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Entries in mpg (11)


Running at 0 mpg

I got home the other night and sat in the car listening to a blog I was playing from my Blackberry (via Bluetooth to MS Sync) for a few minutes. Usually I always turn off the car as soon as I get home, even if I'm going to sit and listen to something for awhile longer.  The car was silent and I must have spaced out because I forgot to turn the car off, even after I had finished listening, and went into the house.   This was at 5pm.

Seven hours later, at 12 midnight, as I was going to bed, I heard my car running outside the window.  I looked and there she was, humming along with the headlights on!  If you leave the car on in Park for a long time it will periodically run the engine to keep the battery charged.

I ran downstairs to turn the car off. When I had gotten home earlier the "tank" average miles per gallon was 43.7 mpg. Amazingly, after running for seven hours straight, the average was only down to 39.7 mpg. I figured out that the car had burned about a half a gallon of gas while it was sitting in my driveway. Not too bad for being on for seven hours!

By the way, I use PodTrapper for Blackberry which is absolutely the best podcast player for BlackBerry!


EV mode stops working

I went to lunch today (2-3 miles from work) and EV mode would not go on even at stoplights.  I even tried turning the car off and on but no luck.  The engine was on the whole time, although at lights it would go into a kind of hibernation where it was on but not doing anything according to the HEV display.  The battery was charged to the max.

The same thing happened on the way back from lunch.  Finally, just as I arrived back at work EV mode started working again.  I wonder if this is what Ford meant in the manual when they wrote on page 7:

Your hybrid high voltage battery may periodically re-condition itself to ensure maximum efficiency.  You may notice slight changes in drivability during this process, but it's an important part of your hybrid's high voltage battery opitmization features.

My great gas mileage of 44.5 for this tank is now 43 :-(


Improved MPG from my Ford Fusion Hybrid

Spring has sprung here in the mid-Atlantic area and it is really helping my mileage figures.  During the winter I went as low as 29-30 mpg, but in the 60-80 degree weather we've had recently my mileage has improved drastically.  In fact, its better than ever. 

Yesterday I set a new record on the way home, which is mostly a 7 mile slog uphill from King of Prussia to Route 30, (the Lincoln Highway, the first transcontinental highway).  Previously my best was 45.5 mpg on Aug 7, 2009.  Yesterday I got 47.8 mpg.

Today, I set a new record on the way to work.  My previous best was 60.4 mpg on July 6, 2009.  Today I got 61.1!

The funny thing is, I'm not even trying that hard. I guess its true when they say that after awhile the mileage gets better - I've got over 7000 miles on my FFH now. The battery seems to be in a high state of charge more often.

Also, my ability to drive for high mpgs has become second nature.  One thing I do now is try to minimize the gas-guzzling moves, keeping the instantaneous mpg indicator above 20mph if possible. I will also minimize battery draw-down situations (going over a hill in EV mode for instance) by applying a little internal combustion engine boost.

Is anyone else experiencing improved MPGs?


Dashboard reset

The other day when I fired up my Ford Fusion Hybrid, I found that the dashboard displays had reverted to their default level of information and that the long term total MPG counter had reset to 0.  I've heard of this happening to others but it had never happened to me before.  I have about 6500 miles on the car.  Could it be mileage related?

Has this happened to you?


Winter killing MPG

Well, winter is here in Philadelphia and its killing my mileage.  I'm hovering right around 29-30 mpg in the 32 degree weather we've been having recently.  It wouldn't be so bad if I drove longer distances but I rarely go more than 7 miles or so and often less.  It takes several miles of running the engine to get it warm, then the engine must continue to turn on occasionally because it cools down quickly in the cold weather, especially if I've got the cabin heater on.

I'm still getting twice the MPG that my Audi would have gotten in this weather, and I'm still loving the car. The front wheel drive is not as great/fun in the snow as the Audi Quattro, but its not too bad either.

Oh, and Happy New Year!!


Fall weather MPG update

It's getting tougher and tougher to maintain 40+ mpg in the colder fall weather here in PA.

I just filled up today.  The last time I filled up was Sep 29 - 5 weeks ago!  For this tank the mpg guage read 40.4 mpg.  It started at 42 mpg and went steadily down as the weather got colder.  It just takes longer for the engine to heat up and my work is only about 7 miles from home.

The guage is still reading high.  My calculated mpg based on gallons used is 38.14 which is 5.6% less than the guage reading of 40.4.

Since shortly after I got the car, my average mpg according to the long term guage is 40.7 and the calculated mpg is 39.14 - a difference of -3.8%.

I'm still loving the car!


Improving the MPG bar graph

The FFH has a bar graph that you can display in the dash that shows MPG over a 10, 20, or 60 minute period.  There are 10 bars so each bar is the average mpg over a period of 1, 2, or 6 minutes:Each bar shows mpg over a 6 minute period

During a recent 600 mile trip to Maine I realized that this gauge is not as useful as it could be.  It frequently happens that there is a bar that has a very low mpg next to a bar with very high mpg.  Imagine the bar graph is set for 10 minutes (so each bar shows mpg during a minute) and the time period for a bar is just beginning.  You happen to be gliding the car in EV mode 50 feet to a stop at a traffic light.  Say this takes 15 seconds.  Then you wait at the light for 45 seconds.  The bar appears and its over 60 mpg because you were in EV mode or stopped for the whole minute.  Yeah!  Then you accelerate from the light and head up a hill for a minute, going 1/2 mile.  The next bar comes up and the mileage is dismal - 20 mpg.  Looking at the bars, you think that's ok - after all, you have one 60 mpg bar next to the 20 mpg bar, the average of those two must be pretty good!

But its not, you only went 50 feet during the first bar's time period.  Since mpg is calculated by dividing miles traveled by gallons used, the overall mpg of those 2 minutes is 20.15 mpg.  The appearance of the bars suggests that you should weight each bar equally, but you should not.

I would love to have the option to display a bar graph based on MPG per mile traveled, rather than per minute traveled.  That would be much more useful.  Each bar would indicate the mpg over a given distance, say 1, 2, or 5 miles.  Each bar would be directly comparable.  You could look at the graph and visually get an idea of your recent mpg by looking at the bar heights.  You would also be able to directly map the bar heights to mileage along your route - eg a low bar might indicate an uphill climb on your commute.


Prius Gen II vs Ford Fusion Hybrid drive test

My daughter has a Gen II Prius (2007).  She's home from school so I took the Prius out on my test loop.  This loop is 14 miles long and I've taken the FFH around it a few times, getting 54-55 mpg.  Its got a mixture of driving conditions - some hills and some relative flats, some over 50 mph stretches, but no highway.  I wanted to see what kind of mileage I could get in the Prius.  While I've driven the Prius before, I've never been able to get more than about 40 mpg in it.  After learning some tricks with my FFH I thought I could do better.

First I did a small drive to warm the Prius up.  I always start my test loop with a warmed up engine so the car is ready to use EV mode.  One of the first things I noticed was the Prius does not have much power in EV mode. Starting from a stop usually turns the engine on. The FFH often has enough power in EV mode to pull away from a stop or go up a small hill.

Prius engine and electric motor driving wheels while battery is chargingThe Prius has an energy flow diagram like the FFH.  The Prius one is easier to read.  The flow lines are color-coded green, orange, red, and dark gray, and clearly indicate flow direction - much easier to interpret than the gradually widening flow lines on the FFH diagram (the FFH energy diagram does indicate intensity of flow by varying the thickness of the flow lines which is nice).

I soon began to miss the gauges in the FFH.  The Energy Monitor screen is the only tool the Gen II Prius has for monitoring what's happening in the drive train. In the Prius it was difficult to predict when the switch from EV mode to "engine on" would From L to R: Battery & engine power, Battery charge indicator, Speedometer, Fuel gauge, Instantaneous MPGoccur because there is no gauge for that, nor is there an indicator for when you are in EV mode. The FFH has a rich set of gauges. Depending on which ones you enable, you can see engine RPM, power draw from the battery and the engine, battery state of charge, instantaneous MPG, when the car is in EV mode, and how much acceleration you can apply in EV mode before the engine turns on.

During my drive I used the "pulse and glide" technique as often as possible.  In the Prius, its easy to move from the "pulse" to the "glide" - there is a sweet spot between taking your foot too far off the throttle and starting to regen and having too much throttle and initiating electric motor assist.  You can easily see when you are gliding using the Energy Monitor because all the flow lines are dark gray.  Getting a good glide going in the FFH is harder - its not as easy to get the exact balance between regen and motor assist. 

The Prius really sings during a glide!  It has less drag than the FFH.  The FFH drag while gliding was explained by Wayne Gerdes as "back EMF build" which I guess means that the electric motors are providing resistance so the FFH is not rolling along completely freely.

The FFH can go faster in EV mode before the engine turns on, 47 mph to the Prius's 42, which means you can pulse up to a higher speed before initiating a glide in EV mode.

The Prius is a great car. I could tell fairly quickly that I would get better mileage, and I did - 70.7 mpg!  I prefer the driving experience of the FFH, but if you're going for MPG the Prius is the one to get.


The myth of pulse and glide revisited

My post on the myth of pulse and glide generated a number of comments here, over on PriusChat, and at CleanMPG.  I thought I'd respond and clear up a few misconceptions with a followup post here.

A number of people thought I was saying that pulse and glide (P&G) doesn't work.  Nothing could be further from the truth.  I continue to learn about this important technique for getting good gas mileage.  The "myth" I was referring to is the common intuition that if you pulse at, say, 20 mpg and glide the same distance at, say, 100 mpg, then you will average out to 60 mpg total. You actually get 33 mpg in that case and don't get 60 mpg until the glide is 5 times longer than the pulse.  If you assume infinite mpg for the glide, you get 60 mpg when the glide is twice as long as the pulse, which is also not particularly intuitive.

Others noted correctly that the glide is usually longer than the pulse, whereas my example stipulated the pulse and glide as being the same length.  The intuition I spoke about above usually arises when people are thinking of equal length pulses and glides, which is why I used equal length pulses and glides in my example. Here is a chart showing the Total MPG for various glide MPGs where the glide is 1, 2, 3, 4, and 5 times as long as the pulse.

Several people made posts or commented on the blog saying that they had gotten great gas mileage (one poster got just over 170 mpg!) while using P&G. That's great! A couple of things to look at in those results, if you really want to understand the long term sustainability of P&G:

  1. Was the battery state of charge (SOC) at the same level before and after the P&G session. If the battery SOC was lower, the P&G is not sustainable over time with that same great gas mileage as the engine will eventually fire up to charge the battery and shorten your glide.
  2. Was it a round trip? Wind and/or downhill slope can make a glide last longer than it would otherwise. By going both directions, those factors are eliminated.

Some posters were concerned about my use of 100 mpg as an estimate of gas mileage for the glide portion. I picked that number because my first P&G experience was with my old 1998 Audi A6 whose engine does not automatically shut off during the glide. These same posters pointed out that in a hybrid like the Ford Fusion Hybrid or Prius, the glide gets infinite mpg. This is where things started to get hot!  This discussion is not bottomed out yet.  For the mathematical model of P&G that I am using, it makes sense to not think of the glide as having infinite mpg.  During the glide there will be battery discharge - from A/C, or lights, or feathering the throttle to extend the glide a little.  Even without these electrical draws, the electric motors still draw some current. You can account for any battery discharge during the glide by reducing the glide's equivalent MPG. Equivalent MPG is a way to think about the true cost of a given transport mode. When discussing all-electric cars, equivalent mpg is used to determine the true cost of moving the car, even though the car is getting infinite mpg in the sense that its not using any fuel at the time its in motion.  Looking at the big picture though, the car has a cost to operate, when its charged at a later time from the electrical grid. So you'll read about the Tesla having an equivalent MPG of 120 or so.   When you glide in a hybrid car but are using the battery, its the same situation. The battery has to be recharged eventually somehow, and this always involves the car's internal combustion engine (brake regeneration can't restore the whole charge on a round trip due to conversion losses).  To keep the mathematical model simple you can select an equivalent mpg for the glide to account for this battery charge loss, which, if you keep doing the P&G for long enough, will eventually cause the engine to run to recharge the battery.  Here is the simple mathematical model I used for my original blog post:

Distance = Glide Distance + Pulse Distance
Glide Gal = Glide Distance/Glide MPG
Pulse Gal = Pulse Distance/Pulse MPG
Total MPG = Distance/(Glide Gal + Pulse Gal)

If you don't like assigning an equivalent MPG to the glide, then set Glide Gal equal to 0.

Thanks to all the posters and commenters for helping me to refine my understanding of P&G, and helping me to develop the pulse equivalent mpg idea.


The myth of pulse and glide

The math behind "pulse and glide" seems intuitive enough.  At least that's what I thought.  Until I started calculating my total MPG for trips to and from work.  On the way to work I have gotten 60 MPG; on the way back I have gotten close to 40 MPG - that's 50 MPG total, right?  I did the math, and my MPG to and from work is only 48 in that case.

How does this apply to pulse and glide? Pulse and glide is a technique where you accelerate moderately to an upper target speed such as 45 MPH, then lay off the accelerator and glide down to a target speed such as 35 MPH, possibly putting the car in neutral during the glide to minimze fuel use.  Then you repeat.  The idea is that the pulse MPG and glide MPG should average out to some pretty good gas mileage.

Let's say you decide to pulse for 30 seconds, then glide for 30 seconds. If you get 20 MPG during the pulse, and 100 MPG during the glide, it seems natural to assume you'll be getting 60 MPG total - after all, 1/2 of the way between 20 MPG and 100 MPG is 60 MPG. This is the myth of pulse and glide. Please bear with me for a little math of pulse and glide ;-)

Here's a chart showing total MPG for a pulse at 20 MPG and a range of glide MPGs from 20 to 150.  At 100 MPG for the glide, the total MPG for the pulse and glide is only 33 MPG, not 60 as we intuited.  At 150 MPG for the glide, the total MPG for the pulse and glide is still only 35! 

The maximum possible total MPG, even if you use no gas at all during the glide, is only 40 MPG.  Why?  During the pulse you get 20 MPG, using G amount of gas and travelling D distance.  Your mileage for the pulse is D/G MPG.  During the glide you use no gas and travel D distance also.  Your total gas usage is G.  Your total distance is 2*D.  Your total mileage is 2*D/G MPG, or twice the pulse MPG.

So, in our example of a 20 MPG pulse and 100 MPG glide, how far must you travel during the glide to get 60 MPG total?  Five times the pulse distance!