80mph, which is what I would've gotten with the gen 1 Volt had it had a usable 14 kWh battery.

 

[obermd]

I believe the Coefficient of Drag for the Gen 1 is lower than the Gen 2. The CoD is the governing factor in efficiency at highway speeds.

 

[FI Spyder]

Both Gen 1 and Gen 2 have two electric motors. Tire pressures? What's the CoD of the two cars? I guess it is possible to tune for best efficiency at lower speeds with a sacrifice at higher speeds.

 

[kishkaru]

That's a great question and an interesting observation. For both gen 1 and gen 2, I set the tire pressures to 40 PSI cold, 42 PSI warmed up.

According to Wikipedia, CoD is:
Gen 1: 0.281
Gen 2: 0.285

It's such a small difference! As an interesting point of comparison:
Bolt EV: 0.320
2016+ Prius: 0.240
Model 3: 0.230

I also have a Bolt EV, and I can personally attest that I can get at least 3 mi/kWh on that brick at 80mph, if not more.

So I'm thinking that there has to be something else at play here, not just CoD. I will say that my fuel tank is full right now (dealer gave me a free full tank), so later on I'll run this range test again with an empty tank. However, I don't think that an empty tank of the fuel is going to help that much. (I usually ran around with my Gen 1 Volt at 25% tank full).

 

[MikeyCVolt]

I have no frame of reference for a gen 1 since I've never had one but my numbers for my 2017 are pretty close to yours. At 70 MPH I get about 2.8 mi/kWH. I can only guess that it's gearing. They may have geared the gen 2 a little lower to get that low end grunt to 30 MPH and they may have also tuned the system to get better ICE mileage and that may have adversely affected battery range at higher speeds. To get 3 mi/kWH I have to keep my speed under about 65 MPH.

Mike

 

[FI Spyder]

A full tank will make a difference when accelerating, at steady speed, no difference. Interesting (surprising) that Bolt is that far out CoD wise. I'm guessing between Gen1/Gen 2 cars there is another factor.

 

[jcanoe]

Perhaps one of your brake pads is making contact with the caliper, creating drag.

 

[kishkaru]

I have no frame of reference for a gen 1 since I've never had one but my numbers for my 2017 are pretty close to yours. At 70 MPH I get about 2.8 mi/kWH. I can only guess that it's gearing. They may have geared the gen 2 a little lower to get that low end grunt to 30 MPH and they may have also tuned the system to get better ICE mileage and that may have adversely affected battery range at higher speeds. To get 3 mi/kWH I have to keep my speed under about 65 MPH.

Mike

That's good to hear Mike. Now I can be assured that this behavior is "normal" for Gen 2. Yes, I do agree that Gen 2 has more low end torque, so what you say about the "gearing" makes sense.

Now if ICE MPG's are better than Gen 1, then I'd consider it a reasonable tradeoff at higher speeds. However, as you can see in the screenshot above, I can only get about 37 MPG at 80mph. On my Gen 1 Volt, I got roughly 35 MPG at these same speeds. So really, the ICE portion has improved a tiny bit, but IMO not enough to compensate for the much lower EV range at highway speed.

 

[jcanoe]

How many EV miles when doing 70 - 75 MPH, see post #14 for chart: https://gm-volt.com/forum/showthread.php?301298-How-many-EV-miles-when-doing-70-75mph/page2

According to the provided chart, at 75 MPH @ 80F you should be able to travel 39 - 40 miles at 75 MPH in a Gen 2 Volt.

Other things to check: tire pressure, road conditions (is it correct to assume the road was not wet?) Mythbusters proved that dirt on a vehicle creates enough drag to lower efficiency by ~10%, maybe more.

 

[kishkaru]

How many EV miles when doing 70 - 75 MPH, see post #14 for chart: https://gm-volt.com/forum/showthread.php?301298-How-many-EV-miles-when-doing-70-75mph/page2

According to the provided chart, at 75 MPH @ 80F you should be able to travel 39 - 40 miles at 75 MPH in a Gen 2 Volt.

Other things to check: tire pressure, road conditions (is it correct to assume the road was not wet?) Mythbusters proved that dirt on a vehicle creates enough drag to lower efficiency by ~10%, maybe more.

Cool, good data! Looks like my results of 36 miles @75mph and 32 miles @80mph is similar to the 60F curve (although my ambient was closer to 65F). The 70/80F curves are promising-- I will need to do another range test on a warmer day to see if I can get the 40 miles @75mph that curve suggests.

Tire pressure as mentioned above was 42 PSI, and the road is all clear. There's no dirt on my vehicle, I assure you, the car's only a few days old

 

[FI Spyder]

The Gen 2 does have more grunt but I attribute that more to the fact that it is about 250 pounds lighter than the Gen1.

 

[VoltAP1]

I’m only at 2500 miles on my gen 2, but it has seemed like efficiency is slowly creeping up as I put some miles on it. Maybe some components need a small amount of wearing in to stabilize on friction/efficiency?

 

[Tomt]

CoD by itself is not really what is important. Total drag (CoD X frontal area) is what really matters.

 

[kishkaru]

CoD by itself is not really what is important. Total drag (CoD X frontal area) is what really matters.

I was able to find an old 2014 C&D article for the Gen 1 Volt which specify the 0.28 CoD and 23.7 square feet frontal area, for a drag area of 6.7 square feet. Unfortunately I was not able to find any info on the frontal area size for Gen 2.

Even then, with the CoD being so close between the two generations, I highly doubt that GM went out of their way to make the frontal area that much less aerodynamic, to yield a 25% hit to efficiency at highway speeds. Most likely electric motor gearing is also at play here.

 

[edk-austin]

New tires generally have more rolling resistance too, until the contact patch wears in.

Interesting discussion. I often use TX SH130 near my house (although not for daily commuting), where the speed limit is 80mph.

 

[wordptom]

Any idea of what could be going on here? Did the switch from 1 electric motor to 2 motors negatively effect the efficiency? Or did GM tune the gen 2 Volt to be more efficient at lower speeds, thus sacrificing efficiency at high speeds? I'm really bummed out about this, as I was hoping to get at least 42 miles of realistic driving range at 80mph, which is what I would've gotten with the gen 1 Volt had it had a usable 14 kWh battery.

There is a definite distinction between Gen 1 and Gen 2 two-motor configuration Electric Mode operation.

Pamela Fletcher, at the time Chief Engineer, Volt Powertrain, mentioned in her videotaped presentation that the 2011 Volt was designed to provide full performance using only MGB, the large motor. MGA, the smaller motor, would be clutched into the drivetrain only in driving conditions where two-motor configuration increases operational efficiency. 70 mph is the "example speed" often cited as when the decreasing efficiency of MGB at high speeds makes it practical to switch to two-motor configuration in Electric Mode. Clutching MGA to the ring gear allows the rpm of MGB to be reduced as both motors provide propulsion torque, improving efficiency enough to provide an additional 1-2 electric miles when cruising at highway speeds.

Note that a request for higher torque (e.g., stepping on the accelerator to pass another car while cruising on the highway) would produce a brief delay while the Gen 1 Volt returns to single-motor operation to provide the requested acceleration.

Electric Mode operation for the Gen 2 Volt is different, in part because the two electric motors are more equal in size (87/47 kW vs 111/55 kW). The design has been modified to use only MGB for low and medium torque demand driving, then provide fast and seamless transition between one and two motor configuration when needed, adding MGA output via the second planetary gear for high torque demand driving at any speed, running the two smaller motors in parallel to get more peak output.

One could speculate that efforts were made in the Gen 2 design to maximize the efficiency of the gas hybrid configuration for freeway cruising speeds, expecting many drivers to have adopted the habit of doing such driving in Hold Mode ("to save the battery for use where it’s more efficient"), with less attention given to Electric Mode driving under the same conditions. Electric Mode highway cruising efficiency is of less concern if such cruising is commonly done using the range extender.

 

[Manitoba Keith]

I recently did a ~2000 km trip, using the "Hold" setting, at speeds of ~75-80 MPH with A/C on. The trip was from Southern Ontario through Michigan, Illinois, Wisconsin, Minnesota and North Dakota. I started with a fully charged battery and noticed the battery was depleting ever so gradually over the 1-1/2 day course of the trip. At the end of the trip, the battery was at about a 50% charge. I know I occasionally forgot to set the "Hold" mode for a minute or two, but not very long. My Gas mileage was 40.339 mpg for the trip.

 

[MikeyCVolt]

I recently did a ~2000 km trip, using the "Hold" setting, at speeds of ~75-80 MPH with A/C on. The trip was from Southern Ontario through Michigan, Illinois, Wisconsin, Minnesota and North Dakota. I started with a fully charged battery and noticed the battery was depleting ever so gradually over the 1-1/2 day course of the trip. At the end of the trip, the battery was at about a 50% charge. I know I occasionally forgot to set the "Hold" mode for a minute or two, but not very long. My Gas mileage was 40.339 mpg for the trip.

Hold mode does "cheat" and dip into the battery without giving back what it took. Even on shorter trips of around 50-60 miles in hold mode, my battery drops .3 to .4 kWH. I find that it'll never replenish that .3 to .4 kWH even if you drive at a steady slower speed for some time. I wish hold mode would actually hold, but in reality it doesn't.

Mike

 

[Jmaj]

That's a great question and an interesting observation. For both gen 1 and gen 2, I set the tire pressures to 40 PSI cold, 42 PSI warmed up.

According to Wikipedia, CoD is:
Gen 1: 0.281
Gen 2: 0.285

It's such a small difference! As an interesting point of comparison:
Bolt EV: 0.320
2016+ Prius: 0.240
Model 3: 0.230

I also have a Bolt EV, and I can personally attest that I can get at least 3 mi/kWh on that brick at 80mph, if not more.

Cd of the production Bolt is .308.
80 mph in my Bolt the power meter reads 30 kw. 2.6 miles/kwh. Density altitude~ 1000 ft. MSL.

 

[kishkaru]

Ah yes, you're right, I stand corrected. The hybridcars article does mention the 0.308 number. However, I'm still able to get well over 3 mi/kWh at 80mph here in CA (much closer to sea level), although I don't know if altitude makes a huge difference for EVs.

There is a definite distinction between Gen 1 and Gen 2 two-motor configuration Electric Mode operation.

Pamela Fletcher, at the time Chief Engineer, Volt Powertrain, mentioned in her videotaped presentation that the 2011 Volt was designed to provide full performance using only MGB, the large motor. MGA, the smaller motor, would be clutched into the drivetrain only in driving conditions where two-motor configuration increases operational efficiency. 70 mph is the "example speed" often cited as when the decreasing efficiency of MGB at high speeds makes it practical to switch to two-motor configuration in Electric Mode. Clutching MGA to the ring gear allows the rpm of MGB to be reduced as both motors provide propulsion torque, improving efficiency enough to provide an additional 1-2 electric miles when cruising at highway speeds.

Note that a request for higher torque (e.g., stepping on the accelerator to pass another car while cruising on the highway) would produce a brief delay while the Gen 1 Volt returns to single-motor operation to provide the requested acceleration.

Electric Mode operation for the Gen 2 Volt is different, in part because the two electric motors are more equal in size (87/47 kW vs 111/55 kW). The design has been modified to use only MGB for low and medium torque demand driving, then provide fast and seamless transition between one and two motor configuration when needed, adding MGA output via the second planetary gear for high torque demand driving at any speed, running the two smaller motors in parallel to get more peak output.

One could speculate that efforts were made in the Gen 2 design to maximize the efficiency of the gas hybrid configuration for freeway cruising speeds, expecting many drivers to have adopted the habit of doing such driving in Hold Mode ("to save the battery for use where it’s more efficient"), with less attention given to Electric Mode driving under the same conditions. Electric Mode highway cruising efficiency is of less concern if such cruising is commonly done using the range extender.

It really sucks for people that bought the Gen 2 Volt (especially Gen 1 owners like me) thinking that they could get up to 42 miles of highway driving range for our round trip commutes, so we never have to use gas at all. I really wish that the fuel economy ratings showed a city/highway EV range (like they do for gas), so we can have a good indication of the car's efficiency and range before we buy it. The single "53 miles of EV range" number is really misleading. Even for experienced EV drivers like myself that knew I'd get nowhere near 53 miles on the freeway, I was still expecting at least 40-42 miles.

It's a definitely a good thing that GM improved the gas generator to be more efficient at highway speeds. And yes, I agree that I'd wholeheartedly use the hold mode to use gas on longer freeway roadtrips. However, if my drive falls within 42 miles roundtrip, I should be able to (and given the choice of) using only EV for the entire highway drive.