Per Hour fuel consumption of the Volt ICE

[fredevad]

Does anyone have any information (or educated guesses) on how much fuel the Volt's ICE will burn per hour?

i.e., I've used my "40 miles" of all electric mode, and continue to drive (city/highway) for another hour. How many gallons of gas have I burned?

Thanks!
Fred

[jrigg]

I'm not awake enough for math, but maybe you are.

Figure that the ICE will provide 50 miles equivalent for every gallon burned. Go from there.

[GearheadGeek]

I think the right answer is "it depends." While there will probably be a figure for how much fuel the ICE will burn per unit time while it's in operation, how much of the time it's in operation for an hour of driving time will depend heavily on driving conditions. In stop/go city driving for an hour at the minimum state of charge, I'm guessing the ICE will actually be running much less than 60 minutes. In flat, steady-state 60 mph cruise after the battery has been depleted to the minimum allowed state of charge, it might run most of the time.

[fredevad]

Thanks for the replies!

jrigg - That's kinda what I'm trying to figure out - except in reverse. I'm trying to calculate my "effective MPG".

As GearheadGeek pointed out - "it depends". Since I know what my normal driving conditions are, I figure that if I know how much gas the ICE will burn per hour when the customer depletion point is reached and the ICE is running 100% of the time, then I can do the math to more accurately determine my operational costs.

For example, I know that a trip I make every 2 weeks is 236 miles round trip, and figure/assume that the first .5 hour (40 miles) are electric only, leaving a balance of 4.5 hours of driving with the ICE running 100% of the time (at a set RPM?).

I found (thanks to this site) the "55 * M / (M - 40)" calculation, however...

My thought is that since the ICE is only generating electricity and not directly powering the wheels, an MPG calculation is invalid to calculate the fuel used. I'm trying to calculate how much gas I'll actually burn during 4.5 hours it's running before I get home to recharge.

Hope this helps clarify my questions. Thanks again,
- Fred

[omnimoeish]

Oh, fun math problem...

I guess that my car burns about 2 gallons per hour on the freeway and gets about 30 mpg, so if you got 50 mpg, that's 5/3rds better gas mileage, and so 2 gallons per hour divided by 5/3 = 1.2 gallons per hour in a Volt. That seems about right since 50 miles * 1.2 = 60 miles which is probably close to your average speed on the freeway. 1.2 gallons of gas right now is about $2.40 per hour. If you are driving .5 hours on electricity (we'll say about $.20) and 4.5 hours on gas, you are burning $10.80 of gas, all together about $11 each way, $22 round trip. As far as gas consumption, you're looking at burning (1.2 gallons per hour for 9 hours) 10.8 gallons of gas.

Now to switch it up and come at it from the miles side. Considering your trip is 236 miles round trip, and you are running all electric for the first 40 miles each way, you are running on gas for 160 miles. 160 miles/50 miles per gallon = 3.2 gallons or $6.40. So that's obviously a lot lower which is understandable that it takes longer than the theoretical I calculated going 60 miles per hour for the full 5 hours. So the real answer is probably somewhere in the middle.

[GearheadGeek]

...

For example, I know that a trip I make every 2 weeks is 236 miles round trip, and figure/assume that the first .5 hour (40 miles) are electric only, leaving a balance of 4.5 hours of driving with the ICE running 100% of the time (at a set RPM?).

...

Remember that on a "real world" drive, even once it's reached the minimum allowable state of charge, the ICE isn't likely to run constantly. You'll also have some charging of the battery with regenerative braking, long downhill runs could allow the ICE to shut off, and the minimal power required to maintain cruising speeds in lots of situations would allow the ICE to build up to its designated maximum state of charge for ICE operation and shut down, allowing the car to run off the battery only for a while.

I'm probably complicating this unnecessarily, but I think that calculations assuming the ICE will be running more or less constantly after the battery depletion point is reached would be overly pessimistic.

[fredevad]

Great responses!

#5 omnimoeish - I guess I'm reluctant to use 50 mpg as a basis for the calculation because 1) that's what I'm trying to calculate and 2) I'm concerned that it is a "safe" marketing number that accounts for all driving styles/conditions/city/highway/etc.

#6 GearheadGeek - I thought I had read somewhere on this site that once the ICE engages, that it's used until the Volt is plugged in and recharged. Maybe I read it wrong or that's changed, it would be great if I could get another 40 miles of electric drive! Anyway, I'm going to assume it stays on for my cost comparisons and I don't have any way to quantify it (at least not until I own a Volt).

Meanwhile - I did a quick search on Google for something like "gallons gas per hour idle" (sorry, had to reboot since and lost the exact text I used to search) and got these numbers as a starting point:
.156 gal/hour
.4 gal/hour
.34 gal/hour

I know that the ICE isn't going to be [comparitively] "idling", but I'm hoping it wouldn't run more than 1500 RPM - please check me if I'm wrong here. So I'm going to use a more conservative number:
.5 gal/hour

So, my calculation comes out to:
4 hours running on ICE @ .5 gal/hour = 4 * .5 = 2 gallons
236 miles using 2 gallons = 236 / 2 = 118 MPG

Now of course, this has to be computed for each trip I would make that would require the ICE to engage.

I'm not trying to challenge GM's 50 mpg. In fact, I think that's a good marketing number so as not to cause an uproar. I already have a spreadsheet worked out to show my gas savings, so I can change the gallons/hour number and get new figures. (Is that too geeky, or am I just excited about the Volt?)

Please feel free to challenge my logic or theories - I want to be as realistic as possible (if not underestimate a little).

[Rooster]

To calculate MPGe (Miles Per Gallon Equivalent), use the following equation:

MPGe = Distance / ((Distance - EV Range)/MPG using ICE)

GM has given us the following values for the Volt:

EV Range = 40 miles
MPG using ICE = 50 MPG

Thus to calculate MPGe for the Volt, use the following equation:

MPGe = Distance / ((Distance - 40)/ 50)

In the case of a 236 miles drive, the MPGe for the Volt is:

MPGe = 236/((236 - 40) / 50) = 60.2 MPG

Easy!

[Rooster]

For those who don't like math, see the attached graph.

[fredevad]

Thanks Rooster, that's seems to be a slightly different calculation than I have seen before. I'll plug that into my spreadsheet and see how it changes things.

I think that I'm not clearly communicating what I'm thinking. Let me try this from a different angle...

Let's say we have 2 identical Volts with depleted batteries sitting side by side on a flat-as-far-as-the-eye-can-see 2 lane Texas road:

• At 10 AM, both cars are driven with the ICE running.
• Volt #1 is driven at a constant speed of 30 miles per hour.
• Volt #2 is driven at a constant speed of 60 miles per hour.
• At 11 AM, the cars are stopped.

Which car used more gas? Why?

Based on how much gas they used (not GM's "50" number), what is the MPG for each Volt?

Thanks again for your thoughts,
- Fred