Determining glide eMPG

WPWoodJr · 07-15-2009, 03:35 PM

Apologies for the length of this post!

In the myth of pulse and glide thread, many have objected to the inclusion of an mpg figure for the glide phase of P&G (for hybrid cars) since the Prius and Ford Fusion Hybrid get "infinite" mpg when gliding. However, due to battery losses during the glide (due to A/C, electric motor current draw, and other factors) the glide uses battery charge which must be recharged later by the engine. To account for this we can assign an "equivalent MPG" (eMPG) amount to the glide. This is the amount of MPG used by the internal combustion engine (ICE) to recharge the battery due to the battery discharge during the glide.

The question becomes, what is the actual glide equivalent MPG for a hybrid? I've worked out an experimental protocol for determining the glide eMPG.

We need to establish values for all variables except glide eMPG. Then we can calculate glide eMPG from the other variables. The variables that we need measured values for are:

Ratio of Glide distance to Pulse distance (eg 4 to 1)
Total Distance
Pulse MPG
Total MPG

Once we have measured those values experimentally we can determine the glide eMPG for the experiment as:

Pulse Distance = Total Distance/(Ratio + 1)
Glide Distance = Total Distance - Pulse Distance
Pulse Gal = Pulse Distance/Pulse MPG
Glide Gal = Total Distance/Total MPG - Pulse Gal
Glide eMPG = Glide Distance/Glide Gal

So, how do we determine these values experimentally? I propose setting up an experiment on a long stretch of level roadway (as long as 5 miles if possible).

Phase 1: Establish baseline Glide to Pulse Ratio
Initially we run a pulse and glide test with a target range of, say, 30 - 45 MPH. The battery should have a high initial state of charge (SOC) so that the ICE does not need to run to recharge the battery. All electrical equipment should be off (A/C, lights, etc) so as not to drain the battery and force the ICE on. Its important to do a true glide, not a glide assisted by EV mode or reduced by regen, so we put the car in neutral during the glide. We measure the average length of the pulse and glide phases. We may determine that the pulse lasts 20 seconds and the glide lasts 80 seconds (4 to 1 ratio of glide to pulse).

Phase 2: P&G test with low battery SOC and electric equipment on
Next, we run the experiment. The battery SOC should be low, very near the point where the engine will turn on to recharge it. The reason is, we want the engine to turn on during the experiment to recharge the battery as necessary. If we started with a full battery, the test would not determine the glide eMPG because we would be running the battery down for a portion of the test.

Turn on any electrical equipment such as lights and A/C used in normal driving. These will be a primary cause of lower pulse eMPG, as will electrical draw from the drivetrain.

We start the experiment by accelerating to 30 MPH, then resetting the trip distance and trip MPG computers. Then we start the pulse and glide routine:

Pulse at 20 MPG to 45 MPH for however long it takes
Glide down to 30 MPH in neutral for however long it takes
If the engine stays on or turns on during the glide, ignore it. Keep the car in the glide until it gets to 30 MPH.
Repeat for the length of the course until the last glide is complete, then note Total MPG and Total Distance

Phase 3: Calculate glide eMPG
Once the experiment has been run, we can calculate the glide eMPG. We know Total MPG and Total Distance from the car's computer. We know the Pulse MPG because we will held that steady at 20 MPG during the experiment. We know the Ratio from the initial test. We can plug these values into the formula above and calculate glide eMPG.

Here's a sample calculation:

Total Distance = 10 miles
Total MPG = 50 MPG
Ratio = 4 to 1
Pulse Distance = 10 / (4 + 1) = 2 miles
Glide Distance = 10 - 2 = 8 miles
Pulse MPG = 20 mpg
Pulse Gal = 2 miles / 20 mpg = 0.1 gal
Glide Gal = 10 miles / 50 mpg - 0.1 gal = 0.1 gal
Glide eMPG = 8 miles / 0.1 gal = 80 mpg

Phase 4: Use pulse eMPG to predict Total MPG in other situations
Armed with glide eMPG for the given experimental conditions (eg A/C on or off, lights on or off, etc) we can predict Total MPG for varying pulse MPGs and glide to pulse ratios as follows:

Glide Gal = Ratio/Glide eMPG
Pulse Gal = 1/Pulse MPG
Total MPG = (Ratio + 1)/(Glide Gal + Pulse Gal)

For example, if your pulse MPG is 25 and glide to pulse ratio is 5:

Glide Gal = 5/80 (80 is eMPG from above example)
Pulse Gal = 1/25
Total MPG = (5 + 1)/(5/80 + 1/25) = 58.5 MPG

Be gentle! These are initial thoughts.

donee · 07-15-2009, 07:59 PM

Hey Bill,

MPG stands for MILES PER GALLON, and ALL Electricity in a stock Prius or Fusion Hybrid is generated ONLY when the engine is runing. REPEAT AFTER ME - THERE IS NO FUEL CONSUMPTION WHEN THE ENGINE IS NOT TURNING ....

I refuse to read more the than the first paragraph of your comments until you live by reality. And the reality here is REPEAT AFTER ME - THERE IS NO FUEL CONSUMPTION WHEN THE ENGINE IS NOT TURNING....

Now, if you want to figure the impact of varying electric loads, factor them AFTER you figure the P+G Mileage. Any computation that includes fuel consumption when the engine is off is WRONG.

ksstathead · 07-15-2009, 08:08 PM

... because you can force SOC to a no change result in testing the P&G benefit.

Measure fuel consumed with P&G and no delta soc.
Calculate average speed under that P&G.
Measure steady state fuel consumed at that average speed over the same course/conditions.
Difference is the savings, if any.

WPWoodJr · 07-15-2009, 10:03 PM

Quote:

Originally Posted by donee

Hey Bill,

MPG stands for MILES PER GALLON, and ALL Electricity in a stock Prius or Fusion Hybrid is generated ONLY when the engine is runing. REPEAT AFTER ME - THERE IS NO FUEL CONSUMPTION WHEN THE ENGINE IS NOT TURNING ....

I refuse to read more the than the first paragraph of your comments until you live by reality. And the reality here is REPEAT AFTER ME - THERE IS NO FUEL CONSUMPTION WHEN THE ENGINE IS NOT TURNING....

Now, if you want to figure the impact of varying electric loads, factor them AFTER you figure the P+G Mileage. Any computation that includes fuel consumption when the engine is off is WRONG.

Hi donee, eMPG is a pretty well-established concept, see

Miles_per_gallon_gasoline_equivalent

. For instance, the electric

Tesla_Roadster

is said to get 120 eMPG.

If you're open to a new way of looking at things, putting a new tool into your arsenal, take a look at the other paragraphs!

WPWoodJr · 07-15-2009, 10:27 PM

Quote:

Originally Posted by ksstathead

... because you can force SOC to a no change result in testing the P&G benefit.

Measure fuel consumed with P&G and no delta soc.
Calculate average speed under that P&G.
Measure steady state fuel consumed at that average speed over the same course/conditions.
Difference is the savings, if any.

Yes, this would measure the advantage of P&G over cruise control, which is not in question and its not the subject of my post.

ken1784 · 07-15-2009, 11:17 PM

Quote:

Originally Posted by WPWoodJr

So, how do we determine these values experimentally?

Following picture was taken at a famous Prius Marathon Run in 2005.

They used the SuperMID M-1 mileage meter.
The 3rd and the 4th line of the picture tells us following data.

Total MPG: 49.51km/L
Total driving time: 4 hours 21 minutes
Total distance: 200.95km
Pulse MPG: 12.83km/L
Pulse ratio: 26% (more preciously 12.83/49.51=25.91%)
Total(=Pulse) fuel usage: 4.058L

Then...

Pulse Distance = Total Distance/(Ratio + 1) = 200.95 * 0.2591 = 52.07km
Glide Distance = Total Distance - Pulse Distance = 200.95 - 52.07 = 148.88km
Pulse liter = Pulse Distance/Pulse MPG = 52.07 / 12.83 = 4.058L
Glide liter = Total Distance/Total MPG - Pulse liter = 200.95/49.51 - 4.058 = 0.000 L
Glide eMPG = Glide Distance/Glide liter = 148.88/0.000 = infinity

Ken@Japan

WPWoodJr · 07-16-2009, 12:08 AM

Quote:

Originally Posted by ken1784

Following picture was taken at a famous Prius Marathon Run in 2005.

They used the SuperMID M-1 mileage meter.
The 3rd and the 4th line of the picture tells us following data.

Total MPG: 49.51km/L
Total driving time: 4 hours 21 minutes
Total distance: 200.95km
Pulse MPG: 12.83km/L
Pulse ratio: 26% (more preciously 12.83/49.51=25.91%)
Total(=Pulse) fuel usage: 4.058L

Then...

Pulse Distance = Total Distance/(Ratio + 1) = 200.95 * 0.2591 = 52.07km
Glide Distance = Total Distance - Pulse Distance = 200.95 - 52.07 = 148.88km
Pulse liter = Pulse Distance/Pulse MPG = 52.07 / 12.83 = 4.058L
Glide liter = Total Distance/Total MPG - Pulse liter = 200.95/49.51 - 4.058 = 0.000 L
Glide eMPG = Glide Distance/Glide liter = 148.88/0.000 = infinity

Ken@Japan

Hi Ken, can you explain how you got pulse ratio? It doesn't look correct to me. Ratio should be the length of glide divided by the length of pulse under high SOC conditions.

ken1784 · 07-16-2009, 12:44 AM

Quote:

Originally Posted by WPWoodJr

Hi Ken, can you explain how you got pulse ratio? It doesn't look correct to me. Ratio should be the length of glide divided by the length of pulse under high SOC conditions.

The pulse ratio is ( (pulse distance) / (total distance) ) X 100%.

Ken@Japan

WPWoodJr · 07-16-2009, 10:41 AM

You seem to be trying to show that glide eMPG is infinite using some existing data with my formula - but your ratio is not calculated according to the experimental protocol. You can't calculate ratio as proportion of MPG contributed by the engine as you did here:

Quote:

Pulse ratio: 26% (more preciously 12.83/49.51=25.91%)

Your approach will always show infinite glide mpg because it has no way to differentiate what proportion of the pulse fuel usage is due to recharging the battery and what proportion is due to getting to the P&G top speed.

Ratio should be experimentally determined as follows:

Quote:

Initially we run a pulse and glide test with a target range of, say, 30 - 45 MPH. The battery should have a high initial state of charge (SOC) so that the ICE does not need to run to recharge the battery. Its important to do a true glide, not a glide assisted by EV mode or reduced by regen, so we put the car in neutral during the glide. We measure the average length of the pulse and glide phases. We may determine that the pulse lasts 20 seconds and the glide lasts 80 seconds (4 to 1 ratio of glide to pulse).

HTMLSpinnr · 07-16-2009, 11:00 AM

I'm not sure I agree w/ the benefit of measuring battery "eMPG" in a Prius unless you're using a Plug-In Hybrid. All energy consumed in the Prius for motive power ultimately comes from gasoline, for which the consumption is fairly easily measured in real time, and average consumption measurable when refilling into a known volume of space.

Running in EV only mode is only using previously stored energy, which will then be replenished through regeneration or charging from the ICE. Until you introduce a second source of power that isn't derived from gasoline, eMPG will remain inappropriate for measuring *overall* Prius efficiency, which is what most of us ultimately care about.

07-15-2009, 03:35 PM	#1
WPWoodJr Member Join Date: Feb 2009 Location: Villanova, PA Posts: 56 My Car: Other Hybrid Model: N/A Package: N/A Thanks: 7 Thanked 4 Times in 4 Posts Friends: 1	Determining glide eMPG Apologies for the length of this post! In the myth of pulse and glide thread, many have objected to the inclusion of an mpg figure for the glide phase of P&G (for hybrid cars) since the Prius and Ford Fusion Hybrid get "infinite" mpg when gliding. However, due to battery losses during the glide (due to A/C, electric motor current draw, and other factors) the glide uses battery charge which must be recharged later by the engine. To account for this we can assign an "equivalent MPG" (eMPG) amount to the glide. This is the amount of MPG used by the internal combustion engine (ICE) to recharge the battery due to the battery discharge during the glide. The question becomes, what is the actual glide equivalent MPG for a hybrid? I've worked out an experimental protocol for determining the glide eMPG. We need to establish values for all variables except glide eMPG. Then we can calculate glide eMPG from the other variables. The variables that we need measured values for are: Ratio of Glide distance to Pulse distance (eg 4 to 1) Total Distance Pulse MPG Total MPG Once we have measured those values experimentally we can determine the glide eMPG for the experiment as: Pulse Distance = Total Distance/(Ratio + 1) Glide Distance = Total Distance - Pulse Distance Pulse Gal = Pulse Distance/Pulse MPG Glide Gal = Total Distance/Total MPG - Pulse Gal Glide eMPG = Glide Distance/Glide Gal So, how do we determine these values experimentally? I propose setting up an experiment on a long stretch of level roadway (as long as 5 miles if possible). Phase 1: Establish baseline Glide to Pulse Ratio Initially we run a pulse and glide test with a target range of, say, 30 - 45 MPH. The battery should have a high initial state of charge (SOC) so that the ICE does not need to run to recharge the battery. All electrical equipment should be off (A/C, lights, etc) so as not to drain the battery and force the ICE on. Its important to do a true glide, not a glide assisted by EV mode or reduced by regen, so we put the car in neutral during the glide. We measure the average length of the pulse and glide phases. We may determine that the pulse lasts 20 seconds and the glide lasts 80 seconds (4 to 1 ratio of glide to pulse). Phase 2: P&G test with low battery SOC and electric equipment on Next, we run the experiment. The battery SOC should be low, very near the point where the engine will turn on to recharge it. The reason is, we want the engine to turn on during the experiment to recharge the battery as necessary. If we started with a full battery, the test would not determine the glide eMPG because we would be running the battery down for a portion of the test. Turn on any electrical equipment such as lights and A/C used in normal driving. These will be a primary cause of lower pulse eMPG, as will electrical draw from the drivetrain. We start the experiment by accelerating to 30 MPH, then resetting the trip distance and trip MPG computers. Then we start the pulse and glide routine: Pulse at 20 MPG to 45 MPH for however long it takes Glide down to 30 MPH in neutral for however long it takes If the engine stays on or turns on during the glide, ignore it. Keep the car in the glide until it gets to 30 MPH. Repeat for the length of the course until the last glide is complete, then note Total MPG and Total Distance Phase 3: Calculate glide eMPG Once the experiment has been run, we can calculate the glide eMPG. We know Total MPG and Total Distance from the car's computer. We know the Pulse MPG because we will held that steady at 20 MPG during the experiment. We know the Ratio from the initial test. We can plug these values into the formula above and calculate glide eMPG. Here's a sample calculation: Total Distance = 10 miles Total MPG = 50 MPG Ratio = 4 to 1 Pulse Distance = 10 / (4 + 1) = 2 miles Glide Distance = 10 - 2 = 8 miles Pulse MPG = 20 mpg Pulse Gal = 2 miles / 20 mpg = 0.1 gal Glide Gal = 10 miles / 50 mpg - 0.1 gal = 0.1 gal Glide eMPG = 8 miles / 0.1 gal = 80 mpg Phase 4: Use pulse eMPG to predict Total MPG in other situations Armed with glide eMPG for the given experimental conditions (eg A/C on or off, lights on or off, etc) we can predict Total MPG for varying pulse MPGs and glide to pulse ratios as follows: Glide Gal = Ratio/Glide eMPG Pulse Gal = 1/Pulse MPG Total MPG = (Ratio + 1)/(Glide Gal + Pulse Gal) For example, if your pulse MPG is 25 and glide to pulse ratio is 5: Glide Gal = 5/80 (80 is eMPG from above example) Pulse Gal = 1/25 Total MPG = (5 + 1)/(5/80 + 1/25) = 58.5 MPG Be gentle! These are initial thoughts. __________________ Visit my Ford Fusion Hybrid blog at http://ffh.squarespace.com Last edited by WPWoodJr; 07-19-2009 at 11:52 PM. Reason: clarification

07-15-2009, 07:59 PM	#2
donee Senior Member Join Date: Aug 2005 Location: Chicagoland Posts: 2,384 My Car: 2006 Prius Model: Package: #2 Thanks: 9 Thanked 84 Times in 75 Posts Friends: 0	Re: Determining glide eMPG Hey Bill, MPG stands for MILES PER GALLON, and ALL Electricity in a stock Prius or Fusion Hybrid is generated ONLY when the engine is runing. REPEAT AFTER ME - THERE IS NO FUEL CONSUMPTION WHEN THE ENGINE IS NOT TURNING .... I refuse to read more the than the first paragraph of your comments until you live by reality. And the reality here is REPEAT AFTER ME - THERE IS NO FUEL CONSUMPTION WHEN THE ENGINE IS NOT TURNING.... Now, if you want to figure the impact of varying electric loads, factor them AFTER you figure the P+G Mileage. Any computation that includes fuel consumption when the engine is off is WRONG.

07-15-2009, 08:08 PM	#3
ksstathead Senior Member Join Date: May 2007 Location: Kansas Posts: 578 My Car: 2010 Prius Model: III Package: Navigation Thanks: 330 Thanked 78 Times in 63 Posts Friends: 0	Re: Determining glide eMPG ... because you can force SOC to a no change result in testing the P&G benefit. Measure fuel consumed with P&G and no delta soc. Calculate average speed under that P&G. Measure steady state fuel consumed at that average speed over the same course/conditions. Difference is the savings, if any.

07-16-2009, 11:00 AM	#10
HTMLSpinnr Gen III Forums Moderator Join Date: Dec 2003 Location: Surprise, AZ (Phoenix) Posts: 4,516 My Car: 2010 Prius Model: IV Package: Solar Roof Thanks: 299 Thanked 549 Times in 258 Posts Friends: 18	Re: Determining glide eMPG I'm not sure I agree w/ the benefit of measuring battery "eMPG" in a Prius unless you're using a Plug-In Hybrid. All energy consumed in the Prius for motive power ultimately comes from gasoline, for which the consumption is fairly easily measured in real time, and average consumption measurable when refilling into a known volume of space. Running in EV only mode is only using previously stored energy, which will then be replenished through regeneration or charging from the ICE. Until you introduce a second source of power that isn't derived from gasoline, eMPG will remain inappropriate for measuring overall Prius efficiency, which is what most of us ultimately care about. __________________ Rick Johnson - Prius Expert/Gen III Forums Moderator His: Gen III 2010 Prius - Blizzard Pearl Prius IV w/ Solar - 15.6k miles - (First in SW US Region! 5/30/2009) - 2010 Photos - MPG Tracking - Mods: Curt Hitch, Toyota Premium Horn, Illuminated Door Sills - Happy 1 Year Birthday! Hers: Gen II 2004 - Tideland Green Pearl #9 - 119.8K miles - Mods: Bridgestone Ecopia Tires, Homemade mesh fog lamp grills Past: Gen I 2002 - Blue Moon Pearl - Traded at 55.1k miles - Mods: Coastal hitch Facebook - Ask an Expert I am not employed by Toyota, and the opinions expressed by me as a Prius Expert are not those of Toyota.

LinkBacks (?) LinkBack to this Thread: http://priuschat.com/forums/gen-iii-2010-prius-fuel-economy/65353-determining-glide-empg.html
Posted By	For	Type	Date
Blog - Ford Fusion Hybrid	This thread	Refback	07-25-2010 01:14 AM

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Glide over 41 mph. Possible?	NeoPrius	Gen II Prius Fuel Economy	13	09-27-2008 11:11 AM
Puls and glide above 41 mph?	lagreca	Gen II Prius Fuel Economy	2	07-17-2007 07:28 AM
Got the Glide!	vdubstress	Gen II Prius Fuel Economy	9	05-24-2007 04:18 PM
Neutral Vs Glide	darkecho	Gen II Prius Fuel Economy	26	09-15-2006 10:57 PM
Pulse and glide	Tadashi	Gen II Prius Fuel Economy	3	06-19-2006 09:17 PM