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Discussion Starter #1
BMW is marketing its soon to launch i8 and they are claiming:

The hybrid system supplements this with power from the electric motor to deliver typical BMW driving pleasure combined with groundbreaking efficiency. Calculated using the EU test cycle for plug-in hybrid vehicles, the average fuel efficiency of the BMW i8 at model launch will be less than 2.5 liters/100 km, which equates to approximately 95 miles per US gallon, with CO2 emissions of less than 59 grams per kilometer.
This sounds great doesn't it? 95 MPG from 231bhp? This is calculated using the EU test cycle for plug-in hybrid vehicles but just what is that cycle and what does it tell us? I have searched and searched and I can't find details of this test anywhere on the internet. It might be there somewhere but I can't find it. If you know of where I can find out just what this test entails then please post. I can find out about MPG testing for petrol and diesel cars and so I suppose it could be similar to that and this is where I start to have problems with MPG testing.

The petrol and diesel tests are over a ridiculously short test. The Urban cycle is over a test distance of about 2.5 miles and the Extra-Urban cycle is over 4.5 miles (see here - http://www.dft.gov.uk/vca//fcb/the-fuel-consumption-testing-scheme.asp). In petrol cars this is way too short a distance and we all know that the published MPG figures for cars hardly ever represents what the car does in real-world driving. But with a plug-in hybrid, that can do the test cycles on electrical power, petrol or a combination of both this kind of testing is pretty meaningless. The MPG figures that come out of the test will vary depending on the ratio battery to petrol used and so any MPG or CO2 figure you see from test of any plug in hybrid car will be totally useless as a guide to what could be expected in the real world.

In fact, the only figures that could possible mean anything at all are 1) the KWh/mile when on battery and 2) the MPG when running on petrol only. Any figures that try to give a figure that combines battery and petrol will be meaningless to any potential owner because that MPG figure will only apply to that specific ratio of battery/petrol.

I would like to see the testing changed for plug in vehicles to be more helpful to prospective owners. We should be able to have the two metrics I mentioned... 1) KWh per mile when on battery only and 2) MPG on petrol ONLY using the existing Urban/Extra Urban/Combined cycles. We can then apply that info to our own driving situation and then we can see what overall MPG we are likely to get.
 

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The best description of the EU test cycle that I know of was on the Ampera blog (http://www.opel-ampera.com/wp_en/2011/1 ... -weighted/). Unfortunately they discontinued the blog and while it is still there they've deleted some of the css so it is almost impossible to read. I'll quote the entire blog article here (and if they don't like that they can ask me to delete it). First the important numbers from an earlier blog post:
The Ampera achieves the following values under the standardized conditions of the NEDC:

Electrical consumption in battery mode (combined): 16.9 kWh/100km
Gasoline consumption in range-extender mode (combined): 5.0 l/100km
Electrical range (combined): 83 km
Now the explanation of how those numbers lead to the 1.2k/100km figure:
Combined and weighted

Dr. Christian Kunstmann - October 28, 2011
After the really important consumption figures were detailed in my last blog post, today I’ll explain how the combined and weighted fuel consumption of 1.2 l/100 km results from that.

The regulation UN ECE R101 applies to vehicles with electric propulsion, especially those that have a battery that can be re-charged over a socket. This does not distinguish between plug-in hybrid-electric vehicles (PHEV) and extended-range electric vehicles (E-REV).

This is also based on the NEDC, which is completed two times:

“Condition A” is the drive during which the battery may be discharged. So in this case, energy from the grid may be completely used up.
“Condition B” is the drive which must end with the battery charge level exactly as high or as low as it was at the beginning of the drive. In this case all energy comes from gasoline.
New European Driving Cycle
The testers measure how much electrical energy has to be recharged and how much gasoline refilled. These are the individual consumption figures I already mentioned. In addition, the range the car covers in the cycle until it can no longer maintain the necessary speed is determined in the NEDC. The result for the Ampera:

Electric range (in NEDC, combined): 83 km
The UN ECE R101 defines how one consumption figure is calculated from the individual consumptions figures. This is supposed to guarantee the comparability of various propulsion concepts, whereby the gasoline consumption figures are not simply averaged, but rather weighted.

Two data points are used for the weighting factor…

Electric range in NEDC
A pre-determined mid-distance driving range of 25 km between two charging procedures
… in a special formula:
CW=RE/(RE+25km)*CA + 25km/(RE+25km)*CB
with:

CW = Consumption weighted (fuel)
RE = Range electric
CA = Consumption “Condition A” (fuel)
CB = Consumption “Condition B” (fuel)

You see that a vehicle with exactly 25 km range has weighting factors of 0.5 each. So in this case fuel consumption is simply calculated by averaging Condition A and Condition B. If a car like the Ampera manages more than 25 km electrically, the lower fuel consumption in Condition A is weighted higher than the consumption in Condition B.

Energy use and efficiency tips
In the fuel consumption (combined and weighted) thus calculated “combined” stands for the combination of urban and extra-urban driving and “weighted” for the mix between Condition A and Condition B. CO2 emissions are expected to be 2,320 grams per litre gasoline.

Results for the Ampera from this calculation are:

Fuel consumption (combined and weighted):
1.2 l/100km
CO2 emissions (combined and weighted):
27 g/km
Manufactures are obligated to always list these two figures. But Ampera customers would probably have difficulties making sense of this hard-to-understand mix. In addition, it’s not clear that the Ampera is designed for pure electric driving and so the combustion engine is not used in Condition A. And this is exactly the difference to a plug-in hybrid that can’t reach its full driving potential on pure electricity with its small electric motor.

The emissions value is, however, important for car tax classification. Cars with less than 50 g/km CO2 emissions are exempt from taxes in Germany for ten years, which obviously also applies to the Ampera.
 

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Paul Churchley said:
I would like to see the testing changed for plug in vehicles to be more helpful to prospective owners. We should be able to have the two metrics I mentioned... 1) KWh per mile when on battery only and 2) MPG on petrol ONLY using the existing Urban/Extra Urban/Combined cycles. We can then apply that info to our own driving situation and then we can see what overall MPG we are likely to get.
How are you going the handle hybrids such as the PiP here? They don't really have a battery-only mode, so any driving even when the battery is charged is likely to result in petrol use. That's what the official test tries to handle by measuring two separate fuel consumption figures and calculating a weighted average.

The official figures are pretty meaningless as shorter journeys will be better and long journeys worse than the weighted average figure, but they do at least give you something to compare just as long as you know what you are comparing (which almost nobody does).
 

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Discussion Starter #5
I don't know much about the PiP. Are you saying that it never operates only on battery? I thought it did but for only 12-15 miles? If it does have a battery only mode then it is just the same as the Ampera... we need KWh/mile and MPG on petrol only. If it never operates on battery then I find it difficult to fathom how to measure its energy use and the slit between battery and petrol.

Am I missing something? MPG and m/100miles is the same... just different ways of saying the same thing isn't it? It might make it easier for people to compare fuel costs though because the variable is gallons and not miles... same metric though.

The whole concept of eMPG I find confusing and difficult to relate to my actual use.

For me this whole area is confusing me. The current metrics don't make sense for PH-EVs and yet the media is relying on them to publicise, for good or bad, the fuel consumption. It is a very muddy area IMO.
 

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I also thought the pip could be driven purely in battery mode for @15 miles before the ice cut in, will take a look what is on the web.

To split mpg and KWh/mile would certainly make it easier to compare the efficiency of each. However my personnel circumstances dictate my fuel costs over the year are taken into account to finance the car. So to get a definitive figure of 234mpg compared to say 50mpg in ice mode gives me a hefty wedge to offset against the lease and extras costs.

As you say the mpg v g/100m figures are the same, just a different way of calculating, however for some it is the perception and ease of visuisation that counts. For instance my son who lives in luxembourg just cannot get to grips with mpg, whereas I still work in old money.

What the answer is, who knows, the debate will continue until we are all running on electric only.

Gary
94.6mpg or 3.0L/100km
 

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According to wikipedia, the plug-in Prius has a battery only mode, but is limited to 62mph in that mode. The other two modes are ECO and Power and in those modes the petrol engine will engage automatically above 62mph (and I believe also if you accelerate too hard). The wikipedia article also says that the EPA 11 mile electric range is for a blended mode using both gas and battery.

This article http://electricvehicle.ieee.org/vehicle ... in-hybrid/ says:
Running in EV mode, the system switches to full hybrid mode under full acceleration, which splits engine power between the drive wheels and the generator, keeping the battery charged – just as in the standard Prius model.
I'm not sure from all that whether in EV mode you are actually limited to 62mph or whether it just means that it is EV only up to that speed and petrol at higher speeds. It certainly implies that unless you explicitly select EV mode it is going to use the engine before the battery is flat unless you keep both speed and acceleration down.

With mpg your electric car has a problem with division by zero. With g/100m or l/100km the scale is linear. Or to put it another way the difference between 30mpg and 40mpg is 2.4l/100km which is a worthwhile difference, but between 200mpg and 210mpg it is only 0.06l/100km which isn't worth bothering about, but people will talk about one car being 10mpg better than another as though the difference means anything without knowing the actual figures.
 

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Discussion Starter #8
taylorgx said:
However my personnel circumstances dictate my fuel costs over the year are taken into account to finance the car. So to get a definitive figure of 234mpg compared to say 50mpg in ice mode gives me a hefty wedge to offset against the lease and extras costs.
But that 234mpg is a completely false figure. At least with the 50mpg figure you know that if you never plug it in that is what you will get and from that you can easily work out what your particular mpg is likely to be based on your daily mileage.

I am not just trying to be difficult... I promise!!! I genuinely done see how that 234mpg figure helps anyone in any way. You can tell nothing from it and to prove it my lifetime is currently standing at 78mpg. How does that relate to the 234mpg figure or any other single combined figure they choose to use?
 

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Paul Churchley said:
I am not just trying to be difficult... I promise!!! I genuinely done see how that 234mpg figure helps anyone in any way. You can tell nothing from it and to prove it my lifetime is currently standing at 78mpg. How does that relate to the 234mpg figure or any other single combined figure they choose to use?
By your own admission, you use your Leaf for shorter journeys and the Ampera mostly for longer journeys. I would say that was very atypical use for an Ampera. My Volt currently thinks its lifetime mileage is 206mpg, it would be more if they hadn't had to replace the battery, but even so that is pretty close to the official figure.

I do agree with you that the 234mpg figure is pretty useless, but I don't agree when you say it is a completely false figure. It is an accurate figure for a particular pattern of use. That pattern of use seems to match mine reasonably closely but it doesn't match yours at all.
 

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Discussion Starter #10
Duncan said:
Paul Churchley said:
I do agree with you that the 234mpg figure is pretty useless, but I don't agree when you say it is a completely false figure. It is an accurate figure for a particular pattern of use. That pattern of use seems to match mine reasonably closely but it doesn't match yours at all.
That is a fair point.

However, I wonder just how typical I am vs how typical you are? I honestly don't know if most owners in the UK are closer to my use pattern than yours. You are correct in that I do use the Leaf for most of our day to day, short, trips and that is why my lifetime mpg is so low and that is why I have this view of the 234mpg, or any combined mpg. It fits only one use pattern.
 

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Paul Churchley said:
Duncan said:
Paul Churchley said:
I do agree with you that the 234mpg figure is pretty useless, but I don't agree when you say it is a completely false figure. It is an accurate figure for a particular pattern of use. That pattern of use seems to match mine reasonably closely but it doesn't match yours at all.
That is a fair point.

However, I wonder just how typical I am vs how typical you are? I honestly don't know if most owners in the UK are closer to my use pattern than yours. You are correct in that I do use the Leaf for most of our day to day, short, trips and that is why my lifetime mpg is so low and that is why I have this view of the 234mpg, or any combined mpg. It fits only one use pattern.
What do you think would be the best way to quantify a use pattern? By that I mean if you were to ask someone some questions about their car use, how easy do you think it would be to come up with some meaningful numbers that you could crunch to produce an approximate guess of the fuel consumption for Volt/Ampera, i3, PiP and so on.

You can tell I'm thinking about wrapping that up in a web app, can't you? WARNING: Brain dump follows...

The sort of thing I'm thinking of is:
How far is your typical daily commute? How many days per week do you commute? How far do you typically drive on other days? How often do you make significantly longer trips?

Or possibly a simpler set of figures that could be used would be annual mileage, any regular patterns (such as typical daily commute and how many days do you commute) then assume the remaining miles are spread evenly across the remaining days.

So I think my typical daily commute is about 30 miles total, 4 days per week and the fifth day I get a lift. Assuming 10k over the last year that would mean an average of 80 miles each weekend, but that's wrong because it's bursty: there's a single trip to Edinburgh and back and a single trip to Cornwall and back to account for plus other odd longer trips. So I'm going to say my annual pattern is 10x100+ mile trips, 208x30 mile commute, 130x30 mile other trips. No that doesn't sound right because I do have not infrequent days when I do 50+. I know the mileage is roughly correct so I'll make it fewer days but more miles (as we do use my wife's car some of the time) and guess at 56 days of 50+ miles for the third figure.

I think that's probably getting into the right ballpark for calculating efficiency without being completely unmanageable: take 3 different ranges (in my case 30, 50, and 100 but they might be configurable), a guess at how many days per year each of those applies within the constraint that they should add up to the expected annual figure, then apply each of those to the official figures for kWh on electric only, range, and non-battery expired mpg. Of course I'm making a million assumptions (such as ignoring days with a long range but a recharge in the middle).

Output could then need to give you all the relevant figures: amount of electricity, petrol, costs for each, mpg, mpge, mpg c02 equiv. and so on.
 

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Discussion Starter #12
I have plugged your figures into a spreadsheet using sensible numbers for the Ampera and I calculate that given the trip profile you describe above your lifetime MPG should be 211mpg...

Given you said...
Duncan said:
Paul Churchley said:
My Volt currently thinks its lifetime mileage is 206mpg
I think that this possibly has legs!

See the spreadsheet on Google Drive here

This is just one sample. I would like to do the same for me and if anyone else would like me to do a spreadsheet for you then just post the journey profiles and I will use this algorithm to try to predict your lifetime MPG. Hopefully we can tune it to be more accurate and then it would be easy to build a web site to incorporate it.
 

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I have just been looking at the top ten green cars on the latest edition of the Green Car Website.
No 1. BMW i3. Fuel cost for 1200 miles. £153
Joint 2. Volt/Ampera. Fuel cost for 1200 miles. £306

It so happens that we put petrol in our car every 400 miles to compare savings with our last car which we filled up every 400 miles.
For the last 1200 miles in which we spent more money on fuel than usual it cost us £137.38
This was £94.32 on petrol and £43.06 on electric which includes all the electric we used in the house in the 20 hours at the lower rate.
I wonder what formula they use to get 306 miles ?
Geoff
 

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234 MPG since last July here :) (196 lifetime due to having to commute on petrol for much of the first two months).


I'd be a fair bit higher if Vauxhall's delivery driver didn't insist on driving it on petrol to and from the garage.


Regarding the PiP: I've driven it on a short test loop and in my opinion it is impossible to accelerate at any reasonable rate without the ICE coming on. There's a Youtube video where some guys experimented to try to keep it EV only and found you need to do 0-60 in 29 seconds.
 

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£306 for 1200 miles? Nonsense.

It's under £50 to fill it with petrol, which will take it 310 miles. So 1200 miles is under 4 tanks or £200. Even on petrol only, it is impossible to be £306.
 

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I just checked checked the website. I may just have got the facts a bit wrong. Now I read it again it says £306 for 12000 miles! This now seems a bit ambitious to me, it may be possible but one would be lucky to achieve this especially in the winter months.
Sorry to post a misleading entry
Geoff
 

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Discussion Starter #18
Well, £306 at say, £1.35/ltr is 226 ltrs.

12,000 miles using 226 ltrs is 53 mpg and that would be on petrol only so I would expect it better than that if it was plugged in at all.

Looks reasonable to me for petrol only use!
 

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Discussion Starter #20
This is why I post my calcs... whoops! Beginner mistake!

So, 241mpg...

So, correct me again please if I am wrong but that is about 50 gallons or 2750 miles at my normal petrol only mpg of about 55. The remaining 9250 miles on battery. Your petrol mpg may vary slightly depending on driving style, speed etc but that should be OK for this purpose IMO.

Possible for sure but how many people will have that exact mix?
 
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