A hypothesis: 75mph is the highway mpg sweet spot

[MaineFusion]

 

This shows the impact of speed on fuel economy. At 60 MPH the car will cycle in and out of EV mode, using the generator to load up the ICE to its peak efficiency when running. Above 62 MPH the hp needed to overcome the wind likely made it rare for it to be more efficient to run with the ICE off. At 70 MPH the wind resistance alone is plenty load for the ICE to be operating in its most efficient range. Above 70 MPH and the ICE starts running inefficiently because the load is too high. See this post for more info on what I'm talking about.

 

A perfect example of this is my ability to consistently do over 60mpg (typically around 63mpg) when driving country roads between 35-45 mph. This past weekend I intentionally puttered around country roads between Saco and Portland Maine to pad extra MPG on in an effort to break the 600mi in one tank barrier. Plus it was a nice day and I really badly wanted to go for a drive.

 

In the course of 100.1 mi with plenty of stops (e.g. engine cooling down), I averaged 61.8mpg and was in EV mode over 70% of the time. When I'm doing normal highway driving I'm lucky to be in EV mode 50% of the time, because when I am in EV mode at highway speeds it requires more electricity to keep the car up to speed.

 

[alpha754293]

I've made reference to my getting above 40mpg with the cruise set at 75mpg, and for me, time and time again it's proven itself to be true. I'm talking about being on a flat road with traffic that's light (or fast) enough that you're not having to slow down all the time due to traffic in front of you. This is with the ICE on, and a perhaps a tiny bit of hybrid assistance. I've noticed that my mileage at 75mph is typically better than my fuel mileage at 65mph. This was true for me before the MPG patch and it remains true now.

 

See my thread about: my fuel economy master equation

 

http://fordfusionhybridforum.com/topic/7331-my-fuel-economy-master-equation/

 

And I think that there's a picture that I have which plots fuel economy [mpg] vs. speed [km/h]. Note: 120 km/h = 75 mph.

[Tommy Juhasz]

What does it mean by having the ICE on?

[GrySql]

What does it mean by having the ICE on?

We have a page with all the tricky acronyms listed, look here.

ICE = Internal Combustion Engine

[jonessoda]

I just did a 700 mile round trip from San Diego to Fresno, which included the Grapevine and got 51.0 on the way up and 51.8 on the way back. Cruise set at 62mph. I can't imagine going any faster would net better mileage.

[dkegel]

I just did a 700 mile round trip from San Diego to Fresno, which included the Grapevine and got 51.0 on the way up and 51.8 on the way back. Cruise set at 62mph. I can't imagine going any faster would net better mileage.

It wouldn't. I haven't read this whole thread but faster always = less mpg. Between wind resistance and higher engine RPM, there's no way you can get better MPG at a higher speed.

 

Dave

[keithsm2]

It wouldn't. I haven't read this whole thread but faster always = less mpg. Between wind resistance and higher engine RPM, there's no way you can get better MPG at a higher speed.

 

Dave

maybe not entirely true.....( just to play devils advocate) you can probably get better MPG at 30 mph than at 2 mph :) there has to be some sweet spot.....( im sure it is less than 65 MPH though)

[dkegel]

maybe not entirely true.....( just to play devils advocate) you can probably get better MPG at 30 mph than at 2 mph :) there has to be some sweet spot.....( im sure it is less than 65 MPH though)

I knew someone would go there. :) I agree.

 

Dave

[keithsm2]

im bored!

[corncobs]

im bored!

You should go to the movies ;-)

[SteveB_TX]

I am lucky to get 35 mpg at a steady 75 mph. NOT a sweet spot for MPG ! :shift:

Edited October 2, 2013 by SteveB_TX

[kuzzi]

Since the PCM update I no longer see 40+ instant when above 70, but day to day driving has improved slightly.

[keithsm2]

You should go to the movies ;-)

Go with me??

[Eddie Sessum]

35.5 at 80 MPH, 43 MPG at 65 MPH. Pre update of course.

How about a post update mpg? :)

[Eddie Sessum]

I am lucky to get 35 mpg at a steady 75 mph. NOT a sweet spot for MPG ! :shift:

 

 

Im about to do a damp road run at 75mph. If you dont see another reply from me tonight post a reply to remind me.

 

Going to middendorfs with the family.

[jeff_h]

Here is a screenshot of a basic (the only kind I make) spreadsheet to compare the averages at different speeds. I emailed this to acdii and hybridbear almost a year ago for their review, just never posted it as I thought it might be confusing or lead to claims that it is BS... note the large term 'HYPOTHETICAL' in there and not meant as an authoritative source of real-world results, was really just intended to generate some conversation on which average speed would get the better MPG -- to me I think the slower speed would yield the slightly better MPG, however there are others who say no way José... and this was also made when the top EV speed was 62 (before 13B07 update) so maybe the numbers should be shifted more apart to show more of a difference. So this is just a basic chart for illustration of the differences over different terrain and how that would average out... and yes I know using zero gas is more than 120 MPG but was trying to keep the range of the graph reader-friendly. So here it is, don't know if it will help with discussion in this thread or not, but what the heck.

 

[acdii]

How about a post update mpg? :)

Well, I havent done any long trips, but I have done a few short ones, and it does get better at 80 now, but no long term facts to prove it. Just going by the instant readout, Its around 40 at 80 now with a full SOC.

 

 

 

BTW some cars do have a sweet spot. I had a Crown Vic that was 68 MPH and got 28 MPG, 4 MPG higher than when going 55. Some ICE have an RPM range where it is the most efficient, and depending on gearing, will place it at a set speed to get the best fuel economy. Since the Hybrid isnt geared, finding the ICE sweet spot is much more difficult and based solely on SOC. The higher the SOC, the less energy required by the ICE to propel since it doesn't need to also charge. So finding the speed at which little EV is used and the ICE runs the most efficient is key.

[alpha754293]

So - before I get too far/deep into this, I seriously and STRONGLY suggest that for those that haven't read my thread about my master equation (which was pre PCM update) - to do so because I plot fuel economy as a function of vehicle speed on a 16.4 km stretch of highway just outside of Windsor, Ontario where it is largely, completely flat. (You can also see pictures of said highway in my gallery).

 

Take a gander at that cuz it probably actually has information that speaks to a lot of what people have already been saying, but it's actually collected data (using just the trip computer and my iPhone camera). Not exactly scientific, but it's enough to capture the jist of the point.

 

 

 

This shows the impact of speed on fuel economy. At 60 MPH the car will cycle in and out of EV mode, using the generator to load up the ICE to its peak efficiency when running. Above 62 MPH the hp needed to overcome the wind likely made it rare for it to be more efficient to run with the ICE off. At 70 MPH the wind resistance alone is plenty load for the ICE to be operating in its most efficient range. Above 70 MPH and the ICE starts running inefficiently because the load is too high. See this post for more info on what I'm talking about.

 

Interesting chart. I think that it's illustrative in general, but I'm not sure if that's entirely true for the Fusion specifically.

 

In engineering terms, power is the integral of force * velocity with respect to time.

Force (in this case would be aerodynamic drag, symbol F sub D (or F_D)) would be 1/2 * air density (rho) * velocity^2 (v) * coefficient of drag (C_D) * frontal area (A).

(The chart above would be true and valid if it listed the values that it used for air density, coefficient of drag, and frontal area. And that also assumes that the air density is constant, which it MOSTLY is.)

 

re: sweet spot (in general)

From what I've been able to tell, it looks like that the sweet spot might actually be < 55 mph. I've tried it at 37 mph (60 km/h), 41 mph (~65 km/h), 44 mph (70 km/h), 50 mph (80 km/h), 56 mph (90 km/h) and obviously, you can use the EV mode more at the lower speeds than towards the higher speeds, especially for longer distances. (Haven't tried re-running the 90 km/h highway test run since the day after the PCM update because at that point, I wanted to see if it was going to switch back into EV mode more often, which it didn't seem to do that unless it got a chance to lift off on the throttle. But on a nearly perfectly flat road, with a nearly constant throttle input; it couldn't switch into EV mode even at 90 km/h.)

 

(For the slower speed tests, I can't use the same stretch of highway because I would be going too slow. I have to use a different county road where I can do the slower speed tests safely.)

 

I have sneaking suspicion that if I can hold it at 60 km/h without ever stopping, I can probably hit 50+ mpg even in my heavy Titanium trimmed FFH.

So, there's SOME truth to the statement that slower = better fuel economy.

But - to the sweet spot. Well, what's sometimes better for fuel economy isn't always very useful to you. Driving at 25 mph might get you AMAZING fuel economy, but driving from Windsor to Toronto (~400 km/250 miles) would take 10 hours at that speed on a trip that would normally take 4 hours on the highway. So..."sweet" (or optimal in engineering terms) also depends on what's practical/reasonable amount of time for you to get to your destination.

 

On a 700 mile drive from Detroit to Boston though - if you drove at 70 mph, it would take you 10 hours. But if you drive 10% faster, it would take you roughly 9.1 hours. Does that extra 54 minutes REALLY make that much of a difference for you? And I'm pretty sure that you're likely going to be consuming more than 10% extra fuel to drive that extra 10% faster. So...was it really worth the fuel to save the time? I'm of the opinion that if it's going to take 10 hours, it's going to take 10 hours.

[hybridbear]

Interesting chart. I think that it's illustrative in general, but I'm not sure if that's entirely true for the Fusion specifically.

 

In engineering terms, power is the integral of force * velocity with respect to time.

 

Force (in this case would be aerodynamic drag, symbol F sub D (or F_D)) would be 1/2 * air density (rho) * velocity^2 (v) * coefficient of drag (C_D) * frontal area (A).

 

(The chart above would be true and valid if it listed the values that it used for air density, coefficient of drag, and frontal area. And that also assumes that the air density is constant, which it MOSTLY is.)

 

re: sweet spot (in general)

From what I've been able to tell, it looks like that the sweet spot might actually be < 55 mph. I've tried it at 37 mph (60 km/h), 41 mph (~65 km/h), 44 mph (70 km/h), 50 mph (80 km/h), 56 mph (90 km/h) and obviously, you can use the EV mode more at the lower speeds than towards the higher speeds, especially for longer distances. (Haven't tried re-running the 90 km/h highway test run since the day after the PCM update because at that point, I wanted to see if it was going to switch back into EV mode more often, which it didn't seem to do that unless it got a chance to lift off on the throttle. But on a nearly perfectly flat road, with a nearly constant throttle input; it couldn't switch into EV mode even at 90 km/h.)

 

(For the slower speed tests, I can't use the same stretch of highway because I would be going too slow. I have to use a different county road where I can do the slower speed tests safely.)

 

I have sneaking suspicion that if I can hold it at 60 km/h without ever stopping, I can probably hit 50+ mpg even in my heavy Titanium trimmed FFH.

 

So, there's SOME truth to the statement that slower = better fuel economy.

 

But - to the sweet spot. Well, what's sometimes better for fuel economy isn't always very useful to you. Driving at 25 mph might get you AMAZING fuel economy, but driving from Windsor to Toronto (~400 km/250 miles) would take 10 hours at that speed on a trip that would normally take 4 hours on the highway. So..."sweet" (or optimal in engineering terms) also depends on what's practical/reasonable amount of time for you to get to your destination.

 

On a 700 mile drive from Detroit to Boston though - if you drove at 70 mph, it would take you 10 hours. But if you drive 10% faster, it would take you roughly 9.1 hours. Does that extra 54 minutes REALLY make that much of a difference for you? And I'm pretty sure that you're likely going to be consuming more than 10% extra fuel to drive that extra 10% faster. So...was it really worth the fuel to save the time? I'm of the opinion that if it's going to take 10 hours, it's going to take 10 hours.

The chart was intended to be illustrative in general. I believe it is from a Prius Forum and is based on the previous generation Prius.

 

Absolutely the sweet spot is less than 55 MPH for peak fuel economy at a sustained speed. It seems that the peak might be around 40 MPH. At that speed the HVB doesn't discharge very quickly to maintain your speed and when the ICE comes on it operates very efficiently to charge the battery and propel the car. At that level you likely see the ICE operating in its most efficient BSFC region. Since you work at Ford, could you ask around and see what BSFC info the engineers who worked on this powertrain have? It would be really interesting to know at what power range the ICE is most efficient. My hypotheses are in my ScanGauge thread, but they're just based on watching the SGII and my observations.

[SteveB_TX]

OMG I have a headache! I know what I am going to do... I am going to drive and enjoy my Z and not worry about squeezing the last drop of gasoline out my tank. :shift: Whew! I feel much better now.

 

Okay, :backtotopic:

[alpha754293]

The chart was intended to be illustrative in general. I believe it is from a Prius Forum and is based on the previous generation Prius.

 

Absolutely the sweet spot is less than 55 MPH for peak fuel economy at a sustained speed. It seems that the peak might be around 40 MPH. At that speed the HVB doesn't discharge very quickly to maintain your speed and when the ICE comes on it operates very efficiently to charge the battery and propel the car. At that level you likely see the ICE operating in its most efficient BSFC region. Since you work at Ford, could you ask around and see what BSFC info the engineers who worked on this powertrain have? It would be really interesting to know at what power range the ICE is most efficient. My hypotheses are in my ScanGauge thread, but they're just based on watching the SGII and my observations.

 

Well, that's why I tried it at the different speeds (60 km/h, 65 km/h, 70 km/h, 80 km/h, etc..). Course, if I REALLY want to be scientific about it, I would be reading out the actual voltages for the high voltage battery pack (which is kinda like the SOC readout, but in MUCH greater detail), and making sure that I get it back up to that level before doing all of the different speed tests. But at that point, I might as well bring my car into a dyno lab and run it from there. While it would ensure that the starting points are identical, it's not very practical in terms of how a car would ACTUALLY drive on the road (in that the starting SOC would NOT be constant) - which is why I didn't bother. Plus I've neither the time, nor the money to actually hire a lab to do that.

 

re: bsfc

Couple of things - 1) since I work for Ford, there's a LOT that I CAN'T say/publish (if it isn't already released into the public domain). It gets a little the "grey" area if I were to run/bench the engine in my own car, collecting the same information, and whether I can publish stuff that I do not get from work directly (for semi-obvious reasons). So there's that. 2) bsfc is a function of engine speed and power, which is often represented in a 2D contour map. So, it depends on the kind of bsfc info you're looking for. Since it's know that it's an Atkinson engine, you might be able to calculate what the theorectical bsfc should be based on just undergrad level thermo. So, if you've taken thermo; you should be able to look up how to do those calculations and run through your thermo analysis. If you haven't, I could probably run it for you, but I might not get around to it for a while - busy with other stuff. You'll have to make certain assumptions about the initlal operating conditions of the engine. I think that I MIGHT have set up an Excel spreadsheet a LONG time ago (for the Otto cycle, but I'm pretty sure that I should be able to modify it for the Atkinson cycle) to run through ONE set of the calculations, and I MIGHT have even wrote a MATLAB script to run through a whole sweep of engine speeds and powers, but I might not have gotten that far actually to code it up for that. I forget.

 

Not particularly "difficult" per se. Like I said, this would have been actually a pretty good assignment for a college Junior that's taking thermo. (Cuz once you run through the calculations once, the rest is modifying the code so that you can have it automatically sweep through it; which for your average consumer laptop nowadays, would make short change of that bit of calculation in a couple of seconds, IF that).

 

re: "It would be really interesting to know at what power range the ICE is most efficient."

Well...honestly. ehhh... I say that because while the IP cluster shows "power", notice that there are no actual absolute values ascribed to that. Also remember that power is used to propel your vehicle forward which it has to overcome rolling resistance, aerodynamic drag, and and graviational resistance (like going uphill for example). Wind (or the wind vector technically) is rolled in with aerodynamic resistance. So, if say you need 60 kW to stay at 100 km/h on flat road with no wind, you're going to obviously slow down (assuming power is held constant) going uphill, or you're also going to slow down if there's a head wind. And so, I say that because now, in order to answer the question "what power range would be most efficient" - the reality is that you don't drive to power, you drive to speed; and you vary the power to hold it at that desired speed. And there's a bit of "faith" (if you will) in trusting that the other engineers here at Ford has written the PCM firmware/software that IT will figure out how to run your engine in the most fuel efficient way possible so that you don't have to really worry about so that you can (a la SteveB_TX]) just drive and enjoy.

 

And the fact that the reality is that most roads AREN'T perfectly flat (unless you only drive a very tiny portion of the roads in the world), and wind DEFINITELY isn't a constant, it'll be very difficult to manually make your car operate in the most efficient power range without affecting something else (like the speed that you're going).

 

And I'm not saying to it to be glib. But that is kinda the reality though. Can you calculate the theorectical bsfcs? Yes. Is it necessarily worthwhile to do so? Mehhh...generally, probably not. It'd be a good to know, but it probably is something that unless you can manually control the eCVT TCM, not very useful (in practical terms) information.

[alpha754293]

OMG I have a headache! I know what I am going to do... I am going to drive and enjoy my Z and not worry about squeezing the last drop of gasoline out my tank. :shift: Whew! I feel much better now.

 

Okay, :backtotopic:

 

It appears that I have that kind of effect on a LOT of people. That's why I just hang in there at 110 km/h (69 mph). Limit in Michigan is 70 mph anyways, and in Ontario, is 100 km/h, so it's not so slow that it takes me FOREVER to get to where I'm going, but it's not so fast that a) it has a significant adverse impact on my fuel economy, and b) prone to being ticketed. It's my "sweet spot".

[acdii]

53 MPH for the ICE. When SOC is at or above 80%, and cruising on ICE @ 53, you can expect to see the instant hovering around the 50 MPG mark. What the overall SS is, dont know, dont really care, the car is getting what it gets for the daily drive I have. Might not be spectacular as some others get, but knowing how my other cars did, this one is doing better on the same trip, and thats all that matters. For trips that are on flatter roads, and have speeds around 45 MPH for the most part, easily in the high 40's low 50's. There is nothing wrong with this car.

[alpha754293]

53 MPH for the ICE. When SOC is at or above 80%, and cruising on ICE @ 53, you can expect to see the instant hovering around the 50 MPG mark. What the overall SS is, dont know, dont really care, the car is getting what it gets for the daily drive I have. Might not be spectacular as some others get, but knowing how my other cars did, this one is doing better on the same trip, and thats all that matters. For trips that are on flatter roads, and have speeds around 45 MPH for the most part, easily in the high 40's low 50's. There is nothing wrong with this car.

 

I might have to start running some tests out on county roads. I still think that the instantaneous gauge isn't very useful because when you're recharging, the instantaneous plummets, so I still don't understand why people use that. The average or the trip computer is probably more telling since it does weighted averaging so when the instantaneous fuel economy plummets when it's charging, it'll get averaged out.

[hybridbear]

re: bsfc

Couple of things - 1) since I work for Ford, there's a LOT that I CAN'T say/publish (if it isn't already released into the public domain). It gets a little the "grey" area if I were to run/bench the engine in my own car, collecting the same information, and whether I can publish stuff that I do not get from work directly (for semi-obvious reasons). So there's that. 2) bsfc is a function of engine speed and power, which is often represented in a 2D contour map. So, it depends on the kind of bsfc info you're looking for. Since it's know that it's an Atkinson engine, you might be able to calculate what the theorectical bsfc should be based on just undergrad level thermo. So, if you've taken thermo; you should be able to look up how to do those calculations and run through your thermo analysis. If you haven't, I could probably run it for you, but I might not get around to it for a while - busy with other stuff. You'll have to make certain assumptions about the initlal operating conditions of the engine. I think that I MIGHT have set up an Excel spreadsheet a LONG time ago (for the Otto cycle, but I'm pretty sure that I should be able to modify it for the Atkinson cycle) to run through ONE set of the calculations, and I MIGHT have even wrote a MATLAB script to run through a whole sweep of engine speeds and powers, but I might not have gotten that far actually to code it up for that. I forget.

 

Not particularly "difficult" per se. Like I said, this would have been actually a pretty good assignment for a college Junior that's taking thermo. (Cuz once you run through the calculations once, the rest is modifying the code so that you can have it automatically sweep through it; which for your average consumer laptop nowadays, would make short change of that bit of calculation in a couple of seconds, IF that).

I'd be interested to see that when you have the time. I'm not an engineer so I have limited understanding of these concepts but I want to understand them better.

 

re: "It would be really interesting to know at what power range the ICE is most efficient."

Well...honestly. ehhh... I say that because while the IP cluster shows "power", notice that there are no actual absolute values ascribed to that. Also remember that power is used to propel your vehicle forward which it has to overcome rolling resistance, aerodynamic drag, and gravitational resistance (like going uphill for example). Wind (or the wind vector technically) is rolled in with aerodynamic resistance. So, if say you need 60 kW to stay at 100 km/h on flat road with no wind, you're going to obviously slow down (assuming power is held constant) going uphill, or you're also going to slow down if there's a head wind. And so, I say that because now, in order to answer the question "what power range would be most efficient" - the reality is that you don't drive to power, you drive to speed; and you vary the power to hold it at that desired speed. And there's a bit of "faith" (if you will) in trusting that the other engineers here at Ford has written the PCM firmware/software that IT will figure out how to run your engine in the most fuel efficient way possible so that you don't have to really worry about so that you can (a la SteveB_TX]) just drive and enjoy.

 

And the fact that the reality is that most roads AREN'T perfectly flat (unless you only drive a very tiny portion of the roads in the world), and wind DEFINITELY isn't a constant, it'll be very difficult to manually make your car operate in the most efficient power range without affecting something else (like the speed that you're going).

 

And I'm not saying to it to be glib. But that is kinda the reality though. Can you calculate the theoretical bsfcs? Yes. Is it necessarily worthwhile to do so? Mehhh...generally, probably not. It'd be a good to know, but it probably is something that unless you can manually control the eCVT TCM, not very useful (in practical terms) information.

I want to know from the perspective of manipulating my pressure on the gas pedal to make the engine run most efficiently. Unlike your comment above I would rather drive to power and kind of do already. On long highway trips I'll set the cruise control, but as I've learned more about how the FFH works I've starting way more driving to power rather than to speed. Since I walk to work I'm rarely driving in traffic so it's not a big deal usually for me to vary my speed up and down when there are few other cars around. Since most of our terrain is rolling hills I try to drive in such a way that I pick up speed going down the hills. Since it's more efficient to turn this gravitational energy into kinetic energy than it is to turn it into electrical energy through regen I prefer to feather the throttle so that the current flow in/out of the HVB is minimal. This way I can minimize efficiency losses. When travelling up a hill I try to keep the power demand low and in EV for awhile and then once my speed drops too much I step on the pedal enough to engage the ICE to recharge and propel me up this hill.

 

It my ScanGauge thread I've started doing some calculations based on the HP reading of the SGII to figure out the kW to tie it to the graph. I've also been able to do that with the amps of current out of the HVB to calculate approximate kWs at certain spots on the car's display. If I had BSFC data of at what kWs the ICE is operating in the optimal BSFC region then I could correlate that data to spots on the display. That would then help me to make sure I keep the ICE operating in that region as often as possible.

 

I agree that you can't operate the car in its most optimum range without affecting speed, but I am willing to change my speed to optimize ICE efficiency. EcoCruise already does this to a certain degree on the highway. While the pedal position doesn't necessarily directly correlate to RPMs like in a gas-only car, it does correlate directly to power demand. If I know what level of power demand is best then I can make my foot work within that area.