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What speed provides the best mpg on a flat road?


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25 replies to this topic

#1 OFFLINE   Drag Limited

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Posted 28 April 2015 - 12:59 PM

I've been a hypermiler for some time and last week just purchased my 2012 FFH. 

 

In my old pickup I could get the best mpg at 42 MPH because it was the slowest I could go in my top gear without bogging down the engine based on my scan gauge. 

 

Because my Fusion has EV mode at a certain mph and a CVT transmission with no "gears" I was wondering what is the most fuel efficient speed to drive for a long distance? 









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#2 OFFLINE   Waldo

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Posted 28 April 2015 - 01:09 PM

Since the hybrid doesn't have the problem of the engine operating outside of it's optimum BSFC zone due to gearing, the best mpg will be obtained at the slowest speed, so 0.01mph.


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#3 OFFLINE   md13ffhguy

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Posted 28 April 2015 - 02:19 PM

Since the hybrid doesn't have the problem of the engine operating outside of it's optimum BSFC zone due to gearing, the best mpg will be obtained at the slowest speed, so 0.01mph.

Is that to say that efficiency trends downward as speed increases from 0? Is there truly no bell curve...?

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#4 OFFLINE   Drag Limited

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Posted 28 April 2015 - 03:09 PM

Since the hybrid doesn't have the problem of the engine operating outside of it's optimum BSFC zone due to gearing, the best mpg will be obtained at the slowest speed, so 0.01mph.

 

I would think that just to keep the engine running / battery at a sustainable level that you would actually be much more "MPG efficient" at a higher speed than .01? it's hard to image going 650 miles at .01 mph haha. 

 

As I mentioned earlier, in my truck with conventional gears, my top gear (4th) was more fuel efficient than the first three. 



#5 OFFLINE   lolder

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Posted 28 April 2015 - 05:46 PM

In a test in 2008, a 2010 FFH was driven by expert hypermilers over 1400 miles on a single 17.5 gallon tank of gas. The average speed was 22 mph and the mpg was 81. There is no "bell curve" of mpg for most eCVT hybrids.

 

At a practical speed, the 2010 FFH gets 50 mpg at 50 mph at 85º F so you could drive 17 hours and 850 miles and have 1/2 a gallon reserve. There would need to be some facilities in the car.


Edited by lolder, 28 April 2015 - 07:57 PM.

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#6 OFFLINE   Automate

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Posted 28 April 2015 - 10:12 PM

Probably not a symmetrical bell curve but certainly 0.01 mph is NOT the most efficient speed.

 

Look at it this way.  We know the car can go 700 miles on a tank.  If driven at 0.01 mph it would take 70,000 hours to go 700 miles.  That is almost 8 years!  We know a tank of gas, even if the car is left in park will not run the car for 8 years.

 

The problem is the car uses some energy just to keep itself on.  Even if you turn off all the optional assessors you still have a lot of electronics to keep powered up such as the power control module, battery control module, transmission control module, OBDII module, anti-lock brakes, DC to DC controller, power steering control module, instrument panel control module, etc. In addition you have the battery cooling fans, and all the energy lost charging and discharging the battery if you never use the ICE to make the car move.

 

My guess is it wouldn't be a single speed.  The most efficient speed would be one speed when running ICE and another speed when running EV mode.  


Edited by Automate, 28 April 2015 - 10:16 PM.

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#7 OFFLINE   Griswald

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Posted 29 April 2015 - 07:10 AM

I think it all depends on what the computer decides is right at the time. Running at 65 mph with the ICE off is pretty efficient, but you can only do that for a mile or so before the computer starts the ICE to recharge. Running at 65 mph with the battery full and the ICE running can get over 50 mpg on gasoline alone, that is really difficult to keep running like that, the computer wants to keep switching to battery.

The best answer seems to be "it depends"

#8 OFFLINE   Waldo

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Posted 29 April 2015 - 07:42 AM

Good point on the accessories load, there is definitely a time factor in the power draw that would mean going a bit faster would be helpful.  But you cannot separate the ICE and EV modes when considering the most efficient speed.  The most efficient speed will be the one in which the ICE runs at it's most efficient to charge the battery to near full, then you deplete the battery until it's nearly empty, then repeat. This must be balanced against the rolling resistance and aero drag forces, which both go up as speed increases.  Now the computer may be programmed to only run the ICE at it's most efficient when actually accelerating the vehicle, so the most efficient way to drive is probably to accelerate up to around 30mph, then immediately coast down to EV idle and idle along until the ICE comes back on again.

 

I assume that the power transfer from battery to motor is most efficient at the lowest loads possible, but if that's not the case, then there may be a certain load in EV mode that's faster than idle speed that is more efficient.

 

The real difference in concept from a conventional vehicle is that in a conventional vehicle you want to run the engine at the point where it is getting the best instantaneous fuel economy.  In the hybrid, when the ICE is running, you do not want to be getting the best instantaneous fuel economy, you want to be running in the optimum BSFC region of the engine, then it will charge the battery faster and turn off sooner.


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#9 OFFLINE   hybridbear

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Posted 29 April 2015 - 07:52 AM

Remember that the OP has a 2012 FFH so his powertrain design and maximum EV speed are different than what most of us know in the 2013+ FFH.

 

 

The real difference in concept from a conventional vehicle is that in a conventional vehicle you want to run the engine at the point where it is getting the best instantaneous fuel economy.  In the hybrid, when the ICE is running, you do not want to be getting the best instantaneous fuel economy, you want to be running in the optimum BSFC region of the engine, then it will charge the battery faster and turn off sooner.

Exactly!


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Current Vehicles

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Previous Vehicles

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#10 OFFLINE   Drag Limited

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Posted 30 April 2015 - 10:41 AM

Yes, I own a 2012 old body style fusion so my numbers will be a little bit different. 

 

I need to do a little research since this is my first Hybrid because "BSFC" talk is over my head.  But if there is a certain mph that I could utilize on long trips for my 2012 then that would be helpful. I'd like to stay at a minimum of 45 mph but I wanted to know if there was a sweet spot for my older hybrid just to keep in the back of my mind. 



#11 OFFLINE   Drag Limited

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Posted 30 April 2015 - 10:47 AM

In a test in 2008, a 2010 FFH was driven by expert hypermilers over 1400 miles on a single 17.5 gallon tank of gas. The average speed was 22 mph and the mpg was 81. There is no "bell curve" of mpg for most eCVT hybrids.

 

At a practical speed, the 2010 FFH gets 50 mpg at 50 mph at 85º F so you could drive 17 hours and 850 miles and have 1/2 a gallon reserve. There would need to be some facilities in the car.

 

Thank you!

 

Would it have a substantial jump in mpg to slow down to 45 mpg to utilize EV mode?

 

I make a 150 mile trip on 4 lane roads (not interstate) that will allow me to drive as slow as 45mph without causing an accident. But 50 MPH would be a very safe speed since most of the road is 55MPH.



#12 OFFLINE   Waldo

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Posted 30 April 2015 - 03:57 PM

Slower is always gong to be better, but let's do a little math.  Say you drive 150 miles at 50mph and get 50mpg.  That's 3 hours and 3 gallons of fuel used.  Say you can drive that same 150 miles at 45mph and get 55mpg.  That's an extra 20 minutes of driving and you will have saved 68 cents worth of fuel (assuming $2.50/gallon)  Is 20 minutes of your time really not worth 68 cents?


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#13 OFFLINE   lolder

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Posted 30 April 2015 - 04:00 PM

Drive at the posted speed limit or faster if most of the traffic is doing so.


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#14 OFFLINE   md13ffhguy

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Posted 30 April 2015 - 06:41 PM

Or, just...

Drive at the posted speed limit (end quote).

Seems to be a bit more efficient that way. Oh, and stay in the right lane. ;)

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#15 OFFLINE   md13ffhguy

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Posted 30 April 2015 - 06:47 PM

Is 20 minutes of your time really not worth 68 cents?


That's a good point, but I was reading this thread to be about what's theoretically possible. This is something I've often wondered... A perfectly straight road that stretches on forever. No wind, no other traffic, etc... At what constant speed can this car travel and cover the greatest distance? Theoretically speaking. My guess it's somewhere in the 45 mph range.

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#16 OFFLINE   Automate

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Posted 30 April 2015 - 07:11 PM

In a test in 2008, a 2010 FFH was driven by expert hypermilers over 1400 miles on a single 17.5 gallon tank of gas. The average speed was 22 mph and the mpg was 81.

 

I would guess these guys did their homework and knew the most efficient speed on ICE and EV.    My guess would be 25 to 35 on ICE and 15 to 20 on EV.  They spent a lot more time on EV so their average was closer to the EV speed.

 

A 2nd generation FFH should be able to do even better with its smaller engine, better aerodynamics and lithium batteries.


Edited by Automate, 30 April 2015 - 07:15 PM.

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#17 OFFLINE   billford

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Posted 30 April 2015 - 07:43 PM

I'm not an engineer, but I think that the engine could be a lot more fuel efficient if it runs at a constant rpm when it needs to run, something like a stationary generator. It could be tuned to run at peak efficiency in a narrow rpm range. Maybe even the 1 litre ecoboost could do the job.

 

But I'm sure It would be objectionable to drivers when they hear the engine running at high rpms especially at a red light on a cold day with the heater on...

 

 

 

 
 
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#18 OFFLINE   Automate

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Posted 30 April 2015 - 07:56 PM

I'm not an engineer, but I think that the engine could be a lot more fuel efficient if it runs at a constant rpm when it needs to run, something like a stationary generator. It could be tuned to run at peak efficiency in a narrow rpm range. Maybe even the 1 litre ecoboost could do the job.

 

That was the first design when GM was developing the Volt.  No mechanical coupling between the engine and the wheels.  The engine runs efficiently but what they found is power losses converting mechanical energy to electrical then sending it into the batteries and then extracting it back out are much greater than the very small losses of a eCVT transmission.



#19 OFFLINE   lolder

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Posted 30 April 2015 - 10:14 PM

Here's a "Fuel Map" of the efficiency of two Prii ICEs measured in "Brake Specific Fuel Consumption", BSFC. http://ecomodder.com...prius_bsfc1.jpg

 

The chart is for the last two Prii ICEs. They try to only operate on the heavy solid lines. You can see for the 2FR-FXE, the current Prius ICE, the engine operates within a region where the BSFC is a very low 220 grams/ Kw-hr from about 1100 to over 3000 rpm. The Ford graphs would have similar shapes but have different values. If you watch your RPM while driving, it will usually be within this range. The lowest BSFC for the 2FR-FXE would probably be about 2100 RPM from the chart and be somewhat less than 220. That's where you would operated if you used it solely as a constant speed generator. As the above poster pointed out Chevy has found that's inefficient and the latest Volt has even more torque directed to the wheels by gears. By variable valve timing and other computer controlled ICE operation manipulation the lowest BSFC areas are over a fairly broad RPM range. This is the heart of all Atkinson ICE eCVT hybrid designs. The Eco Boosts do not have these low BSFCs. They are designed to produce a lot of power per weight and displacement.


Edited by lolder, 30 April 2015 - 10:16 PM.


#20 OFFLINE   Texasota

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Posted 30 April 2015 - 10:30 PM

I'm not an engineer, but I think that the engine could be a lot more fuel efficient if it runs at a constant rpm when it needs to run, something like a stationary generator. It could be tuned to run at peak efficiency in a narrow rpm range. Maybe even the 1 litre ecoboost could do the job.

 

But I'm sure It would be objectionable to drivers when they hear the engine running at high rpms especially at a red light on a cold day with the heater on...

Even more annoying would be the low speeds likely encountered when climbing a mountain or similar long grade.






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